Impact of Topography, Annual Burning, and Nitrogen Addition on Soil Microbial Communities in a Semiarid Grassland

To gain insight into microbial responses to topography, annual burning, and N addition, a field experiment was conducted from April 2005 to December 2009 in a semiarid grassland of northern China. Soil physicochemical properties, microbial biomass, and microbial community composition were measured in 2006 and 2008. A larger ratio of fungi/bacteria was observed in the upper slope than in the lower slope. Interannual climate fluctuation could have modified the effects of topography on microbial biomass and composition. Burning effects on microbial biomass and composition also depended on year, which could be attributed to low fire severity resulting from decreasing fuel load over time or microbial resilience. Nitrogen addition exerted a much stronger influence on microbial biomass in 2008 compared with 2006 and reshaped microbial communities through decreasing the relative proportion of fungal groups [arbuscular mycorrhizal fungi (AMF) and nonmycorrhizal fungi] in 2008. Overall, these results highlight dynamic responses of soil microbial communities to both the intrinsic features (topography) and exogenous disturbances (fire or N deposition) of the semiarid grassland.