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Natural wetlands are mostly nitrogen-limited ecosystems, while reclamation stimulates the loss of nitrogen (N) in soils by shifting the N regime. To investigate the microbial mechanisms of the N regime shift, we first conducted a global meta-analysis to quantify the wetland reclamation impacts on soil mineral N pools and then a field campaign to sample 24 soil cores up to 100 cm depth in a natural wetland and a 23-year cultivated soybean field from the Sanjiang Plain in northeastern China. After wetland reclamation, the N regime was shifted to cause a potential risk of massive N loss in soils; their microbial mechanisms were revealed through metagenomic data. In cropland, the relative abundance of genes involved in nitrification and assimilatory nitrate reduction to ammonia (ANRA) were enriched while those in N fixation, mineralization, denitrification, and dissimilatory nitrate reduction to ammonia (DNRA) were diminished. Wetland reclamation substantially enhanced the relative abundance of genes involved in nitrification (except for genes for ammonia oxidation to NH2OH) and denitrification in surface (0–30 cm) soils but decreased them in subsurface (30–100 cm) soils. After wetland reclamation, the relative abundance of genes involved in denitrification and DNRA significantly reduced in spring and summer, but such patterns were not found in autumn and winter. This change enhanced potential microbial-driven N loss in spring and summer. The metagenomic data serve as surrogate data sources for quantifying soil roles on soil N cycles under land use change. [ABSTRACT FROM AUTHOR]

Natural wetlands store 20–30% of soil organic carbon (C) in terrestrial ecosystems. Wetlands conversion to croplands has caused a massive loss of soil organic C; however, the microbial contribution to this C loss remains elusive. In this study, we took 24 soil cores from a natural wetland and a 23-year cultivated cropland over four seasons to compare the abundance of functional genes encoding 138 enzymes for ten C-cycling pathways classified as organic C degradation, catabolic and anabolic processes. We found that land use-induced C loss was primarily driven by enhanced organic C degradation (∼24% increase) but suppressed microbial C assimilation efficiency, represented by shifted catabolism and declined anabolism (∼22% decrease). The enhanced organic C degradation was advocated by the enriched genes encoding organic C degradation enzymes; the suppressed microbial C assimilation efficiency was upheld by the shifted catabolic processes and inhibited anabolic processes. Compared with natural wetlands, the croplands showed substantial shifts in catabolic processes - 22% suppression of the Embden-Meyerhof-Parnas pathway, 27% decline in the Citrate Cycle, 22% increase in Entner-Doudoroff, and 25% stimulation in pentose phosphate pathways. Meanwhile, the anabolic process, especially the Wood-Ljungdahl pathway, was significantly suppressed (∼55%) in the cropland. The shifts in organic C degradation, catabolism, and anabolism were stronger in spring and summer than in winter and autumn. At the ecosystem level, microbial metabolic quotient was suppressed by 44%, while microbial biomass carbon was boosted by 5% after wetland conversion to cropland, indicating a faster microbial growth rate but lower C use efficiency. Differentiating microbial metabolic processes explained the C loss after wetland cultivation, indicating an urgency to represent microbial metabolism in soil C models. • The organic C degradation was enhanced after wetland conversion. • The catabolic process was shifted to energetically less efficient after conversion. • The anabolic process was depressed in cultivated cropland. • C loss after cultivation was driven by microbial metabolism differentiation. [ABSTRACT FROM AUTHOR]

Background and aims: Nitrogen (N) is often the nutrient limiting the decomposability of shoots from macrophytes in wetlands. We aimed to determine the effects of increasing soil N availability on the decomposition of shoots during the standing-dead phase. Methods: We measured the quality of senesced leaves from graminoids and their subsequent aerial decay under different N addition treatments (Control, 0 kg N ha yr.; N60, 60 kg N ha yr.; N120, 120 kg N ha yr.; N240, 240 kg N ha yr.) in a temperate marsh. Results: Nitrogen addition increased N concentrations in senesced leaves and often increased phosphorus (P) concentrations. The exponential decay constants ( k) of leaves from the N120 and N240 treatments were higher than the control treatment during aerial decay. Nitrogen amounts (in percentage terms) remaining in decaying leaves always significantly decreased after N addition, but the effects on P amounts varied with N addition rates. The nutrient amounts remaining in leaves during the standing-dead phase had negative relationships with the initial nutrient concentrations. Conclusions: Soil N availability exerts remarkable effects on the decay process of standing litters by altering the initial quality, and thus the biogeochemical cycling in temperate wetlands. [ABSTRACT FROM AUTHOR]

• The impacts of wetland conversion on soil microbes vary over season and along soil profile. • The homogenization impact is the strongest in summer and weakest in winter. • Wetland reclamation suppresses microbial diversity in winter, spring, and summer. • The input of fresh carbon in autumn enhances microbial diversity. Soil microbes drive biogeochemical cycles of carbon (C) and nutrients, and land conversion alters soil microbes and thus, affects C and nutrient cycling at the ecosystem level. To evaluate the impacts of wetland reclamation on microbes along soil profiles and over seasons, soil cores (0–100 cm) were collected from a natural wetland and an adjacent cropland that has been continuously cultivated for 23 years in the Sanjiang Plain, northeastern China. Wetland conversion to cropland generally homogenized the microbial abundance along the soil profiles in four seasons (May of spring, July of summer, October of autumn, and December of winter), while the vertical homogenization was the strongest in summer and weakest in winter. After 23 years of cultivation, the abundances of fungi and bacteria in surface soils were significantly reduced by approximately 90% and 80%, respectively, whereas the bacterial abundances increased and fungal abundances decreased in middle and deep soils, suggesting that cultivation impacts on microbial community structure varied along soil layers. Compared to the wetland, total phospholipid fatty acids in the middle and deep soils of the cropland were enhanced by approximately 60–367% in autumn and winter and 30–137 % in spring and summer. Wetland reclamation suppressed the microbial diversity in the surface soils in winter, spring, and summer but stimulated that in autumn, indicating robust enhancements on microbial diversity by the inputs of fresh litter C. The quantifications of microbial abundance and community structure showed the non-uniform impacts of wetland reclamation on microbes along the soil profile and over seasons, which provides valuable information for benchmarking models in simulating microbial roles on C cycling in deep soils at the seasonal scale. [ABSTRACT FROM AUTHOR]

