Functional trait variation and community-weighted means of tree traits can alter soil microbial biomass and community composition

The interactions between plant diversity and the soil microbial community are crucial for maintaining multiple ecosystem functions. However, the underlying mechanisms of how plant diversity impacts the soil microbial biomass and community composition are still unclear. In this study, we used a biodiversity-ecosystem function experiment to explore whether tree species richness affects the soil microbial biomass and community composition through functional trait variation and community-weighted trait means (CWM). The diversity gradient covered 1, 4, 8, 16 and 32 tree species. Phospholipid fatty acid analysis was used to determine the soil's total microbial biomass and composition. We found that an increase in tree species richness decreased the soil's total microbial biomass, with a more pronounced decrease in young forests with a lower functional trait variation in root nitrogen or with resource-conservative traits (i.e., higher CWM of leaf dry matter content). An increase in tree species richness decreased the biomass ratio of gram-positive to gram-negative bacteria but did not significantly affect the fungi to bacteria ratio. The fungi to bacteria ratio significantly decreased with the CWM of leaf nitrogen content and the functional trait variation in leaf dry matter content. Both the functional trait variation and CWM of the plant leaf and root traits related to resource acquisition influenced the strength and direction of the tree species richness effect on the soil microbial community. Our findings indicated that the functional trait variation and CWM of plant functional traits of the tree communities drive the changes in soil microbial biomass and community composition, which in the long-term feedback on ecosystem functions and services they provide.