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Does hydrological reconnection enhance nitrogen cycling rates in the lakeshore wetlands of a eutrophic lake?
... These conflicting results suggest that the response of N-removal rates in river to low C/N water remains unclear. Nitrification and denitrification in aquatic environments are microbial-mediated processes that are impacted by many abiotic factors, such as dissolved oxygen, pH, and availability of C and N (Nizzoli et al., 2018;Seitzinger et al., 2006;Wu et al., 2019;Yao et al., 2018a), and biotic factors, such as vegetation, microbial activity, and diversity (Forshay and Dodson, 2011;Liu et al., 2015;Xiong et al., 2017). Several studies have reported the influence of various environmental factors on N removal in hydrological ecosystems with high water N loading. ...
... For instance, Liu et al. (2018) demonstrated that water physicochemical properties and water nutrients not only directly affected the denitrification rate but also indirectly through influencing the submerged vegetation. However, Wu et al. (2019) observed the direct pathway, rather than indirect pathway of water NH 4 + significantly affecting the sediment nitrification and denitrification. Additionally, a study conducted in Honghu Lake showed that the well-developed submerged vegetation could directly inhibit the growth of nitrifier whereas increase the abundance of denitrifier indirectly via improving the sediment conditions (Wu et al., 2020). ...
... Given the significant correlations between sediment N-removal rates and environmental factors (Table S2), it can be speculated that the increasing tendencies of sediment N-removal rates under the influence of low C/N water might be accounted for by the alteration of multiple environmental factors in the Chuanfang River. It has been widely reported that WPP, WN, SC, and SV can significantly affect the nitrification and denitrification in aquatic environments (Attard et al., 2011;Bru et al., 2011;Enwall et al., 2010;Strauss et al., 2002;Wu et al., 2019;Xiong et al., 2017;Yao et al., 2018a). This finding was confirmed by the results of variance partitioning as well as path analyses in our study (Figs. 4, 5; Table 2). ...
As an intersection that links terrestrial and aquatic ecosystems, river ecosystems are hotspots for nitrogen (N) removal. Nitrification and denitrification processes have been recognized as the primary mechanisms behind permanent N removal in rivers, which can be influenced by a variety of environmental factors. However, in rivers characterized by low carbon to nitrogen (C/N) ratio, the interaction among environmental factors and their effects on the sediment N-removal rates remain to be elucidated. In this study, we conducted six surveys in the Chuanfang River (an urban low C/N ratio river flowing into the Dianchi Lake) to investigate the effects of four categories of environmental variables on sediment N-removal rates (water physicochemical properties, water nutrients, sediment characteristics, and submerged vegetation) and their respective contribution to sediment N-removal rates. The results showed that sediment N-removal rates increased along the river reach, accompanied by a gradual improvement in water quality, sediment fertility, and submerged vegetation. Furthermore, comprehensive results of multiple analysis indicated positive effects of water physicochemical properties (e.g. dissolved oxygen, pH, temperature, etc.), sediment characteristics (e.g. sediment C, N, etc.), and submerged vegetation (e.g. tissue C, N, etc.) on the N-removal rates but emphasized the restriction of water nutrients (e.g. water N, etc.). In addition, the significant negative correlation between nitrification and denitrification in path model signified the decoupled nitrification–denitrification in the Chuanfang River. This study suggested that river ecosystems that received low C/N water had a great N-removal capacity, which could be further enhanced by the improvement of water quality, sediment fertility, and submerged vegetation.
... The lakeshore zone is an ecological transition zone between lacustrine and terrestrial ecosystems [4][5][6]. It is a protective barrier for lakes, functioning as a buffer to intercept runoff pollutants, and hosting various physical, chemical, and biological reactions that remove pollutants for pollution load reduction and lake water quality improvement [7][8][9][10]. Denitrification, a process of permanent nitrogen removal by converting nitrate to nitrogen gas under anaerobic conditions, is one of the main nitrogen removal mechanisms in lakeshore sediments [11][12][13][14]. ...
Denitrification of sediments is an important way to remove reactive nitrogen in lakeshore zones. In this work, we analyzed sediment denitrification patterns across the shore zone of Lake Taihu and explored their underlying mechanisms using flooding simulation experiments. The results showed that denitrification mainly occurred in the upper sediment layer (0–10 cm) and the denitrification rate was highest at the land–water interface (6.2 mg N/m2h), where there was a frequent rise and fall in the water level. Denitrification was weaker in the lakebed sediments (4.6 mg N/m2h), which were inundated long-term, and in the sediments of the near-shore zone (2.3 mg N/m2h), which were dried out for extended periods. Flooding simulation experiments further indicated a strong positive relationship between sediment denitrification rate and flooding frequency. When the flooding occurred once every 3, 6, 9, 12, or 15 days, the denitrification rate reached 7.6, 5.7, 2.8, 0.9, and 0.6 mg N/m2h, respectively. Frequent flooding caused alternating anoxic and aerobic conditions in sediments, accelerating nitrogen substrate supply and promoting the growth and activity of denitrifying bacteria. Based on these findings, we propose a possible strategy for enhancing sediment denitrification by manipulating the water level, which can help guide nitrogen removal in lakeshore zones.
... Sediment carbon concentration and plant abundance were usually predicted the most important factors determining community structure of N-cycling bacteria especially the denitrifier assemblages, because the majority of denitrifiers were aerobic heterotrophs (Bremer et al., 2009;Wu et al., 2019;Ma et al., 2020). These findings were in good agreement with our results. ...
Sediment nitrogen (N) cycling is an important biological removal process for N permanently and driven by N-cycling microbial community. There is a growing interest in interactions between submerged vegetation (SV) and sediment N-cycling bacterial community, because of the close link between rooted aquatic plants and the sediment microbes. However, the effects of SV on the sediment N-cycling bacterial community are still controversial. Furthermore, the discrimination of direct and indirect effects of SV on the N-cycling bacterial community remains unclear. Here, we investigated the biomass and species richness of SV and determined the corresponding environment factors (water quality and sediment properties) in Honghu Lake (China). We also used functional genes as markers to unveil the bacterial diversity and community composition and abundance in lake sediments. Our results showed that biomass and species richness of SV affected the composition, diversity and abundance of sediment N-cycling bacterial communities through improving lake water quality and sediment properties. With the increasing richness and abundance of SV, the diversity of most N-cycling bacterial assemblages including nitrifying, denitrifying and DNRA bacteria decreased, while the abundance increased. However, the anammox bacterial assemblage in sediments showed inverse trends. Sediment carbon vs. nitrogen (C:N) ratio negatively affected the abundance of amoA and nirS + nirK + nosZ bacterial assemblages. Additionally, due to the presence of SV, positive interactions among N-cycling bacterial assemblages were found, such as amoA and nrfA bacterial assemblages. Overall, our findings confirmed the significant effects of SV on the N-cycling bacterial community structure and abundance. Moreover, the direct effects of SV on the N-cycling bacterial community and the indirect effects through altering the sediment C were clarified in our study. Our results casted a new light on the negative effects of high C:N ratio. From the study, we made a conclusion that the better SV develops, the greater nitrogen removal occurs in lake sediments.
Eutrophication is a major global concern in lakes, caused by excessive nutrient loadings (nitrogen and phosphorus) from human activities and likely exacerbated by climate change. Present use of indicators to monitor and assess lake eutrophication is restricted to water quality constituents (e.g., total phosphorus, total nitrogen) and does not necessarily represent global environmental changes and the anthropogenic influences within the lake’s drainage basin. Nutrients interact in multiple ways with climate, basin conditions (e.g., socio-economic development, point-source, diffuse source pollutants), and lake systems. It is therefore essential to account for complex feedback mechanisms and non-linear interactions that exist between nutrients and lake ecosystems in eutrophication assessments. However, the lack of a set of water quality indicators that represent a holistic understanding of lake eutrophication challenges such assessments, in addition to the limited water quality monitoring data available. In this review, we synthesize the main indicators of eutrophication for global freshwater lake basins that not only include the water quality constituents but also the sources, biogeochemical pathways and responses of nutrient emissions. We develop a new causal network (i.e., multiple links of indicators) using the DPSIR (drivers-pressure-state-impact-response) framework that highlights complex interrelationships among the indicators and provides a holistic perspective of eutrophication dynamics in freshwater lake basins. We further review the 30 key indicators of drivers and pressures using seven cross-cutting themes: (i) hydro-climatology; (ii) socio-economy; (iii) land use; (iv) lake characteristics; (v) crop farming and livestock; (vi) hydrology and water management; (vii) fishing and aquaculture. This study indicates a need for more comprehensive indicators that represent the complex mechanisms of eutrophication in lake systems, to guide the global expansion of water quality monitoring networks, and support integrated assessments to manage eutrophication. Finally, the indicators proposed in this study can be used by managers and decision-makers to monitor water quality and set realistic targets for sustainable water quality management to achieve clean water for all, in line with Sustainable Development Goal 6.
Excessive anthropogenic nitrogen discharges have led to increasing nitrogen in aquatic systems, resulting in eutrophication problems. To address this problem, urban wetlands are used to remove nitrogen from wastewater treatment plant tailwater because of their efficient nitrogen removal capabilities. At the same time, high‑nitrogen tailwater can exert numerous impacts on the sediment denitrifier communities and denitrification of urban wetlands, and it is necessary to clarify this impact. In this research, we investigated denitrifier (nirK-, nirS- and nosZ-denitrifier) abundances and potential denitrification rate of 30 sediment samples from two urban wetland rivers in Haizhu National Wetland Park (China). One river called experimental river (ER) receives tailwater from a wastewater treatment plant, and the other called control river (CR) is unaffected by the tailwater. Moreover, we also determined the environmental variables to distinguish their relative effects on the variance of denitrifier abundances and denitrification rate in the two rivers. The results showed that the water quality was significantly different between ER and CR, while sediment property was not significant (p < 0.05). And it is unable to distinguish which has the greater denitrification rate between ER and CR. The redundancy analysis and path analysis indicated that riverine nutrient concentrations (e.g., ambient carbon and nitrogen) and physicochemical parameters (e.g., ambient temperature, dissolved oxygen and pH) were important determinants for denitrifier abundances and denitrification in ER and in CR, respectively. Furthermore, denitrifier abundances had an insignificant impact on denitrification in ER, but a considerable impact on denitrification in CR. The findings of this research can help to further understand how the tailwater affects sediment denitrifier communities and denitrification in urban wetland rivers, and to make better use of urban wetlands as a natural purification method.
