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Abstract

Soils associated with wet and ephemerally wet environments, i.e. wet soils, cover an area greater than 12.1 million km2; inland wetlands deliver at least Int$27.0 trillion in tangible and intangible benefits. However, due to their intimate association with wet environments, wet soils are at risk of degradation during drought events. This review investigates the effects of drought on wet soils, with particular attention to the changes in soil geochemistry and greenhouse gas emissions. It is clear from this review that drought poses a significant threat to wet soils, a threat which can be difficult to determine before an event but which poses a catastrophic risk to some sites. Drought causes oxygen penetration to increase in wet soils, leading to an increase in oxidation of organic matter and reduced inorganic species (e.g. sulfides). Oxidation of these materials can lead to soil acidification, metal mobilization and to negative impacts on water quality. Increased oxygen in the soil profile also affects biogeochemical cycling, with increased production of nitrous oxide and decreased production of methane. Effects of drought differ between peat and mineral soil types and subtypes. Wet soils undergo major chronological transformations and biogeochemical changes in the alteration of environments occurring before, during and after drought conditions. Water conditions (i.e. subaqueous, saturated, unsaturated and resaturated) also play a major role in chronological soil transformations. Soils may not easily recover between severe droughts and instead enter alternative stable states. This review highlights substantial gaps in our understanding of the effects of drought on wet soils and shows that previous studies overrepresent relatively small geographical regions.

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... This was attributed to the declining wetland water table and inadequate ground water recharge of deep wells (used as sources of domestic water) perceived to be linked to the wetland. Therefore, during drought years, the water security of the community was adversely compromised [58]. The effects of drought on water levels were observed to be clear and pronounced, a situation that was also reported in similar studies conducted elsewhere [59]. ...
... The drying of water ponds had resulted in a reduction in fish production. The disappearance of pools of water during drought periods had also been observed to be a common natural phenomenon in the wetlands [60], meaning droughts have been largely affecting water resources in wetlands at various scales, although the impact tends to vary [55,58]. Moreover, 85% of the household respondents revealed that drought had a severe to very severe impact on the amount of thatch grass harvested from the wetland area ( Figure 3). ...
... This was attributed to the fact that crop cultivation is not taking place within the wetland but on the fringes where the community was applying both organic and inorganic fertilisers in their gardens to improve crop yields. However, droughts in some areas were observed to be increasingly impacting on wet soils and leading to their degradation [58], and this may explain why 10% of the households rated the impact of drought on soil fertility as very severe. ...
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The paper assesses local people’s perceptions on the impact of drought on wetland ecosystem services and the associated household livelihood benefits, focusing on the Driefontein Ramsar site in Chirumanzu district, Zimbabwe. Field data were obtained using a questionnaire from 159 randomly selected households, key informant interviews and transect walks. The study findings show that provisioning, regulating and supporting services are severely affected by a high frequency of drought, occurring at least once every two years, compared to cultural services. There is a reduction in water for domestic use and crop farming, pasture for livestock, fish, thatch grass and ground water recharge. Although cultural services such as traditional rain-making ceremonies and spiritual enhancement are largely unaffected by drought, the wetland’s aesthetic value was reported to be diminishing. The habitat and breeding areas of endangered crane bird species were perceived to be dwindling, affecting their reproduction. All the household heads are not formally employed and largely depend on the wetland resources for food and income. However, drought is adversely affecting wetland-based agricultural activities that are key pillars of the households’ economy. Therefore, there is a need for alternative livelihood strategies that enable local communities to adapt to drought impacts without exerting more pressure on the declining wetland resources.
... The inter-annual variation in DOC reported here highlights the role of hydrology as a driver of lateral C flux from soils and peatlands to streams 16,18 , including the potential for drought to restrict DOC inputs to northern aquatic systems 30 , and elsewhere 31,32 . In this case, the riparian forest-stream connection was far more sensitive to drought than was the mire-stream connection, likely reflecting the steep vertical gradient in organic carbon storage for streamside soils, which gives rise to positive relationships between discharge, water table depth, and lateral DOC mobilization 6,33 . ...
... For instance, droughts have been found to decrease phenolic microbial inhibiter compounds in wetlands resulting in increased organic matter decomposition and an increase in carbon loss in peats 22,23 . In addition, droughts increase the temperature and degree of aeration of soils that are normally inundated, upregulating organic matter decomposition 31,47 . Finally, rewetting these exposed soils can also trigger the physical and microbial processes that promote rapid organic matter mineralization 2 . ...
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One likely consequence of global climate change is an increased frequency and intensity of droughts at high latitudes. Here we use a 17-year record from 13 nested boreal streams to examine direct and lagged effects of summer drought on the quantity and quality of dissolved organic carbon (DOC) inputs from catchment soils. Protracted periods of drought reduced DOC concentrations in all catchments but also led to large stream DOC pulses upon rewetting. Concurrent changes in DOC optical properties and chemical character suggest that seasonal drying and rewetting trigger soil processes that alter the forms of carbon supplied to streams. Contrary to expectations, clearest drought effects were observed in larger watersheds, whereas responses were most muted in smaller, peatland-dominated catchments. Collectively, our results indicate that summer drought causes a fundamental shift in the seasonal distribution of DOC concentrations and character, which together operate as primary controls over the ecological and biogeochemical functioning of northern aquatic ecosystems.
... The speed at which the overall trend declines within these regions increases until late 2017, where it continues to rapidly decrease at the same pace. This could be a result of extreme weather events during 2017 and 2018 from which the peatland has yet to recover (Fenner and Freeman, 2011;Stirling et al., 2020;Undorf et al., 2020). If the trend gradients were included in the clustering, a different region, which may not have exhibited such weather events, with different trend gradients would be classed as degraded when compared to the Flow Country. ...
Preprint
Peatlands account for 10% of UK land area, 80% of which are degraded to some degree, emitting carbon at a similar magnitude to oil refineries or landfill sites. A lack of tools for rapid and reliable assessment of peatland condition has limited monitoring of vast areas of peatland and prevented targeting areas urgently needing action to halt further degradation. Measured using interferometric synthetic aperture radar (InSAR), peatland surface motion is highly indicative of peatland condition, largely driven by the eco-hydrological change in the peatland causing swelling and shrinking of the peat substrate. The computational intensity of recent methods using InSAR time series to capture the annual functional structure of peatland surface motion becomes increasingly challenging as the sample size increases. Instead, we utilize the behavior of the entire peatland surface motion time series using object oriented data analysis to assess peatland condition. In a Gibbs sampling scheme, our cluster analysis based on the functional behavior of the surface motion time series finds features representative of soft/wet peatlands, drier/shrubby peatlands and thin/modified peatlands align with the clusters. The posterior distribution of the assigned peatland types enables the scale of peatland degradation to be assessed, which will guide future cost-effective decisions for peatland restoration.
... The concave down shapes of the moisture response curves (Fig. 2) are consistent with other studies. However, the optimum WFPS values obtained here for the peat soils (Fig. 3d) tend to be higher than the ~ 60% moisture levels typically observed for more upland and mineral soils (e.g., 37,45,[63][64][65][66]. Optimum moisture levels for soil respiration emerge from complex interactions among physiochemical (e.g., pore water distribution, oxygen availability, and solute diffusion 67,68 ) and biological factors (e.g., microbial enzyme activity regulation 51 ). ...
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Peat accumulation in high latitude wetlands represents a natural long-term carbon sink, resulting from the cumulative excess of growing season net ecosystem production over non-growing season (NGS) net mineralization in soils. With high latitudes experiencing warming at a faster pace than the global average, especially during the NGS, a major concern is that enhanced mineralization of soil organic carbon will steadily increase CO 2 emissions from northern peatlands. In this study, we conducted laboratory incubations with soils from boreal and temperate peatlands across Canada. Peat soils were pretreated for different soil moisture levels, and CO 2 production rates were measured at 12 sequential temperatures, covering a range from − 10 to + 35 °C including one freeze–thaw event. On average, the CO 2 production rates in the boreal peat samples increased more sharply with temperature than in the temperate peat samples. For same temperature, optimum soil moisture levels for CO 2 production were higher in the peat samples from more flooded sites. However, standard reaction kinetics (e.g., Q 10 temperature coefficient and Arrhenius equation) failed to account for the apparent lack of temperature dependence of CO 2 production rates measured below 0 °C, and a sudden increase after a freezing event. Thus, we caution against using the simple kinetic expressions to represent the CO 2 emissions from northern peatlands, especially regarding the long NGS period with multiple soil freeze and thaw events.
... Rewetting of the cores (T4) significantly (p < 0.01) increased soil dissolved inorganic nitrogen (DIN; DIN = NH 4 + +NO 3 -) concentrations in relation to DIN concentrations at the beginning of the study (figure 1(a)), apart from a non-significant increase of NH 4 + concentrations for C-MH and C-GG. Particularly in soils with high carbon (>40% total carbon) and moisture contents (84%-147% WFPS prior wetting), rewetting increased DIN concentrations 1.4-10.5 fold, providing the substrates for microbial nitrification and denitrification, with consequential NO and N 2 O emissions [9,42,43]. A comprehensive field study in a Californian semiarid grassland showed a significant contribution of NO 3 − forming NO and N 2 O pulses immediately after rewetting, with later involvement of NH 4 + in post-wetting emissions [33]. ...
Article
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Soil emissions of NO and N2O from typical land uses across Lowland and Highland Scotland were simulated under climate change conditions, during a short-term laboratory study. All locations investigated were significant sources of N2O (range: 157-277 µg N2O-N m-2 h-1) and low-to-moderate sources of NO emissions (range: 0.4-30.5 µg NO-N m-2 h-1), with a general tendency to decrease with altitude and increase with fertiliser and atmospheric N inputs. Simulated climate warming and extreme events (drought, intensive rainfall) increased soil NO pulses and N2O emissions from both natural and managed ecosystems in the following order: natural Highlands < natural Lowlands < grazed grasslands < natural moorland receiving high NH3 deposition rates. Largest NO emission rates were observed from natural moorlands exposed to high NH3 deposition rates. Although soil NO emissions were much smaller (6-660 times) than those of N2O, their impact on air quality is likely to increase as combustion sources of NOx are declining as a result of successful mitigation. This study provides evidence of high N emission rates from natural ecosystems and calls for urgent action to improve existing national and intergovernmental inventories for NO and N2O, which at present do not fully account for emissions from natural soils receiving no direct anthropogenic N inputs.
