Global Change Ecology

Global emissions of atmospheric CO2 and tropospheric O3 are rising and expected to impact large areas of the Earth’s forests. While CO2 stimulates net primary production, O3 reduces photosynthesis, altering plant C allocation and reducing ecosystem C storage. The effects of multiple air pollutants can alter belowground C allocation, leading to changes in the partial pressure of CO2 (pCO2) in the soil, chemistry of dissolved inorganic carbonate (DIC) and the rate of mineral weathering. As this system represents a linkage between the longand short-term C cycles and sequestration of atmospheric CO2, changes in atmospheric chemistry that affect net primary production may alter the fate of C in these ecosystems. To date, little is known about the combined effects of elevated CO2 and O3 on the inorganic C cycle in forest systems. Free air CO2 and O3 enrichment (FACE) technology was used at the Aspen FACE project in Rhinelander, Wisconsin to understand how elevated atmospheric CO2 and O3 interact to alter pCO2 and DIC concentrations in the soil. Ambient and elevated CO2 levels were 360±16 and 542±81 ll l , respectively; ambient and elevated O3 levels were 33±14 and 49±24 nl l , respectively. Measured concentrations of soil CO2 and calculated concentrations of DIC increased over the growing season by 14 and 22%, respectively, under elevated atmospheric CO2 and were unaffected by elevated tropospheric O3. The increased concentration of DIC altered inorganic carbonate chemistry by increasing system total alkalinity by 210%, likely due to enhanced chemical weathering. The study also demonstrated the close coupling between the seasonal dC of soil pCO2 and DIC, as a mixing model showed that new atmospheric CO2 accounted for approximately 90% of the C leaving the system as DIC. This study illustrates the potential of using stable isotopic techniques and FACE technology to examine longand short-term ecosystem C sequestration.