Along with wetland loss, the damming effect on hydrological modification in wetland is another less debated and challenging topic, which needs to have urgent attention. The present work intended to investigate the damming effect on the water richness and eco-hydrological condition of the floodplain wetland and its consequent ecological responses in Punarbhaba River Basin of India and Bangladesh. Satellite images derived hydro-period, water presence frequency (WPF), and water depth were generated for developing water richness model in pre (up to 1992) and post dam conditions (1993–2019). The range of variability (RVA) was modelled using time series satellite images based water index or normalized difference water index (NDWI). Based on RVA model, the hydrological failure rate was developed. Depth of water was used for preparing the flow duration curve (FDC) to estimate the eco-hydro-deficit and surplus condition in wetland at spatial scale for pre and post-dam periods. Satellite image based trophic state index models for pre and post dam conditions were constructed to investigate the ecological response of dam on floodplain wetlands. Results of water richness model showed that wetlands area under high wetland water richness zone decreased from 71.83% to 7.65% in the post-dam conditions. Results of hydrological failure rate showed that high failure rate captured 45% of total wetland area in the post-dam period. Results of eco-hydro-deficit exhibited that eco-hydro-deficit areas increased from 11.22% to 52.19% and 35.03%–52.67% respectively in post-dam conditions indicating growing ecological stress. The TSI models showed that most parts of the wetlands were converted from oligotrophic to meso-eutrophic state signifying the qualitative degradation of water and potential ecosystem health. The area under high TSI was observed in the wetland area having eco-hydro-deficit and high hydrological failure rate zones. These characteristics of wetland areas were found at the fringe of wetlands and fragmented smaller wetland units. The study concluded that damming over the Punarbhaba River adversely affected the water security of the floodplain wetlands in terms of modifying the hydrological richness, ecological condition of the wetland habitat, and ecological systems. The findings of the present study could provide a comprehensive research on the monitoring of surface water crisis in the wetlands, which will be the basic foundation to formulate water resource management plans for conservation, management and restoration of wetlands.
Hydrophytes have been widely used to reduce nutrient levels in aquatic ecosystems, but only limited species with high nutrient removal efficiencies have been implemented. Thus, it is necessary to continually explore new candidate species with high nutrient removal efficiencies. To effectively explore the nutrient removal ability of hydrophytes, a new process-based model combining the multiple-quotas approach and nutrient-cycle model was developed. The multiple-quotas approach provides a theoretical framework to conceptually explain the uptake and response of autotrophs to multiple nutrients. The developed process-based model was validated using observational data from microcosm experiments with two emergent hydrophytes, Menyanthes trifoliata and Cicuta virosa. The results showed that both M. trifoliata and C. virosa effectively reduced nitrogen (N) and phosphorus (P) in both water and sediment layers, but M. trifoliata showed a higher removal efficiency for both nutrients than C. virosa, particularly for total ammonia + ammonium-nitrogen (NHx-N) and nitrate-nitrogen (NO3-N) in the sediment layer (M. trifoliata: 0.579–0.976 for NHx-N, 0.567–0.702 for NO3-N; C. virosa: 0.212–0.501 for NHx-N, 0.466–0.560 for NO3-N). In addition, M. trifoliata achieved the maximum removal efficiency for N and P at higher nutrient exposure levels than C. virosa (M. trifoliata: exposure level of 0.725–0.775; C. virosa: exposure level of 0.550–0.575). The developed model well simulated the species-specific growth patterns of hydrophytes depending on the nutrient exposure level as well as the N and P dynamics in the water and sediment layers. The approach adopted in this study provides a useful tool for discovering candidate species to improve hydrophyte diversity and effectively remove nutrients from aquatic ecosystems.
Sediment nitrification plays a vital role in nitrogen (N) biogeochemical cycling and ecological function of an aquatic ecosystem. The relative importance of environmental factors and nitrifying microbial communities in regulating sediment nitrification process has received less attention, especially in aquatic habitats where high N loads are frequently detected. Here, we report the potential nitrification rates of 35 sediment samples from 10 shallow lakes in the Yangtze River basin. The diversity and abundance of ammonia-oxidizing archaea (AOA) and bacteria (AOB) were quantified using archaeal and bacterial amoA genes. The results showed that there was no significant difference in sediment nitrification rates among sites of different trophic state. The nitrification rates were positively related to water chlorophyll-a, sediment N and carbon levels, but not significantly associated with diversity and abundance of ammonia-oxidizing microorganisms and submerged plants. Interestingly, the abundance and diversity of sediment AOB but not AOA communities were significantly influenced by trophic state. In addition, AOB communities were more sensitive to changes in local environments and catchment land uses than the AOA communities. Using path analysis, we found that 55–60% of the indirect effect of catchment land uses on nitrification rates was mediated via sediment N content. Our findings suggest that, although nitrification is a microbial process, variation in sediment nitrification rates in Yangtze lakes is mainly explained by abiotic factors but not by microbial abundance and diversity.
We analysed the spatial and temporal variability of benthic nitrogen fluxes and denitrification rates in a sub-alpine meromictic lake (Lake Idro, Italy), and compared in-lake nitrogen retention and loss with the net anthropogenic nitrogen inputs to the watershed. We hypothesized a low nitrogen retention and denitrification capacity due to meromixis. This results from nitrate supply from the epilimnion slowing down during stratification and oxygen deficiency inhibiting nitrification and promoting ammonium recycling and its accumulation. We also hypothesized a steep vertical gradient of sedimentary denitrification capacity, decreasing with depth and oxygen deficiency. These are important and understudied issues in inland waters, as climate change and direct anthropic pressures may increase the extent of meromixis. Nearshore sediments had high denitrification rates (87 mg m⁻² day⁻¹) and efficiency (~ 100%), while in the monimolimnion denitrification was negligible. The littoral zone, covering 10% of the lake surface, contributed ~50% of total denitrification, while the monimolimnion, which covered 70% of the sediment surface, contributed to < 13% of total denitrification. The persistent and expanding meromixis of Lake Idro is expected to further decrease its nitrogen removal capacity (31% of the incoming nitrogen load) compared to what has been measured in other temperate lakes. Values up to 60% are generally reported for other such lakes. Results of this study are relevant as the combination of anthropogenic pressures, climate change and meromixis may threaten the nitrogen processing capacity of lakes.
Sediment denitrification in lakes alleviates the effects of eutrophication through the removal of nitrogen to the atmosphere as N2O and N2. However, N2O contributes notably to the greenhouse effect and global warming. Human land uses (e.g. agricultural and urban areas) strongly affect lake water quality and sediment characteristics, which, in turn, may regulate lake sediment denitrification and N2O production. In this study, we investigated sediment denitrification and N2O production and their relationships to within-lake variables and watershed land uses in 20 lakes from the Yangtze River basin in China. The results indicated that both lake water quality and sediment characteristics were significantly influenced by watershed land uses. N2O production rates increased with increasing background denitrification rates. Background denitrification and N2O production rates were positively related to water nitrogen concentrations but were not significantly correlated with sediment characteristics and plant community structure. A significant positive relationship was observed between background denitrification rate and percentage of human-dominated land uses (HDL) in watersheds. Structural equation modelling revealed that the indirect effects of HDL on sediment denitrification and N2O production in Yangtze lakes were mediated primarily through lake water quality. Our findings also suggest that although sediments in Yangtze lakes can remove large quantities of nitrogen through denitrification, they may also be an important source of N2O, especially in lakes with high nitrogen content.
1. This study estimated the importance of in situ denitrification in the nitrate buffering capacities of a riparian forest. 2. Spatial and temporal patterns of in situ denitrification were investigated along a riparian catena. 3. Highest rates of in situ denitrification (up to 78 mgN m(-2) day(-1)) were measured in the riparian forest soils in late winter and early spring. Lowest rates (3 mgN m(-2) day(-1)) were measured in summer and autumn. 4. Whatever the season considered, 30 m of riparian buffer strip were enough to remove all the nitrates coming through the groundwater. 5. Rehabilitation of riparian zones with riparian vegetation together with the maintenance of waterlogged conditions induced by riverflow regulation appear to be a good point from which to start the restoration of buffering capacities of river ecosystems against nitrogen loads.
Nitrification in the hyporheic zone of Sycamore Creek, a Sonoran Desert stream, was examined, focusing on the association between respiration and nitrate production. Subsurface respiration in Sycamore Creek is highest in regions of hydrologic downwelling where organic matter derived from the stream surface is transported into the hyporheic zone. Similarly, nitrification was closely related to hydrologic exchange between the surface and hyporheic zone. Nitrification in downwelling regions averaged 13.1 mu gNO(3)-N . L sediments(-1). h(-1) compared with 1.7 mu gNO(3)-N . L sediments(-1). h(-1) in upwelling regions. Hyporheic respiration also varies temporally as a result of flash floods which scour and remove algae from the stream and thus reduce the pool of organic matter to support subsurface metabolism. Nitrification was also significantly affected by flooding; nitrification increased from an average of only 3.0 mu gNO(3)-N . L sediments(-1). h(-1) immediately following floods to 38.5 mu gNO(3)-N . L sediments(-1). h(-1) late in succession. Nitrification was significantly correlated with hyporheic respiration, supporting the hypothesis that nitrification is fueled by mineralization of organic nitrogen to ammonium. The coupling between subsurface respiration and nitrification is one step in a cyclic interaction between surface and hyporheic zones and serves to transform nitrogen from an organic to inorganic form.
Unlucky Lakes
The negative consequences of increased loading of nitrogen and phosphorus into aquatic ecosystems are well known. Management strategies aimed at reducing the sources of these excess nutrients, such as fertilizer runoff or sewage outflows, can largely mitigate the increases in nitrogen and phosphorus levels; however, it is unclear if these strategies are influencing other spects of these ecosystems. Using a global lake data set, Finlay et al. (p. 247 ; see the Perspective by Bernhardt ) found that reducing phosphorus inputs reduced a lake's ability to export reactive nitrogen, exacerbating nitrate pollution.