... The losses are particularly large in wetlands, as 50-87 % of their global extent is currently degraded or has been lost since 1700 AD (Mitsch and Gosselink, 2007;Davidson, 2014). These losses not only result in the extinction of species and typical biodiversity, but also in the loss of vital ecosystem services (Stirling et al., 2020). For example, natural wetlands store a great amount of soil carbon, provide food, fibre and clean drinking water, mitigate flood and drought, prevent downstream pollution, cool their surroundings via evapotranspiration and provide protection against flooding and or storms (Maltby and Acreman, 2011;Mitsch et al., 2015). ...
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The restoration of degraded ecosystems and landscapes is challenging, because returning to the original state is often socio-economically unfeasible. A novel approach is to construct new ecosystems to improve the functioning of degraded landscapes. However, the development of novel ecosystems is largely driven by the pre-construction hydrogeophysical and ecological conditions of the soil. In Lake Markermeer, a deteriorating freshwater lake in the Netherlands, a large archipelago is currently being constructed to boost the ecological functioning of the lake. Hence, islands – with wetlands and with more elevated and dryer areas – have been created to sustain biodiversity and key biogeochemical functions such as nutrient cycling. The islands are constructed from lake-bottom sediments. To study how two potentially important drivers, water level and bioturbation, affect soil characteristics in a novel wetland ecosystem, we experimentally tested the effects of water level (-30, -10 and 5 cm), and bioturbation by earthworms (Lumbricus rubellus) and Tubifex spp. in a microcosm experiment. We demonstrate that a high water level prevents soil subsidence, soil crack formation and carbon dioxide (CO2) emissions, and affects nitrogen cycling. In dryer soils, the presence of earthworms strongly increases CO2 emissions next to reducing soil crack formation, while Tubifex spp. in wetter soils hardly affect soil characteristics. Our findings highlight the important roles of both water level and bioturbation for the functioning of novel soils, which likely affects vegetation development in novel ecosystems. This knowledge can be used to aid the construction and nature development of novel wetlands.
... For instance, droughts have been found to decrease phenolic microbial inhibiter compounds in wetlands resulting in increased organic matter decomposition and an increase in carbon loss in peats 29 . Additionally, droughts increase the temperature and degree of aeration of soils that are normally inundated, upregulating organic matter decomposition 30,31 . Finally, rewetting these exposed soils also trigger the physical and microbial processes that promote organic matter mineralization [Borken and Matzner, 2008]. ...
Preprint
Full-text available
One likely consequence of global climate change is an increased frequency and intensity of droughts at high latitudes. We use a 17-year record from 13 nested boreal streams to examine the direct and lagged effects of summer drought on the quantity and quality of dissolved organic carbon (DOC) inputs from catchment soils. Protracted periods of drought reduced DOC concentrations in all catchments but also led to large pulses of DOC inputs upon rewetting in autumn. Concurrent changes in DOC optical properties and chemical character suggest that seasonal drying and rewetting triggers soil processes that alter the forms of carbon supplied to streams. Contrary to common belief, the clearest drought effects were observed in larger watersheds, whereas responses were most muted in smaller catchments. Collectively, our results reveal an emerging shift in the seasonal distribution of DOC concentrations and character, with potentially far-reaching consequences for northern aquatic ecosystems.
... The compost as an organic matter plays an important role in managing the solubility of some metals. The more organic matter dissolved in soils, the less amount of soluble Al in plants because the dissolved organic matter (DOM) will buffer the free state of Al and Fe with DOM-metal bound (Stirling et al., 2020;Watmough & Orlovskaya, 2015). Zanin et al. (2019) further stated that organic matters are referred to as redox reactive, which can reduce metal ionic compounds, including Fe 3+ . ...
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This study examines the dynamics of essential macro-nutrients for rice cultivation in acid sulfate soils ameliorated with composted oyster mushroom baglog waste. A single factor randomized block design (RBD) was used, and the factors studied include the compost dose of oyster mushroom baglog waste, which consists of 5 treatment levels, namely 0 t ha-1 (control), 5 t ha-1, 10 t ha-1, 15 t ha-1, and 20 t ha-1. Furthermore, this study was carried out from May to September 2021 in the rice fields of the Faculty of Agriculture, Lambung Mangkurat University (ULM), Sungai Rangas Village, Banjar Regency, South Kalimantan. The rice plants were cultivated using an intensification technique, and the compost was applied based on the research treatment for two weeks on prepared land before planting. Also, Bartlett’s test was carried out before analysis of variance, which had a significant effect of P<0.05, and was further tested using Duncan’s Multiple Range Test (DMRT) at a 5% level. The results showed variations in the availability of macro-nutrients at five different growth stages: early planting, full vegetative, early panicle emergence, panicle filling, and harvesting phases. The highest levels of ammonium (NH4+) and nitrate (NH3-) were found in the full vegetative stage, while early planting had the lowest. Also, there was an increase in the available phosphorus (P) from the early planting to the full vegetative stage. The increase in exchangeable potassium (K) occurred at the transition of these stages. These increasing nutrients were due to the addition of the compost. The higher the NH4+, NO3-, available P, and exchangeable K in acid sulfate soils, the more nitrogen (N), P, and K uptake in rice plants. The provision of the compost supplied N, P, and K in available forms and reduced the amount of soluble alumunium (Al) and iron (Fe). Thereby the plant roots absorb the nutrients optimally. Additionally, the compost increased the essential macro-nutrient availability and plant uptake using the rice intensification technique from early planting to harvest.
... Although most attention has focused on the effect of drought on agricultural systems, drought also poses a significant threat to wetland environments such as peatlands (Stirling et al., 2020). Peatlands are important carbon-storing ecosystems in temperate and boreal regions and they are now seen as crucial to climate change mitigation strategies (Leifeld and Menichetti, 2018). ...
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Mosses of the genus Sphagnum are the main components of peatlands, a major carbon-storing ecosystem. Changes in precipitation patterns are predicted to affect water relations in this ecosystem, but the effect of desiccation on the physiological and molecular processes in Sphagnum are still largely unexplored. Here we show that different Sphagnum species have differential physiological and molecular responses to desiccation but, surprisingly, this is not directly correlated with their position in relation to the water table. In addition, the expression of drought responsive genes is increased upon water withdrawal in all species. This increase in gene expression is accompanied by an increase in ABA, supporting a role for ABA during desiccation responses in Sphagnum. Not only do ABA levels increase upon desiccation, but Sphagnum plants pre-treated with ABA display increased tolerance to desiccation, suggesting that ABA levels play a functional role in the response. In addition, many of the ABA signalling components are present in Sphagnum and we demonstrate, by complementation in Physcomitrium patens, that Sphagnum ABI3 is functionally conserved. The data presented here, therefore, support a conserved role for ABA in desiccation responses in Sphagnum.
... Wetland soils containing Fe sulfides are productive ecosystems but pose a major threat to the environment when they fall dry, as for example after drainage or during drought periods (Fanning et al., 2017;Stirling et al., 2020). They are wide-spread throughout the world in coastal and inland areas, e.g. in southern Australia (Fanning et al., 2017). ...
Article
Aeration of wetland soils containing iron (Fe) sulfides can cause strong acidification due to the generation of large amounts of sulfuric acid and formation of Fe oxyhydroxy sulfate phases such as jarosite. Remediation by re-establishment of anoxic conditions promotes jarosite transformation to Fe oxyhydroxides and/or Fe sulfides, but the driving conditions and mechanisms are largely unresolved. We investigated a sandy, jarosite-containing soil (initial pH = 3.0, Eh ~600 mV) in a laboratory incubation experiment under submerged conditions, either with or without wheat straw addition. Additionally, a model soil composed of synthesized jarosite mixed with quartz sand was used. Eh and pH values were monitored weekly. Solution concentrations of total dissolved organic carbon, Fe, S, and K as well as proportions of Fe²⁺ and SO4²⁻ were analysed at the end of the experiment. Sequential Fe extraction, X-ray diffraction, and Mössbauer spectroscopy were used to characterize the mineral composition of the soils. Only when straw was added to natural and artificial sulfuric soils, the pH increased up to 6.5, and Eh decreased to approx. 0 mV. The release of Fe (mainly Fe²⁺), K, and S (mainly SO4²⁻) into the soil solution indicated redox- and pH-induced dissolution of jarosite. Mineralogical analyses confirmed jarosite losses in both soils. While lepidocrocite formed in the natural sulfuric soil, goethite was formed in the artificial sulfuric soil. Both soils showed also increases in non-sulfidized, probably organically associated Fe²⁺/Fe³⁺, but no (re-)formation of Fe sulfides. Unlike Fe sulfides, the formed Fe oxyhydroxides are not prone to support re-acidification in the case of future aeration. Thus, inducing moderately reductive conditions by controlled supply of organic matter could be a promising way for remediation of soils and sediments acidified by oxidation of sulfuric materials.
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Atmospheric nitrous oxide (N 2 O) is a potent greenhouse gas thought to be mainly derived from microbial metabolism as part of the denitrification pathway. Here, we report that in unexplored peat soils of Central and South America, N 2 O production can be driven by abiotic reactions (> 98 %) highly competitive to their enzymatic counterparts. Extracted soil iron positively correlated with in-situ abiotic N 2 O production determined by isotopic tracers. Moreover, we found that microbial N 2 O reduction accompanied abiotic production, essentially closing a coupled abiotic-biotic N 2 O cycle. Anaerobic N 2 O consumption occurred ubiquitously (pH 6.4-3.7), with proportions of diverse clade II N 2 O-reducers increasing with consumption rates. Our findings show denitrification in tropical peat soils is not a purely biological process, but rather a 'mosaic' of abiotic and biotic reduction reactions. We predict hydrological and temperature fluctuations differentially affect abiotic and biotic drivers and further contribute to the high N 2 O flux variation in the region.
Article
This study investigated the spatiotemporal variation of arsenic (As) distribution, species and its behaviour in the aquatic environment changed by extended dry and heavy rainfall periods in the area adjacent an abandoned gold mine, South Korea. As appears to be transported from the mine wastes to Guryong stream through groundwater baseflow and leachate. The oxidation-reduction potential (ORP) conditions changed spatially and temporally following dry and heavy rainfall periods, and rainfalls caused decrease in the stream pH. As mainly existed as arsenates (AsO4³⁻) bound with H, Ca, and Fe in water. In groundwater, the lower the pH and the ORP, the higher the proportions of acid species of arsenate (HAsO4²⁻, H2AsO4⁻) and arsenite (H3AsO3), resulting in increased As mobility and toxicity, respectively. In stream, the primary influencing factor of As variation is the ORP. Under oxidizing conditions, As in stream could precipitate as amorphous FeAsO4·2H2O and FeOOH in the streambed. Then, the ORP decrease could remobilize As by redissolution and desorption of As bound to bed sediments. Thus, streambed sediments acted as a temporary sink-and-source for As, and extension of source areas accompanied with physical transport after heavy rainfalls.
