Soil microorganisms are sensitive to changes in environmental factors and play critical roles in the responses of terrestrial ecosystems to natural or anthropogenic perturbations. This study was conducted to examine interannual variability of soil microbial biomass and respiration in response to topography, annual burning and N addition in a semiarid temperate steppe in northern China. Averaged across the 3 years from 2005 to 2007, soil microbial biomass carbon (MBC), nitrogen (MBN) and respiration (MR) were 31.6, 36.7, and 16.2% greater in the lower than upper slope, respectively. The topographic difference was larger in the drier year, leading to stronger temporal variability of MBC, MBN, and MR in the upper than lower slope. Annual burning stimulated MBC and MBN, on average, by 23.3 and 14.4%, but it suppressed MR. Both the positive responses of soil microbial biomass and the negative responses of MR to burning declined over time. Nitrogen addition reduced MBC and MBN by 17.1 and 8.3% respectively, and the negative effects increased with year. N addition also amplified temporal variability of MBC, MBN and MR. Across the 48 plots, soil microbial biomass and respiration showed positive dependence upon soil moisture and dissolved organic carbon (C). In addition, coefficient variations (CVs) of microbial biomass and respiration were also positively correlated with CVs of soil moisture and aboveground biomass (AGB), suggesting water and plant regulation on the interannual variability of soil microorganisms in response to environmental disturbances in the semiarid temperate steppe. Amplified temporal variability of soil microorganisms under drought, fire and N addition indicates reduced stability of soil microbial community with consequent influences on ecosystem function under environmental change. (C) 2010 Elsevier B.V. All rights reserved.