Contributions of carbonates to soil CO2 emissions

Ramnarine, R., Wagner-Riddle, C., Dunfield, K. E. and Voroney, R. P. 2012. Contributions of carbonates to soil CO2 emissions. Can. J. Soil Sci. 92: 599-607. Carbon dioxide (CO2) is released in soil as a by-product of microbial and root respiration, but soil carbonates may also be a source of CO2 emissions in calcareous soils. Global estimates of inorganic carbon range from 700 to 900 Pg as carbonates stored in soils, representing a significant potential source of CO2 to the atmosphere. While previous studies have focused on the total CO2 efflux from the soil, our goal was to identify the various sources and their contribution to total CO2 emissions, by measuring the isotopic signature of the CO2 emitted from the soil. Calcareous Luvisolic silt loam soil samples were obtained from conventional tillage (CT) and no-tillage (NT) plots in southern Ontario, Canada. Soil samples (root- and residue-free) were laboratory-incubated for 14 d and the isotopic signature of the CO2 (delta C-13(CO2)) released was analyzed using isotope ratio mass spectrometry. Isotopic measurement was essential in quantifying the abiotic CO2 production from carbonates, due to the unique delta C-13 signature of carbonates and soil organic matter. A two-end member mixing model was used to estimate the proportion of CO2 evolved from soil carbonates and soil organic matter decomposition. Analysis of emitted CO2 collected after the 14-d incubation indicate that the proportion of CO2 originating from soil inorganic carbon was 62 to 74% for CT soil samples, and 64 to 80% for NT soil samples. Further work is recommended in the quantification of CO2 emissions from calcareous soils, and to determine the transferability of laboratory results to field studies.