Chapter
A variety of societal challenges, both climatic and non-climatic, can lead to abrupt, and in some cases, irreversible environmental change that adversely impacts human development. One approach to addressing these challenges is to increasingly rely on engineering solutions that are designed and managed to be simple to implement, easy to replicate, and provide predictable outcomes. However, these solutions require significant investments in materials and energy. An alternative approach is using nature-based solutions (NBS) that use ecosystems and the services they provide to address societal challenges in sustainable ways. This chapter will first discuss the various climatic and non-climatic societal challenges leading to environmental degradation and impacting human well-being and socio-economic development. The chapter will then provide an overview of the concept of NBS and the multiple co-benefits they provide.
Article
Droughts are increasing in frequency and severity in many regions of the world and there are uncertainties how recurrent drought will impact acid sulfate soils, which can undergo extreme and persistent acidification (pH < 4) following oxidation. Using column experiments, we induced a 9 week drought/drying and 9 week rewetting phase in two acid sulfate soil profiles with sulfuric (pH < 4) and hypersulfidic materials from the Lower River Murray region of Australia's largest river system. These soils had not yet recovered from the extreme period of the ‘Millennium Drought’ between 2007 and 2010. pH, redox potential, dissolved metals and greenhouse gases (CO2, CH4 and N2O) were measured at multiple depths in each column every 3 weeks during the drying and rewetting phases. The solid phase of the Sulfuric and Hypersulfidic clay soil profiles in both column experiments were analysed for complete acid-base accounting (actual and retained acidity, potential acidity in the form of pyrite, and acid neutralising capacity) and reactive metals at the beginning and end of the experiment. Residual pyrite, present in high concentrations in the Sulfuric clay soil column, oxidised during the drying period, further lowering the pH (<4) and mobilising dissolved metals (Al, Cd, Cu, Ni, Zn). More reactive Fe and Al phases were formed during the drying-rewetting cycle while reactive Mn decreased. The soil pH did not recover (i.e. increase) during the rewetting phase in both the Sulfuric and Hypersulfidic clay soils, likely due to barriers to microbial reduction reactions, although Fe-oxidising bacteria were likely still active as CO2 was released. Acid sulfate soils may not recover in inter-drought periods and more severe impacts can be expected following recurrent droughts.
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Early civilizations have inhabited areas with stable water resources that supported living needs and activities. The Mesopotamian marshes have experienced dramatic changes during the past five decades. The aim of this study is to observe, analyse and report the extent of changes in these marshes from 1972 to 2020. Data from various sources were acquired through Google Earth Engine (GEE) including climate variables, land cover, surface reflectance, and surface water occurrence collections. Additionally, streamflow data was also analysed. Methods were based on diagnostic analysis to monitor and evaluate the causes and results of the total environmental dynamism. Results show a clear wetlands dynamism over time, a decrease of incoming flow to the region due to the damming of upstream tributaries, and a significant loss in marshlands extent, even though no significant long-term change was observed in lumped rainfall from 1982, and even during periods where no meteorological drought had been recorded. Human interventions have disturbed the ecosystems, which is evident when studying water occurrence changes. These show that the diversion of rivers and the building of a new drainage system caused the migration and spatiotemporal changes of marshlands. Nonetheless, restoration plans (after 2003) and strong wet conditions (period 2018–2020) have helped to recover the ecosystems, these have not led the marshlands to regain their former extent. Further studies should pay more attention to the drainage network within the study area as well as the neighbouring regions and their impact on the streamflow that feeds the marshes.
Chapter
Terrestrial biomes in the U.S. can be managed for SOC sequestration. Sequestration for climate change adaptation and mitigation occurs when the soil C inputs are derived from atmospheric carbon dioxide (CO2) fixed by photosynthesis within a biome, and the synthesized SOC is protected and stabilized for long periods of time. Aside soil and land-use management practices, elevated CO2, nitrogen (N) additions, warming, irrigation and increases in biomass but also natural disturbances affect SOC stocks. The SOC sequestration in managed land of forest biomes in the U.S. can be managed by practices including: (i) harvesting, (ii) thinning, (iii) fertilization, (iv) liming, (v) drainage, (vi) irrigation, (vii) tree species selection and (viii) control of understory vegetation, and by managing natural disturbances. Management of stand-replacing disturbances (i.e., fire, insect outbreaks) is particularly promising to enhance SOC sequestration. However, forest management is focused on producing timber by silviculture and, until recently, not on soil management including SOC stocks resulting in limited understanding on how to enhance forest SOC sequestration. Fire strongly affects SOC sequestration in the boreal forest/taiga biome, but it is unclear how recent changes in fire size, severity and intensity together with changes in insect and pathogen outbreaks alter SOC stocks. Importantly, it is not possible to fully control and manage SOC sequestration in boreal U.S. forests because of its scale and remoteness. In contrast, forest management interventions in the temperate coniferous forest biome during harvesting, thinning, reforestation and prescribed burning can potentially enhance SOC sequestration. Reducing the extent of harvested area on a landscape level, N-fertilization, and introduction/favoring faster-growing trees species and those more tolerant of heat or drought are among the management options. The SOC sequestration in both temperate coniferous, and broadleaf and mixed U.S. forest biomes share the same key SOC vulnerabilities associated with harvest and fire. Specifically, recent changes in fire regimes in western U.S. forests are a major concern for the fate of SOC. In the tropical forest biome, hurricanes, typhoons and cyclones may increasingly affect SOC sequestration. Otherwise, SOC sequestration in tropical forests may be enhanced by: (i) fire management, (ii) prevention of grass invasions, (iii) selection of high-SOC species for plantations, (iv) mixed-species plantations, (v) reforestation of burned areas, (vi) grazer density control, (vii) reforestation, (viii) facilitation of N-fixer establishment, (ix) control of soil erosion, (x) selection of high-SOC species or genetic families on degraded soils and for plantations, and (xi) retaining logging residues. The SOC sequestration in the temperate grassland, savanna, and shrubland biome in the U.S. may be enhanced by: (i) improved grazing management, (ii) fertilization, (iii) irrigation, (iv) increasing species diversity, and (v) sowing legumes and improved grass species. In contrast, management activities to increase SOC sequestration in the tundra biome are limited. Terrestrial wetlands in the U.S. are not managed for SOC sequestration. However, restoration of drained peatlands to wetlands, wetland agriculture (‘paludiculture’) and reduction in peat mining may contribute to SOC sequestration. The potential for management of SOC sequestration in deserts and xeric shrublands is limited as plants are often near their physiological limits for temperature and water stress. Among the opportunities to enhance SOC sequestration are restoration of degraded lands and improved grazing management. Management practices to enhance cropland SOC sequestration in the U.S. include: (i) maintaining permanent cropland cover with vegetation (i.e., elimination of summer fallow, use of perennials and cover crops), (ii) protecting the soil from erosion (i.e., reduced tillage or no-till (NT), maintaining residue cover), and (iii) improved nutrient and water management. Irrigation, and applying organic fertilizers and biochar can also contribute to SOC sequestration in U.S. croplands. Human activities, i.e., land clearing, removal of vegetation, and disturbance of soils including adding impervious cover associated with construction activities affect SOC sequestration in settlements and urban areas. However, these soils are not managed for SOC sequestration, and any recommendations on SOC-enhancing soil and land use management practices are premature. This chapter will summarize potential alterations in SOC sequestration by soil and land-use management practices, and the effects of climate and global changes on sequestration processes. The chapter will also present approaches for carbon monitoring and accounting in terrestrial ecosystems in the U.S., and how SOC sequestration in terrestrial biomes is affected by natural disturbances and how sequestration can potentially be enhanced by management interventions.
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Wetlands store a significant proportion of terrestrial carbon, however, when degraded and dry, they can become net carbon emitters. Climatic stressors, such as rising temperature and reduced precipitation, further exacerbate carbon release risks. This study explores incentivizing adoption of constructed wetlands (CW) on agricultural farms for treating effluents and releasing into drying lakes. A payment for ecosystem services (PES) framework is developed to analyze land use allocation decisions of farmers towards adopting CWs on their private farms. Release of treated agricultural wastewater helps a drying lake remain wet under a warming climate preventing release of carbon stored in its soils. Results indicate that PES payments equaling 10,000 to 20,000 INR (150–300 US dollars) per mega litre (ML) can be effective in incentivizing adoption of CWs on farms in India, and their benefits to drying lakes can be significant. Specifically, life of lakes can be prolonged to more than 100 years under such PES based schemes besides resulting in substantial carbon storage in soils. Such PES schemes can be a cost-effective way to not only protect and conserve lakes for their biodiversity and livelihood benefits but also from a carbon mitigation perspective. Results further show that when a social planner allocates land between farming and CW, incorporating the carbon sequestration benefits of lakes and when facing a risk of abrupt and permanent drying of lakes, their adoption rate is higher compared to that of the farmers. When extrapolated, carbon benefits from such PES programs for the entire country could be nearly 15 trillion USD over the next 100 years.
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Purpose The main objective of this research was to examine the effects of inter-annual water regime variation on soil and plant characteristics, and soil–vegetation relationship at the lakeshore wetland of Poyang Lake, China. Materials and methods Six transects which paralleled the lakeshore were established at a typical Carex lakeshore wetland of Poyang Lake. We performed the field investigation and sampling in the autumn growing seasons in a flooding year (2017) and a drought year (2018). Paired t test was carried out to determine the effects of inter-annual water regime variation on various soil and plant parameters. Redundancy analysis was used to examine the relationships of plant species composition with flooding and soil variables in both years. Results and discussion Soil water content (SW), soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), the ratio of SOC to TP (C/P ratio), the ratio of TN to TP (N/P ratio), ammonia nitrogen concentration (NH4-N), and nitrate nitrogen concentration (NO3-N) were 21 ~ 71% higher in the flooding year than in the drought year. Plant Shannon–Wiener indices were significantly lower in the drought year than in the flooding year, whereas aboveground and belowground plant biomass showed opposite patterns. Flooding and soil variables could strongly determine plant communities in both years. Flooding duration was the most important variable among all flooding and soil variables. After eliminating the effects of flooding duration, pH and SW still significantly affected plant community composition in the flooding year, while only NH4-N significantly impacted on plant species distribution in the drought year. Conclusions The results provided direct evidences that the inter-annual water regime variation not only could dramatically modify soil and plant variables but also could change the soil–vegetation relationship in the wetlands. Our studies have great implications for the conservation and restoration of the wetlands, especially for the lake wetlands in the middle and lower reaches of Yangtze River.
