Nearly all of earth's plant life responds favorably to increases in the air's CO2 content by exhibiting enhanced rates of photosynthesis and biomass production. Consequently, these phenomena tend to increase soil carbon contents by increasing root exudation of organic compounds and the amount of plant litter returned to the soil. Thus, one might expect CO2-mediated increases in soil carbon content to impact soil bacterial communities. In this summary, we review the findings of several papers that have studied the effects of elevated CO2 on bacteria.
In a FACE experiment executed for two years on monocultures of ryegrass (Lolium perenne) and white clover (Trifolium repens), Marilley et al. (1999) reported that an atmospheric CO2 concentration of 600 ppm did not impact the total number of bacteria present in bulk soil beneath either species of plant. However, it tended to increase bacterial numbers in the rhizosphere, which consists of soil in close proximity to plant roots. In addition, atmospheric CO2 enrichment altered the profile of bacterial communities in a plant species-dependent manner. In ryegrass, for example, elevated CO2 increased the dominance of Pseudomonas species, which enhance plant growth by many different mechanisms; while in white clover, it increased the dominance of Rhizobium species, which enhance plant growth by making atmospheric nitrogen available for their utilization.
After three-years of differential CO2 treatment in this same FACE experiment, Montealegre et al. (2000) determined the genetic structure of 120 isolates of the symbiotic bacterium Rhizobium leguminosarum associated with roots of white clover. They determined that atmospheric CO2 enrichment favored some of these isolates over others. Moreover, when these isolates were mixed with isolates favored by the ambient CO2 concentration, and when the resulting combination was exposed to elevated CO2 concentrations, the isolates favored by elevated CO2 produced 17% more nodules on roots than isolates favored by ambient CO2 concentrations.
Finally, in an unrelated study, Zak et al. (2000) found that elevated CO2 had no effect on soil microbial biomass, microbial nitrogen mineralization rates, or microbial community composition beneath aspen cuttings exposed to 700 ppm CO2 for two-and-a-half years.
These observations suggest that rising atmospheric CO2 levels will likely allow greater numbers of bacteria to exist within the rhizosphere, most likely due to greater CO2-induced increases in soil carbon contents. They also suggest that CO2-induced shifts in rhizosphere bacterial community composition may occur, possibly helping to optimize plant nutrient acquisition in a species-dependent manner.
References
Marilley, L., Hartwig, U.A. and Aragno, M. 1999. Influence of an elevated atmospheric CO2 content on soil and rhizosphere bacterial communities beneath Lolium perenne and Trifolium repens under field conditions. Microbial Ecology 38: 39-49.
Montealegre, C.M., Van Kessel, C., Blumenthal, J.M., Hur, H.G., Hartwig, U.A. and Sadowsky, M.J. 2000. Elevated atmospheric CO2 alters microbial population structure in a pasture ecosystem. Global Change Biology 6: 475-482.
Zak, D.R., Pregitzer, K.S., Curtis, P.S. and Holmes, W.E. 2000. Atmospheric CO2 and the composition and function of soil microbial communities. Ecological Applications 10: 47-59.