Plant community composition and traits modulate the impacts of drought intensity on soil microbial community composition and function

Terrestrial ecosystems are increasingly threatened by extreme drought events. Soil microbial communities are central to terrestrial ecosystem function via their role in regulating biogeochemical cycling. Consequently, the impact of increasingly intense drought events on soil microbial communities will have knock-on effects for how ecosystems cope with climate change. In an outdoor grassland mesocosm experiment, we determined how increasing drought intensity affects bacterial and fungal community composition, and functioning, during and after drought. We also tested whether plant community resource acquisition strategy (fast- versus slow-strategy plant communities), plant community composition, and plant functional traits mediate soil microbial responses to increasing drought intensity. We found that increasing drought intensity markedly shifted bacterial and fungal community composition, and these effects persisted until the end of the experiment (two months after re-wetting). Bacterial and fungal communities that experienced severe droughts did not return to baseline composition, while those that experienced a mild drought did. Microbial community functioning (potential extracellular enzyme activity) was reduced at peak drought and shortly after re-wetting. While drought intensity effects on bacterial or fungal communities were insensitive to plant community resource acquisition strategy, functional group abundance (aboveground biomass of grass or forb plant species) composition (grass:forb ratio) and leaf traits (leaf dry matter content and leaf nitrogen concentration) explained significant variation in bacterial and fungal community composition during and after drought. Notably, plant community leaf dry matter content and soil nitrogen were the key factors mediating the effect of increasing drought intensity on microbial indicator taxa (ASVs). We conclude that increasing drought intensity affects grassland soil microbial communities during and after drought, and this impact is influenced by plant community composition and functional traits.陆地生态系统正日益受到极端干旱事件的威胁。土壤微生物群落通过调控生物地球化学循环，是陆地生态系统功能的核心驱动者。因此，愈发强烈的干旱事件对土壤微生物群落的影响，将对生态系统应对气候变化的能力产生连锁效应。本研究通过户外草地中宇宙（mesocosm）实验，探究了干旱期间及干旱结束后，干旱强度递增对细菌、真菌群落组成及其功能的影响。本研究同时检验了植物群落资源获取策略（即快策略与慢策略植物群落）、植物群落组成及植物功能性状，是否会介导土壤微生物群落对干旱强度递增的响应。研究结果显示，干旱强度递增显著改变了细菌与真菌的群落组成，且该效应持续至实验结束（即复湿后两个月）。经历极端干旱的细菌与真菌群落未能恢复至初始组成，而经历轻度干旱的群落则可恢复至初始状态。微生物群落功能（即潜在胞外酶活性）在干旱峰值期及复湿后短期内出现下降。尽管干旱强度对细菌或真菌群落的影响不受植物群落资源获取策略的调控，但功能群丰度（禾本科或非禾本科草本植物的地上生物量）、组成（禾草与非禾草的比例）以及叶片性状（叶片干物质含量与叶片氮浓度），能够解释干旱期间及干旱后细菌与真菌群落组成的显著变异。值得注意的是，植物群落叶片干物质含量与土壤氮素，是介导干旱强度递增对微生物指示类群（扩增子序列变体，Amplicon Sequence Variants，ASVs）影响的关键因子。本研究结论为：干旱强度递增会在干旱期间及干旱后影响草地土壤微生物群落，且该影响受植物群落组成与功能性状的调控。