Soil microorganisms are essential for the functioning of terrestrial ecosystems. Although soil microbial communities and functions are linked to tree species composition and diversity, there has been no comprehensive study of how general or context-dependent these relationships are. Here, we examine tree diversity–soil microbial biomass and respiration relationships across environmental gradients using a global network of tree diversity experiments.
Soil samples collected from eleven tree diversity experiments were used to measure microbial respiration, biomass, and respiratory quotient using the substrate-induced respiration method. All samples were measured using the same analytical device, method, and procedure to reduce measurement bias. We used linear mixed-effects models and principal component analysis (PCA) to examine the effects of tree diversity (taxonomic and phylogenetic), environmental conditions, and interactions on soil microbial properties. Abiotic drivers, mainly soil water content, but also soil carbon and soil pH, significantly increased soil microbial biomass and respiration. High soil water content reduced the importance of other abiotic drivers. Tree diversity had no effect on the soil microbial properties, but interactions with phylogenetic diversity indicated that diversity effects are context-dependent and stronger in drier soils. Similar results were found for soil carbon and soil pH. Our results point to the importance of abiotic variables, and especially soil water content, for maintaining high levels of soil microbial functions and modulating the effects of other environmental drivers. Planting tree species with diverse water-use strategies and structurally complex canopies and high leaf area may be crucial for maintaining high soil microbial biomass and respiration. Since higher phylogenetic distance alleviated unfavorable soil water conditions, reforestation efforts accounting for traits improving soil water content or choosing more phylogenetically distant species may assist in increasing soil microbial functions.