Plant diversity and functional identity drive grassland rhizobacterial community responses after 15 years of CO₂ and nitrogen enrichment

Improved understanding of bacterial community responses to multiple environmental filters over long time periods is a fundamental step to develop mechanistic explanations of plant–bacterial interactions as environmental change progresses. This is the first study to examine responses of grassland root-associated bacterial communities to 15 years of experimental manipulations of plant species richness, functional group and factorial enrichment of atmospheric CO₂ (eCO₂) and soil nitrogen (+N). Across the experiment, plant species richness was the strongest predictor of rhizobacterial community composition, followed by +N, with no observed effect of eCO₂. Monocultures of C₃ and C₄ grasses and legumes all exhibited dissimilar rhizobacterial communities within and among those groups. Functional responses were also dependent on plant functional group, where N₂-fixation genes, NO₃₋-reducing genes and P-solubilizing predicted gene abundances increased under resource-enriched conditions for grasses, but generally declined for legumes. In diverse plots with 16 plant species, the interaction of eCO₂+N altered rhizobacterial composition, while +N increased the predicted abundance of nitrogenase-encoding genes, and eCO₂+N increased the predicted abundance of bacterial P-solubilizing genes. Synthesis: Our findings suggest that rhizobacterial community structure and function will be affected by important global environmental change factors such as eCO₂, but these responses are primarily contingent on plant species richness and the selective influence of different plant functional groups.