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Litter pool sizes, decomposition, and nitrogen dynamics in Spartina alterniflora-invaded and native coastal marshlands of the Yangtze Estuary
Abstract
Past studies have focused primarily on the effects of invasive plants on litter decomposition at soil surfaces. In natural ecosystems, however, considerable amounts of litter may be at aerial and belowground positions. This study was designed to examine the effects of Spartina alterniflora invasion on the pool sizes and decomposition of aerial, surficial, and belowground litter in coastal marshlands, the Yangtze Estuary, which were originally occupied by two native species, Scirpus mariqueter and Phragmites australis. We collected aerial and surficial litter of the three species once a month and belowground litter once every 2 months. We used the litterbag method to quantify litter decomposition at the aerial, surficial and belowground positions for the three species. Yearly averaged litter mass in the Spartina stands was 1.99 kg m(-2); this was 250 and 22.8% higher than that in the Scirpus (0.57 kg m(-2)) and Phragmites (1.62 kg m(-2)) stands, respectively. The litter in the Spartina stands was primarily distributed in the air (45%) and belowground (48%), while Scirpus and Phragmites litter was mainly allocated to belowground positions (85 and 59%, respectively). The averaged decomposition rates of aerial, surficial, and belowground litter were 0.82, 1.83, and 1.27 year(-1) for Spartina, respectively; these were 52, 62 and 69% of those for Scirpus litter at corresponding positions and 158, 144 and 78% of those for Phragmites litter, respectively. The differences in decomposition rates between Spartina and the two native species were largely due to differences in litter quality among the three species, particularly for the belowground litter. The absolute amount of nitrogen increased during the decomposition of Spartina stem, sheath and root litter, while the amount of nitrogen in Scirpus and Phragmites litter declined during decomposition for all tissue types. Our results suggest that Spartina invasion altered the carbon and nitrogen cycling in the coastal marshlands of China.
... Previous studies suggest that S. alterniflora has outcompeted many native plants, including Phragmites australis, and greatly threatened the biodiversity of Chinese salt marshes (Ge et al., 2020). Previous studies have found that S. alterniflora has a higher aboveground biomass, larger total litter pool, lower lignin content, higher C/N ratio, and faster litter decomposition rate than the native species P. australis (Liao et al., 2008); thus, it will consequently alter the quantity and quality of litter inputs to the soil and affect belowground biogeochemical processes (Liao et al., 2008). Although the effects of litter input from S. alterniflora on many biogeochemical processes have been well documented (e.g., Wang et al., 2013;Gong et al., 2020;Farrer and Goldberg, 2009), research on the impacts of the litter from invasive S. alterniflora on nitrogen removal processes in salt marshes is scarce. ...
... Previous studies suggest that S. alterniflora has outcompeted many native plants, including Phragmites australis, and greatly threatened the biodiversity of Chinese salt marshes (Ge et al., 2020). Previous studies have found that S. alterniflora has a higher aboveground biomass, larger total litter pool, lower lignin content, higher C/N ratio, and faster litter decomposition rate than the native species P. australis (Liao et al., 2008); thus, it will consequently alter the quantity and quality of litter inputs to the soil and affect belowground biogeochemical processes (Liao et al., 2008). Although the effects of litter input from S. alterniflora on many biogeochemical processes have been well documented (e.g., Wang et al., 2013;Gong et al., 2020;Farrer and Goldberg, 2009), research on the impacts of the litter from invasive S. alterniflora on nitrogen removal processes in salt marshes is scarce. ...
... S. alterniflora was introduced into this marshland in 2001. Since then, this alien species has expanded rapidly, replacing native species and posing a great threat to the coastal aquaculture and native ecosystem (Chen et al., 2004;Liao et al., 2008). ...
Aboveground litter inputs from plants are among the most important pathways for carbon and nutrient fluxes to the soil. Previous studies on the effects of aboveground litter from invasive plants on ecosystem processes have primarily focused on biogeochemical cycling processes such as C and N mineralization, whereas the effects of aboveground litter from invasive plants on nitrogen removal processes are not well understood. In this study, the effects of the aboveground litter of native Phragmites australis and exotic Spartina alterniflora on soil nitrification and denitrification were compared. Results showed that the removal of the aboveground litter of both species had no effect on nitrification or denitrification in the early growth phase. However, after aboveground litter removal in the late growth phase, nitrification and denitrification in the P. australis stands decreased by 41.18% and 25.11%, respectively, whereas no such changes were observed in the S. alterniflora stands. These results indicate that the impacts of aboveground litter on nitrification and denitrification are species-specific. The aboveground litter from indigenous P. australis affected the SOC content and then indirectly affected nitrification or denitrification, and these effects were clearer in the late growth phase. Although other studies have reported that the invasive S. alterniflora have strong impacts on nitrogen removal processes, our study showed that the aboveground litter from S. alterniflora did not alter nitrification or denitrification, which indicates that other pathways may play important roles in nitrogen removal processes than its aboveground litter does.
... Spartina alterniflora, native to America, has successfully invaded the saltmarshes along the east coast of China and replaced native plant species such as Pragmites australis and Scirpus mariqueter, and profoundly affect the invaded ecosystems (Li et al. 2009). The invasion of S. alterniflora enhances soil C and N stocks, which might have resulted from increased net primary production and litter input (Liao et al. 2008;Zhang et al. 2010). Aboveground litter of S. alterniflora often decomposes faster than that of native P. australis (Liao et al. 2008) and induces changes in the composition of microbial communities and their functioning (Yan et al. 2018). ...
... The invasion of S. alterniflora enhances soil C and N stocks, which might have resulted from increased net primary production and litter input (Liao et al. 2008;Zhang et al. 2010). Aboveground litter of S. alterniflora often decomposes faster than that of native P. australis (Liao et al. 2008) and induces changes in the composition of microbial communities and their functioning (Yan et al. 2018). However, C transformation and partitioning from litter of S. alterniflora to soil are still unclear, and thus, the contribution of above-and belowground litter of S. alterniflora to the soil C stock of invaded ecosystems remains unknown. ...
... Through conducting a 153-day laboratory incubation experiment, we aimed at determining short-and medium-term effects of litter type (leaf and root litter) and soil source (soil derived from invasive S. alterniflora and native P. australis communities) on soil respiration and litter-derived C partitioning into different fractions including microbial biomass C (MBC), DOC and SOC. Leaf litter of S. alterniflora with a higher N concentration decomposed faster than its root litter (Liao et al. 2008), and leaf litter decomposed more rapidly in the home soil than in the away soil (Ayres et al. 2009). Therefore, we hypothesized that (i) leaf litter of S. alterniflora has a more pronounced litter-derived C transformation into CO 2 , MBC, and DOC but lower contribution to SOC than its root litter, and (ii) both leaf and root litter of S. alterniflora decompose faster in its own soil than in soil beneath native plant species. ...
Litter types and soil properties can affect litter decomposition rate and litter carbon (C) transformation, consequently regulating soil C cycling. Yet, litter C transformations of leaves and roots of invasive plants are poorly understood, which limits our understanding of the role of plant residues in soil C sequestration during plant invasion. In a laboratory incubation experiment lasting for 153 days, we used two types of soil which were collected from invasive S. alterniflora and native Phragmites australis marshlands, and traced the transformation of ¹³C from leaf and root litter of invasive Spartina alterniflora into CO2, soil-dissolved organic C (DOC), microbial biomass C (MBC), and soil organic C (SOC). The leaf litter of S. alterniflora decomposed faster than root litter, resulting in higher soil respiration and higher transformation of litter-derived ¹³C into CO2, MBC, and DOC. Although the root litter of S. alterniflora decomposed slowly, SOC comprised up to 24% of litter-derived ¹³C. Furthermore, the litter C transformations of S. alterniflora showed a positive home-field advantage effect. Soil respiration, MBC, fractions of litter-derived ¹³C in Gram-negative bacteria, ¹³C recovered in CO2, DOC, and SOC were higher in the soil colonized by S. alterniflora than in the soil colonized by P. australis, with the home-field advantage effect being more pronounced in root than leaf litter treatments. Therefore, litter type and soil source had differential impacts on litter C transformation patterns of S. alterniflora and the root litter of invasive S. alterniflora played an important role in SOC formation and C sequestration in soils from its invaded ecosystems.
... In natural ecosystems, there have been several studies regarding litter decomposition in the air, that found substantial changes in litter mass and nutrient concentrations (e.g. Newell et al. 1989;Liao et al. 2008;Zhang et al. 2014a), and the dew condensation at night is considered as a key factor for the aerial decomposition ( Kuehn et al. 2004;Zhang et al. 2014b). In these previous studies, the litter position was found to significantly influence decomposition rates and nutrient release in the early stage, and such influences varied with litter quality ( Liao et al. 2008;Zhang et al. 2015). ...
... Newell et al. 1989;Liao et al. 2008;Zhang et al. 2014a), and the dew condensation at night is considered as a key factor for the aerial decomposition ( Kuehn et al. 2004;Zhang et al. 2014b). In these previous studies, the litter position was found to significantly influence decomposition rates and nutrient release in the early stage, and such influences varied with litter quality ( Liao et al. 2008;Zhang et al. 2015). Considering the distinct environmental conditions among positions in agroecosystems, therefore, it is essential to fully consider the effects of position on the decomposition of litter tissues with different quality, which would contribute to a comprehensive understanding of energy transfer and nutrient turnover and benefit ecosystem management. ...
... According to the results from previous studies in other ecosystems (e.g. Liao et al. 2008;Zhang et al. 2015), we hypothesized that (1) the litter position would significantly affect decomposition in agroecosystems, and buried litters would decompose at a higher rate due to more appropriate environmental conditions; (2) aerial decomposition would also play an important role in litter decomposition; and (3) there would be interactive effects between the litter quality and position on decomposition rates. ...
Litter decomposition plays a crucial role in controlling carbon (C) cycling and nutrient turnover in agroecosystems. In this study, the litterbag method was used to investigate the mass loss and nitrogen (N) dynamics of maize litters (culms, leaves and sheaths) at aerial, surficial and belowground positions in the initial 191 d of decomposition. For any tissue, the decomposition rates in the air and on the soil surface were similar, but both were less than the decomposition rates below the ground. The sheaths always decomposed at a lower rate than the other two tissues at any position. During decomposition, the N concentrations for all tissues decreased at both the aerial and the surficial positions but increased for belowground leaves and sheaths in the last months. For the N amount, these three tissues generally exhibited a net N release during the experiment irrespective of the position. Overall, position plays a crucial role in controlling early-stage litter decomposition in croplands, and this role will be modified by litter quality. Therefore, further studies on litter decomposition should fully consider the litter position to comprehensively evaluate the biogeochemical cycles in agroecosystems.
... Under natural conditions, the decomposition of standing dead is divided into three processes, namely, leaching and biotic and abiotic processes, and the initial state of decomposition of dead sediment is standing dead, with in situ decomposition being the most critical decomposition stage. Liao et al. [42] evaluated the decomposition rates of three forms of litter-standing dead, surface, and soil in the Yangtze estuary wetlands. They discovered that the standing dead litter had the slowest decomposition rate among the three categories of litter decomposition. ...
... [5,43,44] have demonstrated that plant litter initially releases a large amount of unstable C, which promotes the accumulation of soil organic carbon and accelerates the mineralization of native soil organic carbon faster. Next, glycolysis occurs to integrate soluble low molecular organic acids, and the final step is condensation to stabilize H 2 S or decomposition to produce CO 2 [42]. The degradation of plant litter is generally divided into three processes: first, the leaching of soluble material through precipitation and immersion, followed by microbial decomposition and nibbling by animals in the soil; second, microbial decomposition of structural components such as cellulose, lignin, and hemicellulose; and finally, abiotic processes such as wet and dry alternation, icing, weathering, and thawing. ...
Wetland is a transitional area where terrestrial ecosystems and aquatic ecosystems interact and influence each other, and it is an important ecosystem on the Earth’s surface. Due to the special characteristics of wetland ecology, the decomposition of wetland plant litter is slightly different from litter in forests, grasslands, and meadows and other traditional areas. The role of litter mineralization in the wetland ecological C cycle and the functional role of plant litter have been neglected. This study analyzes the decomposition mechanism and decomposition model of wetland litter material and focuses on the effects of the decomposition process of wetland litter material on the structure of the soil fauna community, decomposition of soil organic matter, sediment properties, and the dynamic changes in the C cycle of the biological system by combining domestic and international studies from recent years. Finally, we propose that the direction of future research on wetland litter decomposition should be to reveal the mechanism of wetland biodiversity and ecology, as well as the ecological correlation between aboveground and belowground biodiversity, with a view to providing a decision-making basis for wetland phytoremediation and wetland wastewater treatment.
... Correlation coefficients between litter mass remaining and decomposition rates and environmental factors [30] ，可能由于 pH 较高的地方能够分解盐地碱蓬凋落物的微生物类型和数量较多， 但有关分解盐地碱蓬和互花米草凋落物的微生物类型研究很少。在凋落物质量因子中， C/N 和木质素含量被认为是最重要的指标 [31] ，直接影响到凋落物分解状况。很多研究发 现凋落物的 C/N 或 C/P 比越低，分解速率越快 [14,32,33] ...
... Liao 等 [32] 仝川等 [14] Xie 等 [28] Sun 等 [17] 本研究 注：本研究中 ...
... In this wetland, standing litter decomposition was inhibited by shrub expansion after 360 days of decomposition. Litter decomposition in the air is often coregulated by biotic (microbial) and abiotic (physical, photochemical, and thermal) degradation in wetlands (Evans et al. 2019;Liao et al. 2008;Kuehn et al. 2011). Compared with open wetlands, the longer daily high RH period and the higher RH in the morning under shrub canopy would promote microbial degradation of standing litter (Kuehn et al. 2004;Zhang et al. 2014a). ...
... In wetlands, standing litter decomposition is predominantly regulated by abiotic degradation such as photodegradation, leaching, and physical fragmentation (Evans et al. 2019;Liao et al. 2008;Kuehn et al. 2011), whereas litter decomposition on the soil surface is controlled by substrate quality and soil microbial composition (Moorhead and Sinsabaugh 2006;Schneider et al. 2012). In this wetland, shrub expansion reduced soil fungal and total PLFAs because of the increased inputs of plant litter with higher concentrations of secondary compounds such as lignin, total polyphenol, and condensed tannins concentrations (Table 1). ...
Background and aimsGraminoid-dominated wetlands have been subjected to widespread shrub encroachment, yet the effect of this shift in species composition on litter decomposition remains unclear, especially in the standing-dead stage.Methods
We collected labile (Deyeuxia angustifolia) and recalcitrant (Carex schmidtii) graminoid leaf litter from a freshwater wetland in northeast China, and used the litterbag method to characterize litter ash-free dry mass (AFDM) loss and nitrogen (N) release in the air and on the soil surface in open wetlands and two shrub islands (Salix floderusii producing phenol-poor litter and Betula fruticosa producing phenol-rich litter) over 360 days of decomposition.ResultsLitter decomposition in the air and on the soil surface responded differentially to shrub expansion. In the air, AFDM loss and N release for both labile and recalcitrant litter were often lower in shrub islands than in open wetlands. On the soil surface, labile litter decomposition was decelerated in the presence of shrubs with phenol-rich litter, but accelerated in the presence of shrubs with phenol-poor litter. Despite the absence of significant differences in N release, recalcitrant litter AFDM loss was lower in shrub islands than in open wetlands after 360 days of decomposition on the soil surface.Conclusions
Shrub encroachment retards litter decomposition in the air, but the effects on litter decomposition on the soil surface vary with shrub type and litter degradability in graminoid-dominated wetlands. Moreover, these findings emphasize that standing litter decomposition should be considered to enhance our understanding of shrub encroachment effects on litter decomposition in these wetlands.
... Coastal wetlands in China have the world's largest invasion area of S. alterniflora (Liu et al., 2017b), which has now spread to over 340 km 2 (Li et al., 2009). Previous studies noted that SOC accumulation accelerated after the introduction of S. alterniflora (Cheng et al., 2006;Liao et al., 2008b;Yang et al., 2017). In our study, the C accumulation rates in the top 20 cm and in the top 40 cm soils after the invasion were lower than those in the top 30 cm soils of a Sheyang salt marsh (0.18-0.24 kg C m −2 yr −1 , Xiang et al., 2015), but higher than those in the top 100 cm soils of Yancheng salt marshes (0.313 kg C m −2 yr −1 , Yuan et al., 2015). ...
... Soil pH directly decreased with S. alterniflora invasion time in each soil profile, as shown by SEM (Fig. 6). With SOC accumulation, soil pH declined correspondingly in this alkaline environment, which in turn facilitated the decomposition of plant litter of invasive species into soil organic matter and the growth of soil microorganisms (Liao et al., 2008b;Grosso et al., 2008;Liang et al., 2017). This explained the indirect pathway by which invasion time influenced SOC storage in the top 40 cm soils (Fig. 6a). ...
... sea-level rise, global warming, eutrophication; see Kirwan and Mudd, 2012;Mueller et al., 2018). Although there is research exploring the range of litter decomposition in different salt marsh species (White et al., 1978;Liao et al., 2008;Montemayor et al., 2011), less is known about the possible effects of the loss (or replacement) of dominant foundation species on salt marsh litter decomposition (but see Liao et al., 2008). Our work suggests that salt marsh litter decomposition and potential C sequestration is strongly linked to the dominant plant species, not only as a consequence of the produced litter quality but also as a consequence of its standing vegetation structure. ...
... sea-level rise, global warming, eutrophication; see Kirwan and Mudd, 2012;Mueller et al., 2018). Although there is research exploring the range of litter decomposition in different salt marsh species (White et al., 1978;Liao et al., 2008;Montemayor et al., 2011), less is known about the possible effects of the loss (or replacement) of dominant foundation species on salt marsh litter decomposition (but see Liao et al., 2008). Our work suggests that salt marsh litter decomposition and potential C sequestration is strongly linked to the dominant plant species, not only as a consequence of the produced litter quality but also as a consequence of its standing vegetation structure. ...
‘Questions’: Due to their efficiency sequestering and storing atmospheric CO2, coastal vegetated systems are known to play a fundamental role in climate change mitigation. While most of the work evaluating carbon sequestration capacity has focused on global change factors that can affect carbon release from plant litter decomposition through changes in (large-scale) environmental conditions, less is known about the possible effects of the loss (or replacement) of dominant species. We hypothesized that dominant marsh plants can influence decomposition not only through changes in liter quality but also through changes in (micro-scale) soil environmental conditions such as humidity, soil temperature or solar radiation.
‘Location’: We performed a field manipulative experiment in a South Western (SW) Atlantic salt marsh, in Argentina
‘Methods’: We simulate a selective disturbance (i.e. removal of the dominant grass species Spartina densiflora) thus allowing removal plots to develop an alternative plant community. To evaluate the effect of the dominant grass species on litter decomposition, we performed a litterbag approach experiment three years after the establishment of the removal plots.
‘Results’: Results showed that the presence of S. densiflora significantly decreased litter decomposition directly by producing less labile litter, but also by effects that seem to be related to its structure as standing dominant vegetation. The experimental removal of S. densiflora led to an alternative plant community, formed by otherwise subordinate species, which is less densely packed, allowing higher soil radiation incidence and elevated midday soil temperature.
‘Conclusions’: Our results suggest that salt marsh litter decomposition, and thus C sequestration, is determined in part by the identity of the dominant plant, not only because of the quality of produced litter but also as a consequence of the structure of vegetation. Changes in species diversity, above all the dominant species in these coastal systems, could have large impacts on carbon turnover and mitigation capacity of these ecosystems.