Despite dramatic increases in nitrogen (N) loading to fresh waters and growing scientific attention on the changing N cycle, measurements of nitrification and denitrification rates in lakes are lacking. In particular, we know little about how these processes vary spatially within a lake, and how this potential spatial variation contributes to a lake's N dynamics. We measured sediment nitrification and denitrification rates at 40 sites in Gull Lake, Michigan (USA), and found that the shallow edge sediments (<2 m deep) of the lake were hot spots of N transformation. Nitrification rates were comparable in sediments at all depths, while sediment denitrification rates were highest in the shallow edge habitat, and lowest in the profundal sediments (<2 m and >10 m deep, respectively). We scaled-up our sediment transformation rates across the lake to illustrate spatial variability in nitrification and denitrification. For whole-lake nitrification, the contribution of shallow edge, littoral, and profundal sediments followed in proportion to lake surface area of each habitat. In contrast, the contribution of each of these areas to whole-lake denitrification was not proportional to their respective surface areas, and instead was equal across the 3 habitat types, with each area contributing roughly 30% of the total N loss via denitrification. Spatially representative characterization of nitrification and denitrification in lentic ecosystems requires incorporation of the spatial variation in these transformations with a particular focus on littoral sediments, and this is often overlooked in studies of lentic N cycling. Furthermore, anthropogenic changes to lake shorelines that influence N cycling in littoral sediments may have a disproportionate effect on whole-lake ecosystem function.
Despite the particular management practices and climate characteristics of the Mediterranean regions, the literature dealing with N budgets in large catchments subjected to Mediterranean conditions is scarce. The present study aims to deepen our knowledge on the N cycle within the Ebro River Basin (NE Spain) by means of two different approaches: (1) calculating a global N budget in the Ebro River Basin and (2) calculating a series of detailed regional budgets at higher geographical resolution. N inputs and outputs were spatialized by creating a map based on the most detailed information available. Fluvial and atmospheric N export was estimated together with N retention. The Ebro River Basin annually receives a relatively high amount of new N (5118 kg N km-2 yr-1), mostly in the form of synthetic fertilizers (50 %). Although it is a highly productive catchment, the net N input as food and feed import is also high (33 %). Only 8 % of this N is finally exported to the delta zone. Several territorial units characterized by different predominant uses (rainfed agriculture, irrigated agriculture and pastures) have differentiated N dynamics. However, due to the high density of irrigation channels and reservoirs that characterize Mediterranean basins, N retention is very high in all of them (median value, 92 %). These results indicate that problems of eutrophication due to N delivery in the coastal area may not be too severe but that high N retention values may instead lead to problems within the catchment, such as pollution of aquifers and rivers, as well as high atmospheric emissions. The most promising management measures are those devoted to reducing agricultural surpluses such as balanced N fertilization and low N livestock feeding.
Isotope pool dilution studies are increasingly reported in the soils and ecology literature as a means of measuring gross rates of nitrogen (N) mineralization, nitrification, and inorganic N assimilation in soils. We assembled data on soil characteristics and gross rates from 100 studies conducted in forest, shrubland, grassland, and agricultural systems to answer the following questions: What factors appear to be the major drivers for production and consumption of inorganic N as measured by isotope dilution studies? Do rates or the relationships between drivers and rates differ among ecosystem types? Across a wide range of ecosystems, gross N mineralization is positively correlated with microbial biomass and soil C and N concentrations, while soil C:N ratio exerts a negative effect on N mineralization only after adjusting for differences in soil C. Nitrification is a log-linear function of N mineralization, increasing rapidly at low mineralization rates but changing only slightly at high mineralization rates. In contrast, NH 4 1 assimilation by soil microbes increases nearly linearly over the full range of mineralization rates. As a result, nitrification is proportionately more important as a fate for NH4 1 at low mineralization rates than at high mineralization rates. Gross nitrification rates show no relationship to soil pH, with some of the fastest nitrification rates occurring below pH 5 in soils with high N mineralization rates. Differences in soil organic matter (SOM) composition and concentration among ecosystem types in- fluence the production and fate of mineralized N. Soil organic matter from grasslands appears to be inherently more productive of ammonium than SOM from wooded sites, and SOM from deciduous forests is more so than SOM in coniferous forests, but differences appear to result primarily from differing C:N ratios of organic matter. Because of the central importance of SOM characteristics and concentrations in regulating rates, soil organic matter depletion in agricultural systems appears to be an important determinant of gross process rates and the proportion of NH4 1 that is nitrified. Addition of 15 N appears to stimulate NH4 1 consumption more than NO3 2 consumption processes; however, the magnitude of the stim- ulation may provide useful information regarding the factors limiting microbial N trans- formations.
Despite the particular management practices and climate characteristics of
the Mediterranean regions, the literature dealing with N budgets in large
catchments subjected to Mediterranean conditions is scarce. The present
study aims to deepen our knowledge on the N cycle within the Ebro River
Basin (NE Spain) by means of two different approaches: (1) calculating a
global N budget in the Ebro River Basin and (2) calculating a series of
detailed regional budgets at higher geographical resolution. N inputs and
outputs were spatialized by creating a map based on the most detailed
information available. Fluvial and atmospheric N export was estimated
together with N retention. The Ebro River Basin annually receives a
relatively high amount of new N (5118 kg N km−2 yr−1), mostly in the form of synthetic fertilizers (50%). Although
it is a highly productive catchment, the net N input as food and feed import
is also high (33%). Only 8% of this N is finally exported to the delta
zone. Several territorial units characterized by different predominant uses
(rainfed agriculture, irrigated agriculture and pastures) have
differentiated N dynamics. However, due to the high density of irrigation
channels and reservoirs that characterize Mediterranean catchments, N
retention is very high in all of them (median value, 91%). These results
indicate that problems of eutrophication due to N delivery in the coastal
area may not be too severe but that high N retention values may instead lead
to problems within the catchment, such as pollution of aquifers and rivers,
as well as high atmospheric emissions. The most promising management
measures are those devoted to reducing agricultural surpluses through a
better balanced N fertilization.
The middle and lower reaches of the Yangtze River basin have the most representative and largest concentration of freshwater lakes in China. However, the size and number of these lakes have changed considerably over the past century due to the natural and anthropogenic impact. The lakes, larger than 10 km(2) in size, were chosen from relief maps and remotely sensed images in 1875, 1950, 1970, 1990, 2000, and 2008 to study the dynamics of lakes in the middle and lower reaches of the Yangtze River basin and to examine the causes and consequences of these changes. Results indicated that there was a dramatic reduction in lake areas, which decreased by 7,841.2 km(2) (42.64 %) during the study period (1875-2008), and the number of lakes in this region changed moderately. Meanwhile, a large number of lakes in the middle and lower reaches of the Yangtze River basin were directly converted into paddy fields, ponds, building lands, or other land-use types over the study period. Therefore, all kinds of lake reclamation should be identified as the major driving factors for the loss of lake in this region. Furthermore, flooding, soil erosion, and sedimentation were also the main factors which triggered lake changes in different periods. Some wetland conservation and restoration projects have been implemented since the 1970s, but they have not reversed the lake degradation. These findings were of great importance to managers involved in making policy for the conservation of lake ecosystems and the utilization of lake resources.
Hydrologic changes associated with urbanization often lead to lower water tables and drier, more aerobic soils in riparian zones. These changes reduce the potential for denitrification, an anaerobic microbial process that converts nitrate, a common water pollutant, into nitrogen gas. In addition to oxygen, denitrification is controlled by soil organic matter and nitrate. Geomorphic stream restorations are common in urban areas, but their effects on riparian soil conditions and denitrification have not been evaluated. We measured root biomass, soil organic matter, and denitrification potential (anaerobic slurry assay) at four depths in duplicate degraded, restored, and reference riparian zones in the Baltimore, Maryland, U.S.A., metropolitan area. There were three main findings in this study. First, although reference sites were wet and had high soil organic matter, they had low levels of nitrate relative to degraded and restored sites and therefore there were few differences in denitrification potential among sites. Evaluations of riparian restorations that have nitrate removal by denitrification as a goal should consider the complex controls of this process and how they vary between sites. Second, all variables declined markedly with depth in the soil. Restorations that increase riparian water tables will thus foster interaction of groundwater nitrate with near-surface soils with higher denitrification potential. Third, we observed strong positive relationships between root biomass and soil organic matter and between soil organic matter and denitrification potential, which suggest that establishment of deep-rooted vegetation may be particularly important for increasing the depth of the active denitrification zone in restored riparian zones.
Removing dams and levees to restore hydrologic connectivity and enhance ecosystem services such as nutrient removal has been an increasingly common management practice. In the present study, the authors assessed geochemical and biological changes following engineered levee breaches that reconnected eutrophic Upper Klamath Lake and Agency Lake, Oregon, USA, to an adjacent, historic wetland that had been under agricultural use for the last seven decades. Over the three-year study, the reconnected wetland served as a benthic source for both macronutrients (dissolved organic carbon [DOC], soluble reactive phosphorus [SRP], and ammonia) and micronutrients (dissolved iron and manganese). The magnitude of those benthic sources was similar to or greater than that of allochthonous sources. The highest DOC benthic flux to the water column occurred immediately after rewetting occurred. It then decreased during the present study to levels more similar to the adjacent lake. Dissolved ammonia fluxes, initially negative after the levee breaches, became consistently positive through the remainder of the study. Nitrate fluxes, also initially negative, became negligible two years after the levee breaches. In contrast to previous laboratory studies, SRP fluxes remained positive, as did fluxes of dissolved iron and manganese. Our results indicate that the timescales of chemical changes following hydrologic reconnection of wetlands are solute-specific and in some cases extend for multiple years beyond the reconnection event. During the present study, colonization of the reconnected wetlands by aquatic benthic invertebrates gradually generated assemblages similar to those in a nearby wetland refuge and provided further evidence of the multiyear transition of this area to permanent aquatic habitat. Such timescales should be considered when developing water-quality management strategies to achieve wetland-restoration goals.
