"Quantifying the carbon storage potential of terrestrial ecosystems and its sensitivity to climate change relies on our ability to obtain observational constraints on photosynthesis and respiration at large scales. Photosynthesis (GPP), the largest CO2 flux from the land surface, is currently estimated with considerable uncertainty. A recent estimate of global GPP was based on an atmospheric budget of the oxygen isotope composition (d18O) of atmospheric CO2 that strongly relies on the oxygen isotope exchange rates with leaf and soil water pools. This isotopic exchange is rapidly catalysed by carbonic anhydrase (CA) in leaves and to a lesser extent in soils. Soil CA activity was neglected in global CO18O studies until the project PI showed recently that CA activity in soils played an important role for determining the magnitude of global GPP using CO18O. The overall goal of SOLCA is to understand better the environmental and ecological causes behind the variability in CA activity observed in soils. A first hypothesis is that soil CA activity responds to thermal and osmotic stresses. This will be tested by probing CA activity of soil monoliths from around the world using a non-invasive gas exchange technique developed by the PI. Because probing soil CA activity from CO18O gas exchange data requires a knowledge of the d18O of soil water and CO2 diffusion processes, we will utilise additional tracers of CA activity: CO17O and carbonyl sulphide (COS) that will also be measured as they follow the same diffusional pathway as CO2 and are also taken up by CA. A second hypothesis is that soil CA activity can be predicted knowing only global indices of the soil microbial community. This will be tested using state-of-the-art molecular techniques to explain changes in CA activity levels. This project will construct novel algorithms for using additional tracers of the global CO2 budget and will lead to a revised estimate of terrestrial GPP."