Wetland conversion to cropland alters the microbes along soil profiles and over seasons

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.