Effects of Elevated Atmospheric CO(2) on Rhizosphere Soil Microbial Communities in a Mojave Desert Ecosystem

The effects of elevated atmospheric carbon dioxide CO₂] on microbial communities in arid rhizosphere soils beneath Larrea tridentata were examined. Roots of Larrea were harvested from plots fumigated with elevated or ambient levels of CO₂] using Free-Air CO₂ Enrichment (FACE) technology. Twelve bacterial and fungal rRNA gene libraries were constructed, sequenced and categorized into operational taxonomical units (OTUs). There was a significant decrease in OTUs within the Firmicutes (bacteria) in elevated CO₂], and increase in Basiomycota (fungi) in rhizosphere soils of plots exposed to ambient CO₂]. Phylogenetic analyses indicated that OTUs belonged to a wide range of bacterial and fungal taxa. To further study changes in bacterial communities, Quantitative Polymerase Chain Reaction (QPCR) was used to quantify populations of bacteria in rhizosphere soil. The concentration of total bacteria 16S rDNA was similar in conditions of enriched and ambient CO₂]. However, QPCR of Gram-positive microorganisms showed a 43% decrease in the population in elevated CO₂]. The decrease in representation of Gram positives and the similar values for total bacterial DNA suggest that the representation of other bacterial taxa was promoted by elevated CO₂]. These results indicate that elevated CO₂] changes structure and representation of microorganisms associated with roots of desert plants.