... These factors suggest that the changes in plant communities induced by invasive species must be considered in understanding the lateral flows of materials and energy in addition to the different nutrient cycling processes between native and exotic species (Rout & Callaway, 2009;Karstens et al., 2016). For example, extensive literature on plant invasion indicates that ecosystem total C (TC) and total N (TN) pools are high in salt marshes (Ehrenfeld, 2003;Liao et al., 2008a), partially due to the intense sedimentary processes (Gao et al., 2018). ...
... An additional question is the specific underlying mechanisms for the joint forcing between tidal activities and invasions (Li et al., 2014). Several previous studies also indicated that the capture and retention of sediment by marshes can significantly enhance ecosystem N pools (Liao et al., 2008a;Rout & Callaway, 2009;Huang et al., 2016). It appears that sediment deposition may be responsible for high NPP; the high biomass of invasive plants may be due to the high litter quality (high N contents and low C:N ratios in litter) associated with lateral relocation among interconnected terrestrial/aquatic ecosystems at water-land interfaces (Ehrenfeld, 2003;Liao et al., 2008b;Yang et al., 2016a). ...
Lateral movements of materials and energy in coastal wetlands, due mainly to tidal activities, have been recognized as key processes in understanding the biogeochemical cycles of ecosystems. However, our understanding of the roles of lateral movement in shaping ecosystem functions remains limited. Here we quantified the effects of lateral sediment transport on total carbon (C, inorganic + organic) and nitrogen (N) pools in plants and soils in two dominant wetland types: invasive Spartina alterniflora (Spartina) marshes and native Phragmites australis (Phragmites) marshes in coastal Shanghai of the Yangtze Estuary. We found that the accreted sediments across the water‐marsh gradients caused by lateral movement resulted in contrasting C and N contents between the two communities. The sediment load and C and N pools in the plants and soils of the Spartina marshes were significantly higher than those in the adjacent Phragmites marshes. The shifts in species composition and community structure not only altered the C and N balance but also enhanced the ecosystem net primary productivity. Our findings highlight the importance of lateral transport in altering ecosystem structure after Spartina invasion. The ecosystem C and N pools were significantly higher in the invaded ecosystems than in the native community. Our study also reveals that the plant density and structures can alter tidal hydrodynamics and the lateral transportations of sediments, which in turn influence ecosystem C and N cycle. The C accumulation processes of the native and invaded marshes were further complicated by the contrasting productivities of the ecosystems.
... The nutrients (such as carbon and nitrogen) increased following the vegetation development from the bare tidal mud flats, and the biogeochemical circulation was altered (Liao et al., 2008;Gao et al., 2019). In our study, there was a significant increase of TOC and TN along the gradient of smooth cordgrass invasion stages (Table 2). ...
... The smooth cordgrass invaded habitats transmitted more root-derived 13 C to the Gram-negative bacterial phospholipid fatty acids than the native habitats with P. australis, such as the Chongming Dongtan salt marsh (Zhang et al., 2017b). The primary productivity was high enough to provide energy for nitrogen-fixing microorganisms, and the invasion significantly enhanced the abundance and diversity of sulfate-reducing bacteria in wetland soils; thus, the food webs of the coastal wetland were changed fundamentally (Liao et al., 2007(Liao et al., , 2008Cui et al., 2017). Not only the microorganism community changed after smooth cordgrass invasion, but also the macrofaunal community needed to adapt to the changes in the environmental properties, such as food resources (Peng et al., 2017). ...
The exotic species smooth cordgrass (Spartina alterniflora) is recognized as an important invasive species in China, introduced about 40 years ago. The consistent smooth cordgrass invasion significantly modified the coastal ecosystem. Understanding the ecological succession and mechanisms of wetland soil ecosystems is essential for biological conservation after the landscape change resulting from the smooth cordgrass invasion. In this study, five different invasion stages of a 16-year smooth cordgrass invasion sequence were identified in a coastal wetland as no invasion, initial invasion, young invasion, mature invasion, and senescing invasion. The succession of macrofaunal communities and environments were investigated along the gradient of invasion stages. The infauna decreased, and the epifauna increased along the invasion sequence. The significant differences of the communities were detected among the mud flats experiencing different invasion stages. The initial and young invasion stages of smooth cordgrass possibly promote the macrofaunal biodiversity, but biodiversity decreased at mature and senescing invasion stages. The ecological effect of smooth cordgrass invasion on macrofauna depended on the species’ traits and the invasion stage. The environmental properties co-varied with invasion stages, and varied significantly among selected habitats. Total organic carbon (TOC), total nitrogen, and the carbon-nitrogen ratio (C/N) strongly related to the smooth cordgrass coverage, stem density, and height. C/N was identified as the key factor for shaping the environment by principal components analysis, and TOC for regulating the macrofaunal community by canonical correspondence analysis. The succession of macrofaunal communities should be considered as a comprehensive response to the variations on environmental properties co-varying with smooth cordgrass invasion in coastal wetlands.
... In our study, lower soil pH, although not significantly, was observed in S. alterniflora stands (Table 1), perhaps due to the greater exudation of organic acids from root systems (Hines 1994;Bu et al. 2015). Besides, relatively high biomass of S. alterniflora can affect root exudates, the source of substrate N, and provide more NO 2 − easily utilized by nitrifier denitrification related microbes (Liao et al. 2008). Stepwise regression analysis also suggests that the variations of C nitrifier denitrification were explained more by soil WFPS and bulk density (Table S2). ...
... Notably, C nitrifier denitrification was also higher at native P. australis stands in summer (Fig. 4). To our knowledge, both P. australis and S. alterniflora have higher biomass and primary productivity, with different seasonal dynamics (Liao et al. 2008). For example, Tong et al. (2011) showed that the belowground biomass in P. australis stands was greater than that of S. alterniflora in summer, but not in winter. ...
Aims
Exotic Spartina alterniflora invasion resulting from anthropogenic activities significantly affects microbial nitrogen (N) transformation and associated nitrous oxide (N2O) emission in coastal wetland soils. However, the responses of soil N2O emission dynamics to plant invasion remain unclear. This study assesses the effects of S. alterniflora invasion on soil N2O potential production and consumption processes.
Methods
We used natural isotope tracing technique to investigate potential N2O production and consumption rates in S. alterniflora invaded and native saltmarsh zones (Phragmites australis, Scirpus mariqueter and bare mudflat) in the Yangtze Estuary.
Results
Soil potential net N2O production rates in summer were lower in S. alterniflora stands than in S. mariqueter and bare mudflat stands, but no significant differences among these saltmarsh habitats occurred during winter. Potential gross N2O production and consumption rates were higher in S. alterniflora and P. australis stands compared to S. mariqueter and bare mudflat stands. The gross consumption proportion in S. alterniflora and P. australis stands was higher, which affected net N2O production. Hydroxylamine (NH2OH) oxidation and nitrifier denitrification contributed 4.52–12.62% and 13.87–21.58% of soil N2O source, respectively, but denitrification was the dominant pathway (69.83–80.09%). S. alterniflora invasion increased the contributions of NH2OH oxidation and nitrifier denitrification to N2O source slightly, but decreased the contribution of denitrification to N2O source. Soil potential N2O production and consumption processes were influenced by water-filled pore space, pH, sulfide, and carbon and N substrates.
Conclusion
Exotic S. alterniflora invasion affected soil N2O dynamics by increasing substrates and altering microenvironments, thus mediating N2O emission from coastal saltmarsh soils.
... Previous studies regarding the decay mostly focused on the decomposition of residues at the soil surface (Gliksman et al. 2018). Nevertheless, in actual ecosystems, a fraction of residues decomposed without soil contact because they were intercepted by understory vegetation during falling or some plant species stood during decay (Romero et al. 2005;Liao et al. 2008;Wang et al. 2017). Specifically, the forest ecosystems in South China often suffer from disturbance of typhoons, which could uproot, snap, break, or defoliate plants (Tong and Yang 2007). ...
... Even in the group of the residue without soil contact, a majority of decomposition still took place (40%~50% C loss for without soil vs. 50%~60% C loss for with soil) (Fig. 1). Previous field experiments recorded consistent results (Potthoff et al. 2005;Romero et al. 2005;Liao et al. 2008), indicating that indigenous microbes already present in residue exerted fundamental roles for decay. Soil contact further enhanced decay, implying the contributions of decomposers or nutrients derived from soil. ...
External nitrogen (N) supply has been testified to exert important impacts on plant residue decomposition. The influence of N may be interactive with soil contact in terrestrial ecosystems. However, the joint mechanisms of decomposition of plant residues driven by soil contact and N addition remain incomplete. Using contrasting residues, including needles of Chinese fir (Cuninghamia lanceolata) (Cl) (relatively hard to degrade) vs. leaves of eucalyptus (Eucalyptus urophylla) (Eu) (relatively easy to degrade), a full factorial experiment was conducted by 360-day experiment to investigate the combined effect of N addition and soil contact on residue decay. As the microbe-manipulated decomposition could leave an imprint on the residue carbon (C) and N stable isotope, variations of the two signatures (δ¹³C and δ¹⁵N) were synchronously monitored. Our results firstly showed that added N sped up initial decomposition, while it played an opposite role in subsequent stage, and soil contact always stimulated decay. Under soil contact condition, we found a markedly more accelerating effect of N addition on decay of Cl than without soil contact. Linking with residue N dynamics, we thought that although N immobilized from soil could not completely meet microbial needs for decay of Cl, this N limitation was just relieved by added N, leading to this synergistic effect. At late decay stage, the N inhibiting influence was partly offset under soil contact condition, and this phenomenon was more dramatic for Eu. Our results lastly revealed that the ¹³C and ¹⁵N signatures mirrored and explained the underlying mechanisms of the above interactions. Overall, we concluded that external N and soil contact could interactively affect decay, depending on plant residue decomposability. These results would be used to accurately predict C sequestration for terrestrial ecosystems under heightened N scenario in the future.
... In addition, the differential metal distribution in live and senescent aboveground tissues and the biomass production of each type of tissues are critical factors that control the amount and rate of release of metals (Caçador et al., 2009;Duarte et al., 2010;Couto et al., 2013;Song and Sun, 2014;Chen et al., 2017). The release of metals back to the environment through decomposition varies with the plant species, the chemical composition of the tissues and environmental factors (Liao et al., 2008;Negrin et al., 2012a;Song and Sun, 2014;Chen et al., 2017). Moreover, detritus produced from senescent aboveground tissues is the base for most marine food webs, resulting in the introduction of metals to higher trophic levels (Moore et al., 2004;Weis and Weis, 2004;Bergamino and Richoux, 2015). ...
... The contents of OM and fine sediments were lower at site 2, where the concentrations of all analyzed metals were also lower. The lower content of OM at site 2 might be associated with the lower biomass of S. alterniflora at this site (see Fig. 6), since the contribution of this species to the soil carbon pool is significant (Liao et al., 2008;Wang et al., 2018). Sediments with a high content of OM and fine sediments are capable of binding metal ions (Kabata-Pendias, 2010), and several studies have shown that intertidal sediments with these characteristics certainly have higher levels of metals (e.g., Reboreda et al., 2008;Wang et al., 2013;Idaszkin et al., 2014Idaszkin et al., , 2017Serra et al., 2017;Simonetti et al., 2017). ...
Salt marshes are capable of reducing metal pollution in coastal waters, but this capacity is highly dependent on the metal, the physico-chemical characteristics of the sediment, the plant species, the production of biomass, the time of the year, etc. The aim of this study was to assess the uptake and accumulation of Pb, Ni, Cu and Zn in Spartina alterniflora from three salt marshes within the Bahía Blanca estuary (BBE), a human-impacted Argentinean system. Metal concentrations in sediments and plants showed the same order at all sites: Zn > Cu > Pb ≥ Ni. The site with lower organic matter and fine sediment content had lower metal concentrations in the sediments, but not a lower metal content in the plant tissues, meaning that the sediment characteristics influenced the metal concentrations in the sediment and their uptake by plants. Despite differences in sediment characteristics between sites, metals were always higher in the belowground tissues than in aboveground ones and, in general, higher in dead than in live tissues. Some metals were accumulated in plant tissues, but not others, and this is dependent on the metal and the sediment characteristics. Allocation patterns of metals in tissues of S. alterniflora were mainly dependent on metal concentrations, determining higher belowground pools, but the aboveground pools were important in some cases due to higher biomass. Partitioning of metals in above or belowground pools determines their fate within the estuarine system, since tissues can decompose in situ (belowground) or be exported (aboveground). Seasonal dynamics were important for some variables but were less noticeable than the differences between sites and tissues. Our results indicate that S. alterniflora from the BBE is efficient in accumulating some metals, despite usually low metal concentrations in sediments and plants. This accumulation capacity has implications for the whole system through the fate of the tissues.
... With changes in soil properties, halophyte succession also affected the activities of enzymes in the soil (Fig. 5). Earlier studies reported that Phragmites residues had a slower decomposition rate due to recalcitrant constituents as reported by Chimney and Pietro (2006) in macrophytes and lower N content (Liao et al., 2008). Contrary to the above observations, the present study observed that the Phragmites invasion chronosequence (10 yr) increased the enzymes (β-glucosidase, N-acetyl β-glucosaminidase, phosphatase and sulphatase) activities in surface soil layers especially. ...
Vegetations of coastal wetlands play a vital role in providing numerous ecological services to the coastal ecosystem and are also key factors affecting the biogeochemical processes in the soil. The acreage of Phragmites australis is expanding and gradually replacing the Suaeda japonica at Suncheon Bay, Republic of Korea. However, the changes in soil microbial community composition caused by Phragmites introduction is poorly characterized. In the present study, soil cores were collected to a depth of 1m (divided into 10 segments of 10 cm) from Suaeda-vegetated, the boundary of Suaeda- and Phragmites-vegetated, Phragmites-vegetated marshes (after 5 and 10 years of the introduction). These samples were analyzed for physico-chemical characteristics, enzyme activities, soil microbial community (phospholipid fatty acids profiling) and abundance of functional genes related to the methane cycle (mcrA, pmoA and dsrA). Results showed that introduction the of Phragmites increased the water content, pH and dissolved organic carbon content in the soil. The contents of total PLFA and microbial group biomarkers of PLFA were observed to be higher in the surface soil and decreased with soil depth. Early introduction of Phragmites (10 yr) sediments had the significantly highest concentrations of total as well as other microbial biomarkers PLFAs compared with other salt marshes. The relative abundance of biomarker PLFAs of Gram-negative bacteria was significantly higher in Phragmites compared to other salt marshes. The abundance of mcrA was significantly highest in Phragmites salt marshes (10 yr) and mixed salt marshes while the abundance of mcrA, pmoA and dsrA were lowest in Suaeda marshes. Our results reflect the chronological effects of the introduction of Phragmites on the physicochemical and microbial community composition which contribute to establishing the relationship between the plant introduction and soil biogeochemical processes in coastal salt marshes.
... In addition to the decomposition of soil organic matter, litter decomposition is another important topic. From an abiotic perspective, the impacts of litter position (Liao et al. 2008), habitat fragmentation (Rippel et al. 2020), salinity, and flooding (Zhai et al. 2020) on litter decomposition have been studied. For example, the decomposition process of belowground litter was more susceptible to oxygen limitation, and the decomposition rate slowed with flooding time; however, flooding could accelerate the breakdown of surficial litter (Yan et al. 2019). ...
Understanding carbon cycling in blue carbon ecosystems is key to sequestrating more carbon in these ecosystems to mitigate climate change. However, limited information is available on the basic characteristics of publications, research hotspots, research frontiers, and the evolution of topics related to carbon cycling in different blue carbon ecosystems. Here, we conducted bibliometric analysis on carbon cycling in salt marsh, mangrove, and seagrass ecosystems. The results showed that interest in this field has dramatically increased with time, particularly for mangroves. The USA has substantially contributed to the research on all ecosystems. Research hotspots for salt marshes were sedimentation process, carbon sequestration, carbon emissions, lateral carbon exchange, litter decomposition, plant carbon fixation, and carbon sources. In addition, biomass estimation by allometric equations was a hotspot for mangroves, and carbonate cycling and ocean acidification were hotspots for seagrasses. Topics involving energy flow, such as productivity, food webs, and decomposition, were the predominant areas a decade ago. Current research frontiers mainly concentrated on climate change and carbon sequestration for all ecosystems, while methane emission was a common frontier for mangroves and salt marshes. Ecosystem-specific research frontiers included mangrove encroachment for salt marshes, ocean acidification for seagrasses, and aboveground biomass estimation and restoration for mangroves. Future research should expand estimates of lateral carbon exchange and carbonate burial and strengthen the exploration of the impacts of climate change and restoration on blue carbon. Overall, this study provides the research status of carbon cycling in vegetated blue carbon ecosystems, which favors knowledge exchanges for future research.
... However, studies have found that 1 mm pore size litter bags did not significantly change the environmental status under natural conditions (Kuehn and Suberkropp, 1998), but the decomposition rate obtained from the study of litter bags may be higher than the actual decomposition rate (Chimney and Pietro, 2006). In the traditional research method, the no-litter bag method, in which litter contact with soil increases the C, N, and P contents of litter (Liao et al., 2008), overestimates the litter decomposition rate, and it is difficult to truly reflect the material cycle of the ecosystem process (Welsch and Yavitt, 2003). The litter bag method is simple to operate and is still the most common method in field experiments. ...
Seaweeds are important components of marine ecosystems and can form a large biomass in a few months. The decomposition of seaweed litter provides energy and material for primary producers and consumers and is an important link between material circulation and energy flow in the ecosystem. However, during the growth process, part of the seaweed is deposited on the sediment surface in the form of litter. Under the joint action of the environment and organisms, elements enriched in seaweed can be released back into the environment in a short time, causing pollution problems. The cultivation yield of seaweed worldwide reached 34.7 million tons in 2019, but the litter produced during the growth and harvest process has become a vital bottleneck that restricts the further improvement of production and sustainable development of the seaweed cultivation industry. Seaweed outbreaks worldwide occur frequently, producing a mass of litter and resulting in environmental pollution on coasts and economic losses, which have negative effects on coastal ecosystems. The objective of this review is to discuss the decomposition process and ecological environmental effects of seaweed litter from the aspects of the research progress on seaweed litter; the impact of seaweed litter on the environment; and its interaction with organisms. Understanding the decomposition process and environmental impact of seaweed litter can provide theoretical support for coastal environmental protection, seaweed resource conservation and sustainable development of the seaweed cultivation industry worldwide. This review suggests that in the process of large-scale seaweed cultivation and seaweed outbreaks, ageing or falling litter should be cleared in a timely manner, mature seaweed should be harvested in stages, and dried seaweed produced after harvest and washed up on shore should be handled properly to ensure the benefits of environmental protection provided by seaweed growth and sustainable seaweed resource development.
... Few studies have compared the decay rate of seaweeds, primarily focused on bloomforming seaweed species; however, fast-growing and opportunistic seaweeds decay faster than slower-growing and long-lived seaweed, such as Fucus (Buchsbaum et al., 1991;Mews et al., 2006;Conover et al., 2016). The seaweed decay rate was primarily affected by species-specific physicochemical and biochemical properties, including C/N, C/P, lignin/N, and N/ P ratios (Li et al., 2013;Zhu et al., 2013;Zhang et al., 2014;Zhou et al., 2018), and cellulose, lignin and hemicellulose contents (Godshalk and Wetzel, 1978;DeBusk and Dierberg, 1984;Liao et al., 2008;Balasubramanian et al., 2012). The low decay rates of seaweeds generally with a high C/N ratio (Zhu et al., 2013). ...
