Changes to belowground carbon dioxide dynamics under experimental CO(2) enrichment of a forest ecosystem

Forest soils constitute a major component of the global carbon cycle. It is likely that increasing atmospheric CO2 as a result of human activity is having a significant effect on soil carbon cycling, and may result in the formation of a long-term sink for excess atmospheric carbon. As part of the Duke Forest Free-Air CO2 Enrichment (FACE) experiment, the effect of a 50% increase in atmospheric CO2 concentration on CO2 dynamics in a loblolly pine (Pinus taeda L.) forest soil was examined.; Soil respiration and the concentration of CO2 in the soil pore space to a depth of 200 cm were measured over a three year period. The d13C of organic carbon in this soil and of soil-respired CO2 was monitored and used to determine the origin of the CO2 in the soil system. Soil solution chemistry, including total cation concentration and alkalinity, was measured and used to calculate the total flux of dissolved inorganic carbon to groundwater. The size and isotopic composition of labile soil carbon pools in the organic horizon and in the surface mineral soil were determined throughout the experiment.; The experimental increase in atmospheric CO2 caused total annual carbon flux from the soil to the atmosphere to increase by 27% and the instantaneous rate of soil respiration to increase by up to 131%. The greater soil respiration was accompanied by higher CO2 concentrations in the soil pore space which, in turn, accelerated the rates of soil acidification and mineral weathering. In soil solution from 200-cm depth, cation concentrations increased by 286% and alkalinity increased by 164%. As a result, there was a 33% greater flux of dissolved inorganic carbon to groundwater under high atmospheric CO2. These changes to soil CO2 dynamics were attributed to increased root and rhizosphere respiration. A shift in the d13C of soil CO2 indicated that root activity contributed 55% of soil respiration at the site and decreased with increasing soil depth. The d13C of soil-respired CO2 was also found be to affected by soil temperature. There was no detectable change to the size of the organic horizon or the pool of labile carbon in the mineral soil. The change in the d13C of these pools under FACE indicated that the mean residence time was 3 and 4 to 6 years, respectively. These results suggest that high atmospheric CO2 increases the fluxes of inorganic carbon in the soil system but may have relatively little effect on soil carbon storage.