Elevated atmospheric CO2 alters microbial population structure in a pasture ecosystem

An increase in concentration of atmospheric CO₂ is one major factor influencing global climate change. Among the consequences of such an increase is the stimulation of plant growth and productivity. Below‐ground microbial processes are also likely to be affected indirectly by rising atmospheric CO₂ levels, through increased root growth and rhizodeposition rates. Because changes in microbial community composition might have an impact on symbiotic interactions with plants, the response of root nodule symbionts to elevated atmospheric CO₂ was investigated. In this study we determined the genetic structure of 120 Rhizobium leguminosarum bv. trifolii isolates from white clover plants exposed to ambient (350 μmol mol^?1) or elevated (600 μmol mol^?1) atmospheric CO₂ concentrations in the Swiss FACE (Free‐Air‐Carbon‐Dioxide‐Enrichment) facility. Polymerase Chain Reaction (PCR) fingerprinting of genomic DNA showed that the isolates from plants grown under elevated CO₂ were genetically different from those isolates obtained from plants grown under ambient conditions. Moreover, there was a 17% increase in nodule occupancy under conditions of elevated atmospheric CO₂ when strains of R. leguminosarum bv. trifolii isolated from plots exposed to CO₂ enrichment were evaluated for their ability to compete for nodulation with those strains isolated from ambient conditions. These results indicate that a shift in the community composition of R. leguminosarum bv. trifolii occurred as a result of an increased atmospheric CO₂ concentration, and that elevated atmospheric CO₂ affects the competitive ability of root nodule symbionts, most likely leading to a selection of these particular strains to nodulate white clover.