Climate and soils together regulate photosynthetic carbon isotope discrimination within C-3 plants worldwide

Aim: Within C-3 plants, photosynthesis is a balance between CO2 supply from the atmosphere via stomata and demand by enzymes within chloroplasts. This process is dynamic and a complex but crucial aspect of photosynthesis. We sought to understand the spatial pattern in CO2 supply-demand balance on a global scale, via analysis of stable isotopes of carbon within leaves (C-13), which provide an integrative record of CO2 drawdown during photosynthesis. Location: Global. Time period: 1951-2011.Major taxa studied: Vascular plants. Methods: We assembled a database of leaf carbon isotope ratios containing 3,979 species-site combinations from across the globe, including 3,645 for C-3 species. We examined a wide array of potential climate and soil drivers of variation in C-13. Results: The strongest drivers of carbon isotope discrimination at the global scale included atmospheric pressure, potential evapotranspiration and soil pH, which explained 44% of the variation in C-13. Addition of eight more climate and soil variables (each explaining small but highly significant amounts of variation) increased the explained variation to 60%. On top of this, the largest plant trait effect was leaf nitrogen per area, which explained 11% of C-13 variation. Main conclusions: By considering variation in C-13 at a considerably larger scale than previously, we were able to identify and quantify key drivers in CO2 supply-demand balance previously unacknowledged. Of special note is the key role of soil properties, with greater discrimination on low-pH and high-silt soils. Unlike other plant traits, which show typically wide variation within sets of coexisting species, the global pattern in carbon stable isotope ratios is much more conservative; there is relatively narrow variation in time-integrated CO2 concentrations at the site of carboxylation among plants in a given soil and climate.