The effects of hydrologic conditions, water quality gradients, and vegetation on nitrous oxide gaseous emissions were investigated
in two identical 1-ha surface-flow created riverine wetlands in Columbus, Ohio, USA. For two years, both wetlands experienced
seasonal (winter-spring) controlled hydrologic flood pulses followed by one year in which they received a steady flow rate
of water. Nitrous oxide fluxes were quantified in a transverse gradient at different elevations (edge plots and high marsh
plots with alternate wet and dry conditions, and low marsh plots and open water plots that were permanently flooded). The
highest average of N2O fluxes was observed in high marsh plots (21.8 ± 2.5 μg-N m−2 h−1), followed by edge plots (12.6 ± 2.5 μg-N m−2 h−1), open water plots (9.9 ± 2.1 μg-N m−2 h−1), and low marsh plots (7.0 ± 4.8 μg-N m−2 h−1). Highest nitrous oxide fluxes were consistently observed in high marsh plots during summer when soil temperatures were ≥
20°C. In permanently flooded plots without vegetation, nitrous oxide fluxes were low, regardless of flood-pulse conditions.
In high marsh plots, water table remained near the soil surface one week after flooding, causing an increase in N2O fluxes (25.9 ± 13.9 μg-N m−2 h−1) compared with fluxes before (2.4 ± 6.4 2.2 μg-N m−2 h−1) and during (6.9 ± 2.2 μg-N m−2 h−1) flooding. In edge plots, nitrous oxide emissions increased during and after the flooding (11.3 ± 3.2 and 7.3 ± 3.3 μg-N
m−2 h−1) compared with fluxes before the flood pulse (4.1 ±1.8 μg-N m−2 h−1). In low marsh and edge zones, no significant (P> 0.05) differences were observed in the seasonal N2O fluxes in the pulsing year versus steady-flow year. Spring N2O fluxes from high marsh plots were significantly (P=0.04) higher under steady-flow conditions (26.2 ± 5.5 μg-N m−2 h−1) than under pulsing conditions (9.6 ± 3.6 μg-N m−2 h−1), probably due to the water table near the surface that prevailed in those plots under steady flow condition. N2O fluxes were higher in plots with vegetation (39.6 ± 13.7 μg-N m−2 h−1) than in plots without vegetation (−3.6 ± 13.7 μg-N m−2 h−1) when plots were inundated; however, when no surface water was present, N2O fluxes were similar in plots with and without vegetation. Implications for large-scale wetland creation and restoration
in the Mississippi River Basin and elsewhere for controlling nitrogen are discussed.
Protozoan abundance, nitrification potential, and related factors in saturated subsurface sediments and the overlying soil were compared at a nonfertilized grassland and an agricultural cropland site. In a 6-week laboratory experiment, DOC, ammonium, and protozoan abundance were manipulated in flasks containing groundwater-sediment slurries. Microbial abundance (protozoa, actively respiring bacteria, and total bacteria) and nutrient concentrations (extractable ammonium and nitrate) were measured.
Results from the soil profile analysis showed that protozoan abundance declined with depth at both sites, but significant numbers (392 cells g−1dw) were found in groundwater sediments at the cropland site. Nitrification potential declined with depth at the grassland site and increased with depth at the cropland site. In the laboratory experiment, treatment responses generally were observed within 3 weeks, but had diminished by 6 weeks. Protozoa reduced bacterial populations through the first 3 weeks, but this effect was not significant by week 6. In the cropland sediments, increased net nitrate production occurred in the two reduced protozoa treatments that received ammonium, suggesting that nitrification was occurring and was limited by ammonium. High protozoan abundance in the cropland sediments increased the nitrate flux response, unless DOC was added; in this case, no response occurred. No such responses were recorded in the grassland sediments.
Apparently, appreciable nitrification can occur in some groundwater sediments, if sufficient ammonium is present and DOC availability is low. Furthermore, nitrification can be enhanced when protozoan abundance is elevated. Finally, our results suggest that surface land use practices can alter subsurface nitrification rates and microbial community structure.
Soil organic matter (SOM) content is a key indicator of soil quality and is correlated to a number of important soil processes
that occur in wetlands such as respiration, denitrification, and phosphorus sorption. To better understand the differences
in the SOM content of created (CW), restored (RW), and paired natural wetlands (NWs), 11 CW/RW-NW pairs were sampled in North
Carolina. The site pairs spanned a range of hydrogeomorphic (HGM) subclasses common in the Coastal Plain. The following null
hypotheses were tested: (1) SOM content of paired CW/RWs and NWs are similar; (2) SOM content of wetlands across different
HGM subclasses is similar; and (3) interactions between wetland status (CW/RW vs. NW) and hydrogeomorphic subclass are similar.
The first null hypothesis was rejected as CW/RWs had significantly lower mean SOM (11.8 ± 3.9%) than their paired NWs (28.98
± 8.0%) on average and at 10 out of the 11 individual sites. The second and third null hypotheses were also rejected as CW/RWs
and NWs in the non-riverine organic soil flat subclass had significantly higher mean SOM content (31.08 ± 14.2%) than the
other three subclasses (8.18 ± 2.5, 11.18 ± 8.2, and 10.38 ± 4.2%). Individual sites within this fourth subclass also had
significantly different SOM content. This indicated that it would be inappropriate to include the organic soil flat subclass
with either the riverine or non-riverine mineral soil flat subclasses when considering restoration guidelines. These results
also suggested that if there is a choice in mitigation options between restoration or creation, wetlands should be restored
rather than created, especially those in the non-riverine organic soil flat subclass.
Lake eutrophication is influenced by both anthropogenic and natural factors. Few studies have examined relationships between
eutrophication parameters and natural factors at a large spatial scale. This study explored these relationships using data
from 103 lakes across China. Eutrophication parameters including total nitrogen (TN), total phosphorus (TP), TN:TP ratio,
chemical oxygen demand (CODMn), chlorophyll-a (Chl-a), Secchi depth (SD), and trophic state index (TSI) were collected for the period 2001–2005. Sixteen natural factors included
three of geographic location, five of lake morphology, and eight of climate variables. Pearson correlation analysis showed
that TP and TSI were negatively related to elevation, lake depth, and lake volume, and positively related to longitude. All
eutrophication parameters, except for CODMn and Chl-a, showed no significant correlation with climate variables. Multiple regression analyses indicated that natural factors together
accounted for 13–58% of the variance in eutrophication parameters. When the 103 study lakes were classified into different
groups based on longitude and elevation, regression analyses demonstrated that natural factors explained more variance in
TN, TP, CODMn, Chl-a, and TSI in western lakes than in eastern lakes. Lake depth, volume, elevation, and mean annual precipitation were the main
predictors of eutrophication parameters for different lake groups. Although anthropogenic impacts such as point- and nonpoint-source
pollution are considered as the main determinants of lake eutrophication, our results suggest that some natural factors that
reflect lake buffer capacity to nutrient inputs can also play important roles in explaining the eutrophication status of Chinese
lakes.
KeywordsHuman activity-Natural factor-Nitrogen-Phosphorus-Water quality
Soil organic matter (OM) is an important feature of natural wetlands (NWs) often lacking in created wetlands (CWs). Some have
suggested that OM amendments be used to accelerate development of edaphic conditions in CWs. Our objective was to investigate
microbial and geochemical responses to compost amendments at a CW. Five levels of amendments were incorporated into drier
and wetter zones of the CW to test two hypotheses: 1) microbial biomass carbon (MBC) and denitrification potential will increase
with increasing levels of amendments; and 2) phosphorus (P) sorption will decrease with increasing levels of amendments. Regression
indicated that pH, MBC, and P sorption had linear relationships, while bulk density (BD) had an exponential relationship with
amendment level. Denitrification enzyme assay (DEA) had highest values at intermediate amendment levels. Analysis of variance
indicated amendment effects for BD, MBC, DEA, and P sorption, and wetness effects for pH and MBC. Amendment levels between
60-180 Mg ha−1 were ideal for microbial development and denitrification, while not sacrificing P sorption, and would be more logistically
and economically feasible than levels of 200–300 Mg ha−1. However, responses to amendments were complex and optimizing amendments for certain functions may detrimentally affect other
functions.
Key Wordscompost-denitrification enzyme assay-microbial biomass carbon-mitigation-organic-amendments-P sorption-soil properties
In a microcosm 15N enrichment experiment we tested the effect of floating vegetation (Lemna sp.) and submerged vegetation (Elodea nuttallii) on denitrification rates, and compared it to systems without macrophytes. Oxygen concentration, and thus photosynthesis,
plays an important role in regulating denitrification rates and therefore the experiments were performed under dark as well
as under light conditions. Denitrification rates differed widely between treatments, ranging from 2.8 to 20.9μmolNm−2 h−1, and were strongly affected by the type of macrophytes present. These differences may be explained by the effects of macrophytes
on oxygen conditions. Highest denitrification rates were observed under a closed mat of floating macrophytes where oxygen
concentrations were low. In the light, denitrification was inhibited by oxygen from photosynthesis by submerged macrophytes,
and by benthic algae in the systems without macrophytes. However, in microcosms with floating vegetation there was no effect
of light, as the closed mat of floating plants caused permanently dark conditions in the water column. Nitrate removal was
dominated by plant uptake rather than denitrification, and did not differ between systems with submerged or floating plants.
KeywordsDenitrification–Ditches–Floating plants–Macrophytes–Nitrogen
Riparian zones have long been considered as nitrate sinks in landscapes. Yet, riparian zones are also known to be very productive ecosystems with a high rate of nitrogen cycling. A key factor regulating processes in the N cycle in these zones is groundwater table fluctuation, which controls aerobic/anaerobic conditions in the soil. Nitrification and denitrification, key processes regulating plant productivity and nitrogen buffering capacities are strictly aerobic and anaerobic processes, respectively. In this study we compared the effects of these factors on the nitrogen cycling in riparian zones under different climatic conditions and N loading at the European scale. No significant differences in nitrification and denitrification rates were found either between climatic regions or between vegetation types. On the other hand, water table elevation turned out to be the prime determinant of the N dynamics and its end product. Three consistent water table thresholds were identified. In sites where the water table level is within -10 cm of the soil surface, ammonification is the main process and ammonium accumulates in the topsoils. Average water tables between -10 and -30 cm favour denitrification and therefore reduce the nitrogen availability in soils. In drier sites, that is, water table level below -30 cm, nitrate accumulates as a result of high net nitrification. At these latter sites, denitrification only occurs in. ne textured soils probably triggered by rainfall events. Such a threshold could be used to provide a proxy to translate the consequences of stream flow regime change to nitrogen cycling in riparian zones and consequently, to potential changes in nitrogen mitigation.
