A field-based method for simultaneous measurements of the ?¹⁸O and ?¹³C of soil CO₂ efflux

International audienceThree approaches for determining the stable isotopic composition (?13C and ?18O) of soil CO efflux were compared. A new technique employed mini-towers, constructed of open-topped piping, that were placed on the soil surface to collect soil-emitted CO2. Samples were collected along a vertical gradient and analyzed for CO2 concentration and isotopic composition. These data were then used to produce Keeling plots to determine the ?18O and ?13C of CO2 emitted from the soil. These results were then compared to the ?18O and ?13C of soil-respired CO2 measured with two other techniques: (1) flux chambers and (2) estimation from the application of the diffusional fractionation factor to measured values of below ground soil CO2 and to CO2 in equilibrium with soil water ?18O. Mini-tower ?18O Keeling plots were linear and highly significant (0.812 > 0.96), in contrast to chamber ?18O Keeling plots, which showed significant curvature, necessitating the use of a mass balance to calculate the ?18O of respired CO2. In the chambers, the values determined for the ?18O of soil respired CO2 approached the value of CO2 in equilibrium with surficial soil water, and the results were significantly ?18O enriched relative to the mini-tower results and the ?18O of soil CO2 efflux determined from soil CO2. There were close agreements between the three methods for the determination of the ?13C of soil efflux CO2. Results suggest that the mini-towers can be effectively used in the field for determining the ?18O and the ?13C of soil-respired CO2