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

Measurements of O-18 in atmospheric CO2 can be used to trace gross photosyn thetic and respiratory CO2 fluxes between the atmosphere and the terrestria l biosphere. However, this requires knowledge of the O-18 signatures attrib utable to the fluxes from soil and leaves. Newly developed methods were emp loyed to measure the O-18 of soil-respired CO2 and depth profiles of near-s urface soil CO2 in order to evaluate the factors influencing isotopic soil- atmosphere CO2 exchange. The O-18 soil-respired CO2 varied predominantly as a function of the O-18 of soil water which, in turn, changed with soil dry ing and with seasonal variations in source water. The O-18 of soil-respired CO2 corresponds to full isotopic equilibrium with soil water at a depth ra nging between 5 and 15 cm. The O-18 of respired CO2, in reality, results fr om a weighted average of partial equilibria over a range of depths. Soil wa ter isotopic enrichment of up to 10 parts per thousand in the top 5 cm did not appear to strongly influence the isotopic composition of the respired C O2. We demonstrate that during measurements "invasion" of atmospheric CO2 ( the diffusion of ambient CO2 into the soil, followed by partial equilibrati on and retrodiffusion) must be considered to accurately calculate the O-18 of the soil-respired CO2. The impact of invasion in natural settings is als o considered. We also have determined the effective kinetic fractionation o f CO2 diffusion out of the soil to be 7.2 +/- 0.3 parts per thousand. High- resolution (1 cm) depth profiles of O-18 of near-surface (top 10 cm) soil C O2 were carried out by gas chromatography-isotope ratio mass spectrometry ( IRMS). This novel technique allowed us to observe the competitive diffusion -equilibration process near the soil surface and to test simulations by a d iffusion and equilibration model of the soil CO2 O-18 content.