SOIL MICROBIAL COMMUNITIES BENEATH POPULUS-GRANDIDENTATA CROWN UNDER ELEVATED ATMOSPHERIC CO2

In most terrestrial ecosystems, the amount of substrate entering the s oil from plant litter production is only sufficient to meet the mainte nance requirements of soil microorganisms, allowing for no net annual growth. However, the rising atmospheric CO2 concentration has the pote ntial to alter such a balance by increasing plant litter production, a nd hence the amount of substrate available for heterotrophic metabolis m in soil. In a recent experiment, we observed that greater belowgroun d plant litter production at elevated atmospheric CO2 significantly in creased the biomass of soil microorganisms in both rhizosphere and non -rhizosphere soil. Because soil microorganisms differ in their ability to convert substrate into biomass, we hypothesized that greater plant litter production at elevated CO2 should shift community composition as fungal populations increase in response to greater substrate availa bility. We used a molecular technique, phospholipid fatty acid (PLFA) analysis, to gain insight into the composition of soil microbial commu nities beneath Populus grandidentata growing at ambient and twice-ambi ent atmospheric CO2. PLFAs extracted from rhizosphere and non-rhizosph ere soil were derivatized and identified using gas chromatography and mass spectrometry. After one growing season the proportions of bacteri al, actinomycetal, and fungal PLFAs were not significantly influenced by elevated atmospheric CO2 in either rhizosphere or non-rhizosphere s oil. However, clear differences were present between microbial communi ties in rhizosphere and non-rhizosphere soil. Although enhanced belowg round plant litter production under elevated atmospheric CO2 increased the biomass of soil microorganisms, we have no evidence to suggest th at such an increase occurred through a shift in community composition, at least in the short term.