Drivers of the Composition of Bulk Soil and Root-Associated Bacterial Communities across a Plant Diversity Gradient

Plant diversity is an important determinant of ecosystem functioning, enhancing its productivity, and is tightly interlinked with the composition and functioning of soil microbiota, which in turn mediate multiple ecosystem processes, such as decomposition of organic matter, carbon sequestration and nutrient cycling. The mechanisms of plant diversity effects on soil microorganisms are not fully understood, increased litter input, altered root exudation and/or soil chemistry are believed to be possible drivers. Although it has been clearly demonstrated that plant species differ in their root-associated microbiomes, it is unclear how these are affected in natural environments where soil microbes are under influence of multiple plant species. Here we studied the effects of three different plant species on their soil bacterial communities along a diversity gradient using 16S rRNA Illumina sequencing. For each plant species, we established 30 plots (ø 40 cm) covering three adjacent localities with plant diversity increasing from very low to very high, where we determined plant community composition, soil pH and nutrient content, shoot and root biomass of the studied plant and the composition of its bulk soil and root-associated bacterial community. Bacterial communities in the bulk soil were strongly shaped by soil pH and nitrogen content, only weakly by plant diversity and not at all by plant species identity. Conversely, the diversity of surrounding vegetation followed by plant species identity were the best predictors of root-associated bacterial communities. Our results suggest that the influence of individual plant, possibly mediated by root exudates or root litter, is restricted to the roots and their immediate surrounding, whereas bacterial communities in the bulk soil are mainly shaped by soil chemistry.