Gracilaria bioremediates heavy metals (Cd, Cr, Pb, Ni, Cu, Zn, Fe, and Mn) and improves water quality in mariculture zones. However, Gracilaria litter produced during the growth and harvest process has become a critical bottleneck problem that limits the sustainable development of the Gracilaria cultivation industry. Experiments of decaying dried (dead) and frozen fresh (falling and dying) G. lemaneiformis and G. lichenosdies were carried out using the litterbag technique under laboratory-controlled and in situ conditions. The results showed that decay rates (k), decomposed time in 50 % (t50) and in 95 % (t95) varied between dried and frozen fresh Gracilaria and were different between G. lemaneiformis and G. lichenosdies. All Gracilaria samples showed an 80 %-90 % weight loss in 15-45 d. The variation in MAIs (accumulation index of metals) between the dried and frozen fresh Gracilaria litters differed significantly and provided evidence that metals could be imported or exported from litter to the environment. Based on our estimates from the 15-45 d experiment, the decay of Gracilaria can release and adsorb heavy metals. The enrichment of Fe, Pb, and Mn was more significant than the release, but the release of Cr, Zn, Cd, Pb, Cu, and Ni was more significant than the enrichment. Heavy metals in Gracilaria litters were accumulated and released simultaneously during decay. The present study simulated and underscores that Gracilaria cultivation intensely influences heavy metals recycled in marine environments It provides a theoretical basis for seaweed management for the sustainable development of the seaweed industry in the mariculture zone.
... Nevertheless, the high SOC content in the soil of S. alterniflora invasion areas may be related to the properties of S. alterniflora and the characteristics of the bacterial communities in coastal wetlands, which is not only associated with one factor. For one thing, previous studies have provided evidence that S. alterniflora had a stronger carbon fixation capacity than other wetland plants and was decomposed more slowly [34,35], which provided sufficient organic matter for bacteria. It partly explained the reason why soil in S. alterniflora invasion areas had higher SOC and was consistent with SOC contents in our study. ...
Spartina alterniflora is considered an invasive species that has affected the biogeochemical circle of carbon in coastal wetlands around the world. Nevertheless, it is still unclear how S. alternation invasion affects the carbon storage capacity of coastal wetlands as carbon pools through bacterial changes. Herein, bacterial communities and soil carbon content in coastal wetland native areas and S. alterniflora invasion areas were detected. It was found that an S. alterniflora invasion brought more organic carbon and resulted in the increase in Proteobacteria in bare flats and Sueada salsa areas. When decomposition capacity was not sufficient, large amounts of organic carbon may be stored in specific chemical forms, such as monosaccharides, carboxylic acids, alcohols, etc. The results have also shown that soil bacterial communities were highly similar between the bare flat and S. alterniflora invasion area, which is extremely conducive to the rapid growth of S. alterniflora. However, an S. alterniflora invasion would decrease total carbon contents and inorganic carbon contents in the Sueada salsa area. This is not conducive to the stability of the soil carbon pool and soil health. These findings may complement, to some extent, the shortcomings of the interaction between S. alterniflora and bacterial communities, and their joint effect on soil carbon storage.
... Plants have direct mutualistic and antagonistic relationships with soil microorganisms [32, 33] as well as indirect impacts on microbes through the transfer of phosynthate to decomposers and nutrient cyclers [34,35]. Invasive species in particular have been noted to changed soil microbial community structure and function [136][137][138]. Conversely, inoculation with various microbes or whole soils has been shown to have bottom-up effects on plant establishment during restoration [24, 31, 32]. ...
While it is now widely accepted that microorganisms provide essential functions in restoration ecology, the nature of relationships between microbial community assembly and ecosystem recovery remains unclear. There has been a longstanding challenge to decipher whether microorganisms facilitate or simply follow ecosystem recovery, and evidence for each is mixed at best. We propose that understanding microbial community assembly processes is critical to understanding the role of microorganisms during ecosystem restoration and thus optimizing management strategies. We examine how the connection between environment, community structure, and function is fundamentally underpinned by the processes governing community assembly of these microbial communities. We review important factors to consider in evaluating microbial community structure in the context of ecosystem recovery as revealed in studies of microbial succession: (1) variation in community assembly processes, (2) linkages to ecosystem function, and (3) measurable microbial community attributes. We seek to empower restoration ecology with microbial assembly and successional understandings that can generate actionable insights and vital contexts for ecosystem restoration efforts.
... Despite this global scenario, there is comparatively poor empirical data testing how changes in composition can affect decomposition. To our knowledge, there are some studies that evaluated the effects of changes in soil decomposer communities (e.g., Zimmer et al. 2004), and changes in plant litter composition (Treplin et al. 2013), but very few evaluating in situ changes in the plant community (Liao et al. 2008). We already went through how herbivory can decrease S. densiflora dominance , especially at recovering patches after disturbance, which, in turn, can allow subordinate plant species to colonize, generating a different and persistent plant community (Daleo et al. 2014). ...
Just as some species are used as model systems in organismal biology (e.g., physiology, genetics), many ecosystems are commonly used as model systems in ecology. Salt marshes, for instance, are great models to perform manipulative field experiments, and thus, were historically used to understand the drivers of community and ecosystem function. Decades of experimental work, indeed, made a strong contribution to community ecology as a discipline, but most of the emerged hypotheses and models were grounded in a few sites. When studies from new sites came onboard, looking to enlarge generalities, their results challenged the prevailing ideas. Here, we review more than 25 years of intense experimentation in South West Atlantic salt marshes, which helped not only to increase the knowledge about salt marsh functioning, but also to expand this knowledge beyond salt marshes helping to refine community and ecosystem function theory. We show that results coming from SW Atlantic marshes significantly contribute to understand 1) the separate and interactive effect of biotic and abiotic stress for species distribution and even for ecosystem stability, 2) the integrated role of species that can function as ecosystem engineers and as consumers, 3) the balance between stochastic and deterministic forces as drivers of community structure and 4) the regulation of cross-ecosystem fluxes. Nevertheless, we believe SW Atlantic salt marshes still have a lot more to offer, not only as conceptual models that help satisfy our intellectual curiosity, but also as key ecosystems that provide valuable benefits to our societies.
... Given that the two study plants have a large mass loss in the early stage of decomposition (Duan et al., 2018;Liao et al., 2008), leaf litter was incubated for 90 days, from 1 August 2017 to 30 October 2017 at DongYing, from 4 August 2017 to 2 November 2017 at ShangHai, and from 29 July 2017 to 27 October 2017 at ZhangZhou. After the incubation, all of the 2430 litterbags were retrieved from the field, of which 87 were damaged and therefore excluded from further analyses (12, 8, and 6 for P. australis at DongYing, ShangHai, and ZhangZhou, respectively; 20, 10, and 31 for S. alterniflora at DongYing, ShangHai, and ZhangZhou, respectively). ...
Litter decomposition is a key process of the carbon cycle in terrestrial and aquatic ecosystems. The dominant conceptual model of litter decomposition assumes that environmental conditions, litter traits, and decomposer composition control litter decomposition in a decreasing order, yet whether this hierarchical model applies to both invasive and native plant species is unknown. Here, by comparing a widespread invasive plant and its native counterpart in Chinese coastal saltmarshes, we aimed to examine whether the hierarchy of factors controlling litter decomposition varies with plant species in the face of plant invasions. Leaf litter of invasive Spartina alterniflora and native Phragmites australis was collected across an 18° latitudinal range to capture wide variation in litter traits. These leaf litter samples were transported to three saltmarsh sites of different latitudes and were incubated in litterbags varying in mesh size (0.1, 2, and 5 mm) to manipulate decomposer composition. After 90‐day incubation, we found a parallel latitudinal pattern in leaf litter decomposition rate (k) between S. alterniflora and P. australis regardless of saltmarsh site and mesh size. Nonetheless, the k value of S. alterniflora was 2.2‐fold higher than that of P. australis. Moreover, there was a shift in the hierarchy of factors controlling k values between S. alterniflora and P. australis: environmental conditions (climate and soil) dominated other factors in P. australis, whereas litter traits contributed more than environmental conditions in S. alterniflora. Overall, our findings show that leaf litter decomposition and its dominant controlling factors across broad geographical ranges can vary with plant invasions, having important implications for managing invasive plants in the context of conserving coastal blue carbon.
... At last, the positive impact on soil water content after S. alterniflora invasion ( Figure 6) affects sulfur content directly through the migration of soluble ions [75] and indirectly by accelerating organism synthesis and litter decomposition [23]. The large leaf area index, plant density, and biomass of S. alterniflora provide shade and reduce water loss for soil [76,77]. Meanwhile, more organic matters introduced by S. alterniflora improve the soil structure and colloid conditions, strengthening the capacity of adsorption and water retention [49]. ...
Nowadays, plant invasion has become a global ecological threat to local biodiversity and ecosystem stability. Spartina alterniflora encroaches on the ecological niches of local species and changes the soil’s nutrient cycle. However, few comprehensive assessments focus on the effects of S. alterniflora invasion. Here, we investigated how soil sulfur changed with spatiotemporal variation and life forms of native species after S. alterniflora invasion and speculated the possible mechanism of the sulfur increase based on the references. The invasion of S. alterniflora increased soil total sulfur by 57.29% and phytotoxic sulfide by 193.29%. In general, the invasion of S. alterniflora enhanced the total plant biomass and soil nutrients, e.g., soil organic carbon, total nitrogen, and soil microbial biomass carbon, further increasing soil sulfur content. The sulfur accumulation caused by S. alterniflora might result in the poisoning of native species. Thus, we hypothesized that the success of S. alterniflora invasion was closely connected with soil sulfur, especially toxic sulfide. Our study suggests that researchers should give more attention to the correlation between S. alterniflora invasion and the soil sulfur increase. More research is needed to investigate the mechanisms of the successful invasion by accumulating phytotoxic sulfide.
... Most of the sediment nutrient contents (e.g., TC, TN, and TP) were considerably higher at the S. alterniflora invasion sites than uninvaded mudflat in this study, which was consistent with previous investigations. A possible reason for this phenomenon might be that the invasive plants S. alterniflora community had higher litter production and aboveground biomass (Liao et al., 2008), which strongly affected the sediment C and N contents (Peng et al., 2011;Yang et al., 2016). Moreover, our study agreed with some studies that reported that NO 3 − -N content in invaded S. alterniflora was higher than native plants, ...
Ecological restoration using native mangrove species (i.e., Kandelia obovata) is a practical approach for controlling Spartina alterniflora invasion in the coastal wetlands. Diazotrophs play a critical role in enhancing productivity of both S. alterniflora and mangrove plant, via providing new nitrogen to the coastal wetlands ecosystems. However, response of the diazotrophic community to S. alterniflora invasion and subsequent mangrove restoration remains unclear. The present study monitored sediment physicochemical properties and diazotrophic communities across a chronosequence of restored mangrove wetland (bare mudflat, invasive S. alterniflora stands, 3-year and 10-year K. obovata restoration areas, and mature K. obovata forests over 30 years). The results showed an orderly succession in the sediment properties and diazotrophic community composition after S. alterniflora invasion and along mangrove restoration chronosequences. Almost all sediment nutrient contents (e.g., TC, TN, and C/N) were significantly (P < 0.05) increased by S. alterniflora invasion, then they decreased sharply in the newly restored mangrove and increased gradually with restoration ages, with the highest values in the mature mangrove. The diazotrophs demonstrated distinct community structure in different seasons. Diazotrophic alpha diversity increased with S. alterniflora invasion and mangrove restoration age in winter. Sulfate-reducing bacteria (SRB) was the dominant diazotrophic group, especially Desulfuromonadales. Moreover, network analyses revealed that SRB groups were the major keystone taxa. The relative abundance of Desulfuromonadales decreased, while the abundance of Chromatiales increased gradually with the S. alterniflora invasion and mangrove restoration age. Redundancy analysis revealed that TC, TN, and TS were the significant (P < 0.05) environmental factors in altering sediment diazotrophic communities in both seasons. These findings expand the current understanding of succession patterns of diazotrophic communities in mangrove restoration and provide new perspectives on the sustainable management of mangrove ecosystems.
... Besides, the specific plant chemical composition, mainly regarding the content of polysaccharides (such as cellulose, hemicellulose), and phenolic compounds (such as lignin), determine litter hardness and affect litter decomposition (Duan et al., 2018). However, few studies have estimated the amount of these compounds in salt marsh plant species, being these studies mainly performed in monocotyledonous species from Poaceae and Cyperaceae families (Duan et al., 2018;Liao et al., 2008;Valiela et al., 1984). To our knowledge, there are no studies performed in succulent Chenopodiaceae halophytes, despite they are the dominant vegetation in Mediterranean climate salt marshes, which include salt marshes of the Mediterranean Basin, but also those of the Pacific coast of central and southern California and SW Australia (Ibañez et al., 2000). ...
Studies of litter decomposition in salt marshes have been mainly focused on the measurement of decomposition rates, being litter quality, the type of microbial decomposers and their extracellular enzyme activity, rarely considered. Moreover, most of these studies have been conducted in Poaceae and Cyperaceae species, being scarce the literature on Chenopodiaceae species, which are abundant in Mediterranean salt marshes. Here we analyse the litter decomposition process of two Chenopodiaceae (Sarcocornia fruticosa and Halimione portulacoides) and one Poaceae (Elytrigia atherica) species, belonging S. fruticosa to a halophilous scrub habitat and the other two to a salt meadow habitat of a Mediterranean salt marsh. For each species, we analysed litter decomposition rates, litter quality, fungal and bacterial biomass and potential extracellular enzymes activities. In order to embrace the spatial heterogeneity, two zones were considered within each habitat.
Litter of E. atherica decomposed 7- and 13-fold slower than those of S. fruticosa and H. portulacoides, respectively, suggesting that this species is the one that would favour most the carbon sequestration into the soil. The different decomposition rates would be explained by the higher initial lignin and cellulose content of E. atherica rather than by the initial carbon and nitrogen content and C/N ratio. Moreover, enzyme efficiency, compared to enzyme activity, better contributes to explain the different decomposition rates observed. Bacteria dominated throughout the litter decomposition process regardless the species, but fungi increased their relevance in the later stages, when the relative lignin litter content increased. Litter decomposition was affected by microhabitat spatial differences, although the responses depended on the species. Hence, flooding (in the habitat of S. fruticosa) or soil texture (in the habitat of E. atherica and H. portulacoides) might have modulated the decomposition process, being H. portulacoides the most sensitive species to the spatial differences of the salt meadow habitat.
... The decay of aquatic plant litter is influenced by species-specific physicochemical, biological and chemical properties. For example, the hemicellulose, cellulose, and lignose concentrations (Liao et al., 2008;Balasubramanian et al., 2012) and the C/N, C/P, lignin/N, and N/P ratios (Li et al., 2013;Zhang et al., 2014) have been studied. Moreover, the N and P concentrations might affect the activity of microbes (Bray et al., 2012), and high C/N ratios generally result in low decomposition rates (Zhu et al., 2013). ...
The seaweed Gracilaria lemaneiformis can bioremediate heavy metals and improve the environmental quality of mariculture zones. However, the seaweed litter that is produced in the growth and harvest processes becomes one of the important bottlenecks and causes secondary pollution that restricts the development of sustainable seaweed cultivation. Seaweeds exist widely in the coastal areas of the world and are cultivated on a large scale in Asia, but their decomposition process is rarely studied. Experiments that compared decaying dry (dead) and fresh (falling and dying) Gracilaria were conducted to quantify the differences in decomposition rates and heavy metal release in different physiological states. The heavy metals in the seawater and sediment were investigated. The litterbag technique under controlled laboratory conditions was used. The results indicated that the decomposition rates (k) and decay times in 50% (t50%) and 95% (t95%) values varied between dry and fresh Gracilaria. Fresh Gracilaria exhibited a weight loss rate of 15%, and the dry weight loss was 44%. The variations in MAIs (accumulation index of metals) and MR (release rate of metals) between the dry and fresh Gracilaria litters differed significantly, which provides evidence that metals are released back into the environment from Gracilaria litters. The contacted sediments could accelerate the heavy metal release from Gracilaria. Based on our estimates obtained from a 45 d experiment, at least 27.5％ of Cd, 16％ of Cu, 60.1％ of Pb, 72.3％ of Zn, 49.4％ of Fe, 38.6％ of Mn, 68.1％ of Cr, and 67.5％ of Ni present in the fresh Gracilaria and 37.4％ of Cd, 46.2％ of Cu, 77.7％ of Pb, 53.7％ of Zn, 42.7％ of Fe, 67.2％ of Mn, 75.1％ of Cr, and 73.5％ of Ni present in the dried Gracilaria were released back into the water when the biomass was left to decay. This study simulates and underscores that Gracilaria has an strong effect on the heavy metal cycles in marine environments and offers a theoretical basis for the development of sustainable seaweed industries in mariculture zones.
... Schaller and Struyf (2013) found that the aquatic litter decomposition process is controlled in the tropics mainly by microbes. Microorganisms in litter can absorb nutrients from the surrounding environment to maintain their activity, which increases the decomposition of nutrients in the litter (Liao et al., 2008). In nutrient-rich environments, soil bacteria thrive and accelerate decomposition (Fierer et al., 2012). ...
Tidal marshes are important recycling areas for biogenic silica (BSi) and macro- and microelements at the land–sea interface and are key locations for examining the decomposition process of wetland plant litter. In this study, in situ decomposition experiments were conducted with Phragmites australis, Cyperus malaccensis, and Spartina alterniflora in the Min River estuary wetland. Litterbags of 0.2-mm mesh size were used to evaluate the litter decomposition process and residual values of BSi and macro- and microelements, including C, N, Cr, Cu, Cd, Zn, Pb, Al, Mn, and Fe over 520 days. The litter decomposition rate significantly differed among species in the following order: C. malaccensis (0.005 d–1) > S. alterniflora (0.004 d–1) > P. australis (0.003 d–1) with BSi release rates of 98.64%, 96.75%, and 97.23%, respectively. Although there were net releases of BSi, C, and N from the three litter species, continuous decrease in the BSi/(C, N) ratio indicated that BSi was removed from the litter much faster than C and N. The accumulation index results showed that Cu, Pb, Al, and Fe were net-accumulated in the litter, whereas Cd, Mn, Cr, and Zn were predominantly released during litter decay. Pearson’s correlation analysis results showed that the amounts of N, Cu, Cd, Pb, Al, and Fe in the litter restrained BSi release with a significant negative correlation. These findings in the Min River estuary have important implications for geochemical cycles within wetland systems and the transport processes of potential nutrients out of the system.
... C sequestration in soils was also limited by litter quality and decomposition under ecosystems (Fierer et al., 2005). Liao et al. (2008b) reported that the decomposition rate of S. alterniflora litter , particularly the belowground root residues, was slower than that of P. australis litter due to the lower litter quality (i.e., higher C:N ratio) of S. alterniflora (Duan et al., 2018). Respiration of roots and microorganisms is another important factor ...
