Effects of tree diversity on soil microbial community in a subtropical forest in Southwest China

Soil microorganisms are crucial for biological diversity and ecosystem processes in terrestrial ecosystems. Plant diversity has been reported to have positive effects on microbial communities. However, further research is required to understand the complex mechanisms that regulate the interactions between plants and soil microorganisms, especially the corresponding variations in soil microbial communities under tree diversity gradients in natural forest ecosystems. Therefore, we conducted a field investigation in a natural subtropical forest and collected 45 soil samples to evaluate how the soil microbial biomass and community diversity (represented by phospholipid fatty acid biomarkers) responded to a tree diversity gradient as well as their associations with plant (root and litter) traits and soil variables. The contents of total PLFAs ranged from 86.4 to 325.5 nmol g−1 soil with an average of 183.2 ± 66.1 nmol g−1 soil across the 45 plots. Tree diversity significantly enhanced microbial biomass and decreased microbial diversity. Hierarchical partitioning analysis revealed that the soil substrates (including inorganic and organic nutrients) were the major determinants of microbial biomass. Structural equation models also demonstrated that the effects of tree diversity on the biomass and diversity of soil microbial communities were mainly dependent on soil substrates and soil water content, respectively. In addition, soil microorganisms may potentially metabolize relatively recalcitrant carbon components under low tree diversity, whereas they tended to utilize labile carbon sources under high tree diversity. Overall, these findings suggested that the soil microbial community biomass and diversity exhibited distinct changes along the tree diversity gradient, and highlighted the key role of soil substrates availability in controlling microbial biomass.