Water rather than nitrogen availability predominantly modulates soil microbial beta-diversity and co-occurrence networks in a secondary forest

Atmospheric nitrogen (N) deposition and changing precipitation regimes greatly affect the structure and functions of terrestrial ecosystems. However, their impacts on the diversity and assembly of soil microbial communities including bacteria, fungi and protists, remain largely unclear. As part of a six-year field experiment in a secondary forest in a warm temperate and subtropical climate transitional zone in China, we aimed to investigate the responses of soil microbial communities to N addition, increased and decreased precipitation. The results showed that N addition had no effect on soil microbial α- or β-diversity, but reduced the complexity of microbial network. Neither increased nor decreased precipitation influenced soil microbial α-diversity, but decreased precipitation rather than increased precipitation elevated bacterial and protistan community dissimilarities (β-diversity), which could have been largely attributed to species replacement processes through reducing soil water availability. In addition, decreased precipitation weakened microbial complexity and stability, but enhanced the node proportion of protists in the co-occurrence network. Our observations suggest the asymmetric responses of soil microbial β-diversity to increased and decreased precipitation, and underscore that water rather than N availability, especially drought condition, plays a predominant role in modulating soil microbial β-diversity. Moreover, the findings imply that global change can strengthen the importance of soil protists and then reshape microbial assembly in forests.