Influence of an Elevated Atmospheric CO₂ Content on Soil and Rhizosphere Bacterial Communities Beneath <i>Lolium perenne</i> and <i>Trifolium repens</i> under Field Conditions

The increase in atmospheric CO₂ content alters C₃ plant photosynthetic rate, leading to changes in rhizodeposition and other root activities. This may influence the activity, the biomass, and the structure of soil and rhizosphere microbial communities and therefore the nutrient cycling rates and the plant growth. The present paper focuses on bacterial numbers and on community structure. The rhizospheres of two grassland plants, <i>Lolium perenne</i> (ryegrass) and <i>Trifolium repens</i> (white clover), were divided into three fractions: the bulk soil, the rhizospheric soil, and the rhizoplane–endorhizosphere. The elevated atmospheric CO₂ content increased the most probable numbers of heterotrophic bacteria in the rhizosphere of <i>L. perenne</i>. However, this effect lasted only at the beginning of the vegetation period for <i>T. repens</i>. Community structure was assessed after isolation of DNA, PCR amplification, and construction of cloned 16S rDNA libraries. Amplified ribosomal DNA restriction analysis (ARDRA) and colony hybridization with an oligonucleotide probe designed to detect <i>Pseudomonas</i> spp. showed under elevated atmospheric CO₂ content an increased dominance of pseudomonads in the rhizosphere of <i>L. perenne</i> and a decreased dominance in the rhizosphere of <i>T. repens</i>. This work provides evidence for a CO₂-induced alteration in the structure of the rhizosphere bacterial populations, suggesting a possible alteration of the plant-growth-promoting-rhizobacterial (PGPR) effect.