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The aim of this paper was to analyse the spatial and temporal patterns and drivers of water quality in a large arid/semi-arid river system (River Murray, Australia) using a long term (1978 − 2015) dataset collected from 24 monitoring sites. The water quality is highly variable, but on average electrical conductivity (EC), pH, turbidity, dissolved and total nutrient, colour and chlorophyll a levels increase with distance downstream from the headwaters to the lower reaches. This is a function of the natural accumulation of dissolved and particulate components and intermittent, mostly diffuse source, pollutant inputs. The Darling tributary inflow increases turbidity, total phosphorus and pH in the main River Murray channel. Based on long-term trend analysis at four representative sites, EC, nutrients and colour showed declining trends on average at most sites except in the headwaters. Increased flow increases concentrations of most quality parameters, although at very high flows decreases in pH, EC, turbidity and oxidized nitrogen were apparent at many sites. The extreme “Millennium” drought (2002 − 2009) period resulted in lowered concentrations of many water quality parameters, indicating retention in the landscape. In the post-drought flooding (2010 − 2012) period a large amount of organic material was mobilised, resulting in much higher peak colour concentrations than when mid-range flooding was more frequent. It is critical that this monitoring program is continued as a Basin-wide water management plan is implemented.
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Background This paper reports on successes and failures experienced over nearly two decades while attempting to remediate degraded scalds typified by iron clogged layersinterspersed with patches of acid sulphate soil on the Eastern Dundas Tablelands in Victoria, Australia. In 2004 (Gardner et al. I Overview Plant Soil 267:51–59, 2004a, Plant Soil 267:85–95, b), it was suggested that redox processes similar to acid sulfate soil reactions were acting to exacerbate and amplify salinity effects in the landscape. Methods Soil permeability to 1 m depth was measured and compared to that at 3-5 m depths. Additional analysis of soils in the discharge zone were undertaken (mineralogy, SEM, XRF, XRD) to seek evidence of typical acid sulfate reactions which had not been found in 2004. Water quality (EC, pH) and hydraulic pressures were measured in 2005- 2006, after shallow trenchs were dug across the site, and revegetation attempted with a range of native species. Following observations of improved growth at leaking peizometers, those with above ground water levels were drilled to allow water to escape. Assessment of revegetation outcomes were conducted in 2006 and 2016. Results and discussion Soil permeability was lower in the upper 1 m layer of the soil than at 3-5m depth, Clear evidence of acid sulfate soil mineralogy was found, however small scale variation was the norm. Clogging of soil macropores was observed, which could be manually cleared. Fracturing the soil to increase discharge either with explosives or excavation of both shallow and deep trenchs failed because the increased discharge was insufficient to overcome evaporation and salt concentration, and redox processes continued which would eventually clog the soil again. Success was achieved with leaking piezometers to 4.5 m depth, which allowed the groundwater to reach the surface without reducing ferric oxides. Rushes planted at the discharge point maintained an oxidised environment thereby halting the redox processes. Evaporation effects were prevented because the increased discharge occurred at a point. This combination allowed a range of native species to flourish and after 10 years, to spread beyond what we estimate the leaking piezometers would support. Conclusion Evaporation causing concentration of salts is very important, but this is linked to the redox processes, which cause soil clogging and reduced permeability, and thus the evaporation and salt issue. Success was achieved when the reduced groundwater could access the surface in an oxidised environment, at a sufficient rate to prevent salt concentration by evaporation. The revegetation has expanded beyond what could be supported by the original discharge, suggesting that the plants are breaking the clogged soil layers and increasing discharge. The successful colonising species typically produce specialised structures such as dauciform and proteoid roots which are able to reduce and chelate iron oxides.
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Coastal forested wetlands provide important ecosystem services such as carbon sequestration, nutrient retention, and flood protection, but they are also important sources of greenhouse gas emissions. Human appropriation of surface water and extensive ditching and draining of coastal plain landscapes are interacting with rising sea levels to increase the frequency and magnitude of saltwater incursion into formerly freshwater coastal wetlands. Both hydrologic change and saltwater incursion are expected to alter carbon and nutrient cycling in coastal forested wetlands. We performed a full factorial experiment in which we exposed intact soil cores from a coastal forested wetland to experimental marine salt treatments and two hydrologic treatments. We measured the resulting treatment effects on the emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) over 112 days. Salinity effects were compared across four treatments to isolate the effects of increases in ionic strength from the impact of adding a terminal electron acceptor (SO4²⁻). We compared control treatments (DI addition), to artificial saltwater (ASW, target salinity of 5 parts per thousand) and to two treatments that added sulfate alone (SO4²⁻, at the concentration found in 5 ppt saltwater) and saltwater with the sulfate removed (ASW-SO4²⁻, with the 5 ppt target salinity maintained by adding additional NaCl). We found that all salt treatments suppressed CO2 production, in both drought and flooded treatments. Contrary to our expectations, CH4 fluxes from our flooded cores increased between 300 and 1200% relative to controls in the ASW and ASW-SO4²⁻ treatments respectively. In the drought treatments, we saw virtually no CH4 release from any core, while artificial seawater with sulfate increased N2O fluxes by 160% above DI control. In contrast, salt and sulfate decreased N2O fluxes by 72% in our flooded treatments. Our results indicate that salinization of forested wetlands of the coastal plain may have important climate feedbacks resulting from enhanced greenhouse gas emissions and that the magnitude and direction of these emissions are contingent upon wetland hydrology.
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The accumulation of arsenic (As) in rice grain is a public health concern since As is toxic to humans; in particular, inorganic As can cause many chronic diseases including cancer. Rice crops are prone to accumulating As, in part, due to the anaerobic soil conditions triggered by the traditional continuously flooded irrigation practice. The objective of this study was to determine how the severity and the timing (i.e. crop stage) of a single soil drying period impact total As concentration and As speciation within the rice (both white and brown) grain, compared to a continuously flooded (CF) control. Drying the soil until the perched water table reached 15 cm below the soil surface (same severity as in the "Safe Alternate Wetting and Drying"), which in this study corresponded to a soil (0-15 cm) water potential of ~0, did not decrease grain As concentrations, regardless of timing. Drying the soil to Medium Severity [MS: soil (0-15 cm) water potential of -71 kPa] or High Severity [HS: soil (0-15 cm) water potential of -154 kPa] decreased total As by 41-61%. However, inorganic As did not always decrease because the severity and the timing of soil drying affected As speciation within the grain. Overall, the soil had to be dried to HS and/or late in the growing season (i.e., at booting or heading instead of at panicle initiation) to decrease inorganic As concentration in the rice grain. This study indicates that the imposition of a single soil drying period within the growing season can mitigate As accumulation in rice grain, but it depends on the severity and timing of the drying period. Further, irrigation management affects As speciation within the rice grain and this must be considered if regulations on inorganic As are based on a percentage of total As measured.
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Freshwater availability is changing worldwide. Here we quantify 34 trends in terrestrial water storage observed by the Gravity Recovery and Climate Experiment (GRACE) satellites during 2002-2016 and categorize their drivers as natural interannual variability, unsustainable groundwater consumption, climate change or combinations thereof. Several of these trends had been lacking thorough investigation and attribution, including massive changes in northwestern China and the Okavango Delta. Others are consistent with climate model predictions. This observation-based assessment of how the world's water landscape is responding to human impacts and climate variations provides a blueprint for evaluating and predicting emerging threats to water and food security.
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Peatlands and peaty riparian zones are major sources of dissolved organic matter (DOM), but are poorly understood in terms of export dynamics and controls thereof. Thereby quality of DOM affects function and behavior of DOM in aquatic ecosystems, but DOM quality can also help to track DOM sources and their export dynamics under specific hydrologic preconditions. The objective of this study was to elucidate controls on temporal variability in DOM concentration and quality in stream water draining a bog and a forested peaty riparian zone, particularly considering drought and storm flow events. DOM quality was monitored using spectrofluorometric indices for aromaticity (SUVA254), apparent molecular size (SR) and precursor organic material (FI), as well as PARAFAC modeling of excitation emission matrices (EEMs). Indices for DOM quality exhibited major changes due to different hydrologic conditions, but patterns were also dependent on season. Stream water at the forested site with mineral, peaty soils generally exhibited higher variability in DOM concentrations and quality compared to the outflow of an ombrotrophic bog, where DOM was less susceptible to changes in hydrologic conditions. During snowmelt and spring events, near-surface protein-like DOM pools were exported. A microbial DOM fraction originating from groundwater and deep peat layers was increasing during drought, while a strongly microbially altered DOM fraction was also exported by discharge events with dry preconditions at the forested site. This might be due to accelerated microbial activity in the peaty riparian zone of the forested site under these preconditions. Our study demonstrated that DOM export dynamics are not only a passive mixing of different hydrological sources, but monitoring studies have to consider that DOM quality depends on hydrologic preconditions and season. Moreover, the forested peaty riparian zone generated the most variability in headwater DOM quantity and quality, as could be tracked by the used spectrofluorometric indices.
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Rising global temperatures may increase the rates of soil organic matter decomposition by heterotrophic microorganisms, potentially accelerating climate change further by releasing additional carbon dioxide (CO2) to the atmosphere. However, the possibility that microbial community responses to prolonged warming may modify the temperature sensitivity of soil respiration creates large uncertainty in the strength of this positive feedback. Both compensatory responses (decreasing temperature sensitivity of soil respiration in the long-term) and enhancing responses (increasing temperature sensitivity) have been reported, but the mechanisms underlying these responses are poorly understood. In this study, microbial biomass, community structure and the activities of dehydrogenase and β-glucosidase enzymes were determined for 18 soils that had previously demonstrated either no response or varying magnitude of enhancing or compensatory responses of temperature sensitivity of heterotrophic microbial respiration to prolonged cooling. The soil cooling approach, in contrast to warming experiments, discriminates between microbial community responses and the consequences of substrate depletion, by minimising changes in substrate availability. The initial microbial community composition, determined by molecular analysis of soils showing contrasting respiration responses to cooling, provided evidence that the magnitude of enhancing responses was partly related to microbial community composition. There was also evidence that higher relative abundance of saprophytic Basidiomycota may explain the compensatory response observed in one soil, but neither microbial biomass nor enzymatic capacity were significantly affected by cooling. Our findings emphasise the key importance of soil microbial community responses for feedbacks to global change, but also highlight important areas where our understanding remains limited.
