Extreme drought slightly decreased soil labile organic C and N contents and altered microbial community structure in a subtropical evergreen forest

It is predicted that climate extremes such as drought will become more frequent and prolonged by the end of this century, so it is vital that we improve our understanding of the effects of extreme drought on the dynamics of soil labile C and N contents and microbial communities in terrestrial ecosystems. Using a 70% rainfall reduction manipulation experiment to simulate extreme drought in a subtropical evergreen forest of eastern China, we collected soil samples over all four seasons and analyzed the dynamic changes in soil labile organic C and N contents and microbial communities via high-throughput sequencing during the period from June 2016 to February 2017. Soil labile organic C and N contents were represented by soil extractable organic C (EOC) and extractable organic N (EON) contents, microbial biomass C (MBC) and microbial biomass N (MBN) contents. We also measured soil potential CO2 and N2O emissions by using laboratory incubation under drought conditions. The results showed that drought significantly decreased soil pH, while it slightly decreased soil EOC and EON contents, and MBC and MBN contents by 13.4%, 15.7%, 11.1% and 15.2% respectively, though it had no significant effects on these compared with the control plots. The dominant bacterial phyla across all soils were Proteobacteria (32.52-41.30%), Acidobacteria (34.47-42.64%) and Actinobacteria (6.52-8.16%). Drought greatly altered the soil microbial community structure underlying soil C and N cycling: (1) drought significantly increased the relative abundance of Acidobacteria, which was associated with lower soil pH in the drought plots; (2) drought had a significantly lower relative abundance of Proteobacteria, which was supported by the lower soil labile organic C and N contents under drought. Redundancy analysis showed that the indirect drought-induced effects of soil EOC and EON contents from two sampling times played a weightier role in influencing the patterns of soil microbial communities than the direct drought-induced effects of soil moisture. We found that drought significantly decreased soil potential CO2 and N2O emissions, confirming that drought decreased soil microbial activity. Overall, our results suggest that extreme drought slightly decreased soil labile C and N contents, and altered the microbial community structure, mainly through indirect drought-induced pathways.