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

Diversity-function relationships are well known for producers, and these are also known to be influenced by consumers at higher trophic levels. However, these are not well known for microbial decomposers and decomposition processes in soil. Further, it also remains unknown whether and how consumers such as large mammalian herbivores, who are a major feature across more than one-third of the world’s terrestrial realm, influence soil microbial decomposer diversity-function relationships. We used a14-year old long-term herbivore-exclusion experiment to answer two questions: (a) whether microbial functions vary with microbial diversity (both species richness and composition), and (b) whether herbivores alter the diversity-function relationships among microbial decomposers. We measured functional richness, and functional dispersion from the utilization profiles of 30 metabolic substrates. Alongside, we also measured species richness and species composition of soil microbes. We analysed soils from n=10 paired grazed-and-fenced plots, and sampled them three times during the growth season of 2019. Data were from 60 16S rDNA gene amplicon sequences with 7.6 million reads covering 1937 operational taxonomic units (OTU) across 47 phyla (of which 924 OTUs were identifiable to genus level), and 1800 catabolic profiles. Functional diversity in soil was positively related to microbial community composition, but not to species richness. This parallels the diversity-function relationships between producers and production; it also challenges the prevailing notion of functional redundancy in hyper-diverse soil microbial communities. Since certain combinations of species can outperform others, global change factors which can alter microbial communities may also impact decomposition processes and services. But this coupling between diversity and functions was unaffected by experimental herbivore-exclusion, indicating resilience and resistance among decomposers. Structural equation models suggested that the strength of this relationship is favoured by availability of soil moisture. These also showed 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 between diversity and functions was generalizable and robust once 16 or more functions were quantified. While ecosystem functions and services derived from microbial decomposers have intrinsic resilience and resistance, they respond strongly to variability in water availability. Decomposition in grazing ecosystems may be particularly susceptible to how microbial community composition responds to rising precipitation variability under ongoing and projected climate change. Highlights Diversity-function relationships for microbial decomposers remain uncertain. Microbial diversity is positively related to functions This relationship is robust to herbivore-exclusion. Microbial diversity-function relationship can be susceptible to altered precipitation