Tree diversity effects on soil microbial biomass and respiration are context-dependent across forest diversity experiments

Aim Soil microorganisms are essential for the functioning of terrestrial
ecosystems. Although soil microbial communities and functions may be
linked to tree species composition and diversity, there has been no
comprehensive study of how general these potential relationships are, or
if they are context-dependent. Here, we examine tree diversity–soil
microbial biomass and respiration relationships across environmental
gradients using a global network of tree diversity experiments. Location
Global Time Period 2013 Major Taxa Studied Soil microorganisms Methods
Soil samples collected from eleven tree diversity experiments in four
biomes 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 PCA to examine the effects of tree diversity (taxonomic and
phylogenetic), environmental conditions, and interactions on soil
microbial properties. Results Abiotic drivers, mainly soil water content,
but also soil carbon and soil pH, significantly increased soil microbial
biomass and respiration. Optimal soil water content reduced the importance
of other abiotic drivers. Tree diversity alone 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. Main
conclusions 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 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.