Contribution of carbonate weathering to the CO₂ efflux from temperate forest soils

Temperate forests provide favorable conditions for carbonate bedrock weathering as the soil CO₂ partial pressure is high and soil water is regularly available. As a result of weathering, abiotic CO₂ can be released and contribute to the soil CO₂ efflux. We used the distinct isotopic signature of the abiotic CO₂ to estimate its contribution to the total soil CO₂ efflux. Soil cores were sampled from forests on dolomite and limestone and were incubated under the exclusion of atmospheric CO₂. Efflux and isotopic signatures of CO₂ were repeatedly measured of cores containing the whole mineral soil and bedrock material (heterotrophic respiration + CO₂ from weathering) and of cores containing only the mineral top-soil layer (A-horizon; heterotrophic respiration). An aliquot of the cores were let dry out during incubation to assess effects of soil moisture. Although the δ¹³C values of the CO₂ efflux from the dolomite soil cores were within a narrow range (A-horizon -26.2 ± 0.1 ‰; whole soil profile wet -25.8 ± 0.1 ‰; whole soil profile dry -25.5 ± 0.1 ‰) the CO₂ efflux from the separated A-horizons was significantly depleted in ¹³C when compared to the whole soil profiles (p = 0.015). The abiotic contribution to the total CO₂ efflux from the dolomite soil cores was 2.0 ± 0.5 % under wet and 3.4 ± 0.5 % under dry conditions. No abiotic CO₂ efflux was traceable from the limestone soil cores. An overall low contribution of CO₂ from weathering was affirmed by the amount and ¹³C signature of the leached dissolved inorganic carbon (DIC) and the radiocarbon signature of the soil CO₂ efflux in the field. Together, our data point towards no more than 1-2 % contribution of abiotic CO₂ to the growing season soil CO₂ efflux in the field.