Shifts in microbial diversity through land use intensity as drivers of C-cycling in soil

It is well established that land use intensification alters the biomass and structure of soil microbial communities. The impact of land use intensity on soil microbial diversity (i.e. richness and evenness) and consequences for functioning, however, is poorly understood. Here, we coupled molecular characterizations of microbial diversity with measurements of C mineralization on soils obtained from three locations across Europe each representing a gradient of land use intensities under various soil and environmental conditions. Bacterial and fungal diversity were characterized by high throughput sequencing of ribosomal genes. C-cycling activities (i.e., native soil organic matter, plant residues mineralization, priming effect) were measured by quantifying 12C- and 13C-CO2 release after soils had been amended, or not, with 13C-labelled wheat residues. Variation partitioning analysis was used to rank biological and physicochemical soil parameters according to their relative contribution to these activities. Across all three locations, microbial diversity was greatest at intermediate levels of land use intensity, suggestive of a hump-shaped relationship between microbial diversity and land use intensity. This relationship indicates that optimal management of soil microbial diversity might not be achieved under the least intensive agriculture. Microbial richness was the best predictor of C-cycling activities, explaining 42, 50 and 80% of the variation in basal, total and residue mineralization, respectively, and the relative involvement of bacterial vs. fungal diversity varied according to the activity. Altogether, our study provides evidence that there is scope for changes in land use management to benefit microbial diversity with reward for carbon cycling in soil.