Wetland reclamation homogenizes microbial properties along soil profiles

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.