Coastal wetlands are among the most productive ecosystems and store large amounts of organic carbon (C) – the so termed “blue carbon”. However, wetlands in the tropics and subtropics have been invaded by smooth cordgrass (Spartina alterniflora) affecting storage of blue C. To understand how S. alterniflora affects SOC stocks, sources, stability, and their spatial distribution, we sampled soils along a 2500 km coastal transect encompassing tropical to subtropical climate zones. This included 216 samplings within three coastal wetland types: a marsh (Phragmites australis) and two mangroves (Kandelia candel and Avicennia marina). Using δ¹³C, C: nitrogen (N) ratios and lignin biomarker composition we traced changes in the sources, stability and storage of SOC in response to S. alterniflora invasion. The contribution of S. alterniflora‐derived C up to 40 cm accounts for 5.6%, 23% and 12% in the P. australis, K. candel and A. marina communities, respectively, with a corresponding change in SOC storage of +3.5, −14 and −3.9 t C ha‐1. SOC storage did not follow the trend in aboveground biomass from the native to invasive species, or with vegetation types and invasion duration (7–15 years). SOC storage decreased with increasing mean annual precipitation (1000–1900 mm) and temperature (15.3–23.4 ℃). Edaphic variables in P. australis marshes remained stable after S. alterniflora invasion and so, their effects on SOC content were absent. In mangrove wetlands, however, electrical conductivity, total N and phosphorus, pH and active silicon were the main factors controlling SOC stocks. Mangrove wetlands were most strongly impacted by S. alterniflora invasion and efforts are needed to focus on restoring native vegetation. By understanding the mechanisms and consequences of invasion by S. alterniflora, changes in blue C sequestration can be predicted to optimize storage can be developed.
... Climate warming may significantly impact the biogeochemical cycling of estuarine wetlands (Liao et al., 2008). Notably, higher sea levels will increase the frequency of flooding in estuarine wetlands (Bai et al., 2014). ...
The main factors controlling plant litter decomposition rates are litter quality and environmental factors. We investigated how different salinity and inundation conditions influence the decomposition rate and how litter quality affects dynamic change during the decomposition process in the Yellow River Delta wetland in northeastern China. To do this, we designed a field experiment using the litter bag method to study two selected dominant halophyte species (Phragmites australis and Suaeda salsa). We found litter decomposed faster when inundated across a range of salinities. Both the water chemical oxygen demand and NH3-N concentration were inversely correlated with salinity. The ratio of nitrogen to phosphorous showed an upward trend during the process of decomposition. The decomposition rate increased during the later stage of the process. Under inundation conditions, the decomposition rate of only S. salsa showed a positive correlation with salinity. The study results suggest that moderately higher salinity would increase anaerobic decomposition owing to the combined effect of salinity and inundation. The implications of our findings may be used to further assess the impact of environmental parameters and litter quality on the decomposition rate in estuarine wetland and can help determine a strategy for wetland reparation and remediation.
... In this study, the litter N and S concentrations generally showed an increasing trend during decomposition in different invasion stages; the increase of N and S concentrations in litter during decomposition may be due to the rapid leaching of dissolved matter and the immobilization of external microorganisms (Gessner 2001;Li et al. 2014). Previous studies have shown that microorganisms can absorb nutrients from the surrounding environment to maintain their activity, which in turn increases the decomposition of nutrients in litter (Liao et al. 2008;Zhang et al. 2012). In this study, the N and S concentrations in C. malaccensis (CM and CM′) were generally higher than those in S. alterniflora (SA and SA′), which might have been related to the variations in litter mass remaining, residue nutrient content, and environmental factors (Fig. 3b, c). ...
Plant invasion can affect nutrient cycling by changing the quantity and quality of litter entering the environment. To determine the effect of an alien species (Spartina alterniflora) invasion on the decomposition rates and nutrient dynamics of litter in the Min River estuary, three communities with differing levels of invasion, namely, a Cyperus malaccensis community (before invasion, BI stage), a S. alterniflora community (after invasion, AI stage), and a C. malaccensis–S. alterniflora community (during invasion, DI stage), were studied using the space-for-time substitution method. Results showed that the decomposition of C. malaccensis was 59.79% faster than that of S. alterniflora, which was mainly related to the great variations in the C/N and lignin. Compared with S. alterniflora, the N (nitrogen) and S (sulfur) concentrations of litter in C. malaccensis were significantly higher. The C, N, and S stocks increased as the C. malaccensis was being invaded or after complete invasion by S. alterniflora, which might be ascribed to the higher mass remaining in S. alterniflora. Compared with S. alterniflora in DI stage, the higher C/N and C/S ratios might explain the higher C, N, and S stocks in S. alterniflora in AI stage. In summary, the invasion of S. alterniflora reduces the decomposition rate and nutrient release of litter.
... Plant invasion has been associated with differences in litter decomposition and in nutrient use, uptake and cycling between invasive and native plants (Liao et al., 2007(Liao et al., , 2008a(Liao et al., , 2008bHolly et al., 2009;Martina et al., 2016;Duke et al., 2015;Zhang et al., 2016;Sardans et al., 2017). Plant invasion in nutrient-rich ecosystems has usually been associated with higher rates of litter decomposition of invasive than native species (Allison and Vitousek, 2004;Arthur et al., 2012;Hickman et al., 2013;Aragon et al., 2014). ...
Invasion of plants in wetland ecosystems is often associated with changes in litter decomposition and in nutrient use, uptake and cycling between invasive and native plants. We studied litter decomposition rates, N and P release and elemental composition and stoichiometry during the invasion of Phragmites australis and Spartina alterniflora into native Cyperus malaccensis wetlands in the Minjiang River estuary (China). Aboveground litter in mono-specific stands decomposed faster for Cyperus malaccensis than for Spartina alterniflora and for Phragmites australis. Cyperus malaccensis litter decomposed slower under the stands of both invasive species. In contrast, the litter of both invasive species decomposed faster under Cyperus malaccesis stands. We observed that the invasion of these species was associated with an increased rate of aboveground litter decomposition and large absolute amounts of C, N and P released from the litter when litter from invasive species was mixed with that of native species. Our results suggest that the large nutrient release from litter during early stages of the invasion favored invasive species with larger size and higher nutrient-uptake capacity than the native species.
... At the time of its introduction, the development of S. alterniflora salt marsh in China provided considerable ecological benefits, especially by capturing excess sediments from watershed erosion and promoting siltation and land-building by resisting wind and wave erosion. Other benefits of this ecosystem are, fixing CO 2 to mitigate global warming (Zhang et al., 2010), providing habitat for wild animals (Wu et al., 2011), accumulating nutrients (Liao et al., 2008) in the sediments, and controlling pollution by purifying the water (Nalla et al., 2012). However, S. alterniflora has continuously replaced native plants, which have inundation tolerances that overlap with it, e.g. ...
An integrated emergy and economic evaluation of a 7-step ecological engineering system (7-step EES) located in Jiangsu Province, China was carried out. This 7-step EES was constructed to fully utilize existing S. alterniflora production through a low waste, clean production process and to help control the expansion of this invasive grass. First, the above ground S. alterniflora biomass removed for invasion control is used as the feedstock for the extraction of a bio-mineral liquid and its encapsulation, both of which provide a health supplement to treat gout and angiocardiopathy patients. Next, the residues left after extraction are used as a medium for mushroom cultivation, and then for raising earthworms. Finally, the remaining residues become the main component of a microbe enriched organic fertilizer, which is then returned to the soil. A suite of emergy indices were constructed on multiple temporal and spatial scales to examine the operation of the whole system and its subsystems. The results showed that the temporal sustainability of the 7-step EES was relatively high at different spatial scales, especially at the local scale as indicated by its EISDLS (1.55E+03). Full implementation of the 7-step EES for S. alterniflora currently residing on the coast of China can result in a potential economic output that is greater than 2% of the national GDP, with an economic output/input ratio over 29. In addition, the 7-step EES had an environmental loading ratio lower than that of the regional system. Sensitivity analysis showed that decreasing the cost of the purchased inputs needed to make the capsules is the key problem that needs to be solved to optimize the system, both for its sustainability and economic viability, on all spatial and temporal scales.
... One possible explanation could be the different decomposition rate of plant litter. S. alterniflora has a greater amount of litter mass but lower decomposition rate than P. australis (Liao et al., 2008). P. australis has rhizome-root systems that can transport oxygen internal and out into the soil (Armstrong and Armstrong, 1988). ...
Climate warming and plant invasions are among the most serious threads to biodiversity and stability in terrestrial ecosystems. A particular challenge is to predict how soil microbial communities and edaphic factors will change under future conditions. Despite evidence that soil microbial community structure can be shifted by experimental warming and plant invasion, few studies were conducted to quantify the interactive effect of both factors. Here, we compared soil microbial diversity, bacterial community, and soil physiochemical properties in both surface (0–5 cm) and subsurface (5–20 cm) soils of two-year open-top chambers (OTCs) warming and Spartina alterniflora (S. alterniflora) invasion treatments with no warming and Phragmites australis (P. australis) treatments. Results showed that surface soil water content, total organic carbon, total nitrogen (TN), and the ratio of TN to total phosphorous (N:P) were significantly decreased by experimental warming, while pH was significantly enhanced. Soil TN was significantly enhanced in the mixed community of P. australis and S. alterniflora compared with single species community. Soil TP was gradually decreased with the proportion of S. alterniflora increasing by experimental warming. Soil bacterial α diversity was not changed by either experimental warming or S. alterniflora invasion. Experimental warming significantly shifted soil bacterial community, whereas S. alterniflora invasion and their interaction showed no impact. Redundancy analysis showed soil bacterial community structure was highly correlated with soil edaphic factors (i.e. pH, TN, and NH4⁺-N). Prediction of microbial function profiles revealed that several bacterial functions, especially carbon and nitrogen metabolism pathways, were adjusted to tolerate warming. Overall, our results demonstrated that experimental warming significantly altered soil bacterial community and physiochemical properties of S. alterniflora and P. australis communities, and should be considered in ecologically sustainable development of Chongming Island and wetland ecosystem.
... In addition, a large number of burrowing crabs exist in coastal salt marshes, accelerating litter breakdown and returning into soil ( Wang et al., 2010 ). Therefore, plant litters may be transferred from the aboveground to the belowground, and the microenvironment will alter the decomposition process ( Liao et al., 2008 ). Previous studies have focused on the placements of litters, such as the standing, surface and buried in soil from forests, wetlands and agricultural ecosystems ( Holland and Coleman, 1987 ;Moore et al., 2005 ;Zhang et al., 2014 ). ...
A 200-day tethered litter experiment was performed to evaluate the effects of two dimensions of hydrological connectivity (lateral and longitudinal, two perpendicular dimensions) and three placement treatments (surface, semi-buried, and buried) on Suaeda salsa litter decomposition and nutrient release. We used the hydrodynamic condition to represent hydrological connectivity. Mass loss in semi-buried and buried treatments was higher than the surface treatment (p < 0.05), and the former two were basically the same except for the area farthest from the tidal creek. The enhancement of lateral hydrological connectivity significantly accelerated mass loss under the surface and semi-buried treatments (p< 0.05). Sulfur (S) and nitrogen (N) concentrations in the surface treatment were higher than another two treatments in most cases. N concentrations increased with enhancing lateral hydrological connectivity, while potassium (K) concentrations showed a decreasing trend under stronger hydrological connectivity. Phosphorus (P) and S concentrations were not affected by hydrological connectivity. The amount of nutrient release increased with the enhancement of the lateral hydrological connectivity in the surface and semi-buried treatments except for N, and the longitudinal hydrological connectivity only influenced K under the surface treatments. The amount of nutrient release under the surface treatments was lower than another two treatments (p < 0.05). Our findings suggest that the lateral hydrological connectivity has a greater impact on mass loss and nutrient release than the longitudinal hydrological connectivity. If Suaeda. salsa litters are excessively exposed to soil, decomposition processes would increase rapidly and the effects of hydrological connectivity might become minimal.
... Vertical variation in decomposition rates has also been reported in some ecosystems. Liao et al. (2008) found that decomposition rates of aerial leaf litter of some species were slower than those of ground litter in the same species in grasslands. Erickson et al. (1985) and Jomura et al. (2005) found that aerial woody litter decayed more slowly than litter on the ground surface in Douglas-fir forest and temperate broad-leaved forest, respectively. ...
Non-commercial thinning is known to produce aerial needle litter. The vertical position of litter mass is important for understanding material cycling in thinned forests because aerial litter decomposes more slowly than ground litter. The decomposition
processes of aerial and ground litter and the physical falling of aerial litter to the ground determine the dynamics of remaining
amount and vertical distribution of litter. Furthermore, decomposition in each position and aerial litter fall may depend on meteorological factors. We aimed to estimate the dynamics of remaining amount of litter by linking litter fall and decomposition
rates and developed Bayesian models to clarify the effects of meteorological factors on these processes. We monitored decomposition and aerial litter fall of a Cryptomeria japonica needle litter for 42 months after thinning. The model results suggested
that temperature was a determining factor for temporal variation in the litter decomposition rates, while precipitation was the
main determining factor for temporal variation in the fall rate of aerial litter. Temperature and precipitation were positively correlated with decomposition and aerial litter fall, respectively. Our linked model suggested that aerial litter fall contributed only
slightly to remaining amount of total litter during the 42 months after thinning, but affected the vertical distribution of litter
considerably. The model also indicated that the above meteorological factors contribute notably to the dynamics of remaining
amount of total litter and the vertical distribution. These results suggest that our model has the potential to be an effective tool
for understanding material cycling in non-commercially thinned stands in temperate regions.
... The study ended in late fall, and the temperature was colder. Faster decomposition rates under warm conditions can result in higher release of organic acid (Liao et al. 2008), faster consumption of DO by microorganisms, higher release of C, N, and P, leading to even lower DO concentrations, and higher TP and NH 3 -N concentrations in the water (Wu et al. 2017). However, the TP and NH 3 -N concentrations in the added fungi treatments decreased slower than those in the blank treatments over the course of the study, which was likely because the transfer of TP and NH 3 -N from water to litter was improved by the adsorption effect related to the fungi. ...
Litter decomposition is a complex process that is influenced by many different physical, chemical, and biological processes. Environmental variables and leaf litter quality (e.g., nutrient content) are important factors that play a significant role in regulating litter decomposition. In this study, the effects of adding fungi and using different mesh size litter bags on litter (Populus tomentosa Carr. and Salix matsudana Koidz.) decomposition rates and water quality were investigated, and investigate the combination of these factors influences leaf litter decomposition. Dissolved oxygen (DO), chemical oxygen demand (COD), total phosphorus (TP), and ammonia-nitrogen (NH3-N) were measured during the 112-day experiment. The salix leaf litter (k = 0.045) displayed faster decomposition rates than those of populous leaf litter (k = 0.026). Litter decomposition was initially slow and then accelerated; and by the end of the experiment, the decomposition rate was significantly higher (p = 0.012, p < 0.05) when fungi were added to the treatment process compared to the blank, and litter bags with different mesh sizes did not influence the decomposition rate. The variations in the decomposition rates and nutrient content were influenced by litter quality and a number of environmental factors. The decomposition rate was most influenced by internal factors related to litter quality, including the N/P and C/P ratios of the litter. By quantifying the interact effect of environment and litter nutrient dynamic, to figure out the revetment plant litter decomposition process in a wetland system in biological physical and chemical aspects, which can help us in making the variables that determine decomposition rates important for assessing wetland function.
... The metabolic activity of the microbial community was found to rise directly, responding to increased initial litter N content, while the inhibition of decomposition by lignin is attributable to its chemical structure which makes it resistant to microbial attack (Hemminga and Buth, 1991). Phragmites australis roots may have relatively low C:N and lignin : N ratios compared to Spartina species (Liao et al., 2008), facilitating root decomposition. Likewise, roots of Avicennia marina are described as having lower C:N ratios than both Ceriops tagal and Bruguiera gymnorhiza (Huxham et al., 2010). ...
Mangroves and saltmarshes are ‘blue carbon’ ecosystems and studies on their carbon (C) dynamics have become highly topical. In these coastal wetlands, the aboveground biomass sequesters C and allocates C to belowground roots. A significant portion of aboveground biomass is transferred to the sediment as litterfall. Belowground roots and their exudates are ultimately decomposed by microbial communities. As the product of root decomposition, CO2 is released from the sediment surface and organic C (OC) contributes to sediment C burial. In addition, the decomposition of litter and sediment organic material also contributes to CO2 efflux from the sediment surface. The patterns of belowground C dynamics in blue C ecosystems remain unclear, especially sediment C burial, root C decomposition, and the relative contributions of different sources to sediment CO2 efflux. Further, sediment CO2 efflux and C storage have typically been examined separately in past research. Improved understanding of these patterns will help consolidate the theory of C cycling in coastal wetlands and facilitate effective ‘blue C’ management. My thesis aims to fill these gaps via systematic quantitative reviews and synthesis of literature data, field surveys and laboratory microcosm experiments.
Studies on C stock in saltmarsh sediments have increased since the previous major review (Chmura et al., 2003). However, uncertainties exist in estimating global carbon storage in these vulnerable coastal habitats, thus hindering the assessment of their importance. Combining direct data and indirect estimation, my thesis compiled studies involving 143 sites across the southern and northern hemispheres, and provides an updated estimate of the global average carbon accumulation rate (CAR) at 244.7 g C m-2 yr-1 in saltmarsh sediments. Based on region-specific CAR and estimates of saltmarsh area in various geographic regions between 40°S to 69.7°N, total CAR in global saltmarsh sediments is estimated at ~ 10.2 Tg C yr-1. Latitude, tidal range and elevation appear to be important drivers for CAR of saltmarsh sediments, with considerable variation among different biogeographic regions. The data indicate that while the capacity for carbon sequestration by saltmarsh sediments ranked the first amongst coastal wetland and forested terrestrial ecosystems, their carbon budget was the smallest due to their limited and declining global areal extent. However, some uncertainties remain for this global estimate owing to limited data availability.
One of the aims of this thesis was to determine the drivers of root decomposition and its role in C budgets in mangroves and saltmarsh: the patterns of root decomposition, and its contribution to C budgets, in mangroves and saltmarsh. The impact of climatic (temperature and precipitation), geographic (latitude), temporal (decay period) and biotic (ecosystem type) drivers was explored using multiple regression models. Best-fit models explain 50% and 48% of the variance in mangrove and saltmarsh root decay rates, respectively. A combination of biotic, climatic, geographic and temporal drivers influence root decay rates. Rainfall and latitude have the strongest influence on root decomposition rates in saltmarsh. For mangroves, forest type is the most important; decomposition is faster in riverine mangroves than other types. Mangrove species Avicennia marina and saltmarsh species Spartina maritima and Phragmites australis have the highest root decomposition rates. Root decomposition rates of mangroves were slightly higher in the Indo-west Pacific region (average 0.16 % day-1) than in the Atlantic-east Pacific region (0.13 % day-1). Mangrove root decomposition rates also show a negative exponential relationship with porewater salinity. In mangroves, global root decomposition rates are 0.15 % day-1 based on the median value of rates in individual studies (and 0.14 % day-1 after adjusting for area of mangroves at different latitudes). In saltmarsh, global root decomposition rates average 0.12 % day-1 (no adjustment for area with latitude necessary). The global estimate of the amount of root decomposing is 10 Tg C yr-1 in mangroves (8 Tg C yr-1 adjusted for area by latitude) and 31 Tg C yr-1 in saltmarsh. Local root C burial rates reported herein are 51-54 g C m-2 yr-1 for mangroves (58-61 Tg C yr-1 adjusted for area by latitude) and 191 g C m-2 yr-1 for saltmarsh. These values account for 24.1-29.1% (mangroves) and 77.9% (saltmarsh) of the reported sediment C accumulation rates in these habitats. Globally, dead root C production is the significant source of stored sediment C in mangroves and saltmarsh.