Abstract: Wetlands in mid-high latitude regions play a vital role in climate change, due to their high organic carbon density. Nonetheless, the stabilization of soil organic carbon (SOC) and its fractions in these regions is largely unknown. The objectives of this study were to examine the changes in SOC and its fractions along a latitudinal gradient and to analyze the influencing factors in the southern margin of the permafrost region on the Eurasian continent. Topsoil (0–20cm) was collected from five wetlands along a latitudinal gradient in Northeast China. We analyzed SOC and some labile fractions. Our results demonstrated that wetland SOC and its labile fractions concentration declined with decreasing latitude. The Permafrost regions had greater organic carbon content than the regions with seasonally frozen ground. The light fraction organic carbon and particulate organic carbon accounted for 5–83% and 21–32%, respectively, of the SOC and were particularly enriched in the permafrost region. Microbial biomass carbon and dissolved organic carbon showed a similar decreasing trend to that of SOC. At the same latitude, vegetation affected SOC stock and the dynamics of its labile fractions. Therefore, wetlands in mid-high latitudes contain a large carbon pool, and carbon stock varies with latitude. Although the labile fractions were higher in the permafrost region, their activities were lower, due to low temperature and poor nutrient status. Under global warming, the labile carbon pool may be mobilized and contribute to the greenhouse effect. Moreover, vegetation differences should be considered in obtaining an accurate carbon calculation. [Copyright &y& Elsevier]

Abstract: Wetland ecosystems store a large amount of organic carbon (C) in soils, due to the slow decomposition rates of plant litter and soil organic matter. Increased nitrogen (N) availability induced by human activities and global warming may accelerate litter decomposition and affect soil organic C dynamics in wetlands. In the present study, we investigated the effect of N addition on decomposition of Calamagrostis angustifolia litters from freshwater marshes in the Sanjiang Plain of Northeast China under field and laboratory conditions. First, we assessed the changes in initial litter chemical composition and subsequent decomposition following three years of N addition at the rate of 24gNm−2 year−1 under field conditions. Our results showed that N addition increased litter N concentration and decreased C/N ratio, and thus stimulated litter decomposition. Secondly, we examined the effect of increased N availability (0, 25, 50 and 100mgNg−1 litter) on litter decomposition under laboratory conditions. Increased exterior N availability also enhanced microbial respiration and increased litter mass loss under both waterlogging and non-waterlogging conditions. In addition, waterlogging conditions inhibited microbial respiration and suppressed litter mass loss. These findings demonstrated that N addition increased litter decomposition rates through improved litter quality and enhanced microbial activity in freshwater marshes of Northeast China. This implies that increased N availability accelerates litter decomposition rates, and thus may cause substantial losses of soil C and diminish and even reverse the C sink function of wetlands in the Sanjiang Plain of Northeast China. [Copyright &y& Elsevier]

• Wetland reclamation homogenized microbial biomass along soil profile. • Cultivation narrowed the C:N:P:S stoichiometry in soils and microbial biomass. • Cultivation promoted bacterial abundance in 30–80 cm soil profile. Wetlands store approximately one-third of soil carbon (C) in terrestrial ecosystems, and the loss of C in wetlands has been accelerated by reclamation. However, how wetland reclamation affects microbial properties along soil profiles remains unclear. In this study, we sampled 100 cm soil cores from a wetland and an adjacent cropland that has been cultivated for 23 years to evaluate the impacts of land conversion on soil microbial community structure and soil element concentrations in the Sanjiang Plain, Northeastern China. The C and nitrogen (N) concentrations in microbial biomass declined exponentially with depth in both the wetland and cropland with different magnitudes. Continuous cultivation for 23 years tended to homogenized C, N, phosphorus (P), and sulfur (S) contents in soils and microbial biomass along the soil profile. Compared with the wetland, C and N in cropland soils were lower, and microbial biomass C, N, P and S were lower in surface soils (0–30 cm), while higher in both middle soils (40–70 cm) and deep soils (70–100 cm). Cultivation narrowed the C:N:P:S stoichiometry in soils and microbial biomass, with large changes in surface soils and minor changes in deep soils. The Bray-Curtis dissimilarity test confirmed the large changes in surface soils while minor changes in deep soils. After 23-years of cultivation the abundances of fungi and bacteria were significantly reduced by approximately 90% and 78% in surface soils, but the bacterial abundance was enhanced by approximately 2–3 times in middle soils, leading to a decreasing fungi:bacteria ratio along soil profile in cropland. Soil TC and pH were the predominant factors controlling the microbial composition in wetland. The homogenizing impact of wetland reclamation on microbial properties along soil profiles suggests the different responses of microbial and soil elements to human activities, indicating a critical need of differentiating microbes from soils when examining soil elements under a changing environment. [ABSTRACT FROM AUTHOR]