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Climate change induced drying and flooding may alter the redox conditions of organic matter decomposition in peat soils. The seasonal and intermittent changes in pore water solutes (NO<sub>3</sub><sup>−</sup>, Fe<sup>2+</sup>, SO<sub>4</sub><sup>2−</sup>, H<sub>2</sub>S, acetate) and dissolved soil gases (CO<sub>2</sub>, O<sub>2</sub>, CH<sub>4</sub>, H<sub>2</sub>) under natural water table fluctuations were compared to the response under a reinforced drying and flooding in fen peats. Oxygen penetration during dryings led to CO<sub>2</sub> and CH<sub>4</sub> degassing and to a regeneration of dissolved electron acceptors (NO<sub>3</sub><sup>−</sup>, Fe<sup>3+</sup> and SO<sub>4</sub><sup>2−</sup>). Drying intensity controlled the extent of the electron acceptor regeneration. Iron was rapidly reduced and sulfate pools ~ 1 mmol L<sup>−1</sup> depleted upon rewetting and CH<sub>4</sub> did not substantially accumulate until sulfate levels declined to ~ 100 μmoll<sup>−1</sup>. The post-rewetting recovery of soil methane concentrations to levels ~ 80 μmoll<sup>−1</sup> needed 40–50 days after natural drought. This recovery was prolonged after experimentally reinforced drought. A greater regeneration of electron acceptors during drying was not related to prolonged methanogenesis suppression after rewetting. Peat compaction, solid phase content of reactive iron and total reduced inorganic sulfur and organic matter content controlled oxygen penetration, the regeneration of electron acceptors and the recovery of CH<sub>4</sub> production, respectively. Methane production was maintained despite moderate water table decline of 20 cm in denser peats. Flooding led to accumulation of acetate and H<sub>2</sub>, promoted CH<sub>4</sub> production and strengthened the co-occurrence of iron and sulfate reduction and methanogenesis. Mass balances during drying and flooding indicated that an important fraction of the electron flow must have been used for the generation and consumption of electron acceptors in the solid phase or other mechanisms. In contrast to flooding, dry-wet cycles negatively affect methane production on a seasonal scale but this impact might strongly depend on drying intensity and on the peat matrix, whose structure and physical properties influence moisture content.
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The hydrologic transport of dissolved organic carbon (DOC) represents both a primary energetic loss from and a critical energetic link between ecosystems. Coastal freshwater wetlands serve as a primary source of DOC to estuaries; historically the magnitude and timing of DOC transfers has been driven by water movement. Extensive agricultural development throughout the coastal plain of the southeastern US has hydrologically connected much of the landscape via canals to facilitate drainage. The resulting large-scale loss of topographic relief and reduced mean elevation is interacting with increasingly frequent and severe droughts to facilitate the landward movement of seawater through the highly connected artificial drainage networks. The resulting changes in hydrologic regime and salinity are each expected to reduce DOC export from coastal freshwater wetlands, yet their individual and combined impacts are not well understood. Here we show that repeated saltwater incursion during late summer droughts substantially decreased DOC concentrations in surface water (from ~40 to ~18 mg/L) from a mature and a restored forested wetland in the coastal plain of North Carolina, USA. These declines in DOC concentration reduced annual export of DOC to the estuary by 70 % and dampened storm fluxes by 76 %. We used a long-term experiment with intact soil columns to measure the independent and combined effects of drought, salinity, and sulfate loading as potential drivers of the large changes in DOC concentration. We found that soil drying and salinization each reduced DOC similarly (20 % reduction by drought alone, 29 % by salinization) and their combined effect was additive (49 % reduction in salinization + drought treatments). Our results demonstrate that, well in advance of significant sea-level rise, drought and relatively low levels of saltwater incursion (<6 ppt) are already significantly altering the timing and magnitude of dissolved organic carbon flux between coastal forested wetlands and downstream estuaries.
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Saturated acid sulfate soils with hypersulfidic material are productive wetland soils, but when they dry, they generate large amounts of sulfuric acid due to oxidation of pyrite to form sulfuric material (pH <4) and consequently sulfuric soils. After re-saturation of sulfuric soils and thus the re-establishment of reduced conditions, activity of sulfate reducing bacteria (SRB) can lead to a renewed formation of Fe sulfides and pH increase. Many SRB are heterotrophic and require sufficient available organic matter; however, little is known about OC consumption and changes of the composition of organic substrates during the amelioration process. To investigate remediation of a sandy, OC-poor sulfuric soil (initial pH = 2.5), short-term anoxic incubation experiments over a period of approx. 10 weeks were conducted after re-submerging under controlled laboratory conditions. We tested different organic matter quantities between 10% up to 200% of the native soil OC content. Besides wheat straw, we used lactate additions to test if this selectively promotes the activity of SRB, and thus, accelerates sulfate reduction and pH neutralization. The results showed that OC additions of ≥50% of native soil OC content and pre-adjustment of pH to values ≥5.0 were necessary to subsequently enable microbial reduction reactions to occur, which increased the pH to values ≥5.5. OC additions of ≥100% instead of 50% of native soil OC as wheat straw led to quicker changes of redox and pH values, to slightly higher microbial activity as indicated by CO2 release, and to higher proportions of newly-formed mineral-associated OC. The addition of OC as lactate solution to promote specifically SRB was only successful in combination with wheat straw addition. Here, the presence of lactate led to the quickest changes of pH and redox values and resulted in pH ≥7 and redox values ≤ −300 mV due to an active microbial population. Our results indicate that a diverse microbial community is more important for successful remediation than a selective promotion of SRB.
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In this study, we have re-estimated the 2011 global monetary values of natural wetland ecosystem services using new information on the areas of different coastal and inland wetland classes, and included estimates for forested wetlands. The 2011 global monetary value of natural wetland ecosystem services is now estimated at Int$47.4 trillion per year, 43.5% of the value of all natural biomes. Despite forming only ,15% of global natural wetland area, coastal wetlands are estimated to deliver 43.1% (Int$20.4 trillion per year) of the total global ecosystem services monetary value of all natural wetland classes. There is a need to further refine these value estimates by factoring in other determinants of wetland ecosystem service monetary value, by disaggregating unit monetary values to each wetland class and by updating unit monetary values with more recent sources, especially for ecosystem services with no, or few, value estimates.
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In eastern Australia the development of hypoxic blackwater/floodwater and its detrimental consequences are more common in summer than winter. This study examined the effect of temperature on the development of hypoxic conditions which was determined as biochemical oxygen demand (BOD) in floodwater when pasture grass (a source containing labile organic carbon) was inundated. Labile dissolved organic carbon (DOC) in blackwater is one of the main factors that contribute to the development of hypoxic conditions. Temperature plays a key role on the microbial mineralization of labile DOC and hence the development of hypoxic conditions. Prolonged flooding at different seasons (having different temperatures) was simulated in the laboratory by incubating fresh pasture grass cuttings with river water and soil at three different temperatures (20 °C, 27.5 °C and 35 °C) for 20 days. Although this study shows that elevated ambient temperatures can result in more rapid development of hypoxic conditions during the first week of flood peak, it is evident that blackwater formed at relatively moderate ambient temperatures (e.g. 20 °C) has a similar potential to deoxygenate the receiving water bodies, especially after one week's duration of flood peak.
Book
The Australian Soil Classification provides a framework for organising knowledge about Australian soils by allocating soils to classes via a key. Since its publication in 1996, this book has been widely adopted and formally endorsed as the official national system. It has provided a means of communication among scientists and land managers and has proven to be of particular value in land resource survey and research programs, environmental studies and education. Classification is a basic requirement of all science and needs to be periodically revised as knowledge increases. This Second Edition of The Australian Soil Classification includes updates from a working group of the National Committee on Soil and Terrain (NCST), especially in regards to new knowledge about acid sulfate soils (sulfidic materials). Modifications include expanding the classification to incorporate different kinds of sulfidic materials, the introduction of subaqueous soils as well as new Vertosol subgroups, new Hydrosol family criteria and the consistent use of the term reticulate. All soil orders except for Ferrosols and Sodosols are affected by the changes.
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Since the mid-20th century, Mediterranean lagoons have been affected by eutrophication, leading to significant changes in primary producers. In the early 2000s, management actions have been implemented to reduce nutrient inputs with the aim to achieve a good ecological status as requested by the EU water framework directive. As a result of these actions, a sharp decline in nutrient loads has been recorded in several lagoons leading to an oligotrophication of the water column. The analyses of a long-term data set (1998−2015) of 21 polyhaline and euhaline lagoons with contrasting trophic status allowed us to infer a general scheme for the changes in macrophyte assemblages during the oligotrophication process. Placing hypertrophic and oligotrophic conditions end to end, we inferred that the general pattern for the re-oligotrophication trajectory in Mediterranean coastal lagoons is described by the following sequence, with regime shifts between each state: (1) bare non-vegetated sediments, phytoplankton-dominated state; (2) opportunistic macroalgae; (3) seagrass and perennial macroalgae dominated state. However, we did not observe the latter regime shift for the most eutrophicated lagoons, which, so far, remained stuck in the opportunistic macroalgae state. So far, the shift from dominance of opportunistic macroalgae to a system dominated by seagrasses was only observed in a single lagoon where seagrasses had never completely disappeared, which possibly relates to resilience. More generally, the conditions favoring regime shifts from opportunistic macroalgae to seagrasses are still poorly understood. In conclusion, we describe a generic pattern for re-oligotrophication of Mediterranean coastal lagoons, although a full recovery from highly eutrophied to oligotrophic conditions may require more than a decade and may include conditions that remain so far poorly recognized.
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Soil microbial communities mediate soil feedbacks to climate; a thorough understanding of their response to increasing temperatures is therefore central to predict climate-induced changes in carbon (C) fluxes. However, it is unclear how microbial communities will change in structure and function in response to temperature change and to the availability of organic C which varies in complexity. Here we present results from a laboratory incubation study in which soil microbial communities were exposed to different temperatures and organic C complexity. Soil samples were collected from two land-use types differing in climatic and edaphic conditions and located in two regions in southwest Germany. Soils amended with cellobiose (CB), xylan, or coniferyl alcohol (CA, lignin precursor) were incubated at 5, 15 or 25 °C. We found that temperature predominantly controlled microbial respiration rates. Increasing temperature stimulated cumulative respiration rates but decreased total microbial biomass (total phospholipid fatty acids, PLFAs) in all substrate amendments. Temperature increase affected fungal biomass more adversely than bacterial biomass and the temperature response of fungal biomass (fungal PLFAs, ergosterol and ITS fragment) depended upon substrate quality. With the addition of CB, temperature response of fungal biomass did not differ from un-amended control soils, whereas addition of xylan and CA shifted the fungal temperature optima from 5 °C to 15 °C. These results provide first evidence that fungi which decompose complex C substrates (CA and xylan) may have different life strategies and temperature optima than fungal communities which decompose labile C substrate (CB). Gram-positive and gram-negative bacteria differed strongly in their capacity to decompose CB under different temperature regimes: gram-positive bacteria had highest PLFA abundance at 5 °C, while gram-negative bacteria were most abundant at 25 °C. Bacterial community composition, as measured by 16S rRNA gene abundance, and PLFAs showed opposite temperature and substrate decomposition trends. Using multivariate statistics, we found a general association of microbial life strategies and key members of the microbial community: oligotrophic Alphaproteobacteria and Acidobacteria were associated with complex substrates and copiotrophic Actinobacteria with labile substrates. Our study provides evidence that the response of C cycling to warming will be mediated by shifts in the structure and function of soil microbial communities.
