Global Change Ecology

Atmospheric CO2 and O3 concentrations are increasing due to human activity and both trace gases have the potential to alter C cycling in forest ecosystems. Because soil microorganisms depend on plant litter as a source of energy for metabolism, changes in the amount or the biochemistry of plant litter produced under elevated CO2 and O3 could alter microbial community function and composition. Previously, we have observed that elevated CO2 increased the microbial metabolism of cellulose and chitin, whereas elevated O3 dampened this response. We hypothesized that this change in metabolism under CO2 and O3 enrichment would be accompanied by a concomitant change in fungal community composition. We tested our hypothesis at the free-air CO2 and O3 enrichment (FACE) experiment at Rhinelander, Wisconsin, in which Populus tremuloides, Betula papyrifera, and Acer saccharum were grown under factorial CO2 and O3 treatments. We employed extracellular enzyme analysis to assay microbial metabolism, phospholipid fatty acid (PLFA) analysis to determine changes in microbial community composition, and polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE) to analyze the fungal community composition. The activities of 1,4-b-glucosidase (+37%) and 1,4,-b-N-acetylglucosaminidase (+84%) were significantly increased under elevated CO2, whereas 1,4-b-glucosidase activity ( 25%) was significantly suppressed by elevated O3. There was no significant main effect of elevated CO2 or O3 on fungal relative abundance, as measured by PLFA. We identified 39 fungal taxonomic units from soil using DGGE, and found that O3 enrichment significantly altered fungal community composition. We conclude that fungal metabolism is altered under elevated CO2 and O3, and that there was a concomitant change in fungal community composition under elevated O3. Thus, changes in plant inputs to soil under elevated CO2 and O3 can propagate through the microbial food web to alter the cycling of C in soil.