Measurement of ¹⁸O/¹⁶O in the soil‐atmosphere CO₂ flux

Measurements of 18 O in atmospheric CO 2 can be used to trace gross photosynthetic and respiratory CO 2 fluxes between the atmosphere and the terrestrial biosphere. However, this requires knowledge of the 18 O signatures attributable to the fluxes from soil and leaves. Newly developed methods were employed to measure the 18 O of soil‐respired CO 2 and depth profiles of near‐surface soil CO 2 , in order to evaluate the factors influencing isotopic soil‐atmosphere CO 2 exchange. The 18 O of soil‐respired CO 2 varied predominantly as a function of the 18 O of soil water which, in turn, changed with soil drying and with seasonal variations in source water. The 18 O of soil‐respired CO 2 corresponds to full isotopic equilibrium with soil water at a depth ranging between 5 and 15 cm. The 18 O of respired CO 2 , in reality, results from a weighted average of partial equilibria over a range of depths. Soil water isotopic enrichment of up to 10‰ in the top 5 cm did not appear to strongly influence the isotopic composition of the respired CO 2 . We demonstrate that during measurements “invasion” of atmospheric CO 2 (the diffusion of ambient CO 2 into the soil, followed by partial equilibration and retrodiffusion) must be considered to accurately calculate the 18 O of the soil‐respired CO 2 . The impact of invasion in natural settings is also considered. We also have determined the effective kinetic fractionation of CO 2 diffusion out of the soil to be 7.2 ± 0.3‰. High‐resolution (1 cm) depth profiles of 18 O of near‐surface (top 10 cm) soil CO 2 were carried out by gas chromatography—isotope ratio mass spectrometry (GC‐IRMS). This novel technique allowed us to observe the competitive diffusion‐equilibration process near the soil surface and to test simulations by a diffusion and equilibration model of the soil CO 2 18 O content.