Soil CO2 dynamics, acidification, and chemical weathering in a temperate forest with experimental CO2 enrichment

Soils constitute a major component of the global carbon cycle that will be affected by anthropogenic additions of CO 2 to the atmosphere. As part of the Duke Forest Free‐Air CO 2 Enrichment (FACE) experiment, we examined how forest growth at elevated (+200 ppmv) atmospheric CO 2 concentration affects CO 2 dynamics in the soil. Soil respiration and the concentration of CO 2 in the soil pore space to a depth of 200 cm were measured over a 3‐year period. Soil CO 2 production was linked to soil acidification and mineral weathering by measuring changes in the composition of the soil solution, including alkalinity, Si, and major cations. The total flux of dissolved inorganic carbon to groundwater was then calculated from field measurements. The FACE fumigation gas contained a unique 13 C signature that labeled newly fixed carbon, which was monitored in the soil system. As a result of CO 2 enrichment, annual soil respiration increased by 27% and was accompanied by higher CO 2 concentrations in the soil pore space. These changes to soil CO 2 dynamics were most likely the result of increased root and rhizosphere respiration, as suggested by the changes to the δ 13 C of soil CO 2 . Increased soil CO 2 under FACE accelerated the rates of soil acidification and mineral weathering. Thus an increase of 55% in atmospheric CO 2 concentration over 2 years resulted in a 271% increase in soil solution cation concentration, a 162% increase in alkalinity, and a 25% increase in Si concentration at 200–cm depth. The flux of dissolved inorganic carbon to groundwater increased by 33%, indicating a negative feedback to changes in atmospheric CO 2 that could regulate the global carbon cycle over geological time.