Elevated air carbon dioxide concentrations increase dissolved carbon leaching from a cropland soil

Increasing leaching losses of carbon from soils due to accelerated weathering and increasing concentrations of dissolved carbon as a result of intensified soil respiration are suspected to provide a negative feedback on rising atmospheric CO2 concentrations. We tested this hypothesis by studying concentrations of dissolved carbon and groundwater recharge at the Braunschweig free air carbon dioxide enrichment (FACE) experiment under winter wheat and winter barley. Dissolved carbon concentrations under elevated atmospheric CO2 and ambient conditions were rather similar and not consistently higher under FACE. An analysis of d13C signatures suggested that dissolved organic and inorganic carbon contained 9–29% (DOC) and 26–49% (DIC) of ‘‘new’’ carbon originating from CO2 added to the FACE rings. Dissolved inorganic carbon additionally contained 15–42% of carbonate-derived C. A 15% reduction in evapotranspiration under elevated CO2 increased groundwater recharge by 60 mm or 55%, which was the main driver for an observed 81% increase in dissolved carbon leaching from 2.7 to 4.9 g C m-2 year-1 at 90 cm depth. Our results suggest that future changes of dissolved carbon leaching losses will be mainly governed by changes in climate and groundwater recharge and to a lesser extent by increasing dissolved carbon concentrations.