Intensification of catchment agriculture has increased nutrient loads and accelerated eutrophication in some lakes, often resulting in episodic harmful algal blooms or prolonged periods of anoxia. The influence of catchment agriculture on lake sediment denitrification capacity as a nitrogen (N) removal mechanism in lakes is largely unknown, particularly in contrast to research on denitrification in agricultural streams and rivers. We measured denitrification enzyme activity (DEA) to assess sediment denitrification potential in seven monomictic and three polymictic lakes that range in the proportion of agriculture in the catchment from 3 to 96% to determine if there is a link between agricultural land use in the lake catchment and sediment denitrification potential. We collected sediment cores for DEA measurements over 3 weeks in austral spring 2008 (October–November). Lake Okaro, with 96% catchment agriculture, had approximately 15 times higher DEA than Lake Tikitapu, with 3% catchment agriculture (232.2 ± 55.9 vs. 15.9 ± 4.5 μg N gAFDM−1 h−1, respectively). Additionally, sediment denitrification potential increased with the proportion of catchment in agriculture (R
2 = 0.85, P < 0.001). Our data suggest that lakes retain a high capacity to remove excess N via denitrification under increasing N loads from higher proportions of catchment agriculture. However, evidence from the literature suggests that despite a high capacity for denitrification and longer water residence times, lakes with high N loads will still remove a smaller proportion of their N load. Lakes have a denitrification potential that reflects the condition of the lake catchment, but more measurements of in situ denitrification rates across lake catchments is necessary to determine if this capacity translates to high N removal rates.
Human activities have greatly increased the transport of biologically available nitrogen (N) through watersheds to potentially sensitive coastal ecosystems. Lentic water bodies (lakes and reservoirs) have the potential to act as important sinks for this reactive N as it is transported across the landscape because they offer ideal conditions for N burial in sediments or permanent loss via denitrification. However, the patterns and controls on lentic N removal have not been explored in great detail at large regional to global scales. In this paper we describe, evaluate, and apply a new, spatially explicit, annual-scale, global model of lentic N removal called NiRReLa (Nitrogen Retention in Reservoirs and Lakes). The NiRReLa model incorporates small lakes and reservoirs than have been included in previous global analyses, and also allows for separate treatment and analysis of reservoirs and natural lakes. Model runs for the mid-1990s indicate that lentic systems are indeed important sinks for N and are conservatively estimated to remove 19.7 Tg N year-1 from watersheds globally. Small lakes (<50 km2) were critical in the analysis, retaining almost half (9.3 Tg N year -1) of the global total. In model runs, capacity of lakes and reservoirs to remove watershed N varied substantially at the half-degree scale (0-100%) both as a function of climate and the density of lentic systems. Although reservoirs occupy just 6% of the global lentic surface area, we estimate they retain ~33% of the total N removed by lentic systems, due to a combination of higher drainage ratios (catchment surface area:lake or reservoir surface area), higher apparent settling velocities for N, and greater average N loading rates in reservoirs than in lakes. Finally, a sensitivity analysis of NiRReLa suggests that, on-average, N removal within lentic systems will respond more strongly to changes in land use and N loading than to changes in climate at the global scale.
Globally, shallow lakes have suffered from excessive nitrogen (N) loading due to increased human activities in catchments, resulting in water quality degradation and aquatic biodiversity loss. Sediment denitrification, which reduces nitrate (NO3(-)) to N gaseous products, is the most important mechanism for permanent N removal in freshwater lakes. However, the relative contribution of abiotic and biotic factors to the sediment denitrification is highly variable. Here, we determined the unamended denitrification rate and nitrous oxide (N2O) production rate of 74 sediment samples from 22 eutrophic lakes in the Yangtze River basin. We also quantified the diversity and abundance of denitrifying communities using nirK and nirS genes. The results of variance partitioning analyses showed that water physicochemical properties (e.g., dissolved oxygen) and nutrients (e.g., NO3(-) concentration) but not denitrifier communities and submerged vegetation were the major factor groups predicting denitrification and N2O production rates. Path analyses further revealed that water physicochemical properties and nutrients could affect denitrification and N2O production rates both directly and indirectly, and the direct effects were considerably higher than the indirect effects mediated through changes in sediment characteristics, denitrifier communities and submerged vegetation. These findings suggest that the dominant N removal process in Yangtze lakes is largely regulated by abiotic factors rather than diversity and abundance of denitrifiers and submerged macrophytes. Additionally, the findings in this study are helpful in developing a targeted strategy to assess and enhance the N removal capability of eutrophic lakes in China.
Decline of submerged vegetation is one of the most serious ecological problems in eutrophic lakes worldwide. Although restoration of submerged vegetation is widely assumed to enhance ecological functions (e.g., nitrogen removal) and aquatic biodiversity, the evidence for this assumption is very limited. Here, we investigated the spatio-temporal patterns of sediment potential nitrification, unamended denitrification and N2O production rates along a vegetation gradient in the Lake Honghu, where submerged vegetation was largely restored by prohibiting net-pen aquaculture. We also used five functional genes as markers to quantify the abundance of sediment nitrifying and denitrifying microorganisms. Results showed that unvegetated sediments supported greater nitrification rates than rhizosphere sediments of perennial or seasonal vegetation. However, the absence of submerged vegetation had no significant effect on denitrification and N2O production rates. Additionally, the abundance of functional microorganisms in sediments was not significantly different among vegetation types. Season had a strong effect on both nitrogen cycling processes and microbial abundances. The highest nitrification rates were observed in September, while the highest denitrification rates occurred in December. The temporal variation of sediment nitrification, denitrification and N2O production rates could be due to changes in water quality and sediment properties rather than submerged vegetation and microbial abundances. Our findings highlight that vegetation restoration in eutrophic lakes improves water quality but does not enhance sediment nitrogen removal rates and microbial abundances. Therefore, for reducing the N level in eutrophic lakes, major efforts should be made to control nutrients export from terrestrial ecosystems.
Riparian wetlands play a critical role in retaining nitrogen (N) from upland runoff and improving river water quality, mainly through biological processes such as soil denitrification. However, the relative contribution of abiotic and biotic factors to riparian denitrification capacity remains elusive. Here we report the spatio-temporal dynamics of potential and unamended soil denitrification rates in 20 wetlands along the Han River, an important water source in central China. We also quantified the abundance of soil denitrifying microorganisms using nirK and nirS genes. Results showed that soil denitrification rates were significantly different between riparian and reservoir shoreline wetlands, but not between mountain and lowland wetlands. In addition, soil denitrification rates showed strong seasonality, with higher values in August (summer) and April (spring) but lower values in January (winter). The potential and unamended denitrification rates were positively correlated with edaphic conditions (moisture and carbon concentration), denitrifier abundance, and plant species richness. Path analysis further revealed that edaphic conditions could regulate denitrification rates both directly and indirectly through their effects on denitrifier abundance. Our findings highlight that not only environmental factors, but also biotic factors including denitrifying microorganisms and standing vegetation, play an important role in regulating denitrification rate and N removal capacity in riparian wetlands.
Riparian zones play an important role in reducing nitrogen (N) loading to rivers and streams primarily through soil denitrification which reduces nitrate (NO3-) to nitrous oxide (N2O) and dinitrogen (N2) gases. Although the relationships between local environments and soil denitrification are well understood, relatively little is known about the indirect effects of landscape factors (e.g., catchment agriculture) on the soil denitrification of riparian zones. In this study, we used the acetylene block technique to measure the denitrification potential and net N2O production of soils collected from 62 riparian sites in 15 subtropical rivers of varying catchment land uses. The results indicated that, among the local factors studied, the soil moisture, organic matter and NO3- concentrations were positively associated with both the denitrification potential and N2O production rate. Agricultural riparian zones had a denitrification potential (2.81±1.01ngNg-1h-1) significantly higher than forested riparian zones (0.66±0.24ngNg-1h-1). Additionally, the riparian denitrification potential increased with the percentage of agriculture in the catchments (R=0.53, P<0.05). Structural equation modeling revealed that the indirect effects of catchment agriculture on the riparian denitrification potential and N2O production rate were mediated primarily through soil NO3-. Our findings suggest that, compared to forested riparian zones, agricultural riparian zones have greater potential to remove N from polluted runoff. The conversion of original vegetation to agricultural lands in catchments may have a profound impact on the soil N cycles and NO3- removal capacity of riparian zones.
The creation and restoration of wetlands is widely seen as a critical tool for replacing ecosystem functions lost by historic wetland destruction. However, studies have shown that these wetlands often take hundreds of years to achieve the functions for which they are restored. We used controlled field-scale manipulations in four recently restored depressional freshwater wetlands in western New York to investigate the impact of organic amendments of differing lability on the soil and vegetative development during the first 3 yr. Results showed that the addition of soil amendments to wetland plots stimulates development of key soil properties that are critical for wetland functioning. In particular, initial increases in soil C and decreases in bulk density in topsoil and biochar amended plots were still present 3 yr after restoration. Plant biomass recovered quickly and had reached levels of comparable natural wetlands within 2 yr, irrespective of amendments. Amendments did not influence plant diversity. Site differences, however, did influence plant diversity and different sites hosted different numbers and types of species. Two years after restoration, both desirable native wetland species and undesirable weedy species had colonized each site. Results of this research reveal that organic amendments can improve key soil properties critical for wetland functioning. The strength of treatment effects and the development of the plant community, however, are highly influenced by initial site conditions. These results confirm the importance of focusing on both hastening soil development via amendments and careful site selection in restoration design.
Through retaining runoff and pollutants such as heavy metals from surrounding landscapes, ponds around a lake play an important role in mitigating the impacts of human activities on lake ecosystems. In order to determine the potential for heavy metal accumulation of submerged macrophytes, we investigated the concentrations of 10 heavy metals (i.e., As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) in water, sediments, and submerged macrophytes collected from 37 ponds around the Dianchi Lake in China. Our results showed that both water and sediments of these ponds were polluted by Pb. Water and sediments heavy metal concentrations in ponds received urban and agricultural runoff were not significantly higher than those in ponds received forest runoff. This result indicates that a large portion of heavy metals in these ponds may originate from atmospheric deposition and weathering of background soils. Positive relationships were found among heavy metal concentrations in submerged macrophytes, probably due to the coaccumulation of heavy metals. For most heavy metals, no significant relationships were found between submerged macrophytes and their water and sediment environments. The maximum concentrations of Cr, Fe and Ni in Ceratophyllum demersum were 4242, 16,429 and 2662mgkg(-1), respectively. The result suggests that C. demersum is a good candidate species for removing heavy metals from polluted aquatic environments.
