Precipitation regime drives warming responses of microbial biomass and activity in temperate steppe soils

Although numerous warming experiments have examined the impacts of elevated temperature on soil microbial activities, most are based on responses from a single site. To investigate how precipitation regime regulates warming effects on the carbon cycle, we conducted manipulative warming experiments in desert steppe, typical steppe, and meadow steppe along a precipitation gradient in northern China. Soil temperature, moisture, microbial biomass C (MBC), N (MBN), and microbial respiration were measured from 2006 to 2009. Soil moisture was significantly reduced by warming in the typical steppe but not affected in the desert and meadow steppe. Across the 4 years, warming decreased MBC and microbial respiration in the desert and typical steppe but not in the meadow steppe. The magnitude of warming-induced reductions in MBC and microbial respiration declined as site precipitation increased. Across the three sites, the changes in soil MBC, MBN, and microbial respiration were all positively correlated with annual precipitation and changes in belowground net primary productivity. Our results suggest that precipitation regime controls the response of soil microbial activity and biomass to warming, possibly by regulating soil moisture and substrate availability. With increasing precipitation, the stimulatory effects of warming on soil microbial activity and biomass outweigh the inhibitory effects due to declining soil moisture.