Microbial community composition and function beneath temperate trees exposed to elevated atmospheric carbon dioxide and ozone

We hypothesized that changes in plant growth resulting from atmospheric CO₂ and O₃ enrichment would alter the flow of C through soil food webs and that this effect would vary with tree species. To test this idea, we traced the course of C through the soil microbial community using soils from the free-air CO₂ and O₃ enrichment site in Rhinelander, Wisconsin. We added either ¹³C-labeled cellobiose or ¹³C-labeled N-acetylglucosamine to soils collected beneath ecologically distinct temperate trees exposed for 3 years to factorial CO₂ (ambient and 200 µl l^-1 above ambient) and O₃ (ambient and 20 µl l^-1 above ambient) treatments. For both labeled substrates, recovery of ¹³C in microbial respiration increased beneath plants grown under elevated CO₂ by 29% compared to ambient; elevated O₃ eliminated this effect. Production of ¹³C-CO₂ from soils beneath aspen (Populus tremuloides Michx.) and aspen-birch (Betula papyrifera Marsh.) was greater than that beneath aspen-maple (Acer saccharum Marsh.). Phospholipid fatty acid analyses (¹³C-PLFAs) indicated that the microbial community beneath plants exposed to elevated CO₂ metabolized more ¹³C-cellobiose, compared to the microbial community beneath plants exposed to the ambient condition. Recovery of ¹³C in PLFAs was an order of magnitude greater for N-acetylglucosamine-amended soil compared to cellobiose-amended soil, indicating that substrate type influenced microbial metabolism and soil C cycling. We found that elevated CO₂ increased fungal activity and microbial metabolism of cellobiose, and that microbial processes under early-successional aspen and birch species were more strongly affected by CO₂ and O₃ enrichment than those under late-successional maple.