Stream ecotones, specifically the lateral floodplain and subsurface
hyporheic zone, can be important sites for nitrogen (N) removal via
denitrification, but their role in streams with constructed floodplains
has not been examined. We studied denitrification in the hyporheic zone
and floodplains of an agriculturally influenced headwater stream in
Indiana, USA, that had floodplains added as part of a "two-stage ditch"
restoration project. To examine the potential for N removal in the
hyporheic zone, we seasonally measured denitrification rates and nitrate
concentrations by depth into the stream sediments. We found that nitrate
concentration and denitrification rates declined with depth into the
hyporheic zone, but denitrification was still measureable to a depth of
at least 20 cm. We also measured denitrification rates on the restored
floodplains over the course of a flood (pre, during, and
post-inundation), and also compared denitrification rates between
vegetated and non-vegetated areas of the floodplain. We found that
floodplain denitrification rates increased over the course of a
floodplain inundation event, and that the presence of surface water
increased denitrification rates when vegetation was present. Stream
ecotones in midwestern, agriculturally influenced streams have
substantial potential for N removal via denitrification, particularly
when they are hydrologically connected with high-nitrate surface water.
Greater connectivity to stream surface water may result in greater inputs of allochthonous nutrients that could stimulate internal nitrogen (N) and phosphorus (P) cycling in natural, restored, and created riparian wetlands. This study investigated the effects of hydrologic connectivity to stream water on soil nutrient fluxes in plots ( = 20) located among four created and two natural freshwater wetlands of varying hydrology in the Piedmont physiographic province of Virginia. Surface water was slightly deeper; hydrologic inputs of sediment, sediment-N, and ammonium were greater; and soil net ammonification, N mineralization, and N turnover were greater in plots with stream water classified as their primary water source compared with plots with precipitation or groundwater as their primary water source. Soil water-filled pore space, inputs of nitrate, and soil net nitrification, P mineralization, and denitrification enzyme activity (DEA) were similar among plots. Soil ammonification, N mineralization, and N turnover rates increased with the loading rate of ammonium to the soil surface. Phosphorus mineralization and ammonification also increased with sedimentation and sediment-N loading rate. Nitrification flux and DEA were positively associated in these wetlands. In conclusion, hydrologic connectivity to stream water increased allochthonous inputs that stimulated soil N and P cycling and that likely led to greater retention of sediment and nutrients in created and natural wetlands. Our findings suggest that wetland creation and restoration projects should be designed to allow connectivity with stream water if the goal is to optimize the function of water quality improvement in a watershed.
Stream restoration often aims at mitigating nutrient pollution in aquatic ecosystems. However, despite recent research efforts, effects of restoration practices on in-stream nitrogen removal remain unclear. In this study, denitrification rates as well as factors controlling denitrification in unrestored and restored sections of two Danish streams (S1 and S2) were compared. The 15N isotope pairing technique was used to measure denitrification in situ. Denitrifier presence was analyzed by denaturing gradient gel electrophoresis (DGGE) and quantitative PCR of nitrite reductase (nirK and nirS) and nitrous oxide reductase (nosZ) genes. Denitrification rates were highly variable, with denitrification rates of 3106 μmol N m−2 h−1 in the unrestored section of S1, but no detectable denitrification in the restored section of S1, whereas in S2 restored and unrestored sections had similar denitrification rates of around 250 μmol N m−2 h−1. These large differences in denitrification rates were mainly due to differences in hydrologic conditions and sediment characteristics. High nitrate fluxes from upwelling groundwater created denitrification hotspots in the unrestored section of S1. Moreover, a lack of organic matter in the restored section of S1 likely caused a low abundance of denitrifiers and consequently no detectable denitrification. Our results indicate the importance of hydrology and sediment organic matter for stream nitrogen dynamics, which should be considered in restoration design.
This study focuses on the microbial N cycle in the acid soil of a beech forest that falls in the upper range of the N saturation continuum. Our objectives were: (1) to quantify microbial N cycling under long-term N-saturated and limed conditions and (2) to determine the factors controlling the differences in microbial N cycling. Our study site has a long history of high N deposition: ≥25 kg N·ha-1·yr-1 since measurements began in 1971. This was further enhanced by 11 yr (1983-1993) of fertilization (140 kg ammonium sulfate-N·ha-1·yr-1) to create an N saturation plot. Another plot was limed with 30 Mg/ha dolomitic limestone in 1982. In 1999-2000, gross rates of microbial N cycling were measured using 15N pool dilution techniques. Despite the chronic high N deposition, the control plot showed a tightly coupled microbial N cycle; NH4+ and NO3- immobilization rates were comparable to gross N mineralization and nitrification rates, respectively. These were supported by low levels of NH4+, NO3-, and dissolved organic N (DON) in percolate. Liming increased gross N mineralization and nitrification rates but did not cause similar increases in microbial biomass or NH4+, and NO3- immobilization rates. In addition, NO3- immobilization rates were somewhat less than gross nitrification rates; relatively high levels of NO3- and DON in percolate were also observed. The N-saturated plot suggested an uncoupled microbial N cycle; NH4+ immobilization rates were lower than gross N mineralization rates, and NO3- immobilization rates were somewhat less than gross nitrification rates. These were corroborated by high levels of NH4+, NO3-, and DON in percolate. The reduced NH4+ and NO3- immobilization rates in the N-saturated plot could be attributed to the measured decreases in microbial biomass, and the low microbial biomass was likely due to decreases in the supply of labile C. Our study demonstrates that while hydrological N input/output budgets can indicate whether or not a forest ecosystem is in a state of N saturation, the microbial N cycle can provide quantitative information on key processes that govern N losses.
Intact soil cores from three tropical rainforest sites on the Atherton Tablelands, Australia, were sampled at different hygric seasons to determine the effects of soil temperature and soil moisture on gross nitrification using the barometric process separation technique (BaPS). Parameterization experiments revealed that gross nitrification was positively correlated to increases in soil temperature, but negatively correlated to increased rates of water-filled pore space (WFPS) because of simulated rainfall. Pronounced seasonal variations of gross nitrification rates were observed at all three sites with lowest values during the dry season (1.9-9.7 mg NH4+-N m-2 h-1) and highest values during the transition period between dry to wet season (14.8-27.6 mg NH4+-N m-2 h-1). Highest nitrification activities were found for two sites characterized by a narrow C/N ratio and a high total C content in the mineral soil, whereas the site with a wider C/N ratio and lower C content in the soil showed significantly lower nitrification rates. Gross nitrification was positively correlated to in situ N2O-emission rates indicating that nitrification is a key regulating process of N2O-production and emission in these tropical soils.
As the role of soil properties in the development of created wetlands (CWs) has not received adequate attention in regulatory or scientific communities, this study was conducted to evaluate the development of soil properties in 11 CWs in Virginia ranging from 4 to 16 yr since creation. Six of the 11 sites received at least 15 cm of topsoil (TS) while the other five sites received no topsoil (No TS). Cores collected from wet, intermediate, and dry positions at each site were analyzed for moisture, bulk density (Db), soil organic matter (SOM), texture, water-holding capacity (WHC), P sorption index (PSI), and microbial biomass C (MBC). Both positions along the hydrologic gradient and topsoil status were hypothesized to be significant factors in explaining the variability of the measured soil properties. Soil moisture decreased significantly while Db increased significantly from wet to dry zones. Moisture, WHC, and PSI were all significantly elevated in certain zones of TS compared with No TS sites. Soil organic matter had significant Spearman correlations with all other measured soil properties, revealing that this parameter was an important indictor of soil quality. In addition, sites with high mean moisture, SOM, and PSI values all received TS; conversely, the site with the lowest mean moisture and SOM content did not receive TS. Thus, amending CW soils with TS appeared to be an effective strategy for increasing soil moisture, WHC, and PSI. Whenever possible, practices such as TS or organic amendments should be employed, especially if wetland creation involves excavation into subsoils with low SOM and high Db.
Ammonia oxidation plays a pivotal role in the cycling and removal of nitrogen in aquatic ecosystems. Recent findings have expanded the known ammonia-oxidizing prokaryotes from Bacteria to Archaea. However, the relative importance of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) in nitrification is still debated. Here we showed that, in two large eutrophic lakes in China (Lake Taihu and Lake Chaohu), the abundance of AOA and AOB varied in opposite patterns according to the trophic state, although both AOA and AOB were abundant. In detail, from mesotrophic to eutrophic sites, the AOA abundance decreased, while the AOB increased in abundance and outnumbered the AOA at hypertrophic sites. In parallel, the nitrification rate increased along these trophic gradients and was significantly correlated with both the AOB abundance and the numerical ratio of AOB to AOA. Phylogenetic analysis of bacterial amoA sequences showed that Nitrosomonas oligotropha- and Nitrosospira-affiliated AOB dominated in both lakes, while Nitrosomonas communis-related AOB were only detected at the eutrophic sites. The diversity of AOB increased from mesotrophic to eutrophic sites and was positively correlated with the nitrification rate. Overall, this study enhances our understanding of the ecology of ammonia-oxidizing prokaryotes by elucidating conditions that AOB may numerically predominated over AOA, and indicated that AOA may play a less important role than AOB in the nitrification process of eutrophic lakes.
