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.

Results/Conclusions

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 CO_2. 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.