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The accessibility of iron (Fe) species for microbial processes is dependent on solubility and redox state, which are influenced by complexation with dissolved organic matter (DOM) and water-extractable organic matter (WEOM). We evaluated the complexation of these pools of organic matter to soluble Fe(II) and Fe(III) in the slightly acidic Schlöppnerbrunnen fen and subsequent effects on Fe(II) oxidation and Fe(III) reduction. We found the majority of soluble Fe(II) and Fe(III) is complexed to DOM. High-resolution mass spectrometry identified potential complexing partners in peat-derived water extracts (PWE), including compound classes known to function as ligands or electron shuttles, like tannins and sulfur-containing compounds. Furthermore, we observed clear differences in the stability of Fe(II)- and Fe(III)-DOM, with more labile complexes dominating the upper, oxic layers (0–10 cm) and more stable complexes in lower, anoxic layers (15–30 cm). Metal isotope-coded profiling identified a single potential chemical formula (C42H57O13N9Fe2) associated with a stable Fe-DOM complex. Fe(III) reduction and Fe(II) oxidation incubations with Geobacter sulfurreducens PCA and Shewanella oneidensis MR-1 or Sideroxydans CL-21, respectively, were used to determine the influence of Fe-DOM complexes on Fe cycling rates. The addition of PWE led to a 2.3-fold increase in Fe(III) reduction rates and 0.5-fold increase in Fe(II) oxidation rates, indicating Fe-DOM complexes greatly influence microbial Fe cycling by potentially serving as electron shuttles. Molecular analyses revealed Fe(III)-reducing and Fe(II)-oxidizing bacteria co-exist across all depths, in approximately equal proportions (representing 0.1–1.0% of the total microbial community), despite observed changes in redox potential. The activity of Fe(III)-reducing bacteria might explain the presence of the detected Fe(II) stabilized via complexation with DOM even under oxic conditions in upper peat layers. Therefore, these Fe(II)-DOM complexes can be recycled by microaerophilic Fe(II)-oxidizers. Taken together, these results suggest Fe-DOM complexation in the fen accelerates microbial-mediated redox processes across the entire redox continuum.
Article
Through Earth’s history, drought has been a common crisis in terrestrial ecosystems; in human societies, it has caused famines and become one of the Four Horsemen of the apocalypse. As the global hydrological cycle intensifies with global warming, deeper droughts and rewetting will alter, and possibly transform, ecosystems. Soil communities, however, seem more tolerant than plants or animals are to water stress—the main effects, in fact, on soil processes appear to be limited diffusion and the limited supply of resources to soil organisms. Thus, the rains that end a drought not only release soil microbes from stress but also create a resource pulse that fuels soil microbial activity. It remains unclear whether the effects of drought on soil processes result from drying or rewetting. It is also unclear whether the flush of activity on rewetting is driven by microbial growth or by the physical/ chemical processes that mobilize organic matter. In this review, I discuss how soil water, and the lack of it, regulates microbial life and biogeochemical processes. I first focus on organismal-level responses and then consider how these influence whole-soil organic matter dynamics. A final focus is on how to incorporate these effects into Earth System models that can effectively capture dry–wet cycling. Expected final online publication date for the Annual Review of Ecology, Evolution, and Systematics Volume 49 is November 2, 2018. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Article
When acid sulfate soils with hypersulfidic material dry, oxidation of pyrite can cause strong acidification due to formation of sulfuric (pH < 4) material. Re-saturation of acid sulfate soils containing sulfuric material can lead to reformation of pyrite and pH increase through activity of sulfate reducing bacteria, which require available organic carbon (OC). In the Lower River Murray region in Australia, a clayey acid sulfate soil had acidified during the severe "Millennium" drought between 2007 and early 2010. We investigated why it has not recovered for over a decade after being reflooded. We hypothesized that the low quality and availability of OC limits the activity of sulfate reducing bacteria. A long-term anoxic incubation experiment was conducted to test if OC additions can help to overcome OC limitation. Small-scale incubation vessels were used, allowing investigating general biochemical phenomena under controlled laboratory conditions. Pre-incubated acid sulfate soil with sulfuric material (approx. pH 3.5) was submerged and pre-adjusted to pH 5.0. We used different rates of wheat straw and cattle manure application to test different organic matter quantities and qualities. Both substrates were added in two portions, at the beginning of the experiment and after 190 days. With every addition, we added two different amounts of organic matter (2 mg g −1 soil and 9 mg g −1 soil), equivalent to approx. 10% and 50% of the native soil OC content. A control treatment without OC addition was also included. CO 2 production as well as redox and pH values were monitored weekly over a year. At the start and the end of incubation, we determined OC concentrations and the proportion of available, non mineral-associated OC. OC composition was analyzed by solid-state 13 C NMR spectroscopy to assess its chemical degradation. The pH values increased rapidly in treatments with high OC supply, reaching pH ≥ 6.0 within 3 weeks after the second OC addition. Treatments with low OC additions showed slower pH increases, reaching values between pH 5.5 and 6.0 after one year. The control treatment had pH values < 5.0 at the end of the experiment. After one year of anoxic incubation, the control treatment lost 10% of the native OC. Treatments with OC additions lost between 13 and 19% of total OC (native + added OC), with higher percentage loss in treatments with high OC additions. Highest losses were observed for the non mineral-associated OC fraction (up to 69% in treatments with high OC additions), with up to 20% being converted to the mineral-associated OC. OC composition changed little compared to the start of the experiment, showing slightly reduced proportions of carbohydrates (≤10% loss) and slightly higher proportions of lipids and lignin. Best remediation success was achieved by adding 50% of the native soil OC as wheat straw, resulting in fast pH neutralization, strongly reducing conditions, and decreased sulfur and iron concentrations in the soil solution. However, the amount of bioavailable OC was reduced to one third after the incubation period. Repeated OC addition is therefore recommended to keep the total amount of bioavailable, undecomposed plant residues high and to ensure long-term remediation success.
Article
We compiled available data and information on the global and regional areas (Ramsar regions), and changes in area, of 22 classes of marine or coastal and inland wetlands. From those classes for which there is information, inland natural surface wetlands (forming ,77% of total surface wetland extent) are dominated by non-forested peatlands, marshes and swamps on alluvial soils, with peatlands forming ,33% of natural inland wetlands. The smaller area of marine or coastal wetlands (,10% of total wetland extent) is dominated by unvegetated tidal flats and saltmarshes. Largest areas of human-made wetlands for which there is information are rice paddy and water storage bodies, with a much smaller area of tropical oil palm and pulpwood plantations. These human-made wetlands are all increasing in area. The reported decline in global natural wetland area is occurring across almost all classes of inland and marine or coastal natural wetlands. Total global wetland area estimated from these wetland classes is between 15.2 Â 10 6 and 16.2 Â 10 6 km 2 , similar to recent global wetland area estimates derived from remote sensing. Given the considerable data gaps for area of wetland classes, even the most recent other estimates of global wetland extent are likely to be underestimates.
Article
Herein we review estimates of global and regional wetland area from ‘bottom-up’ approaches of site or national wetland inventories and ‘top-down’ approaches from global mapping and remote sensing. The trend for increasing wetland extent reported in the literature over time is a consequence of improved mapping technologies and methods rather than a real increase in wetland area, because a continuing trend for natural wetland loss and conversion is documented over the same time period. The most recent high-resolution estimate of global wetland area is in excess of 12.1�106 km2, of which 54% is permanently inundated and 46% is temporarily inundated. Globally, 92.8% of continental wetland area is inland and only 7.2% is coastal. Regionally, the largest wetland areas are in Asia (31.8%), North America (27.1%) and Latin America and the Caribbean (Neotropics; 15.8%), with smaller areas in Europe (12.5%), Africa (9.9%) and Oceania (2.9%). It is likely that estimates of global wetland area published to date persist in underestimating the true wetland area. The ‘grand challenge’ of a global inventory integrating all types of permanent and temporary wetlands at high spatial resolution has yet to be fully achieved.
Article
Soils can be sources or sinks of carbon depending on the balance between carbon inputs from plants and losses from the decomposition of soil organic matter (SOM). A good understanding of the temperature sensitivity of SOM decomposition is critical for forecasting whether soils in a warming world will lose or gain carbon, and therefore accelerate or mitigate the rate of increasing atmospheric carbon dioxide (CO2) concentration. We provide new evidence to show that the response of SOM decomposition to temperature may be constrained by substrate availability to microbial decomposers. We used laboratory incubations of a grassland soil to compare the temperature sensitivity of SOM decomposition with unmodified substrate availability with that of the same soil in which substrate availability was reduced by adding allophane, a clay-size mineral with a high capacity for binding SOM. In the soil with no added allophane, the decomposition rate increased about 7-fold over the temperature range from 1 to 40 °C. With added allophane, decomposition rate increased only about 3-fold over the same temperature range. We then used a non-disruptive, natural abundance isotopic technique at our field site to partition total soil respiration into CO2 efflux from newly released, ¹³C-depleted SOM (root respiration and rhizosphere decomposition) from CO2 efflux from older ¹³C-enriched SOM from the decomposition of more stable SOM. We found no increase in the decomposition rate of the ¹³C-enriched pool of SOM between 11 and 28 °C. That finding contrasts with most previous studies that have generally reported strong increases in SOM decomposition with temperature. We hypothesised that the large temperature sensitivity observed in laboratory incubations was due to substrate becoming readily available as a result of the disturbance involved in collecting soil samples. In undisturbed field conditions, the limiting step for the decomposition of the more stable SOM pool may be the rate at which decomposable substrate becomes available for decomposition. Our findings will have important implications for the feedbacks between soil carbon storage and the rate of increase in atmospheric CO2 concentration mediated by global warming.