Mangroves are blue carbon ecosystems that sequester significant carbon but release CO2, and to a lesser extent CH4, from the sediment through oxidation of organic carbon or from overlying water when flooded. Previous studies, e.g. Leopold et al. (2015), have investigated sediment organic C (SOC) content and CO2 flux separately, but did not provide a holistic perspective for both components of blue carbon. Based on field data from a mangrove in southeast Queensland, Australia, this thesis used a structural equation model to elucidate (1) the biotic and abiotic drivers of surface SOC (10 cm) and sediment CO2 flux; (2) the effect of SOC on sediment CO2 flux; and (3) the covariation among the environmental drivers assessed. Sediment water content, the percentage of fine-grained sediment (<63μm), surface sediment chlorophyll and light condition collectively drive sediment CO2 flux, explaining 41% of their variation. Sediment water content, the percentage of fine sediment, season, landform setting, mangrove species, sediment salinity and chlorophyll collectively drive surface SOC, explaining 93% of its variance. Sediment water content and the percentage of fine sediment have a negative impact on sediment CO2 flux but a positive effect on surface SOC content, while sediment chlorophyll is a positive driver of both. Surface SOC was significantly higher in Avicennia marina (2994±186 g m-2, mean+ SD) than in Rhizophora stylosa (2383± 209 g m-2). SOC was significantly higher in winter (2771±192 g m-2) than in summer (2599±211 g m-2). SOC significantly increased from creek-side (865±89 g m-2) through mid (3298±137 g m-2) to landward (3933±138g m-2) locations. Sediment salinity was a positive driver of SOC. Sediment CO2 flux without the influence of biogenic structures (crab burrows, aerial roots) averaged 15.4 mmol m-2 d-1 in A. marina stands under dark conditions, lower than the global average dark flux (61 mmol m-2 d-1) for mangroves.
CO2 flux is a critical component of the global C budget. While CO2 flux has been increasingly studied in mangroves, few studies partition components contributing to the flux. Partitioning CO2 flux sources helps constrain C budgets. We developed the combined 13C stable isotope labelling and closed chamber technique. The technique was used to partition CO2 efflux from the seedlings of mangrove Avicennia marina in laboratory microcosms, with a focus on sediment CO2 efflux. The result showed that (1) canopy was the chief component of ecosystem CO2 efflux, (2) the degradation of sediment organic matter was the major component of sediment CO2 efflux, followed by root respiration and litter decomposition via isotope mixing models. There was a significant relationship between δ13C values of CO2 released at the sediment-air interface and both root respiration and sediment organic matter decomposition. The findings provide the relative contribution of different components to ecosystem and sediment CO2 efflux, and thus can partition the sources of ecosystem respiration and sediment C mineralization in mangroves.
My thesis demonstrates that saltmarshes play a disproportionately important role in sediment C accumulation (244.7 g C m-2 yr-1) although their global area extent is declining, and saltmarsh CAR varies with latitude, tidal range, elevation and biogeographic regions. Dead root production contributes a significant proportion to sediment C in both mangroves and saltmarsh, and the updated global estimate of root C decomposition (8 Tg C yr-1) in mangroves reflects an underestimate in the reported value (5 Tg C yr-1). In a sub-tropical mangrove forest, SOC content and CO2 flux are influenced by sediment physio-chemical properties, and/or species, light conditions. Sediment water content and the percentage of fine sediment have contrasting effect on both but sediment chlorophyll is a positive driver of both, and these factors may be highlighted in ‘blue C’ management of this mangrove. In establishing mangroves (Avicennia marina), the result of a laboratory microcosm experiment shows that canopy was the major source of ecosystem CO2 efflux. The decomposition of roots contributes more to sediment CO2 efflux in comparison with sediment organic matter, which is in contrast to the pattern in mature mangroves.
... Invasive plants can alter ecosystem processes, but our ability to predict where those impacts will be most severe in an invaded landscape is limited. For example, invasive species can alter soil biogeochemical processes by changing litter inputs (Liao et al. 2008), the timing and location of nitrogen (N) released from decomposing litter (Laungani and Knops 2009), soil N transformations (Asner andBeatty 1996, Hawkes et al. 2005), and soil carbon losses (Litton et al. 2008, Strickland et al. 2010, 2011. The magnitude and direction of invader impacts on these processes can vary widely between sites (Warren et al. 2012). ...
Invasive plant species can alter critical ecosystem processes including nitrogen transformations, but it is often difficult to anticipate where in an invaded landscape, these effects will occur. Our predictive ability lags because we lack a framework for understanding the multiple pathways through which environmental conditions mediate invader impacts. Here, we present a framework using structural equation modeling to evaluate the impact of an invasive grass, Microstegium vimineum (M.v.), on nitrogen cycling based on a series of invaded sites that varied in invader biomass and non‐M.v. understory biomass, tree basal area, light availability, and soil conditions. Unlike previous studies, we did not find an overall pattern of elevated nitrate concentrations or higher nitrification rates in M.v.‐invaded areas. We found that reference plot conditions mediated differences in mineralization between paired invaded and reference plots at each site through indirect (via M.v. biomass), direct, and interactive pathways; however, the strongest pathways were independent of M.v. biomass. For example, sites with low reference soil nitrate and high non‐M.v. understory biomass tended to have faster mineralization at 5–15 cm in invaded plots. These findings suggest that more attention to reference conditions is needed to understand the impact of invasive species on soil nitrogen cycling and other ecosystem processes and that the greatest impacts will not necessarily be where the invader is most abundant.
The decomposition of standing litter is a vital but easily neglected process. Most studies always focused on the decomposition of leaf litter on the surface of soil or sediment, whereas the decomposition of leaf and non-leaf litter in the air is often overlooked. A field experiment was conducted in the Yellow River Delta to investigate the decomposition of leaf and non-leaf (culm and sheath) litter (Phragmites australis) in the air and on the ground. The results showed that the litter on the ground decomposed faster than the standing litter due to its larger enzyme activities, and the remaining mass of litter on the soil surface was 73–87% of the standing litter. The culm litter had the largest mass remaining among three types of litter owing to the lowest enzyme activities and the largest initial C/N and C/P of the culm litter. Concerning the dynamics of nutrient, nitrogen and phosphorus in leaf and non-leaf litter were first released and then enriched at two decomposition interfaces. Of the three types of litter, the culm litter had the highest N remaining after 360 days of decomposition due to the largest initial C/N of the culm litter. Our findings emphasize the importance of standing litter decomposition in the wetlands of the Yellow River Delta, and suggest that the decomposition of non-leaf (culm and sheath) litter of emergent macrophytes should not be ignored in wetlands.
Standing litter decomposition generally plays important roles in biogeochemical cycling in wetlands. By contrast with dew, the role of solar radiation receives less attention in humid ecosystems. Here, we determined aerial decomposition dynamics of five litters in a freshwater wetland in the Yangtze-Huaihe basin. Moreover, we analyzed the role of dew and sunlight in the decomposition of Phragmites australis culm and leaf based on a simulation experiment. After 360 days of field decomposition, the mass loss ranged from 31.1% to 80.7%, significantly relating to litter nutrient, cellulose, and lignin concentrations. In the simulation experiment, the sunlight treatment rather than the dew treatment facilitated mass loss of culm, and the mass loss in the dew + sunlight treatment was greater than the sunlight treatment. For the leaf, both sunlight and dew treatments increased mass loss, and the mass loss in the dew + sunlight treatment was greater relative to that in the dew and the sunlight treatments. In addition, the mass loss was not significantly different between the sterilization and the control treatments, irrespective of tissue. However, the sterilization + sunlight treatment increased the mass loss of culm, despite no significant effects on the leaf. These findings confirm the importance of standing litter in biogeochemical cycling of freshwater wetlands in the Yangtze-Huaihe basin. Moreover, this study highlights the important role of sunlight and its combined action with dew in regulating litter decomposition in humid ecosystems, where the specific role varies with litter quality.
Soil organic carbon (SOC) in coastal wetlands, also known as ‘blue C’, is an essential component of the global C cycles. To gain a detailed insight into blue C storage and controlling factors, we studied 142 sites across ca. 5000 km of coastal wetlands, covering temperate, subtropical and tropical climates in China. The wetlands represented 6 vegetation types (Phragmites australis, mixed of P. australis and Suaeda, single Suaeda, Spartina alterniflora, mangrove (Kandelia obovata and Avicennia marina), tidal flat) and 3 vegetation types invaded by S. alterniflora (P. australis, K. obovata, A. marina). Our results revealed large spatial heterogeneity in SOC density of the top 1‐meter ranging 40–200 Mg C ha‐1, with higher values in mid‐latitude regions (25–30° N) compared to those in both low‐ (20° N) and high‐ latitude (38–40° N) regions. Vegetation type influenced SOC density, with P. australis and S. alterniflora having the largest SOC density, followed by mangrove, mixed P. australis and Suaeda, single Suaeda and tidal flat. SOC density increased by 6.25 Mg ha‐1 following S. alterniflora invasion into P. australis community, but decreased by 28.56 and 8.17 Mg ha‐1 following invasion into K. obovata and A. marina communities. Based on field measurements and published literature, we calculated a total inventory of 57 ×106 Mg C in the top 1‐meter soil across China’s coastal wetlands. Edaphic variables controlled SOC content, with soil chemical properties explaining the largest variance in SOC content. Climate did not control SOC content, but had a strong interactive effect with edaphic variables. Plant biomass and quality traits were a minor contributor in regulating SOC content, highlighting the importance of quantity and quality of OC inputs and the balance between production and degradation within the coastal wetlands. These findings provide new insights into blue C stabilization mechanisms and sequestration capacity in coastal wetlands.
Litter plays an important role in plant invasion. Our previous studies have found that litter had two opposite effects as Spartina alterniflora invading native plant Phragmites australis community. Litter stimulated the growth of S. alterniflora on the edge of the community, while it inhibited the growth in the old community. Accordingly, we hypothesized that litter may play different roles in different stages of plant invasions. We selected three types of communities: P. australis monoculture (PAM), S. alterniflora monoculture (SAM), S. alterniflora – P. australis mixture (SPM) and conducted a comparative study by keeping and removing litter at Chongming Dongtan in the Yangtze River estuary, China. Our results revealed that litter removal significantly increased light intensity in SAM, and enhanced it in SPM, but had no significant influence on it in PAM. The effect of litter on light intensity resulted in a higher chlorophyll content and higher plant height of S. alterniflora monoculture after litter removal, which eventually led to a higher single weight and aboveground biomass for PAM and SAM after litter removal, and to a higher aboveground biomass for S. alterniflora in SPM. Litter removal significantly enhanced the dominance of S. alterniflora relative to P. australis in SPM. We concluded that the effects of invasive plants' litter on the invasion processes depend on the stem density of invasive plants relative to native plants. If an invasive species had a higher plant density, its litter may not have a shelter effect on native species, but could restrict its own growth.
Leaf litter is an important nutrient source for benthic communities within mangrove ecosystems and in adjacent areas. Exotic Spartina alterniflora invasion to the eastern coastlines of China has been shown to alter trophic relationship of benthos in mangrove wetlands, but little is known about potential effect of exotic liter decomposition on food sources of benthic macrofauna. We took the advantage of significant differences in the δ13C values of exotic S. alterniflora (C4 plant) from a native mangrove species Kandelia obovata (C3 plant) to investigate leaf carbon assimilation by polychaetes during litter decomposition. We found that exotic S. alterniflora leaf litter decomposed more slowly than that of native mangrove species at the mangrove site (P = 0.003), but there was no significant difference between them at the salt marsh site (P = 0.270). The polychaetes assimilated major organic matter from K. obvata and POM (>90%) at the mangrove site. But the polychaetes assimilated major organic matter from S. alterniflora (>60%) at the salt marsh site. However, after adding S. alterniflora leaf litter at the mangrove site, polychaetes assimilated more carbon from S. alterniflora (>45%) than that from K. obovata (<25%). Furthermore, the significant carbon assimilation only occurred within a short period time (1–2 months) during decomposition. Our results suggested that Spartina invasion can change food sources of local mangrove polychaetes through utilizing the organic matter released from litter decomposition, but such assimilation of exotic foods is of short time nature.
The standing dead phase is an important stage in the decomposition of emergent vegetation in marsh wetlands, yet few studies have examined how intrinsic litter traits constrain rates of standing litter decomposition or fungal colonization across plant tissue types or species. To address broad constraints on the decomposition of standing dead litter, we conducted a systematic survey of emergent standing dead decomposition studies that measured decay rates and/or fungal biomass, and litter % lignin, carbon:nitrogen (C:N) and/or carbon:phosphorus (C:P). Across 52 datasets, litter of low C:N and C:P ratios exhibited increased decomposition rates (r = −0.737 and −0.645, respectively), whereas % lignin was not significantly correlated with decomposition rates (r = 0.149). Mixed‐effects models for litter decomposition rates indicated significant effects of litter molar C:N and C:N + lignin as an additive model, with the former providing marginally better support. Litter % lignin, however, was strongly negatively correlated with fungal biomass (r = −0.669), indicating greater fungal colonization of low‐lignin litter, and not correlated with C:N (r = −0.337) and C:P (r = −0.456) ratios. The best‐supported model predicting fungal biomass was litter molar C:N, with the C:N + lignin additive model also showing significant effects. Fungal carbon‐use efficiency (CUE) also had a strong negative correlation with % lignin (r = −0.604), molar C:N (r = −0.323) and C:P (r = −0.632) across datasets. Our study demonstrates the constraining effects that litter stoichiometry and % lignin elicit on decomposition of standing dead litter and fungal colonization, respectively. These findings improve our understanding of biogeochemical cycling and prediction of the fates of C and nutrients in wetlands.
Carbon, nitrogen, and phosphorus—nutrient and restrictive elements for plant growth and important components of the plant body—are mainly transferred and exchanged between plants and the soil environment. Changes in the carbon, nitrogen, and phosphorus eco-stoichiometry greatly impact the growth and expansion of Spartina alterniflora, and understanding these changes can reveal the nutrient coordination mechanism among ecosystem components. To explore the relationship between leaf and soil eco-stoichiometry and determine the key soil factors that affect leaf eco-stoichiometry, we collected leaf and soil samples of S. alterniflora at different tidal levels (i.e., 1, 3, and 5 km away from the coastline) in a coastal wetland in the Yancheng Elk Nature Reserve, Jiangsu province. We measured the leaf and soil carbon, nitrogen, and phosphorus contents and ratios, as well as the soil salinity and soil organic carbon. The results revealed the following. (1) The leaf stoichiometric characteristics and soil properties of S. alterniflora differed significantly between tidal levels; for example, total carbon, nitrogen, soil organic carbon were detected at their highest levels at 3 km and lowest levels at 5 km. (2) Significant correlations were detected between the leaf stoichiometric characteristics and soil characteristics. Additionally, nitrogen limitation was evident in the study area, as indicated by the nitrogen–phosphorus ratio being less than 14 and the soil nitrogen–phosphorus ratio being less than 1. (3) Soil salinity and the soil carbon–nitrogen ratio were shown to be the key factors that affect the eco-stoichiometric characteristics of S. alterniflora. These findings furthered our understanding of the nutrient distribution mechanisms and invasion strategy of S. alterniflora and can thus be used to guide S. alterniflora control policies formulated by government management departments in China.
Increasing human activities are causing global changes, such as elevated atmospheric nitrogen deposition and increased biological invasions. Both atmospheric nitrogen deposition and biological invasions are recognized as increasingly prominent features of ecological landscapes throughout the world, and their interactions affect the structure and function of global ecosystems. Thus, there is considerable interest in understanding the mechanism of the invasion of non-native plants under atmospheric nitrogen deposition, specifically in terms of global nitrogen cycling and its potential contribution to the ongoing global change in coming decades. We reviewed the invasiveness of non-native plants under atmospheric nitrogen deposition. We discuss gaps in this research topic in an effort to guide further research.
The rapid invasion of the plant Spartina alterniflora in coastal wetland areas can threaten the capacity of their soils to store carbon (C), nitrogen (N), and sulfur (S). In this study, we investigated the spatial and temporal distribution patterns of C, N and S of both soil and (native and invasive) plants in four typical coastal wetlands in the core area of the Yancheng National Nature Reserve, China. The results show that the invasive S. alterniflora greatly influenced soil properties and increased soil C, N and S storage capacity: the stock (mean ± standard error) of soil organic carbon (SOC, (3.56 ± 0.36) kg/m3), total nitrogen (TN, (0.43 ± 0.02) kg/m3), and total sulfur (TS, (0.69 ± 0.11) kg/m3) in the S. alterniflora marsh exceeded those in the adjacent bare mudflat, Suaeda salsa marsh, and Phragmites australis marsh. Because of its greater biomass, plant C ((1193.7 ± 133.6) g/m2), N ((18.8 ± 2.4) g/m2), and S ((9.4 ± 1.5) g/m2) storage of S. alterniflora was also larger than those of co-occurring native plants. More biogenic elements circulated in the soil-plant system of the S. alterniflora marsh, and their spatial and temporal distribution patterns were also changed by the S. alterniflora invasion. Soil properties changed by S. alterniflora’s invasion thereby indirectly affected the accumulation of soil C, N and S in this wetland ecosystem. The SOC, TN, and TS contents were positively correlated with soil electrical conductivity and moisture, but negatively correlated with the pH and bulk density of soil. Together, these results indicate that S. alterniflora invasion altered ecosystem processes, resulted in changes in net primary production and litter decomposition, and increased the soil C, N and S storage capacity in the invaded ecosystems in comparison to those with native tallgrass communities in the coastal wetlands of East China.
Our understanding of fine-root decay processes is derived almost exclusively from litterbag studies. However, preparation of roots for litterbag studies and their subsequent decay within litterbags represent major departures from in situ conditions. We hypothesized that litterbag studies misrepresent fine-root decay and nutrient release rates during decomposition. To test these hypotheses, we developed a new intact-core technique that requires no a priori root processing, retains natural rhizosphere associations, and maintains in situ decay conditions. Using both litterbags and intact cores, we measured annual decay rates and nitrogen release from newly senesced fine roots of silver maple, maize, and winter wheat. After one year, mass loss was 10–23% greater, and nitrogen release was 21–29% higher within intact cores. Differences appeared to result from litterbag-induced
alterations to decomposer dynamics and from unavoidable changes to fine-root size-class composition within litterbags. Our results suggest that fine-root decay and nutrient turnover
occur significantly faster than estimated from litterbag studies. By minimizing disturbances to roots, soil, and rhizosphere associates prior to root decay, the intact-core technique provides an improved alternative for measuring fine-root decomposition.
