Concentrations of atmospheric CO2 and tropospheric O3 will continue to increase during this century and are likely to alter terrestrial C cycling. We sampled the top 20 cm of the mineral soil in three northern temperate forest communities at the AspenFACE site over an 11-year fumigation with elevated CO2 (520 μl l-1) and/or elevated O3 (55 nl l-1) in order to understand dynamics in this important terrestrial C pool. In addition, because the CO2 fumigation was conducted with 13C-depleted CO2, we were able to monitor the effect of ambient and elevated O3 on the formation of new soil C.
After 11 years, there was no significant main effect of CO2 or O3 on the total soil C pool. However, within the community containing only Populus tremuloides Michx., elevated CO2 caused a significant decrease in soil C. Together with observations in other studies of increased litter inputs at this site, this suggests decomposition has increased under elevated CO2. In addition, by the end of the 11-year study period 20-30% of the C in the soil was isotopically “new”. Elevated O3 initially reduced the formation of new soil C under elevated CO2, but this effect gradually disappeared until the pools of new C in the two O3 treatments were equal. The trends in new soil C formation under elevated CO2 and O3 mirrored those in fine root biomass. Both main results are contrary to predictions, highlighting the need for future research.