Book
The Global Change Ecology and Wetlands book series will highlight the latest research from the world leaders in the field of climate change in wetlands. Global Change and the Function and Distribution of Wetlands highlights information of importance to wetland ecologists. The chapters include syntheses of international studies on the effects of drought on function and regeneration in wetlands, sea level rise and the distribution of mangrove swamps, former distributions of swamp species and future lessons from paleoecology, and shifts in atmospheric emissions across geographical regions in wetlands. Overall, the book will contribute to a better understanding of the potential effects of climate change on world wetland distribution and function.
Article
Soil cadmium (Cd) contamination and drought stress are among the main issues hindering global food security. Biochar has been used to reduce metal uptake by plants and water stress mitigation, but long-term residual effects of biochar under Cd stress at different moisture levels needs to be investigated. A following rice (Oryza sativa L.) was grown after wheat on Cd-contaminated soil amended with different levels of biochar (0, 3.0, and 5.0%, w/w). Thirty five days old plants were irrigated with three moisture levels including zero drought as a control (1-2 cm water layer on soil), mild drought (MD, 50% of soil water holding capacity, WHC), and severe drought (SD, 35% of soil WHC) for an accompanying 35 days. Plant height, biomass and photosynthesis were reduced whereas oxidative stress increased under MD and SD than control in un-amended soil while opposite trends were observed in plants grown in biochar amended soil. At the same biochar addition, Cd concentrations in seedlings were lower in continuous flooding than MD and SD treatments. The biochar supply reduced the bioavailable Cd in the soil whereas increased the soil EC and pH than the control treatment. In conclusion, continuous flooding plus residual biochar can be strategized in mitigating Cd-contamination in paddy soils and decreased Cd concentrations in rice which may reduce the potential risks to humans.
Article
This paper describes the occurrences, mineralogical assemblages and environmental relevance of iron-rich precipitates derived from acidic (pH<4) waters containing dominantly schwertmannite from a diverse range of six physical settings across the Lower Murray Reclaimed Irrigation Area (LMRIA) in Australia, comprising: (1) suspended flocculated precipitates in ponded drain water, (2) moist coatings or pastes in drying ponds and drains, (3) hard cemented crusts and aggregates amongst Phragmites roots and stems, (4) dry coatings on concrete and wooden structures, (5) dry coatings on surface soils and vegetation and (6) suspended flocculated precipitates in the mixing zone of drain discharge into the River Murray. Schwertmannite formed in these acid drain environments following exposure and oxidation of deep (~0.5→3.5m) clayey hypersulfidic material (pH>4) that dried, cracked and acidified to form deep sulfuric materials (pH<4) due to river and groundwater levels falling by nearly 2m during the latter part of the Millennium Drought (2007 to 2010). Reflooding events occurred between 2011 and 2015. All samples displayed X-ray diffraction (XRD) patterns typical for schwertmannite. In some samples, additional weak reflections from small amounts of jarosite, natrojarosite, gypsum, hexahydrite, konyaite and halite indicated deposition under variable pH conditions and sulfate concentrations due to different flow or evaporation stages. SEM images indicated that morphological and compositional features of schwertmannite were dominated by: (1) framboid-like spheroidal clusters with Fe/S ratio>5 that were preserved after crystallization and likely formed by dissolution of pyrite and microbial oxidation of Fe²⁺ by acidophilic bacteria and (2) fibrous spheres (0.3-3μm) with filamentous morphology and a high degree of porosity. Speciation calculations (PHREEQC) using the dissolved metal and major ion concentrations in drain waters supported the XRD results as the saturation index (SI) exceeded zero for schwertmannite in many drains. The precipitates contained high concentrations of metals (Al>Cu>As>Zn>Pb>Co) and nutrients (e.g. P) due to co-precipitation/scavenging of these elements during the formation of schwertmannite. There was also spatial variability in concentrations of metal(loids) in precipitates between drains. A conceptual model explains and summarizes the morphological properties, mineralogy, geochemistry and environmental processes influencing the formation and relative stabilities of schwertmannite-rich precipitates from six diverse physical settings. The environmental relevance, which has significant implications for rehabilitation options is shown in three perspectives: (1) the conditions for schwertmannite formation have persisted in irrigation drains for over 7years, (2) the ability for schwertmannite-rich precipitates to reveal acid sulfate conditions and therefore act as a mineralogical indicator in irrigation systems and (3) the pollution potential of metals and metalloids scavenged by schwertmannite-rich precipitates.
Article
The severe Millennium Drought (2007-2010) left an area of over 5000. ha in the Lower Murray River (South Australia) dried, cracked and acidified as river and groundwater levels fell nearly 2. m. In this study, we examined irrigated agricultural areas and an adjacent natural wetland for comparison, which were both affected by the drought. Approximately 3. m deep soil cores were collected along transects in three sections of the Lower Murray Reclaimed Irrigation Area (LMRIA) on multiple occasions between 2011 and 2015 and an adjacent natural wetland in 2007. Soil properties measured included pH, reduced inorganic sulfur (RIS, pyrite), titratable actual acidity (TAA), retained acidity, acid neutralising capacity (ANC), X-ray diffraction analyses and scanning electron microscopy. A series of explanatory soil-regolith hydro-toposequence models were developed during the pre-drought period, drying period, and subsequent wetting/reflooding post-drought period. These models indicate that prior to draining of the natural wetlands for irrigated agriculture the region cycled between wetting and flushing, and partial drying conditions in response to seasonal and climatic cycles causing the build-up of hypersulfidic material to be kept in check by oxidation of pyrite during dry periods/droughts and removal during scouring floods. As the region became managed for navigation and irrigation by installing barrages and locks, pyrite began to build-up. The extreme lowering of the water table during the Millennium Drought resulted in deep oxidation of sulfides in anaerobic hypersulfidic material to depths >. 3.5. m in the previously saturated irrigated pastures and within 50. cm of the soil surface in the natural wetlands. Oxidation and acidification between 0.5 and 3.5. m of Hypersulfidic clayey soils was enhanced by the formation of large cracks up to 3.5. m deep. Rewetting and flooding after the drought caused mobilization of sulfuric acid, soluble sulfates, ferrous iron, nutrients and metals with transport into the River Murray. Our findings highlight that irrigated areas formed deeper sulfuric materials (>. 3.5. m) than in adjacent natural wetlands (<. 1. m) due to the difficulties in the management of water tables in irrigation areas because of the installation of high levee banks and deep drains. Maintaining water tables on agricultural soils via irrigation and subsequent drainage will promote the rapid formation of deep (>. 3.5. m) acid sulfate soils with sulfuric material containing extensive retained acidity (jarosite), which can persist for decades or longer.
Article
This study was conducted to evaluate the effect of salinity and sodicity on respiration and dissolved organic matter dynamics in salt-affected soils of different texture. Four non-saline and non-sodic soils differing in texture (4, 13, 24 and 40% clay, referred to as S-4, S-13, S-24 and S-40) were leached with 1M NaCl and 1M CaCl2 solutions resulting in EC1:5 (1:5 soil:water ratio) between 0.4 and 5.0 dS m⁻¹ with two levels of sodicity [SAR1:5 < 3 (non-sodic) and 20 (sodic)]. After adjusting the water content to levels optimal for microbial activity which differed among the soils, this resulted in four ranges of osmotic potential in all soils: control, > −0.55, −0.62 to −1.62 and −2.74 to −3.0 MPa. Finely ground mature wheat straw was added (20 g kg⁻¹) to stimulate microbial activity. At a given EC1:5, cumulative respiration was lower in the lighter textured than the heavier textured soils. Cumulative respiration decreased with decreasing osmotic potential to a similar extent in all soils, with a greater decrease on day 40 than on day 10. Cumulative respiration was greater at SAR1:5 20 than SAR1:5 < 3 only at osmotic potentials between −0.62 and −1.62 MPa on day 40. In all soils and at both sampling times, concentrations of dissolved organic C and N were higher at the lowest osmotic potential (−2.74 to −3.0 MPa) compared to the controls without salt addition. It can be concluded that when comparing soils of different texture, osmotic potential is a better parameter to evaluate the effect of salinity on dissolved organic matter and microbial activity than EC1:5.
Article
When acid sulfate soils containing hypersulfidic material (pH > 4) dry, oxidation of pyrite causes strong acidification with the formation of sulfuric material (pH < 4), which may release high concentrations of metals and metalloids. Re-submerging of sulfuric material can lead to re-formation of pyrite and pH increase to re-form hypersulfidic and hyposulfidic materials due to the action of sulfate-reducing bacteria. However, low availability and/or low biodegradability of organic carbon (OC) may limit the activity of sulfate reducers in re-saturated sulfuric material. Our study investigated the content and composition of OC with specific emphasis on the proportion of readily available, non mineral-associated OC. Samples were taken from a non-acidifying pasture topsoil with hyposulfidic material and two re-submerged subsoils with hypersulfidic material derived from river sediments in South Australia. The sites experienced drying at depths between 0.5 and 4.5 m with severe acidification (pH < 4) during the Millennium drought from 2007 to early 2010. After re-submerging, sulfuric material at one site recovered to neutral pH values, whereas the other site remained acidic. Samples were analysed for total OC content and the proportion of available, non mineral-associated OC. Chemical composition of bulk soil OC and available fractions was determined by solid-state ¹³C NMR spectroscopy and neutral sugar analyses. The OC composition of re-submerged sulfuric material was generally characterised by small proportions of easily degradable carbohydrates and proteins, but high proportions of hardly degradable lignin and lipids. Lowest amounts of available OC fractions and lowest proportions of carbohydrates and proteins were found in hypersulfidic material which is still acidic. This indicates that slow pH recovery rates can be ascribed to low proportions of biodegradable OC. The OC composition can be explained by: (I) sedimentation of organic materials which were already highly biodegraded during formation of river sediments, and (II) selective preservation of lignin and lipids due to permanent waterlogging. Thus, the organic material is characteristic for wetlands, but hardly usable as substrate for microbes and may retard sulfate reduction and pH neutralisation of re-submerged sulfuric material.
Article
Literature about the historical and current recognition of kinds, names and classification for, overall processes (sulfidization and sulfuricization) that form and conditions that induce the formation of potential, active and post-active acid sulfate soils are reviewed to set the stage for papers presented elsewhere in this special issue of Geoderma that contains some of the papers presented at the Eighth International Acid Sulfate Soils Conference held at the University of Maryland, College Park, MD, USA, July 17–23, 2016. Mention is made and examples cited of environmental problems such as AMD (acid mine drainage) and ARD (acid rock drainage), fish kills in waters receiving drainage from acid sulfate soils, even from ones considered post-active, and special land reclamation and management practices to produce crops on land disturbed (often deeply) by humans (including engineers poorly educated about acid sulfate soils) or by natural causes (sea level rise or fall, or extreme drought, such as the recent “Millennium Drought” in the Murray-Darling Basin of Australia. The global distribution of acid sulfate soils is considered, recognizing that post-active ones are extensive, with many likely currently unrecognized, although features in some of them, such as silcrete and ferricrete, likely owe their origin to acid sulfate weathering phenomena. Most coastal subaqueous soils are influenced by sulfidization and constitute sulfidic materials that if dredged and deposited in upland disposal areas become exposed to aerobic conditions and are likely to give rise to active acid sulfate soils with sulfuric horizons.
