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 CO2 to the atmosphere. As part of th e Duke Forest Free-Air CO2 Enrichment (FACE) experiment, we examined how fo rest growth at elevated (+200 ppmv) atmospheric CO2 concentration affects C O2 dynamics in the soil. Soil respiration and the concentration of CO2 the soil pore space to a depth of 200 cm were measured over a 3-year period. So il CO2 production was linked to soil acidification and mineral weathering b y measuring changes in the composition of the soil solution, including alka linity, Si, and major cations. The total flux of dissolved inorganic carbon to groundwater was then calculated from field measurements. The FACE fumig ation gas contained a unique C-13 signature that labeled newly fixed carbon , which was monitored in the soil system. As a result of CO2 enrichment, an nual soil respiration increased by 27% and was accompanied by higher CO2 co ncentrations in the soil pore space. These changes to soil CO2 dynamics wer e most likely the result of increased root and rhizosphere respiration, as suggested by the changes to the delta C-13 of soil CO2. Increased soil CO2 under FACE accelerated the rates of soil acidification and mineral weatheri ng. Thus an increase of 55% in atmospheric CO2 concentration over 2 years r esulted in a 271% increase in soil solution cation concentration, a 162% in crease in alkalinity and a 25% increase in Si concentration at 200-cm depth . The flux of dissolved inorganic carbon to groundwater increased by 33%, i ndicating a negative feedback to changes in atmospheric CO2 that could regu late the global carbon cycle over geological time.