[Responses of soil potential carbon/nitrogen mineralization and microbial activities to extreme droughts in a meadow...

The mineralization of soil carbon (C) and nitrogen (N) is a critical process in the cycling of C and N in terrestrial ecosystems, which is strongly controlled by water availability. In this study, we collected soil samples in a 3-year extreme drought experiment in a meadow steppe in Inner Mongolia, freeze-dried these samples, and measured the potential C and N mineralization rates and water sensitivity of soil microorganism by incubating soils under soil water contents (SWC) of 3%, 8%, 13%, 18%, 25% and 35%. The results showed that averaged across different SWC, the extreme drought treatment of reducing 66% precipitation in growing season significantly increased potential N mineralization rate by 14.2%, but did not affect the potential C mineralization. Extreme drought significantly increased soil microbial biomass N and soil dissolved organic C by 26.8% and 26.9%, respectively. In both the control (natural rainfall) and extreme drought treatment, the potential C and N mineralization and microbial biomass C and N increased with SWC in the incubation, which was possibly caused by the enhanced substrate diffusion. Extreme drought also promoted the initial pulse response of C mineralization, implying the enhanced microbial response to water availability. Extreme drought significantly reduced the ratio of the potential soil C mineralization to the potential N mineralization, suggesting that extreme drought might weak the coupling of soil C and N. Extreme drought could cause different responses to soil water availability between soil C and N cycling. Extreme drought could enhance microbial response to increasing water availability, weak coupling between soil C and N, with consequences on nutrient cycling and primary productivity in the meadow steppe of northern China.