Article
Acid sulfate soil leachates deteriorate the aquatic ecosystems of their recipient waters around the world. In Finland, AS soils are located mainly on the coast of the Baltic Sea, where rivers and estuaries suffer from acid leachates and waters do not meet with the criteria of good water quality set by the EU. Field drainage of cultivated AS soils is attributable to leaching of acidity, but regardless of various mitigation measures, the acidity of discharge water in these areas has not decreased significantly. In order to better understand the pathways involved in the formation of acidity, the redox status of 56 Finnish AS soil fields was examined using redox potential and pH data measured down to 2 m. The findings indicated that the oxidation of soils has occurred at depths below the drainage pipes, with the median being at a depth of 1.6 m. In fields cultivated for a long time, soil texture had a stronger effect on the depth of the redox interface than the drainage method; open ditch drainage and subsurface drainage; oxidation being faster in sandy and silty soils than in clayey soils. The isostatic land uplift also seems to affect the depth of the redox interface in the long run. Most of the studied fields had been cultivated for at least 30 years prior to the study. However, the pH values of the soils were still very low, probably due to actual and retained acidity. The prevention of oxidation of sulfidic materials in subsoils is important, but measures for neutralizing the acidity are needed. Without them it seems that the leaching of acidity will continue and may decrease only slowly. However, severe droughts during summers and the reclamation of unripe AS soils for any purpose will increase the leaching of acidity.
Article
Irrigation with arsenic contaminated groundwater in the Bengal Delta may lead to As accumulation in the soil and rice grain. The dynamics of As concentration and speciation in paddy fields during dry season (boro) rice cultivation were investigated at 4 sites in Bangladesh and West Bengal, India. Three sites which were irrigated with high As groundwater had elevated As concentrations in the soils, showing a significant gradient from the irrigation inlet across the field. Arsenic concentration and speciation in soil pore water varied temporally and spatially; higher As concentrations were associated with an increasing percentage of arsenite, indicating a reductive mobilization. Concentrations of As in rice grain varied by 2-7 fold within individual fields and were poorly related with the soil As concentration. A field site employing alternating flooded-dry irrigation produced the lowest range of grain As concentration, suggesting a lower soil As availability caused by periodic aerobic conditions.
Article
Pyrite in acid sulfate soils can get oxidised during drought resulting in severe soil and water acidification (pH < 4).The frequency and severity of drought and flooding is increasing in many regions of the world due to climate change but there has been limited research on the ability of acid sulfate soils to recover from these events. We studied the recovery of heavy clay soils in the Lower Murray River (South Australia) irrigated agricultural areas over a 5 year period (2011–2015). The heavy clay acid sulfate soils in this region dried, cracked and acidified due to river and groundwater levels falling by nearly 200 cm during the 2007–2010 severe “Millennium” drought followed by reflooding events between 2011 and 2015. Approximately 300 cm deep soil cores were collected from three locations along a transect in 2011, 2012, 2013, and 2015. The soil properties measured were pH, reduced inorganic sulfur (RIS, pyrite), titratable actual acidity (TAA), retained acidity, and acid neutralising capacity. Soil pH showed very little change over the post-drought period with a very acidic (pH 3.5–4.5) layer at approximately 100–225 cm depth in all three soil profiles. In this acidic layer there also were substantial amounts of TAA (up to 200 mol H+ tonne−1 dry weight) and retained acidity (up to 70 mol H+ tonne−1 dry weight) in the form of the Fe oxyhydroxy sulfate mineral jarosite. There was limited reformation of RIS. To assess why the sulfuric material in the acid sulfate soils has not recovered post-drought we conducted (i) laboratory incubation experiments with and without organic matter amendment, and (ii) modelling of the flushing of acidity from the soil due to irrigation, rainfall and drainage. Based on the field and laboratory results the causes of slow recovery appear to be: (i) lack of available organic carbon and too low a pH to enable microbial reduction reactions that generate alkalinity, ii) slow flushing of acidity due to the low hydraulic conductivity in the heavy clay layers with the main zone of below the drain depth, and (iii) slow dissolution of the sparingly soluble jarosite mineral, which is likely buffering the sub-surface soil layers at approximately pH 4. The implications are that acid sulfate soils with sulfuric materials have long recovery times following droughts and impacts are likely to increase in the future.
Article
Extreme climate events are predicted to become more frequent and intense. Their ecological impacts, particularly on carbon cycling, can differ in relation to ecosystem sensitivity. Peatlands, being characterized by peat accumulation under waterlogged soil, can be particularly sensitive to climate extremes if the climate event increases soil oxygenation. However, a mechanistic understanding of peatland responses to persistent climate extremes is still lacking, particularly in terms of aboveground-belowground feedback. Here we present the results of a transplantation experiment of peat mesocosms from high to low altitude in order to simulate, during three years, a mean annual temperature c. 5°C higher and a mean annual precipitation c. 60% lower. Specifically, we aim at understanding the intensity of changes for a set of biogeochemical processes and their feedback on carbon accumulation. In the transplanted mesocosms, plant productivity showed a species-specific response depending on plant growth forms, with a significant decrease (c. 60%) of peat moss productivity. Soil respiration almost doubled and Q10 halved in the transplanted mesocosms in combination with an increase of activity of soil enzymes. Spectroscopic characterization of peat chemistry in the transplanted mesocosms confirmed the deepening of soil oxygenation which, in turn, stimulated microbial decomposition. After three years, soil carbon stock increased only in the control mesocosms whereas a reduction of mean annual carbon accumulation of c. 30% was observed in the transplanted mesocosms. Based on the above information, a structural equation model was built to provide a mechanistic understanding of the causal connections between peat moisture, vegetation response, soil respiration and carbon accumulation. This study identify in the feedback between plant and microbial responses the primary pathways explaining the reduction of carbon accumulation in response to recurring climate extremes in peat soils. This article is protected by copyright. All rights reserved.
Article
We present 6.5 years of eddy covariance measurements of fluxes of methane (FCH4) and carbon dioxide (FCO2) from a flooded rice paddy in Northern California, USA. A pronounced warming trend throughout the study associated with drought and record high temperatures strongly influenced carbon (C) budgets and provided insights into biophysical controls of FCO2 and FCH4. Wavelet analysis indicated that photosynthesis (gross ecosystem production, GEP) induced the diel pattern in FCH4, but soil temperature (Ts) modulated its amplitude. Forward stepwise linear models and neural networking modeling were used to assess the variables regulating seasonal FCH4. As expected due to their competence in modeling nonlinear relationships, neural network models explained considerably more of the variance in daily average FCH4 than linear models. During the growing season, GEP and water levels typically explained most of the variance in daily average FCH4. However, Ts explained much of the interannual variability in annual and growing season CH4 sums. Higher Ts also increased the annual and growing season ratio of FCH4 to GEP. The observation that the FCH4 to GEP ratio scales predictably with Ts may help improve global estimates of FCH4 from rice agriculture. Additionally, Ts strongly influenced ecosystem respiration, resulting in large interannual variability in the net C budget at the paddy, emphasizing the need for long-term measurements particularly under changing climatic conditions.
Article
In response to large reductions in sulphur (S) emissions over the past 30 years, sulphate (SO42-) concentrations in precipitation at Plastic Lake in south-central Ontario, Canada, have declined by more than 70%. More recent decreases in NOx emissions have similarly led to a reduction in nitrate deposition (NO3-) and consequently the pH of bulk precipitation has increased by approximately 0.8 pH units since 1980. Despite the large decrease in acidic deposition, chemical recovery of the streams, as measured by an increase in pH and decrease in aluminum (Al), has been much less than expected, primarily due to losses of base cations from the shallow, base-poor soils. While nitrogen (N) is almost totally retained within the terrestrial catchment, S export continues to exceed inputs measured in bulk deposition and during the early part of the record approximately 70% of the anions in streams were buffered by calcium (Ca2+) and magnesium (Mg2+) compared with only 60% in 2011/12. In the wetland-draining stream (PC1), peak depressions in stream pH and peaks in SO42- and Al concentration in the fall are significantly positively correlated with annual drought days defined as the number of days when stream flow ceases. Even though reductions in SO2 and NOx emissions in Canada have resulted in large improvements in precipitation chemistry, the combined influence of soil acidification and climate-mediated biogeochemical processes occurring in wetlands cause acidification-related issues to persist. Forecasting the longer-term response of soils and surface waters in light of these observations is required to fully assess the need for further emission reductions.
Article
The quantitative impact of intense drought and rewetting on gas exchange in ombrotrophic bogs is still uncertain. In particular we are lacking studies investigating multitudes of sites with different soil properties and nitrogen (N) and sulfur (S) deposition under consistent environmental conditions. We explored the timing and magnitude of change in CO2 (Respiration, Gross Primary Production - GPP, and Net Exchange - NE) and CH4 fluxes during an initial wet, a prolonged dry (~100 days) and a subsequent wet period (~230 days) at 12°C in 14 Sphagnum peat mesocosms collected in hollows from bogs in the UK, Ireland, Poland, and Slovakia. The relation of N and S deposition with GPP, respiration and CH4 exchange was investigated. Nitrogen deposition increased CO2 fluxes and GPP more than respiration, at least up to about 15 kg N ha(-1) yr(-1) . All mesocosms became CO2 sources during drying and most of them when the entire annual period was considered. Response of GPP to drying was faster than of respiration and contributed more to the change in NE; the effect was persistent and few sites recovered "predry" GPP by the end of the wet phase. Respiration was higher during the dry phase but did not keep increasing as WT kept falling and peaked within the initial 33 days of drying; the change was larger when differences in humification with depth were small. CH4 fluxes strongly peaked during early drought and water table decline. After rewetting, methanogenesis recovered faster in dense peats but CH4 fluxes remained low for several months, especially in peats with higher inorganic reduced sulfur content, where sulfate was generated and methanogenesis remained suppressed. Based on a range of European sites the results support the idea that N and S deposition and intense drought can substantially affect greenhouse gas exchange on the annual scale This article is protected by copyright. All rights reserved.