Previous studies have examined the effects of soil osmotic potential (Ψs) on net rates of mineralization and nitrification. Because net rates represent the difference between gross production and consumption processes, it is unclear which process is being affected. We used an 15N isotopic dilution method to evaluate the effects of Ψs on gross rates of nitrification, ammonification, NH+4 assimilation, and NO-3 assimilation, and net rates of nitrous oxide production in a Penoyer sandy loam at field capacity. To avoid creating specific ion toxicities that normally do not occur in this soil, we used a chemical equilibrium model to predict how solute concentrations in the soil solution change during evapo-concentration; then we used solutions containing these mixtures of solutes to create individual Ψs treatments. A nitrification potential assay was also performed to determine the effect of Ψs on nitrification rates at high substrate concentrations. In soil slurries with elevated NH+4 concentration (1110 μM), nitrification rates declined exponentially with reduced Ψs (increased salt concentration); however, in soil samples incubated at field capacity without added NH+4 (9.7 μM, or 2 mg N kg -1), the gross nitrification rate was independent of Ψs. The differential response between slurries and soil at field capacity was attributed to differences in NH+4 concentrations, and indicated that the effects of Ψs were secondary to NH+4 concentrations in controlling nitrification rates. Nitrification rates in slurries declined more when a single salt (K2SO4) was used than when the mixture of salts that more closely approximated the solute composition predicted to occur in the field was used to lower Ψs. This suggests that nitrifying bacteria are capable of adapting to specific ion toxicities. Gross rates of ammonification declined exponentially with decreased Ψs between 0 and -500 kPa but were independent of Ψs at potentials of -500 to -1750 kPa. Rates of microbial assimilation of NO-3 exceeded NH+4 assimilation by a factor of 4, indicating that under NH+4 limited conditions substantial NO-3 assimilation can occur. Microbial assimilation of both NH+4 and NO-3 declined exponentially with decreased Ψs, and were insignificant at <-1500 kPa Ψs. Because NO-3 assimilation declined more rapidly than gross nitrification, net nitrification rates actually increased with declining Ψs. Rates of nitrous oxide (N2O) production were also inversely correlated with Ψs. Our results indicate that in previous studies, measurement of net rates, use of inappropriate salts, and addition of substrate may have resulted in over-estimation of the adverse effects of low Ψs on rates of N-transformations.
The large freshwater lakes of the world are an extremely valuable resource, not only because 68% of the global liquid surface fresh water is contained in them, but because of their importance to the economies, social structure, and viability of the riparian countries. This review provides decision makers with the knowledge of large lakes ([greater-than-or-equal] 500 km2) essential to establishing policies and implementing strategies compatible with sustainable development. This is achieved by considering the present state of the lakes, the extent of changes and factors causing them, long-term consequences of these changes, major threats and possible states of the lakes into the year 2025. Case studies of lakes are presented, namely the St Lawrence Great Lakes of North America as representatives of glacial scour lakes of North America, northern Europe and Asia, and the African Great Lakes as representatives of tropical tectonic lakes. Lake Baikal is also included because it is unique for its species, great age, and largest single volume of liquid surface fresh water. The Aral Sea is further included because of the ecological disaster following diversion of water away from its basin. The major impacts on large lakes are diversions, eutrophication, invasive species, land-use change, overexploitation of resources, and pollution. These impacts can or do affect all the representative lakes, but to varying degree. The St Lawrence Great Lakes have been severely impacted by eutrophication, land-use change, overfishing, invasive species and pollution. Eutrophication has been reversed for these lakes and constraints are now in place on land use change, such as shoreline alteration and destruction of wetlands. With the demise of most commercial fishing, overfishing is no longer as important. Invasive species have become a major problem as increasingly non-indigenous species gain access to the lakes. Pollution continues as a major impact. These problems are likely to continue and seriously impact use of the resources as well as bring about changes in the biota. Among the African Great Lakes, invasive species are a major problem in Lake Victoria, and eutrophication associated with land-use change and overexploitation of resources is a growing problem. Many endemic species have been lost and many are threatened, so that species associations will have changed by 2025. The Aral Sea continues to disappear and in the future, the remaining largest part of it will continue to become increasingly saline and eventually disappear. A small body of water will remain as a freshwater lake with a productive, although small, fishery. Lake Baikal shows evidence of pollution in the southern basin and is likely to be impacted by land-use changes, primarily logging. Some non-indigenous species are present, but so far, they are not a major problem. Overexploitation of resources in the watershed could lead to adverse impacts on inshore waters. Overfishing has been recognized and appears under control. The major threat to Baikal is continued and growing pollution. Climate change and pollution are global problems that will affect all lakes, large and small. At present, while some warming has occurred, climate change appears not to have impacted large lakes. Present studies on the Laurentian Great Lakes predict possible major impacts. Pollution, especially from persistent toxic substances such as PCBs, is a global problem. Diversion of water out or away from large lakes will become more of a threat as global human population growth continues and water supplies from rivers and ground water become depleted.
Adequate characterization of labile organic carbon (LOC) is essential to the understanding of C cycling in soil. There has been very little evaluation about the nature of LOC characterizations in coastal wetlands, where soils are constantly influenced by different redox fluctuations and salt water intrusions. In this study, we characterized and compared LOC fractions in coastal wetland soils of the Mississippi River deltaic plain using four different methods including 1) aerobically mineralizable C (AMC), 2) cold water extractable C (CWEC), 3) hot water extractable C (HWEC), and 4) salt extractable C (SEC), as well as acid hydrolysable C (AHC) which includes both labile and slowly degradable organic C. Molecular organic C functional groups of these wetland soils were characterized by (13)C solid-state nuclear magnetic resonance (NMR). The LOC and AHC increased with soil organic C (SOC) regardless of wetland soil type. The LOC estimates by four different methods were positively and significantly linearly related to each other (R(2)=0.62-0.84) and with AHC (R(2)=0.47-0.71). The various LOC fractions accounted for ≤4.3% of SOC whereas AHC fraction represented 16-49% of SOC. AMC was influenced positively by O/N-alkyl and carboxyl C but negatively by alkyl C, whereas CWEC and SEC fractions were influenced only positively by carboxyl C but negatively by alkyl C in SOC. On the other hand, HWEC fraction was found to be only influenced positively by carbonyl C, and AHC positively by O/N-alkyl and alkyl C but negatively by aromatic C groups in SOC. Overall these relations suggested different contributions of various molecular organic C moieties to LOC in these wetlands from those often found for upland soils. The presence of more than 50% non-acid hydrolysable C suggested the dominance of relatively stable SOC pool that would be sequestered in these Mississippi River deltaic plain coastal wetland soils. The results have important implications to the understanding of the liability and refractory character of SOC in these wetlands as recent studies suggest marsh SOC to be an important C source in fueling hypoxia in the northern Gulf of Mexico.
A meta-analysis was conducted on 136 data sets of denitrification rates (DR) recorded both during the period of highest water temperature and monthly in five types of aquatic ecosystems: oceans, coastal environments, estuaries, lakes and rivers. There was a gradual increase of DR from the ocean to rivers and lakes at both scales, with the rivers showing the highest DR variability. Denitrification peaked during summertime and showed highest seasonal variability in lakes and rivers. High concentrations of nitrate and interstitially-dissolved organic carbon as well as low oxygen concentration in the overlying water enhanced DR both during summer and at a seasonal scale whereas total phosphorus did at the seasonal scale only. There was a positive linear relationship between overlying nitrate and DR over the range of 1–970µmol NO3 (r
2=0.86, P=0.001). DR in lakes and rivers might reach values doubling those in the more denitrifying terrestrial ecosystems (e.g. agrosystems). Discrepancies in DR and its controlling factors between site-specific studies and this meta-analysis may arise from environmental variability at two, often confounded, scales of observation: the habitat and the ecosystem level. Future studies on denitrification in aquatic environments should address the topic of spatial heterogeneity more thoroughly.
Wetland ecosystems in agricultural areas often become progressively more isolated from main water bodies. Stagnation favors the accumulation of organic matter as the supply of electron acceptors with water renewal is limited. In this context it is expected that nitrogen recycling prevails over nitrogen dissipation. To test this hypothesis, denitrification rates, fluxes of dissolved oxygen (SOD), inorganic carbon (DIC) and nitrogen and sediment features were measured in winter and summer 2007 on 22 shallow riverine wetlands in the Po River Plain (Northern Italy). Fluxes were determined from incubations of intact cores by measurement of concentration changes or isotope pairing in the case of denitrification. Sampled sites were eutrophic to hypertrophic; 10 were connected and 12 were isolated from the adjacent rivers, resulting in large differences in nitrate concentrations in the water column (from −2 h−1) were up to two orders of magnitude higher than rates measured in isolated wetlands (2–231 μmol N m−2 h−1), suggesting a strong regulation of the process by nitrate availability. These rates were also significantly higher in summer (9–1,888 μmol N m−2 h−1) than in winter (2–365 μmol N m−2 h−1). Denitrification supported by water column nitrate (DW) accounted for 60–100% of total denitrification (Dtot); denitrification coupled to nitrification (DN) was probably controlled by limited oxygen availability within sediments. Denitrification efficiency, calculated as the ratio between N removal via denitrification and N regeneration, and the relative role of denitrification for organic matter oxidation, were high in connected wetlands but not in isolated sites. This study confirms the importance of restoring hydraulic connectivity of riverine wetlands for the maintenance of important biogeochemical functions such as nitrogen removal via denitrification.
Floodplain restoration has been advocated as a means to restore several ecological services associated with floodplains: water quality improvement, fish rearing habitat, wildlife habitat, flood control, and groundwater recharge. A history of agricultural encroachment on the lower Cosumnes River has resulted in extensive channelization and levee construction. In fall 1998, an experimental floodplain was established by breaching a levee in order to restore the connection between the main channel and its historic floodplain. In this study, we examined how effective this newly restored floodplain was at processing nitrate (NO
3−) before reentering the main channel downstream. Two methods were used to examine nitrate loss. In December 2001, we determined denitrification potentials of the floodplain soils before seasonal flooding inundated the floodplain. Next, we conducted a series of field soil column (mesocosm) experiments from March to June 2002 to study NO
3−-N loss from the overlying water in both sandy and clayey soils and at three levels of NO
3−-N (ambient, +1 mg N l−1, +5 mg N l−1). In addition, we examined NO
3−-N loss from mesocosms with water only to determine if loss was related primarily to soil or water column processes. Denitrification potentials were highly variable ranging from 1.6 to 769 ng N2O–N cm−3 h−1. In addition, the denitrification potential was highly correlated with the amount of bioavailable carbon indicating that carbon was a limiting factor for denitrification. Nitrate-N loss rates from mesocosms ranged from 2.9 to 21.0 μg N l−1 h−1 over all treatments and all 3 months examined. Significant loss of NO
3−-N (60–93%) from the water only treatment only occurred in June when warmer temperatures and solar radiation most likely increased NO
3−-N uptake by phytoplankton. When scaled up, potential NO
3−-N loss from the restored floodplain represented 0.6–4.4% of the annual N load from the Lower Cosumnes River during a typical wet year and ~24% during a dry year. During dry water years, the effectiveness of the floodplain for reducing nitrate is limited by the amount of N supplied to the floodplain. Results from this study suggest that restored floodplains can be an effective NO
3− sink.
