We repeatedly sampled the surface mineral soil (0-20 cm depth) in three northern temperate forest communities over an 11-year experimental fumigation to understand the effects of elevated carbon dioxide (CO(2)) and/or elevated phyto-toxic ozone (O(3)) on soil carbon (C). After 11 years, there was no significant main effect of CO(2) or O(3) on soil C. However, within the community containing only aspen (Populus tremuloides Michx.), elevated CO(2) caused a significant decrease in soil C content. Together with the observations of increased litter inputs, this result strongly suggests accelerated decomposition under elevated CO(2.) In addition, an initial reduction in the formation of new (fumigation-derived) soil C by O(3) under elevated CO(2) proved to be only a temporary effect, mirroring trends in fine root biomass. Our results contradict predictions of increased soil C under elevated CO(2) and decreased soil C under elevated O(3) and should be considered in models simulating the effects of Earth's altered atmosphere.