Precipitation regime drives soil microbial responses to warming in temperate steppes

Abstract:

Although numerous warming experiments have been done to examine the impacts of elevated temperature on soil microbial actives, most of them were based on responses from a single site. To investigate how precipitation regime regulate warming’s effects on carbon cycle, field manipulative warming experiments were conducted at 3 types of steppes (desert, typical and meadow steppe) along a precipitation gradient in northern China. Soil temperature, moisture, dissolved organic C (DOC), inorganic nitrogen (N) concentration, microbial biomass C (MBC), N (MBN) and respiration (MR) were measured once a year from 2006 to 2009. The results showed that soil moisture was significantly decreased in the typical steppe whereas not affected in the desert and meadow steppe, respectively. Across the 4 years, warming decreased MBC and MR in the desert and typical steppe while did not affect them in the meadow steppe. The magnitude of reductions in warming-induced MBC and MR declined with increasing precipitation gradient at a regional scale. Across the precipitation gradient, all changes in soil MBC, MBN and MR were positively correlated with both annual precipitation and changes in belowground net primary productivity (BNPP), suggesting that soil microbial responses to warming may be regulated by annual precipitation and substrate availability. However, the lab-incubation revealed that soil moisture is more important in regulating soil microbial activities than substrate across the 3 steppes. In addition, soil microbial responses to warming showed year-to-year variations during the first 4 years coincided with the fluctuations in annual precipitation across the 3 steppes. Our results suggested that precipitation regime controls the spatial and interannual responses of soil microbes to warming, mainly by regulating soil moisture and substrate availability. With the increase in precipitation, the positive responses of soil microbes to warming started to outweigh the negative impacts caused by the decline of soil moisture.