Stream macrophytes are often removed with their sediments to deepen stream channels, stabilize channel banks, or provide habitat
for target species. These sediments may support enhanced nitrogen processing. To evaluate sediment nitrogen processing, identify
seasonal patterns, and assess sediment processes relative to stream load, we measured denitrification and nitrification rates
in a restored third- to fourth-order agricultural stream, Black Earth Creek, Wisconsin, and estimated processing over a 10km
reach. Our results show that sediments with submerged and emergent macrophytes (e.g., Potomageton spp. and Phalaris arudinacea) support greater denitrification rates than bare sediments (1.12μmol Ng−1h−1 vs. 0.29). Sediments with macrophytes were not carbon limited and organic matter fraction was weakly correlated to denitrification.
The highest denitrification potential occurred in macrophyte beds (5.19μmol Ng−1h−1). Nitrification rates were greater in emergent beds than bare sediments (1.07μgNml−1day−1 vs. 0.35) with the greatest nitrification rates during the summer. Total denitrification removal in sediments with macrophytes
was equivalent to 43% of the nitrate stream load (463.7kgN day−1) during spring and nitrification in sediments with macrophytes was equivalent to 247% of summer ammonium load (3.5kgNday−1). Although the in-channel connectivity to nitrogen rich water was limited, actual stream nitrogen loads could increase with
removal of macrophytes. Macrophyte beds and supporting fringing wetted areas are important if nitrogen management is a concern
for riparian stream restoration efforts.
KeywordsNitrate–Ammonium–Pollution–Restoration–Ecosystem services–Macrophyte–Denitrification–Stream–Nitrification–Sediment–Wet fringe
The short-term response of soil denitrification to reduced aeration was studied using the acetylene inhibition method for the assay of denitrification. Two distinct phases of denitrification rate were observed. An initial constant rate, termed phase I, was not decreased by chloramphenicol, was increased slightly or not at all by organic carbon amendment, and lasted for 1–3 h. Phase I was attributed to the activity of pre-existing denitrifying enzymes in the soil microflora. Following phase I the denitrification rate increased; chloramphenicol inhibited this increase. In soils without organic-C amendment a second linear phase, termed phase II, was attained after 4–8 h of anaerobic incubation. The linearity of this phase was attributed to the full derepression of denitrifying enzyme synthesis by the indigenous population and to the lack of significant growth of denitrifiers. Phase I rate was dependent on the initial or in situ aeration state of the soil sample; phase II was not. Therefore, phase I may be more directly related to field denitrification rates.Denitrification rate changes following water saturation of soils in aerobic atmospheres were also examined. Rates were greatly increased by wetting but only after a lag of several hours. Our interpretation is that following wetting of natural soils, anaerobic or partially anaerobic conditions are established by respiration and reduced O2 diffusion rate; this first eliminates O2 inhibition then derepresses the synthesis of denitrifying enzymes. Although denitrifying enzymes are apparently present even in relatively dry soils, their activity is low until O2 inhibition is eliminated. From this evidence we reason that most N is lost from soils during brief periods beginning a few hours after irrigation or a rainfall.
The effects of three submersed plant species (Elodea canadensis Mixch., Myriophyllum alterniflorum L. and Lobelia dortmanna L.) on bacterial activity and microbial biomass in ambient sediment were measured under laboratory conditions with special reference to organic carbon release through the plant roots. The effect of Lobelia on denitrification potential in sediment was also assessed. Organic carbon was released into the sediment by all the three plant species: of the photosynthetically fixed 14C-carbon, Elodea released 14%, Myriophyllum 4% and Lobelia 2% during the 7-day experiment. However, regardless of the carbon supply, Elodea and Myriophyllum did neither enhance the bacterial activity nor the microbial biomass in the sediment, whereas Lobelia had a positive influence on both the variables, as well as on denitrification potential within deeper root zone.
The current U.S. wetland mitigation policy of “no net loss” requires that a new wetland be created to replace any natural wetland destroyed under development pressures. This policy, however, may be resulting in a net loss of carbon‐based wetland functions. We evaluated the ability of created wetlands to accumulate carbon and to mitigate loss of carbon‐based functions in natural wetlands with variable hydrology. Potential limiting factors to carbon accumulation within created systems included soil aggregation, texture, and bulk density. Rates of soil development and the time required for created wetlands to accumulate the amount of carbon found in natural wetlands were estimated by an exponential model.
Soils collected from five created (ages 3–8 years) and four natural freshwater marshes, located in central Ohio, USA, were analyzed for soil organic carbon (SOC), mineralizable soil carbon ( C min ), water‐stable aggregates (WSA), particle‐size fractions (PSD), and bulk density. Peak‐standing aboveground plant biomass was also quantified. Created wetlands contained significantly less plant biomass, SOC, and C min than natural wetlands (α ≤ 0.05; false discovery rate). Soil physical properties also differed significantly between created and natural wetlands, with fewer macroaggregates, more microaggregates, more silt–clay (0–5 cm only), and higher bulk density in created wetlands (α ≤ 0.05; false discovery rate). Carbon content was positively correlated with macroaggregate content and negatively correlated with microaggregate content, silt‐clay fraction, and bulk density.
Fit of SOC data to the exponential model indicated that a newly created wetland would require 300 years to sequester the amount of SOC contained in a natural wetland. At this rate of carbon accumulation, a mitigation ratio of 2.7:1 (area) would be necessary for successful mitigation over a 50‐year time period. However, other trajectories fit the data equally well and suggested area mitigation ratios of 2.2:1 (logistic) to 4.4:1 (linear regression) to 5.1:1 (exponential regression). Whether created wetlands are on a trajectory toward natural wetland carbon function, however, remains uncertain. Until gaps in the data are filled and a trajectory verified, the best mitigation policy will be a conservative one, with a restrictive permitting process and high mitigation ratios (5.1:1 minimum).
Wetland restoration is increasingly used as a strategy both to address historical wetland losses and to mitigate new wetland impacts. Research has examined the success of restored wetlands for avifaunal habitat, plant biodiversity, and plant cover; however, less is known about soil development in these systems. Soil processes are particularly important as soil organic matter (SOM), cation exchange capacity (CEC), and other properties are directly linked to wetland functions such as water quality improvement. This research compared soil development processes and properties of 30 palustrine depressional wetlands of four different age classes (approximately 5, 14, 35, and 55 years since restoration) located in central New York (USA). Five natural wetlands were used as references. This chronosequence included wetlands 27 years older than previously conducted studies, making it the longest reported database available. Replicated soil cores from each site were analyzed for SOM, bulk density (D(b)), CEC, and concentrations of nutrients and other chemical constituents. Decomposition rate and aboveground plant and litter biomass were measured as key contributors to soil development. The results indicate that some soil properties critical for water quality functions take decades or centuries to reach natural reference levels. Of particular importance, in the top five centimeters of soil, SOM, D(b), and CEC achieved <50% of reference levels 55 years after restoration. Soil development processes in these depressional wetlands appear to be driven by autochthonous inputs and by internal processes such as litter decomposition and are not accelerated in the initial phase of development by allochthonous inputs as has been documented in coastal salt marshes and riverine floodplains. While monitoring generally focuses on the initial establishment phase of restored ecosystems, our findings indicate that the later autogenic phase strongly influences development trajectories for important wetland soil properties. Therefore, the role of different successional phases in determining long-term trajectories of ecosystem development should be considered in restoration design, research, and monitoring. This research highlights areas for improving the field of restoration through understanding of successional processes, increased efforts to jump-start soil development, longer-term monitoring programs, and greater focus on soil components of restored wetlands.
Denitrification is the most important mechanism of nitrogen retention in aquatic systems. Research into the spatial variability of sediment denitrification has been relatively rare. Here, we use the N2 flux technique to measure sediment denitrification rates at 19 littoral and 1 profundal site in Lake Memphremagog. Littoral denitrification rates were highly variable with an average rate of 111 mumol N m-2 h-1. Littoral denitrification rates were positively related to temperature (r2 = 0.66, p < 0.01), % organic matter (r2 = 0.31, p < 0.05) and macrophyte biomass density and negatively related to depth. These results in combination with an analysis of the literature and a predictive model created from literature data relating site depth and denitrification rates show that the littoral zone dominates whole lake denitrification.
Riparian zones have been found to function as "sinks" for nitrate (NO3-), the most common groundwater pollutant in the U. S., in many areas. The vast majority of riparian research, however, has focused on agricultural watersheds. There has been little analysis of riparian zones in urban watersheds, despite the fact that urban areas are important sources of NO3- to nitrogen (N)-sensitive coastal waters in many locations. In this study, we measured stream incision, water table depths, and pools, production (mineralization, nitrification), and consumption (denitrification) of NO3- in urban soils. Samples were taken from soil profiles (0-100 cm) of three forested urban and suburban zones and one forested reference riparian zone in the Baltimore, Maryland metropolitan area. Our objectives were to determine (1) if stream incision associated with urbanization results in lower riparian water tables, and (2) if pools, production, and consumption of NO3- vary systematically with stream incision and riparian water table levels. Two of the three urban and suburban streams were more incised and all three had lower water tables in their riparian zones than the forested reference stream. Urban and suburban riparian zones had higher NO3- pools and nitrification rates than the forested reference riparian zone, which was likely due to more aerobic soil profiles, lower levels of available soil carbon, and greater N enrichment in the urban and suburban sites. At all sites, denitrification potential decreased markedly with depth in the soil profile. Lower water tables in the urban and suburban riparian zones thus inhibit interaction of groundwater-borne NO3- with near surface soils that have the highest denitrification potential. These results suggest that urban hydrologic factors can increase the production and reduce the consumption of NO3- in riparian zones, reducing their ability to function as sinks for NO3- in the landscape.