Microbial Community and Functional Responses to Rainfall Manipulations in a Prairie Soil.

Climate change scenarios predict that variability in precipitation will increase across the North American Great Plains, meaning higher-intensity rainfall events will occur at lower frequency, with extended drought periods between rainfalls. These soils support large C stocks and productive agriculture, as well as diverse microbial communities that help control soil C sequestration and fertility.  Changes in microbial C dynamics and community composition under more variable soil water availability conditions are not well understood. This study examines soil microbial responses to increased rainfall variability in a native prairie soil, providing a reference point for responses in nearby agroecosystems.  We measured soil microbial community diversity and function under an experimentally altered precipitation regime at the Konza Prairie in Kansas that provides a long-term replicated field experiment that maintains extended between-rainfall intervals with unchanged total growing season precipitation. We sampled field soils with “Ambient” and “Altered” precipitation history before, during, and after rainfall events.  Results indicate greater microbial carbon use efficiency in Altered soils, concurrent with increased biomass C:N ratio and fungal:bacterial PLFA ratio in drier soils; however, ribosomal gene sequence data showed only minor community responses.  This suggests that (i) coarse changes in relative abundance of microbial domains (bacteria/fungi) dominate within-domain taxonomic responses, and/or (ii) physiological shifts within microbial communities are more likely than microbial community turnover with increased variability in soil water availability.  Microbial gene expression showed increased relative abundance of mRNAs from oxidative phosphorylation and bacterial cellobiose transport genes immediately following rainfall, concurrent with increased bulk soil cellulolytic activity, suggesting rapid microbial processing of labile and polymeric soil C. Overall, our results suggest that microbes are sensitive to extended seasonal drying and rainfall pulses, resilient to shifts in precipitation frequency over long time scales, and express a variety of physiological strategies to cope with drought stress in prairie soils.