The oxygen isotope composition of CO2 in the atmosphere is strongly influenced by the oxygen isotope exchange of CO2 with leaf and soil water pools across the Earth’s surface. Until recently it was thought that this isotopic exchange was considerably slower in soils compared with leaves. However field estimates have shown that soil isotopic exchange may be up to 1000 times faster than previously estimated for some ecosystems, because of the enzyme carbonic anhydrase. In this talk we will present seasonal gas exchange observations from a pine forest in South West France that demonstrate the important contribution of the soil oxygen isotope signal at the ecosystem scale. We found that over the entire season the oxygen isotope signal of ecosystem CO2 exchange, derived from nocturnal Keeling plots, was most similar to the oxygen isotope composition of the soil CO2 efflux. Using the isotope-enabled ecosystem model MuSICA, and the isotopic signals observed in leaf, stem and soil chambers we could estimate the nocturnal flux rate contribution of each component to the net ecosystem exchange over the season. In addition we investigated the temporal activity of soil carbonic anhydrase activity over the growing season and to what extent it varied with soil surface CO2 concentration, temperature and soil moisture. Conclusions from this analysis will be presented and recommendations for future experimental studies made.