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

Abstract

We hypothesized that changes in plant growth resulting from atmospheric CO 2 and O 3 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 CO 2 and O 3 enrichment site in Rhinelander, Wisconsin. We added either 13 C-labeled cellobiose or 13 C-labeled N- acetylglucosamine to soils collected beneath ecologically distinct temperate trees exposed for 3 years to factorial CO 2 (ambient and 200 µl l –1 above ambient) and O 3 (ambient and 20 µl l –1 above ambient) treatments. For both labeled substrates, recovery of 13 C in microbial respiration increased beneath plants grown under elevated CO 2 by 29% compared to ambient; elevated O 3 eliminated this effect. Production of 13 C-CO 2 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 ( 13 C-PLFAs) indicated that the microbial community beneath plants exposed to elevated CO 2 metabolized more 13 C-cellobiose, compared to the microbial community beneath plants exposed to the ambient condition. Recovery of 13 C 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 CO 2 increased fungal activity and microbial metabolism of cellobiose, and that microbial processes under early-successional aspen and birch species were more strongly affected by CO 2 and O 3 enrichment than those under late-successional maple.