Smooth cordgrass Spartina alterniflora Loisel., a perennial rhizomatous grass native to the Atlantic and Gulf coasts of North America, spreads rapidly in estuaries and coastal salt marshes in the Pacific coast of North America, Europe, New Zealand and China, and has caused considerable effects on the invaded regions. We here describe a comprehensive account of its biology and ecology, and discuss the management of this invasive plant. S. alterniflora was intentionally introduced to China in 1979 for the purposes of erosion check, soil melioration and dike protection. However, its rapid elongation rates, high leaf area indices, high photosynthetic rates, long photosynthetic season and clonal growth make S. alterniflora an aggressive competitor with native salt marsh plants in the coastal regions in China. The estimates made for the year 2002 show that S. alterniflora covered 112000 hectares throughout the eastern China, from Guangxi (21º N) to Tianjin (39º N), and is still spreading rapidly in the east coast of China. The successful invasion of S. alterniflora in non-native ranges is obviously the result of the interactions between its great invading ability and a high invasibility of the invaded ecosystems, which is further facilitated by human activities. On the basis of its population trend and potential impact on native ecosystems, S. alterniflora was officially placed on the list of most harmful invasive alien plants (nine species) in China in 2003. S. alterniflora invasions in the salt marshes have multiple effects on the abiotic and biotic properties and the functioning of the invaded ecosystems, including conversion of mudflats to Spartina meadows, loss of shorebirds' foraging habitats, alteration of ecosystem processes (e.g. carbon and nitrogen cycling), decrease in abundance of native species, degradation of native ecosystems and their functions, and considerable economic loss. It is predicted that the environmental changes driven by human activities in the coastal regions (e.g. eutrophication, sea level rise and saltwater intrusion) may favour its further invasions in coastal ecosystems in the future. Like other invasive species, it is quite difficult, expensive and even impossible to eradicate S. alterniflora once it has successfully invaded the coastal ecosystems. Obviously, further intentional introductions of S. alterniflora should be banned in China, and effective control
The short height form of Spartina alterniflora observed in inland areas of Louisiana Gulf Coast marshes is caused by toxic concentrations of sulphide, a result of slightly lower elevation and subsequently lower sediment redox potential than the adjacent productive streamside marsh. Sulphide may limit growth by preventing N uptake and root development. Total sulphide concentrations as high as 250 mu g g -1 of sediment and free sulphide concentrations in the order of 150 mu g g -1 were found in the inland sediment. The inland sediment contained higher NH 4+-N concentrations at depth than the streamside sediment. Maximum NH 4+-N concentrations of 60 and 16 mu g N g -1 of dry soil was observed in the inland and streamside sediment, respectively. Redox potential was higher at the surface sediment of the more productive streamside marsh compared to the adjoining inland marsh. The higher redox potential is mainly due to the higher elevation of the streamside area that permitted greater drainage. This in turn allowed the development of more oxidized sediment containing lower sulphide levels.-from Authors
Our understanding of fine-root decay processes is derived almost exclusively from litterbag studies. However, preparation of roots for litterbag studies and their sub- sequent decay within litterbags represent major departures from in situ conditions. We hypothesized that litterbag studies misrepresent fine-root decay and nutrient release rates during decomposition. To test these hypotheses we developed a new intact-core technique that requires no a priori root processing, retains natural rhizosphere associations, and main- tains in situ decay conditions. Using both litterbags and intact cores, we measured annual decay rates and nitrogen release from newly senesced fine roots of silver maple, maize, and winter wheat. After one year, mass loss was 10-23% greater, and nitrogen release was 21-29% higher within intact cores. Differences appeared to result from litterbag-induced alterations to decomposer dynamics and from unavoidable changes to fine-root size-class composition within litterbags. Our results suggest that fine-root decay and nutrient turnover occur significantly faster than estimated from litterbag studies. By minimizing disturbances to roots, soil, and rhizosphere associates prior to root decay, the intact-core technique provides an improved alternative for measuring fine-root decomposition.
Seasonal patterns of aboveground plant mass and the depth distribution of live roots, rhizomes, and dead belowground organic matter were measured for Spartina alterniflora and Spartina cynosuroides in Georgia tidal marshes. Peak live aboveground biomass was 1.6 × higher for S. cynosuroides than for S. alterniflora. Live biomass was 2.4 × more belowground than aboveground for S. cynosuroides and 1.7 × for S. alterniflora. Rhizomes made up 76 and 87% of live belowground biomass during the year. Mirrored patterns of biomass accumulation and loss in above‐ and belowground tissues during the year suggest the importance of seasonal storage and redistribution of organic matter.
Belowground production was measured with a technique that partially accounts for midseason decomposition. Total plant production was estimated to be 7,620 g dry mass·m ⁻² ·yr ⁻ ’ for S. alterniflora and 7,708 for S. cynosuroides. Belowground production was roughly 1.6 × aboveground production. Turnover rates for belowground live material were 1.42 · yr ⁻¹ for S. cynosuroides and 3.22·yr ⁻¹ for S. alterniflora. The fate of root and rhizome material, including the extent to which such material enters the estuarine or nearshore food webs, is not clear.
We investigated the source and composition of free and protein-bound amino acids during the decomposition ofSpartina alterniflora Loisel in laboratory percolators and in a field experiment in the Great Sippewissett Marsh (Falmouth, Massachusetts). In
the percolator experiment, 50% of the nitrogen (N) could be extracted fromS. alterniflora litter in 16 d. This extract consisted of dissolved free amino acid N (28%), suspended protein amino acid N (16%), inorganic
N (12%), and nitrogen from unidentified compounds (44%). Much of the free amino acid nitrogen was utilized by detrital microorganisms,
resulting in a greater loss of suspended protein amino acid (SPAA) nitrogen from the biologically active percolator due to
microbials biomass. Suspended microbial mass accounted for at least 50% of the SPAA washed out of the biologically active
percolator. In the field study, 38% of the original litter nitrogen was leached fromS. alterniflora litter in litterbags during the first 13 d. After this initial leaching period, the concentration (41% to 69% of total nitrogen)
and composition of most amino acids bound in the litter did not change over the 23-month period of the experiment. Increases
in microbial protein did not account for increases in total nitrogen which occurred during the decomposition of the litter.
Similarly, adsorbed ammonium did not appear to be responsible for this increase.
Aboveground and belowground biomass ofSpartina alterniflora were harvested during the period of peak aerial biomass from six sites along a latitudinal gradient ranging from Georgia
to Nova Scotia. An equation relating live aboveground to live belowground biomass for short-form plants was formulated, using
data collected in Delaware marshes. When data from the other sites were substituted into the equation, the mean live belowground
biomass it predicted was within 15% of the value determined by harvesting at four of the five sites. At all sites, short-form
plant live belowground biomass was concentrated in the upper 10 cm. Dead belowground biomass was located mostly in the top
15 cm in southern marshes, but was more evenly distributed with depth in northern marshes. Results were more ambiguous for
tall-form plants, probably because of greater spatial variability in biomass distribution, and greater seasonal biomass dynamics.
Spartina alterniflora, smooth cordgrass, invading San Francisco Bay, California (USA), is attacked by high densities of a plant hopper, Prokelisia marginata, and a mirid bug, Trigonotylus uhleri. Both herbivores are sap-feeders. We investigated the impact of these herbivores on S. alterniflora's growth rate, vegetative spread, and seed production by manipulating herbivore densities in the field and in a greenhouse.
Herbivore densities in the field peaked in early fall, with P. marginata averaging more than 300 individuals per mature culm of S. alterniflora (about 100,000 per m2) and T. uhleri densities exceeding 10 per culm (about 3,000 per m2). Field reductions of herbivore densities by approximately 70% with insecticidal soap did not result in greater vegetative
growth rates or lateral spread of plants; plants grew vigorously with the highest densities of insects. In the greenhouse
study, conducted with seedlings, herbivory significantly reduced plant mass and tiller number in some but not all replicate
herbivory treatments. In both field and greenhouse, there were significant differences between some clones' growth rates independent
of herbivory. Inflorescence production in the field was not affected by reduced-herbivory treatments. Seed set was low under
conditions of both natural and reduced herbivory, averaging 0.4%. Despite densities of P. marginata and T. uhleri that are much higher than typically observed in areas where S. alterniflora is native, herbivory by these particular insects appears to have little impact and in unlikely to limit S. alterniflora’s spread through San Francisco Bay. *** DIRECT SUPPORT *** A01BY070 00009
Whether plant invasion increases ecosystem carbon (C) stocks is controversial largely due to the lack of knowledge about differences
in ecophysiological properties between invasive and native species. We conducted a field experiment in which we measured ecophysiological
properties to explore the response of the ecosystem C stocks to the invasion of Spartina alterniflora (Spartina) in wetlands dominated by native Scirpus mariqueter (Scirpus) and Phragmites australis (Phragmites) in the Yangtze Estuary, China. We measured growing season length, leaf area index (LAI), net photosynthetic rate (Pn), root
biomass, net primary production (NPP), litter quality and litter decomposition, plant and soil C and nitrogen (N) stocks in
ecosystems dominated by the three species. Our results showed that Spartina had a longer growing season, higher LAI, higher Pn, and greater root biomass than Scirpus and Phragmites. Net primary production (NPP) was 2.16kgCm−2y−1 in Spartina ecosystems, which was, on average, 1.44 and 0.47kgCm−2y−1 greater than that in Scirpus and Phragmites ecosystems, respectively. The litter decomposition rate, particularly the belowground decomposition rate, was lower for Spartina than Scirpus and Phragmites due to the lower litter quality of Spartina. The ecosystem C stock (20.94kgm−2) for Spartina was greater than that for Scirpus (17.07kgm−2), Phragmites (19.51kgm−2) and the mudflats (15.12kgm−2). Additionally, Spartina ecosystems had a significantly greater N stock (698.8gm−2) than Scirpus (597.1gm−2), Phragmites ecosystems (578.2gm−2) and the mudflats (375.1gm−2). Our results suggest that Spartina invasion altered ecophysiological processes, resulted in changes in NPP and litter decomposition, and ultimately led to enhanced
ecosystem C and N stocks in the invaded ecosystems in comparison to the ecosystems with native species.
The Yangtze River estuary is an important ecoregion. However, Spartina alterniflora, native to North America, was introduced to the estuary in the 1990s through both natural dispersal and humans and now it is a dominant species in the estuarine ecosystems, with its invasions leading to multiple consequences to the estuary. S. alterniflora had great competitive effects on native species, including Scirpus mariqueter and Phragmites australis, and could potentially exclude the natives locally. The presence of S. alterniflora had little influence on the total density of soil nematodes and macrobenthonic invertebrates, but significantly altered the structure of trophic functional groups of nematode and macrobenthonic invertebrate communities. The conversion of mudflats to Spartina meadows had significant effects on birds of Charadriidae and Scolopacidae, which might be attributable to the reduction of food resources and the physical alterations of habitats for shorebirds. S. alterniflora invasions increased the primary productivity of the invaded ecosystems, and altered carbon and nitrogen cycling processes. Our studies focused mainly on the effects of S. alterniflora invasions on the structure of native ecosystems; thus further studies are clearly needed to investigate how ecosystem functioning is affected by the modification of the structure of estuarine ecosystems by S. alterniflora invasions.
Standing litter of emergent macrophytes often forms a major portion of the detrital mass in wetland habitats. Microbial assemblages inhabiting this detritus must adapt physiologically to daily fluctuations in temperature and water availability. We examined the effects of various environmental conditions on the concentrations of osmoregulatory solutes (polyols and trehalose) and the respiratory activities of fungal assemblages inhabiting standing litter of the freshwater emergent macrophyte Juncus effusus. Under field conditions, the concentrations of osmolytes (polyols plus trehalose) in fungal decomposers were negatively correlated with plant litter water potentials (r = -0.75, P < 0.001) and rates of microbial respiration (r = -0.66, P < 0.001). The highest concentration of osmolytes (polyols plus trehalose) occurred in standing litter exposed to desiccating conditions (range from wet to dry, 0.06 to 0.68 mumol . mg of fungal biomass). Similar fluctuations in polyol and trehalose concentrations were observed in standing litter wetted and dried under laboratory conditions and for four predominant fungal decomposers of J. effusus grown individually on sterilized Juncus leaves. These studies suggest that fungal inhabitants associated with standing litter of emergent macrophytes can adjust their intracellular solute concentrations in response to daily fluctuations in water availability.
The carbon balance in terrestrial ecosystems is determined by the difference between inputs from primary production and the return of carbon to the atmosphere through decomposition of organic matter. Our understanding of the factors that control carbon turnover in water-limited ecosystems is limited, however, as studies of litter decomposition have shown contradictory results and only a modest correlation with precipitation. Here we evaluate the influence of solar radiation, soil biotic activity and soil resource availability on litter decomposition in the semi-arid Patagonian steppe using the results of manipulative experiments carried out under ambient conditions of rainfall and temperature. We show that intercepted solar radiation was the only factor that had a significant effect on the decomposition of organic matter, with attenuation of ultraviolet-B and total radiation causing a 33 and 60 per cent reduction in decomposition, respectively. We conclude that photodegradation is a dominant control on above-ground litter decomposition in this semi-arid ecosystem. Losses through photochemical mineralization may represent a short-circuit in the carbon cycle, with a substantial fraction of carbon fixed in plant biomass being lost directly to the atmosphere without cycling through soil organic matter pools. Furthermore, future changes in radiation interception due to decreased cloudiness, increased stratospheric ozone depletion, or reduced vegetative cover may have a more significant effect on the carbon balance in these water-limited ecosystems than changes in temperature or precipitation.
Plant species affect natural ecosystems through interactions between environmental and genetic factors. The importance of plant species in controlling decomposition is now well-established through its influence on litter quality, which affects mass loss and nutrient release. At the same time, direct species effects are often confounded with indirect site effects due to the ecophysiological responses of plants to environmental variability. We evaluated the intrinsic effects of species on litter quality and decomposition, comparing 14 native perennial grass species from three different grassland ecosystems in North and South America. Plants were grown under controlled greenhouse conditions to eliminate any indirect effects of climate on litter quality, and senescent material of leaf litter and roots were collected. The initial litter nutrient quality and the carbon quality were assessed, and decomposition was determined over a period of one year by placing litterbags in a common grassland site. In spite of constant growth conditions, species' litter showed broad and significant differences in N, P and lignin concentration, as well as C:N ratio, with the greatest differences occurring between C(3) and C(4) species and leaf litter and root material. In addition, decomposition was significantly different among species and between leaf litter and roots within species, with constants (k) ranging from 1.50 to 3.65 year(-1) for leaf litter, and 0.51-1.82 year(-1) for roots. These results highlight the fact that, independent of climate or edaphic changes due to human activity, changes in plant species or in allocation patterns among plant organs in grassland ecosystems could have a large effect on carbon turnover. At the same time, the way in which intrinsic species characteristics affect decomposition demonstrates a large degree of functional convergence among species from grasslands of North and South America.
Litter decomposition provides the primary source of mineral nitrogen (N) for biological activity in most terrestrial ecosystems.
A 10-year decomposition experiment in 21 sites from seven biomes found that net N release from leaf litter is dominantly driven
by the initial tissue N concentration and mass remaining regardless of climate, edaphic conditions, or biota. Arid grasslands
exposed to high ultraviolet radiation were an exception, where net N release was insensitive to initial N. Roots released
N linearly with decomposition and exhibited little net N immobilization. We suggest that fundamental constraints on decomposer
physiologies lead to predictable global-scale patterns in net N release during decomposition.
Decomposition dynamics of two salt marsh species, Spartina alterniflora tall (SAT) form and Spartina patens (SP), were investigated at Mud Cove, near Manahawkin, Ocean County, New Jersey. Decomposition rates were determined monthly over 371 days by measuring the amount of material lost from plastic net litter bags. Litter bags containing SP material were placed in both SP and SAT vegetation zones; litter bags containing SAT material were also placed in both of these zones. The material was analyzed for ash content, total carbon, Kjeldahl nitrogen, caloric content, fat, and crude fiber.
Final weight loss values were as follows: SAT in SAT zone 72.4%, SP in SAT zone 56.9%, SAT in SP zone 46.7%, and SP in SP zone 26.4%. When SAT and SP material were placed in the same location (SAT or SP sites), the SAT material decomposed at a greater rate. If the same vegetation type (SAT or SP) were placed in both SAT and SP locations, the material at the SAT location decomposed at a greater rate than similar material in the SP location. The results indicate that while environmental characteristics (e.g., flooding) at a site influence the rate at which these two species will decompose, Spartina patens is inherently more resistant to decomposition.
The capacity of cetyl trimethylammonium bromide to dissolve proteins in acid solution has been utilized in development of a method, called acid-detergent fiber method (ADF), which is not only a fiber determination in itself but also the major preparatory step in the determination of lignin. The entire procedure for determining fiber and lignin is considerably more rapid than presently published methods. Compositional studies show ADF to consist chiefly of lignin and polysaccharides. Correlations with the new fiber method and digestibility of 18 forages (r = —0.79) showed it to be somewhat superior to crude fiber (r = —0.73) in estimating nutritive value. The correlation of the new lignin method and digestibility was —0.90 when grass and legume species were separated.
Researchers interested in accurately describing natural microbial participation in the decay of portions of vascular plants must try to avoid altering genuine conditions of decay via their methods (Swift et al., 1979; Boulton and Boon, 1991; Newell, 1994). This is an old refrain (e.g., Park, 1974, on the elusive balance between particle retention and shredder admittance using litterbags), but one that continues to go unheeded (e.g., a paper published in 1992 in a leading aquatic-science journal; methods: green shoots of grass cut, dried, and placed in litterbags on or buried in salt-marsh sediment). “The challenge of ecology immediately reveals the correlated dangers, for it is singularly easy to fall into error through a failure to describe accurately the various parts of the system and to appreciate their possible significance. An error at this stage may lead to the development of inapposite experimental techniques, so that the final synthesis must inevitably fail. A constant temptation besetting the ecologist is that of loose thinking to enable him to gloss over intractable parts of his study” (Griffin, 1972).
This study investigated the use of satellite remote sensing for the mapping of vegetation in the intertidal zone of the Shanghai region. A Landsat5-TM multi-spectral satellite image, dated August 2, 2003, was used. The image was processed using the ERDAS Imagine 8.6 image processing software. A subset of the Landsat image was geometrically corrected. Information on the spectral properties of the vegetation were integrated with expert systems knowledge to produce a classification for the intertidal. vegetation. An extensive field survey in situ was then carried out to verify and revise the results of the image classification. The revised results of the classification were integrated using a GIS platform and the spatial distribution and quantities of the intertidal plant communities were analyzed. The results showed that the total area of the intertidal vegetation in the Shanghai region was 21302.1hm 2 and the vegetation was mostly composed of Phragmites australis communities, Scirpus mariqueter communities and Spartina alterniflora communities. There was a distinct zonation in the distribution of the intertidal vegetation communities along a gradient of altitude. The results of this research on the intertidal vegetation in Shanghai will provide a sound basis for resource management and planning, preservation of biodiversity, sustainable development and utility of wetlands in the region.
Flooding of nearly 3 yr was ineffective in removing common reed litter, whereas 1 yr of inundation would be sufficient to remove most hybrid cattail Typha glauca, whitetop rivergrass Scholochloa festucacea and hardstem bulrush Scirpus lacustris glaucus litter. Flooding common reed Phragmites australis litter resulted in net loss of nutrients available within the wetland because of uptake of nutrients by microbes associated with the litter. Flooding cattail had little effect on nutrients available even after 3 yr of inundation. Whitetop and bulrush litter released most of their nutrients quickly. -from Authors
Although freshwater shorelines occupy extensive areas of the temperate zone, we still have few conceptual models for pattern and process in shoreline vegetation. This study uses multivariate vegetation data to describe vegetation-environment relationships in a set of riverine wetlands and then explores general relationships between pattern and process. Samples were collected from five marshes along the Ottawa River (eastern Canada) (n = 94 sample units). Detrended correspondence analysis was used to described major gradients, and TRWINSPAN was used to classify vegetation types. TWINSPAN produced four major classes dominated by Sparganium eurycarpum, Eleocharis smallii, Scirpus americanus, and Typha latifolia. Within each class, two associations could be recognized, differing in the degree to which one species managed to dominate the vegetation. Ordination showed that these vegetation types were arranged along two major axes: a standing crop and litter gradient, and a water depth gradient. Species richness was greatest just above the late August waterline in Eleocharis smallii vegetation that had low fertility, intermediate total biomass (250g/m^2) and low littermass (30 g/m^2). Very high biomass (>400g/m^2) was observed where indices of high fertility and low disturbance coincided. Low species richness in this Typha-dominated vegetation is thought to be a result of competitive exclusion. Exposure to waves, ice, and flowing water produced a fertility gradient. The least fertile sites had small evergreen species such as Eriocaulon septangulare and Ranunculus flammula. These species possessed traits associated with Grime's "stress tolerator" strategy. The three main factors controlling vegetation competition were water depth, the effects of spring flooding in removing litter, and the fertility gradient produced by waves and flowing water. These were incorporated into a conceptual model including both patterns and processes observed along the Ottawa River.
