Contribution of carbonate weathering to the CO efflux from temperate forest soils.

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