Community composition, and not species richness, of microbes influences decomposer functional diversity in soil

Diversity-function relationships in producers, and how these are influenced by consumers, are well known. However, these are not well known for microbial decomposers in soil. It is also unknown whether and how consumers such as large mammalian herbivores influence soil microbial decomposer diversity-function relationships. We used a 14-year-old herbivore-exclusion experiment in the Trans-Himalayan drylands of northern India to address whether microbial functions vary with microbial diversity (both species richness and composition), and whether herbivores alter the diversity-function relationships. We analyzed soils from n = 10 paired grazed-and-fenced plots three times during the growth season of 2019. Data were from 16S rDNA gene amplicon sequencing of 7.6 million reads covering 1937 operational taxonomic units (OTU) of bacteria across 47 phyla (of which 924 OTUs were identifiable to genus level), and 1800 catabolic profiles across 30 substrates related to carbon metabolism. We found that functional diversity was positively related to microbial community composition, but not to species richness; it was also unaffected by large mammalian herbivore-exclusion to indicate resilience and resistance. This positive relationship between community composition and functional diversity challenges the prevailing notion of functional redundancy in hyper-diverse soil microbial communities since certain combinations of species could outperform others and determine decomposition processes and services. Structural equation models suggested that the strength of this relationship is favoured by availability of soil moisture. Microbial functions varied more strongly with temporal variables (e.g., seasonality) than with spatial variables (e.g., edaphic factors such as soil texture and pH). Although interpretations can be constrained by which and how many functions are investigated, the relationship was generalizable and robust once 16 or more functions were quantified. Decomposition in drylands may be particularly susceptible to how the identity of the microbial species, and not the number of species, responds to rising precipitation variability under ongoing and projected climate change.