Aboveground and belowground net primary productivity estimates of the dominant angiosperms from two tidal marshes along the Delaware Bay estuary are reported. Ash, carbon and nitrogen content of the aboveground and belowground components were also determined. Annual net aboveground production was determined by the peak live standing crop method and the Smalley (1958) method. Aerial production of tall Spartina alterniflora Loisel. at the Canary Creek salt marsh, employing the Smalley technique, was 1487 g m-2 yr-1, while production of short S alterniflora, S. patens (Ait.) Muhl. and Distichlis spicata (L.) Greene was, 654 g m-2 yr-1, 1147 g m-2 yr-1, and 785 g m-2 yr-1, respectively. At the brackish water Blackbird Creek marsh, Phragmites australis (Cav.) Trin. had the highest aboveground production (2940 g m-2 yr-1), followed by S. patens (1089 g m-2 yr-1) and short S. alterniflora (916 g m-2 yr-1) Belowground primary production was determined by the annual increment (max-min) method, with respective estimates of 6.5 kg m-2 yr-1, 5.0 kg m-2 yr-1, and 3.3 kg m-2 yr-1, for tall S alterniflora, short S alterniflora and S. patens, at the Canary Creek marsh. Belowground estimates at the Blackbird Creek marsh were within this range.
In situ decomposition of roots and rhizomes of the marsh plants, Juncus roemerianus and Spartina cynosuroides was investigated using litter bags. The decomposition rate was greatest in the top 10 cm (20% mass loss/yr) of the marsh soil. There was no apparent decomposition below 20 cm depth. Belowground tissues of S. cynosuroides decomposed faster than those of J. roemerianus during the first 4 mo. The rhizome decomposition rate of 27%/yr (mass loss) was faster than the 16%/yr of the roots of J. reomerianus. There was no difference between the composition rate of mixed root and rhizome materials between experiments initiated in winter and those started in the spring. This indicates a relatively constant decomposition rate during the year in the 0-10 cm soil zone. There was no apparent trend in the hydrogen, carbon, phosphorus, nitrogen, or caloric content changes of the decomposing roots and rhizomes during the study.
Macrophytes in the littoral zone contribute with a large fraction of the primary production in many lakes. To examine the effect of solar radiation on the degradation of macrophyte detritus, we incubated senescent leaves of Phragmites australis in shallow water in shade, under full solar radiation, and under filters removing either UVB or both UVA and UVB. Several parameters were measured during two months to monitor the composition and quantity of particulate (POM) and dissolved (DOM) organic matter derived from the leaves. Generally there were no or only small differences between the different radiation treatments, which differed markedly from the shaded microcosms. Hence, more DOM was leached from exposed detritus, corresponding to a higher loss of POM than in the shade. However, the fluorescence of the DOM, an indicator of the concentration of humic matter, was higher in the shade and in microcosms only receiving photosynthetically active radiation (PAR) than in microcosms exposed to UVA or both UVA and UVB, indicating that PAR is not effective in the photolysis of humic matter. This experiment indicates that solar radiation promotes the leaching of DOM from littoral detritus. Not only UV radiation, but also PAR is effective in this process.
The influence of Spartina alterniflora marshes and detritus availability on the spatial structure and time variability of macrobenthic associations was investigated in Paranaguá Bay, a subtropical estuary in southern Brazil. Four sampling sites were established along the salinity and exposure gradient of Paranaguá Bay in the winter and summer of 1992. At each site, 5 samples (0.02 m^2corer) were taken in the salt marshes and adjacent unvegetated tidal flats. Sediment composition, salt marsh structure (stem height and density, live and dead above- and below-ground biomass of S. alterniflora) and vegetal detritus biomass were determined to assess whether they can affect the dynamics of macrobenthic invertebrates. The development of salt marshes and the amount of detritus, the latter mainly originated from adjacent mangroves, were determined by the energy and salinity gradients along the bay. Plant above- and below-ground components, together with detritus biomass, exert a strong influence on the composition and distribution of macrofauna in Paranaguá Bay. Salt marshes support higher densities and species number of macrobenthic invertebrates. Multivariate analyses showed that oscillations of different fractions of Spartina alterniflora, as well as detritus availability, affect the different components of macrobenthic associations in a variable degree. Spatial and temporal shifts of infaunal organisms appear to be more evident than those of mobile epibenthic forms.
Standing dead plant litter of emergent macrophytes often forms a considerable portion of the detrital mass in wetland habitats. We examined the effects of varying environmental conditions on the diel respiratory activity (rate of CO2 evolution] of microbial assemblages associated with standing dead Litter of the emergent macrophyte Juncus effusus L. from a small freshwater wetland in central Alabama, USA. Availability of water was a major factor affecting rates of respiration in standing litter. Under field conditions, rates of CO2 evolution from plant litter fluctuated greatly, with the highest rates occurring at night and in the early morning hours, coinciding with increasing relative humidity (>90%) and plant litter water potentials (>-1.0 MPa). Rates ranged from 2 to 285 mu g CO2-Cg(-1) organic mass h(-1) over 24 h. Daily CO2 flux from microbial decomposers inhabiting standing litter ranged between 1.37 and 3.35 g C m(-2) d(-1). After controlled laboratory additions of water, rates of CO2 evolution from standing litter increased significantly, with sustained maximal rates occurring within 5 min after being wetted (>100 mu g CO2-Cg(-1) organic mass h(-1)). Results of these investigations establish that J. effusus litter is colonized by active microbial decomposers while in the standing dead phase. Furthermore, these microbial assemblages make significant contributions to ecosystem metabolism and may represent an important CO2 output not previously recognized or incorporated in estimates of CO2 flux from wetland habitats.
Most predictions of plant species effects on ecosystems are based on single traits(e.g. litter chemistry) or suites of related traits(functional groups). However, recent studies demonstrate that predictions of species effects are improved by considering multiple traits. In order to develop this multiple trait approach, it is critical to understand how these multiple traits vary in relation to one another among species. The ecosystem effects of traits that strongly covary can likely be summarised by one of these traits. In contrast, it will be necessary to determine ecosystem processes of specific trait combinations for those traits that vary independently across species.
In the field, I established monocultures of eight herbaceous species common in California annual grasslands. Plant species significantly differed in their litter quantity and quality, live biomass, and effects on soil labile C, soil temperature, and soil moisture. Species effects on moisture and temperature were only significant at the times of the growing season when each of these limited plant and microbial activity. Some of these traits correlated with one another, such as litter biomass and species effects on soil temperature during the winter. However, for the most part, plant species exhibited unique combinations of these traits. For example, species with similar litter chemistry had the largest differences in plant biomass, soil moisture, and soil labile C. Species rankings for many traits varied seasonally. The independent variation of these traits suggests that predictions of plant species effects on ecosystems will likely be enhanced by an understanding of how ecosystem effects of plant traits may vary depending on the combination of traits.
Intertidal decaying stems and leaf sheaths of Phragmites australis were randomly collected and their mycota examined. Sixty one species of fungi were associated with the decaying stems and leaf sheaths, including Antiptodera spp., Lignincola laevis, Phornatospora phragmiticola and Zopfiella latipes. The following new species are described, Halosarpheia phragmiticola, Massarina phragmiticola, Phomatospora phragmiticola and Cytoplacosphaeria phragmiticola. The fungal communities associated with decaying Phragmites australis permanently submerged in the gei wai (tidal shrimp farms) differ from those in the intertidal region. The diversity of these fungi are discussed in relation to the biodiversity of fungi in mangrove communities in Hong Kong and with those fungi of other salt marsh communities.
While environmental characteristics (eg. flooding) at a site influence the rate at which these two species will decompose, S. patens is inherently more resistant to decomposition than S. alterniflora. -from Authors
We examined the effects of environmental conditions on the microbially mediated CO 2 evolution from standing-dead litter (leaf blades, leaf sheaths, and culms) of the common reed, Phragmites australis (Cav.) Trin. ex Steud., in two temperate littoral freshwater marshes. Water availability was the major factor affecting CO 2 evolution rates. In the laboratory, microbial assemblages responded rapidly to controlled additions of water, with large increases in CO 2 evolution occurring within five minutes after wetting of litter (e.g., leaf blades: 10–295 g CO 2 -C·(g ash-free dry mass [AFDM]) 1 ·h 1 . Under field conditions, CO 2 evolution in the absence of precipitation exhibited a pronounced diel periodicity, with the highest rates occurring during periods of increased water availability resulting from a temperature-induced rise in relative humidity (95%) and corresponding litter water potential (2.0 MPa) during nighttime. For example, in October, rates of CO 2 evolution over a 24-h cycle ranged from 5 to 223 g CO 2 -C·(g AFDM) 1 ·h 1 for leaf blades and from 10 to 155 g CO 2 -C·(g AFDM) 1 ·h 1 for leaf sheaths. Maximum rates of CO 2 evolution from sheaths were consistently lower than those for leaf blades (by 25%), but were typically an order of magnitude higher than those observed from culm litter (e.g., 1.0–18 g CO 2 -C·(g AFDM) 1 ·h 1 over a diel cycle in August) exposed to identical en-vironmental conditions. Much of the differences in maximum CO 2 evolution rates from different litter types were related (r 2 0.72) to differences in litter associated fungal biomass (leaf blades 34–74 mg dry mass/g AFDM, leaf sheaths 16–67 mg dry mass/g AFDM, and culms 2–7 mg dry mass/g AFDM), which was estimated from litter ergosterol concentra-tions. Based on measured stocks of standing-dead plant litter, estimated daily CO 2 flux from standing-dead shoots ranged between 51 and 570 mg C/m 2 of wetland surface area. These values translate into a roughly estimated annual carbon mineralization equivalent to a mean of 8% (leaf blades), 29% (leaf sheaths), and 3% (culms) of net aboveground plant pro-duction. These data provide compelling evidence that microbial decomposition of plant litter in the aerial standing-dead phase can contribute appreciably to overall carbon flux from marshes to the atmosphere, even in cool temperate climates, where most wetlands occur.
Like most plants with perennial rhizomes and annual shoots, Spartina alterniflora does not abscise its leaves. This brings into question the appropriateness of commonly used litterbag methods for study of natural decomposition of S. alterniflora shoots. We compared decay phenomena for S. alterniflora leaves that were either (i) tagged and left in the natural standing position or (ii) cut and placed in litterbags on the marsh sediment at the same site. Rates of loss of mass and leaf area were similar between standing and cut leaves, but distinct differences were detected between leaf types for the carbon to nitrogen ratio and measures of microbial mass and activity. For example, bacterial standing crop was 4 to 67 times greater for cut leaves (though bacterial cells were 9 to 22 times less active (3H-thymidine incorporation/cell)), and fungal living standing crop (ergosterol content) was 5 to 9 times greater for standing leaves. In areas such as southern temperate zones, where shoots are not pushed onto the sediment by ice and snow, it is better to avoid altering the natural positioning of leaves that are not abscised in studies of shoot decay because (i) standing-dead shoots can harbour active microbial assemblages and (ii) shunting directly to the sediment phase can yield an artificial image of microbial–chemical dynamics.
Plant species are hypothesized to impact ecosystem nitrogen cycling in two distinctly different ways. First, differences in nitrogen use efficiency can lead to positive feedbacks on the rate of nitrogen cycling. Alternatively, plant species can also control the inputs and losses of nitrogen from ecosystems.
Our current understanding of litter decomposition shows that most nitrogen present within litter is not released during decomposition but incorporated into soil organic matter. This nitrogen retention is caused by an increase in the relative nitrogen content in decomposing litter and a much lower carbon-to-nitrogen ratio of soil organic matter. The long time lag between plant litter formation and the actual release of nitrogen from the litter results in a bottleneck, which prevents feedbacks of plant quality differences on nitrogen cycling. Instead, rates of gross nitrogen mineralization, which are often an order of magnitude higher than net mineralization, indicate that nitrogen cycling within ecosystems is dominated by a microbial nitrogen loop. Nitrogen is released from the soil organic matter and incorporated into microbial biomass. Upon their death, the nitrogen is again incorporated into the soil organic matter. However, this microbial nitrogen loop is driven by plant-supplied carbon and provides a strong negative feedback through nitrogen cycling on plant productivity. Evidence supporting this hypothesis is strong for temperate grassland ecosystems. For other terrestrial ecosystems, such as forests, tropical and boreal regions, the data are much more limited. Thus, current evidence does not support the view that differences in the efficiency of plant nitrogen use lead to positive feedbacks.
In contrast, soil microbes are the dominant factor structuring ecosystem nitrogen cycling. Soil microbes derive nitrogen from the decomposition of soil organic matter, but this microbial activity is driven by recent plant carbon inputs. Changes in plant carbon inputs, resulting from plant species shifts, lead to a negative feedback through microbial nitrogen immobilization. In contrast, there is abundant evidence that plant species impact nitrogen inputs and losses, such as: atmospheric deposition, fire-induced losses, nitrogen leaching, and nitrogen fixation, which is driven by carbon supply from plants to nitrogen fixers. Additionally, plants can influence the activity and composition of soil microbial communities, which has the potential to lead to differences in nitrification, denitrification and trace nitrogen gas losses. Plant species also impact herbivore behaviour and thereby have the potential to lead to animal-facilitated movement of nitrogen between ecosystems.
Thus, current evidence supports the view that plant species can have large impacts on ecosystem nitrogen cycling. However, species impacts are not caused by differences in plant quantity and quality, but by plant species impacts on nitrogen inputs and losses.
Using comparative analysis of the rates of key processes, we have docu-mented the net effect of a shift in plant species composition on nitrogen cycles with the example of the rapid expansion of Phragmites australis (common reed) and its replacement of short grasses (e.g., Spartina patens) in coastal marshes of the eastern United States. In this study, we measured nitrogen (N) uptake by marsh plants, N adsorption from the water column by litter, changes in N content of litter, sediment N mineralization, nitrification, and nitrate consumption in adjacent plots dominated either by P. australis or by historically dominant S. patens. Rates of individual processes were generally greater in P. australis than in S. patens, but the magnitude of difference varied greatly among processes. Seasonal measurements of standing stock nitrogen in plant tissue indicate that P. australis took up 60% more N than did S. patens, and annual rates of N immobilization were nearly 300% greater in P. australis litter than in S. patens litter. The greater demand for N in P. australis plots, however, was apparently compensated for by increased rates of N supply; mineral-ization rates in P. australis sediments were nearly 300% greater than those in sediments with S. patens. Rates of nitrate reduction (dissimilatory and assimilatory) were 300% greater in P. australis sediments. Whereas P. australis clearly sequestered more N in live and dead biomass than did S. patens, the presence of P. australis also stimulated the microbial production of inorganic N. This compensation of increased N demand with increased N supply suggests that the net nitrogen budget (inputoutput) of brackish tidal marshes is not immediately altered by the replacement of S. patens with P. australis. However, the greater magnitude of internal N cycling in P. australis communities is likely to influence the mobility of N pools, thereby altering pathways of N export.
Summary • Large pools of dead wood in mangrove forests following disturbances such as hurricanes may influence nutrient fluxes. We hypothesized that decomposition of wood of mangroves from Florida, USA (Avicennia germinans, Laguncularia racemosa and Rhizophora mangle), and the consequent nutrient dynamics, would depend on species, location in the forest relative to freshwater and marine influences and whether the wood was standing, lying on the sediment surface or buried. • Wood disks (8–10 cm diameter, 1 cm thick) from each species were set to decompose at sites along the Shark River, either buried in the sediment, on the soil surface or in the air (above both the soil surface and high tide elevation). • A simple exponential model described the decay of wood in the air, and neither species nor site had any effect on the decay coefficient during the first 13 months of decomposition. • Over 28 months of decomposition, buried and surface disks decomposed following a two-component model, with labile and refractory components. Avicennia germinans had the largest labile component (18 ± 2% of dry weight), while Laguncularia racemosa had the lowest (10 ± 2%). Labile components decayed at rates of 0.37–23.71% month−1, while refractory components decayed at rates of 0.001–0.033% month−1. Disks decomposing on the soil surface had higher decay rates than buried disks, but both were higher than disks in the air. All species had similar decay rates of the labile and refractory components, but A. germinans exhibited faster overall decay because of a higher proportion of labile components. • Nitrogen content generally increased in buried and surface disks, but there was little change in N content of disks in the air over the 2-year study. Between 17% and 68% of total phosphorus in wood leached out during the first 2 months of decomposition, with buried disks having the greater losses, P remaining constant or increasing slightly thereafter. • Newly deposited wood from living trees was a short-term source of N for the ecosystem but, by the end of 2 years, had become a net sink. Wood, however, remained a source of P for the ecosystem. • As in other forested ecosystems, coarse woody debris can have a significant impact on carbon and nutrient dynamics in mangrove forests. The prevalence of disturbances, such as hurricanes, that can deposit large amounts of wood on the forest floor accentuates the importance of downed wood in these forests. Journal of Ecology (2005) doi: 10.1111/j.1365-2745.2005.00970.x
1. The effects of solar radiation on bacterial and fungal growth on aquatic macrophyte detritus were studied in a microcosm experiment. Senescent leaves of Phragmites australis were incubated for 63 days in shallow water in the shade under photosynthetically active radiation (PAR) together with ultraviolet radiation, or under filters removing either ultraviolet B (UVB) or both UVB and ultraviolet A (UVA).
2. Bacterial abundance and bacterial 3H-leucine incorporation in the water were measured, together with α- and β-D-glucosidase activity. In addition, bacterial abundance and fungal biomass associated with the litter were measured.
3. The results indicate that both PAR and UVA affect the micro-organisms involved in the decomposition of leaf litter. The α/β-D-glucosidase activity ratio was less than one in irradiated and more than one in shaded microcosms, suggesting a change in the substrate dissolved organic matter composition towards more β- than α-glycosidic linkages as a result of solar radiation.
4. Microcosms receiving UVB displayed a significantly higher β-D-glucosidase activity than shaded microcosms, and those exposed to PAR or PAR + UVA, demonstrating the potential importance of UVB radiation.
5. The free-living bacteria tended to be dominated by filamentous forms in microcosms subject to solar radiation, especially PAR, and attached microbial communities showed a greater tendency to be dominated by bacteria in irradiated microcosms than in shaded microcosms.
1 The effects of precipitation on litter quality and surface litter decomposition were evaluated across a natural rainfall gradient, ranging from 500 to 5500 mm mean annual precipitation, on the island of Hawai’i.
