Rare soil microbial taxa regulate the negative effects of land degradation drivers on soil organic matter decomposition

Land degradation drivers, including loss in vegetation and eutrophication, are expected to impact soil biodiversity and functions in drylands world-wide. Soils contain both common and rare microbial taxa that drive multiple soil functions. Yet, little is known about how these microbial taxa influence the impacts of land degradation drivers on ecosystem functions. Obtaining this information is essential to determine whether rare taxa need to be protected, or if protecting only common taxa would be enough to sustain and protect ecosystem functions and services. Here, we conducted an experiment to investigate the effects of N-enrichment and vegetation loss (plant removal), which are two major land degradation drivers in semi-arid grasslands, on the diversities of common and rare soil bacterial and fungal taxa and soil function [soil organic matter (SOM) decomposition] in a long-term experiment. Six years after N-enrichment and vegetation loss, we found that N-enrichment decreased the alpha diversities of common and rare soil bacteria and rare soil fungi, while vegetation loss only decreased the alpha diversity of rare soil fungi. Both N-enrichment and vegetation loss altered the community composition of common and rare bacteria and fungi, except for the lack of response of common soil fungi to the vegetation loss. Moreover, both structural equation modelling and variation partitioning analyses show that land degradation drivers reduce SOM decomposition, and these were also indirectly associated with changes in the diversity of rare microbial taxa, especially that of bacteria. Synthesis and applications. Collectively, this work shows that land degradation can have negative impacts on soil biodiversity and functions, and the rare microbial taxa indirectly regulate the impacts of land degradation on ecosystem functioning. These results indicate that the rare microbial taxa can be used as one of the ecological indicators for identifying land degradation in the semi-arid grasslands. These findings are essential to understand the future impacts of desertification and land degradation on rare microbial taxa–function relationships in global drylands