Drivers of the composition of active rhizosphere bacterial communities in temperate grasslands

The active bacterial rhizobiomes and root exudateprofiles of phytometers of six plant species growing in central European temperate grassland communities were investigated in three regions located up to 700 km apart, across diverse edaphicconditions and along a strong land use gradient. The recruitment process from bulk soilcommunities was identified as the major direct driver of the composition of active rhizospherebacterial communities. Unexpectedly, the effect of soil properties, particularly soil texture, water content, and soil type, strongly dominated over plant properties and the composition of polar root exudatesof the primary metabolism. While plant species-specific selection of bacteria was minor, the RNA-based composition of active rhizospherebacteria substantially differed between rhizosphereand bulk soil. Although other variables could additionally be responsible for the consistent enrichment of particular bacteria in the rhizosphere, distinct bacterial OTUs were linked to the presence of specific polar root exudatesindependent of individual plant species. Our study also identified numerous previously unknown taxa that are correlated with rhizospheredynamics and hence represent suitable targets for future manipulations of the plant rhizobiome.