2 Litter of Metrosideros polymorpha from the driest (500 mm) and wettest (5500 mm) extremes of the gradient was decomposed at five sites along the precipitation gradient. Metrosideros polymorpha litter collected from all sites was also decomposed at a single common site (2500 mm), and M. polymorpha litter collected from each site was decomposed at the site of collection (in situ).
3 Initial litter quality varied significantly with precipitation at the site. Leaf mass per area (LMA) and phosphorus content decreased, while lignin and nitrogen content both increased, with increasing site precipitation.
4 Mass loss of all common litter types increased with increasing precipitation, as did in situ decomposition, which gave k-values ranging from 0.22 to 1.06. When decomposition at the same site was evaluated, litter from the driest site decomposed up to 2.5 times more rapidly than wet-site litter.
5 Decomposition at the common site showed an inverse correlation between inherent decomposability of litter (i.e. litter quality) and precipitation at the site from which litter originated, with k-values ranging from 0.71 for litter from the driest site to 0.28 for litter from the wettest site. Decomposability correlated strongly and inversely with initial lignin concentrations.
6 Proportional nutrient release from in situ litter was slower than mass loss, but nitrogen and phosphorus mineralization generally increased with increasing precipitation.
7 At the common site, there was an inverse relationship between rate of nutrient release and precipitation at the site where litter was collected. The systematic variation in litter decomposability due to changes in annual precipitation appears to buffer the direct effects of precipitation on carbon and nutrient turnover.
Most theory and empirical research on exotic invasions is based on the assumption that problematic exotics are much more abundant in the regions where they invade than in the regions where they are native. However, the overwhelming majority of studies on exotic plants have been conducted solely within the introduced range. With few exceptions, ecologists know surprisingly little about the abundance, interaction strengths and ecosystems impacts of even the best‐studied exotics in their native range.
We argue that taking a biogeographical approach is key to understanding exotic plant invasions. On a descriptive level, unambiguous quantification of distributions and abundances of exotics in native and introduced ranges are crucial. Experiments conducted at a biogeographical scale are also necessary to elucidate the mechanisms that enable highly successful exotics to occur at substantially higher abundance in their introduced vs. native communities.
We summarize the leading hypotheses for exotic plant success. We assert that tests of these major hypotheses for invasions (the natural enemies, evolution of invasiveness, empty niche and novel weapons hypotheses) require comparative biogeographical approaches.
In addition to focusing on comparative work in the native and introduced range, we also suggest other approaches that could yield important insight into processes that influence exotic success.
Increased understanding of invasions has the potential to provide unique insight into fundamental ecological theory, including that on individualistic‐holistic structure, the role of trophic interactions in population regulation, and the importance of co‐evolution in communities.
Invasive plants are one of the most serious threats to native species assemblages and have been responsible for the degradation of natural habitats worldwide. As a result, removal of invasive species and reestablishment of natural vegetation have been attempted in order to restore biodiversity and ecosystem function. This study examined how native arthropod assemblages, an abundant and functionally important group of organisms in many ecosystems, are affected by the incursion of the invasive wetland plant Phragmites australis and if the restoration of the native vegetation in brackish Spartina alterniflora marshes results in the reestablishment of the arthropod community. The invasion of Phragmites into a coastal Spartina marsh in southern New Jersey seriously altered arthropod assemblages and trophic structure by changing the abundance of trophic groups (detritivores, herbivores, carnivores) and their taxonomic composition. Herbivore assemblages shifted from the dominance of external free-living specialists (e.g., planthoppers) in Spartina to concealed feeders in Phragmites (stem-feeding cecidomyiids). Moreover, free-living arthropods in Phragmites became dominated by detritivores such as Collembola and chironomids. The dominant marsh spiders, web-building linyphiids, were significantly reduced in Phragmites habitats, likely caused by differences in the physical environment of the invaded habitats (e.g., lower stem densities). Thus, trophic structure of arthropod assemblages in Phragmites, as seen in the large shifts in feeding guilds, was significantly different from that in Spartina. Removal of Phragmites with the herbicide glyphosate resulted in the rapid return of Spartina (≤5 yrs). Moreover, return of the dominant vegetation was accompanied by the recovery of most original habitat characteristics (e.g., live and dead plant biomass, water flow rate). The arthropod assemblage associated with Spartina also quickly returned to its preinvasion state and was not distinguishable from that in uninvaded Spartina reference sites. This study provides evidence that the reestablishment of native vegetation in areas previously altered by an invasive plant can result in the rapid recovery of the native arthropod assemblage associated with the restored habitat.
In tidal marshes of the northeast US, replacement of native cattail (Typha angustifolia) by the common reed (Phragmites australis) is widespread, and reed is often the target of removal efforts. Reed sequesters nearly twice the amount of nitrogen per
unit marsh area in living aboveground tissue compared to cattail. Microbial decay processes immobilize additional nitrogen
or return this organic nitrogen to the pool of inorganic nitrogen. We compared microbial growth during decay of standing and
fallen litter of cattail and reed. Shoots of both plants were collected at the time of peak live biomass and then periodically
throughout litter decomposition. Litter was analyzed for mass loss, nitrogen content, and biomass and production of fungi
and bacteria. There were statistically significant but small differences in litter-associated microbial biomass and production
between these two plants. Microbial production on both litter types was dominated by fungi, accounting for >99% of the total.
Living fungal biomass (estimated from ergosterol) associated with reed and cattail litter averaged 6.1 and 8.2 mg fungal C/g
litter dry mass, respectively, and fungal nitrogen accounted for roughly 25% of the total nitrogen associated with litter.
Detrital nitrogen standing stocks/m2 were greater for reed than cattail throughout the first 2.5 years of decay. Therefore, the ability of reed litter to support
decomposer growth is only somewhat lower and nitrogen retention is greater than for one of the plants it replaces. These differences
are probably insufficient to argue for aggressive control of reed in tidal wetlands.
N2 fixation associated with the epiphytic community on standing dead Spartina alterniflora shoots was examined in both a natural and transplanted salt marsh in North Carolina. Acetylene reduction (AR) assays were
conducted over a 24-mo period to estimate N2 fixation rates on standing dead stems and leaves. In the natural salt marsh, mean AR rates ranged from 0.5 nmol C2H4 cm−2 h−1 to 14 nmol C2H4 cm−2 h−1, while in the transplanted marsh mean AR rates ranged from 1 nmol C2H4 cm−2 h−1 to 33 nmol C2H4 cm−2 h−1. Diel AR activity of epiphytic communities in both marshes varied seasonally. Midday incubations yielded higher AR rates
than nighttime incubations in the spring, while midday incubations in late summer and fall generally yielded AR rates equal
to or lower than nighttime incubations. Desiccation during low tides occasionally repressed AR activity, although AR rates
quickly rebounded with wetting. AR activity was localized in the epiphytic community, rather than in the underlying Spartina stem material. Based on the measured AR rates and the density of standing dead stems, the annual input of new N to the natural
salt marsh via epiphytic N2 fixation is estimated to be 2.6 g N m−2 yr−1. The estimate of annual input of new N to the transplanted marsh is 3.8 g N m−2 yr−1. These estimates should be added to previous estimates of N2 fixation in marsh sediments to estimate the total contribution of new nitrogen to salt marsh nitrogen budgets.
The recent expansion of Phragmites australis (common reed) from the marsh-upland interface into high marsh zones provides an opportunity to assess the impact of individual
plant species on biomass production and decomposition in salt marshes. Seasonal harvests of aboveground and belowground biomass
demonstrate that annual production of P. australis is approximately three times greater for aboveground biomass, two times greater for belowground biomass, and 30% lower in
root: shoot ratio than neighboring populations of S. patens. Whole-plant litter (stems and leaves) also decomposes at a much slower annual rate for P. australis (k=0.25) than S. patents litter (k=0.57). By crossing litter type with site of litter decomposition, I found these plant species to influence decay
rates through litter type and not through their effects on marsh surface conditions (e.g., temperature, sedimentation rates).
Based on these calculations, annual rates of carbon accumulation in the peat of high marshes are likely to increase 5-fold
once P. australis becomes established due to its greater rates of biomass production and residence time in infrequently flooded brackish marshes.
Although it is generally acknowledged that invasions by exotic plant species represent a major threat to biodiversity and ecosystem stability, little attention has been paid to the potential impacts of these invasions on nutrient cycling processes in the soil. The literature on plant–soil interactions strongly suggests that the introduction of a new plant species, such as an invasive exotic, has the potential to change many components of the carbon (C), nitrogen (N), water, and other cycles of an ecosystem. I have reviewed studies that compare pool sizes and flux rates of the major nutrient cycles in invaded and noninvaded systems for invasions of 56 species. The available data suggest that invasive plant species frequently increase biomass and net primary production, increase N availability, alter N fixation rates, and produce litter with higher decomposition rates than co-occurring natives. However, the opposite patterns also occur, and patterns of difference between exotics and native species show no trends in some other components of nutrient cycles (for example, the size of soil pools of C and N). In some cases, a given species has different effects at different sites, suggesting that the composition of the invaded community and/or environmental factors such as soil type may determine the direction and magnitude of ecosystem-level impacts. Exotic plants alter soil nutrient dynamics by differing from native species in biomass and productivity, tissue chemistry, plant morphology, and phenology. Future research is needed to (a) experimentally test the patterns suggested by this data set; (b) examine fluxes and pools for which few data are available, including whole-site budgets; and (c) determine the magnitude of the difference in plant characteristics and in plant dominance within a community that is needed to alter ecosystem processes. Such research should be an integral component of the evaluation of the impacts of invasive species.
This study examined the mass loss, fungal biomass, and nutrient dynamics of standing Phragmitesaustralis leaf blades during senescence and early decay in littoral reed stands of two hardwater lakes. Green living leaves were tagged at defined canopy heights in early autumn (late August or early September) and periodically collected until all leaf blades had fallen off the parent shoot. Samples were analysed for leaf dry mass remaining, fungal biomass associated with leaves (ergosterol concentrations), and nitrogen and phosphorus concentrations. Considerable mass loss of leaves occurred in the standing position (up to 28%). Nitrogen and phosphorus concentrations of leaves decreased substantially with time (by 39–77%), indicating that a major portion of these nutrients was translocated to the rhizome during senescence. Fungal biomass associated with leaves increased during the study period, reaching an estimated maximum of about 40 mg g−1 of leaf dry mass. Fungal biomass was negatively correlated with leaf N and P concentrations. The observed patterns of leaf mass loss, nutrient dynamics, and fungal biomass were consistent with the successive senescence and death of leaves from the shoot base to its tip. The results of this study point to a notable mass loss of P.australis leaf blades in the standing position, which appears to be mediated by both plant and microbial processes. Nutrient dynamics, in contrast, appear to be largely governed by plant processes.
The South Florida Water Management District has constructed large treatment wetlands (stormwater treatment areas (STAs)) to reduce total phosphorus concentrations in agricultural runoff before this water enters the Everglades. An important component of nutrient removal and storage in these systems is incorporation of nutrients into aquatic macrophytes and burial of this biomass in the sediments. However, decomposition of plant biomass before burial returns nutrients to the water column and may reduce STA treatment efficiency. As part of research on biogeochemical control of STA performance, we conducted a summer (July–September) and a long-term (12-month) experiment (February–February) that measured decomposition rates and release of chemical constituents from dominant aquatic macrophytes in a constructed wetland located in south Florida. The rank order of mean decomposition rates was Najas/Ceratophyllum (0.0568 d−1) > Pistia (0.0508 d−1) > Eichhornia (0.0191 d−1) > submerged Typha (0.0059 d−1) > aerial Typha (0.0008 d−1). Summer decomposition rates were generally higher than rates from the long-term experiment, which suggested a temperature effect. Decomposition rates were negatively correlated with litter C:N and C:P molar ratios and cellulose and lignin content and positively correlated with N and P content. There was no significant difference in decomposition rates among sampling stations despite the fact that there was a decreasing gradient in water column inorganic phosphorus and nitrogen concentrations at these sites. Relatively little of the initial P mass remained in the litter of all species, except Typha, by the end of both experiments. First-order decomposition models derived using nonlinear regression generally had explanatory power, i.e. accounted for variance, comparable to more complex decreasing-coefficient models. Decomposition rates for the species examined in this study were within the range of published values when comparisons were made either by species or by plant group.
We examined patterns of biomass accumulation and tissue concentrations of five metals—mercury, copper, zinc, chromium, lead—and two elements—carbon and nitrogen—to determine differences in net metal accumulation and distribution between Phragmites australis (common reed) and Spartina alterniflora (cord grass) which were growing intermingled in a contaminated low marsh. Data were collected at 2-month intervals across a growing season (April–October, 1999). Although they comprise only 5–15% of whole plant biomass for both species, roots consistently contained 70–100% of the whole plant metal burdens for both S. alterniflora and P. australis (shoot:root ratio <0.42). Stems and rhizomes had low and similar concentrations between plant species throughout the summer. Leaves of S. alterniflora, however, had consistently greater concentrations of Hg and Cr than those of P. australis. In contrast, the micronutrients Cu and Zn were enriched in P. australis leaf tissue in October, compared to S. alterniflora. Pools of metal in aboveground biomass were similar between plant species, but throughout the season S. alterniflora allocated more of this burden to leaf tissue than P. australis, which allocated more of the aboveground burden to stem tissue, a recalcitrant tissue with lower concentrations but greater biomass. The consistently higher concentrations and total pools of Hg and Cr in S. alterniflora leaf tissue and higher Zn and Cu in P. australis may result from differences in leaf phenology, root-influenced metal availability, or transport of dissolved metals. Because S. alterniflora shifts more of its Hg and Cr load into highly decomposable leaf tissues (as opposed to recalcitrant stems, roots, and rhizomes) this pathway of metal bioavailability would be reduced when S. alterniflora is replaced by P. australis.
Root growth increased during the early growing season in Spartina alterniflora salt marsh plots. While fertilization with nitrogenous fertilizer did not affect initial growth, a marked decrease in root biomass followed the spring peak particularly where nutrient doses were highest. A sharp reduction in roots occurred in enriched areas covered by Spartina patens , although, as with S. alterniflora , aboveground biomass increased. Roots disappeared during autumn leaving rhizomes as the only part of the plants to overwinter. The maximum standing crop for roots was 0–2 cm deep, for rhizomes 2–5 cm. Net annual underground production was calculated from annual increments in dead matter belowground. Total production, underground and aboveground, exceeds that of any marine vegetation, ranging from 3,900 to 6,600 g m ‒2 yr ‒1 in S. alterniflora areas and 3,200 to 6,200 g m ‒2 yr ‒1 in S. patens areas. Fertilization increased production particularly aboveground where dead plant parts are subject to export.
We examined biomass accumulation, tissue concentrations of lead (Pb), and net uptake of Pb in Phragmites australis (common reed) and Spartina alterniflora (salt cord grass) grown under greenhouse conditions in sediment of different Pb concentrations. Sediment and newly emerged ramets of each plant species were collected in April 1999 from Tuckerton, NJ, a relatively clean salt marsh. One-gallon pots were filled with either control sediment (29 microg g(-1) Pb) or Pb-added sediment (68 microg g(-1) Pb), and the sediment moisture was kept saturated along with controlled additions of additional nutrients. At harvest in October, whole plant biomass was 60-85% greater for pots with P. australis than pots with S. alterniflora and a 40-70% reduction in biomass in response to the addition of Pb was observed for both species. In the high Pb treatments, both concentrations and pools of Pb were greater in the leaves of S. alterniflora than in leaves of P. australis at the end of the growing season. In both species, Pb concentrations were higher in lower leaves than upper leaves. The addition of Pb into experimental pots led to over an 800% increase in Pb standing stock for both species. In S. alterniflora, however, significantly more of this pool was allocated to aboveground biomass (leaves and stems) than to belowground biomass (roots and rhizomes). This difference in allocation was more profound at the higher sediment Pb concentration (Pb-added pots). This fundamental difference between the species in response to Pb contamination indicates that metal export into food webs or the water column should be greater in stands of S. alterniflora than in P. australis. These results suggest that in Pb-contaminated, and possibly all metal-contaminated sediments, the replacement of S. alterniflora with P. australis may reduce metal bioavailability by sequestering a greater proportion of its metal burden in belowground tissues which are likely to be permanently buried.
To investigate the decay rate and metal uptake in litter from two species of wetland plants, leaves and stems of senescent Spartina alterniflora and Phragmites australis (P) were obtained from the Hackensack Meadowlands (NJ, USA) in October 1998, and their initial metal contents were determined. Two types of S. alterniflora were obtained, one set from a natural site (NS) and one from a restored site (RS). Leaves and stems were placed in separate litterbags, and samples of each type were reciprocally transplanted into each of the three collection sites (NS, RS, and P) as well as in the laboratory, where they were alternately dried and wetted. Litterbags were retrieved from the field at four six-month intervals and after one year from the laboratory. Annual decay coefficients were greater for leaves than for stems. Stems of P. australis initially decomposed more slowly (37-63% remaining) than those of S. alterniflora (23-53 % remaining), but after two years, decay was comparable (8-40% remaining for both species). Decomposition was slower at the RS site than at the other field sites, and it was slowest in the laboratory. Metal concentrations initially were lower in stems than in leaves, and Cr, Pb, and Zn were lower in P. australis than in S. alterniflora. In the field, large increases (10- to 100-fold) in metal concentrations rapidly obliterated any initial differences between plant species. Metal concentrations in leaves rose more quickly and remained greater than in stems. For example, Cu approached 300 microg/g in leaves but was less than 200 microg/g in stems. In contrast to the modest rise in metal concentrations in the leaf tissue at the more contaminated RS site (Zn rose to approximately 200 microg/g in sediments containing approximately 400 microg/g), Cu and Zn concentrations in leaf litter at the P and NS sites increased to levels exceeding those in the surrounding sediment (Zn rose to approximately 500 microg/g in sediments containing approximately 200 microg/g). Temporal changes in metal pools (grams of metal per litterbag) were not discernable because of the negative correlation of mass remaining and metal concentrations as well as because of the great variability of metal concentrations within each treatment. Decomposition and the accumulation of metals may be influenced more by differences between tissue types than by species or sediment metal concentrations.
Physiological traits that contribute to the establishment and spread of invasive plant species could also have impacts on ecosystem processes. The traits prevalent in many invasive plants, such as high specific leaf areas, rapid growth rates, and elevated leaf nutrient concentrations, improve litter quality and should increase rates of decomposition and nutrient cycling. To test for these ecosystem impacts, we measured initial leaf litter properties, decomposition rates, and nutrient dynamics in 11 understory plants from the Hawaiian islands in control and nitrogen + phosphorus fertilized plots. These included five common native species, four of which were ferns, and six aggressive invasive species, including five angiosperms and one fern. We found a 50-fold variation in leaf litter decay rates, with natives decaying at rates of 0.2-2.3 year(-1) and invaders at 1.4-9.3 year(-1). This difference was driven by very low decomposition rates in native fern litter. Fertilization significantly increased the decay rates of leaf litter from two native and two invasive species. Most invasive litter types lost nitrogen and phosphorus more rapidly and in larger quantities than comparable native litter types. All litter types except three native ferns lost nitrogen after 100 days of decomposition, and all litter types except the most recalcitrant native ferns lost >50% of initial phosphorus by the end of the experiment (204-735 days). If invasive understory plants displace native species, nutrient cycling rates could increase dramatically due to rapid decomposition and nutrient release from invasive litter. Such changes are likely to cause a positive feedback to invasion in Hawai'i because many invasive plants thrive on nutrient-rich soils.