The validity of ecological hypotheses concerning aboveground organisms for soil microbial biomass and diversity across soil depths on the Mongolian Plateau

Although many empirical hypotheses have been proposed and tested to explain the biomass and diversity of aboveground organisms, these hypotheses have not been tested for soil microorganisms across soil depths. Here, we determined the ability of hypotheses concerning physiological tolerance, plant energy, soil energy, and habitat heterogeneity to explain the variance in microbial biomass (as determined by phospholipid fatty acid) and diversity (as determined by high-throughput sequencing) in surface (0–20 cm) and deep (40–60 cm) soil layers on the Mongolian Plateau. We found that aboveground net primary production (ANPP) and plant species richness steeply declined as aridity values increased. Physiological tolerance and habitat heterogeneity explained the spatial variation in ANPP while physiological tolerance and soil energy explained the spatial variation in plant diversity. The relationships between aridity and microbial variables differed between soil layers or microbial groups. Aridity values had hump-shaped relationships with surface-soil bacterial biomass or diversity but had linear and negative relationships with surface-soil fungal biomass or diversity. Aridity values were linearly and negatively related with deep-soil bacterial or fungal biomass but were unrelated with deep-soil bacterial or fungal diversity. We also found the ability of the ecological hypotheses to explain the variances in microbial variables (biomass and diversity) diffed between soil layers or microbial groups on the Mongolia Plateau. The surface-soil bacterial biomass was mainly associated with plant energy and physiological tolerance while surface-soil bacterial diversity was mainly associated with habitat heterogeneity and physiological tolerance; surface-soil fungal biomass and diversity were mainly associated with habitat heterogeneity and physiological tolerance. The deep-soil bacterial biomass and diversity were mainly associated with physiological tolerance and soil energy; deep-soil fungal biomass was mainly associated with plant energy and physiological tolerance while deep-soil fungal diversity was mainly associated with soil energy and habitat heterogeneity. Our results showed that the major ecological hypotheses traditionally applied to plant communities can explain much of the variation in soil microbial biomass and diversity. These findings that identifying ability of ecological hypotheses to explain the variance in both plant and microbial biomass and diversity will be crucial importance to understand biodiversity assembly under ongoing environmental changes.