Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ17O of atmospheric CO2

Understanding the processes that affect the triple oxygen isotope composition of atmospheric CO2 during gas exchange can help constrain the interaction and fluxes between the atmosphere and the biosphere. We conducted leaf cuvette experiments under controlled conditions using three plant species. The experiments were conducted at two different light intensities and using CO2 with different Delta O-17. We directly quantify the effect of photosynthesis on Delta O-17 of atmospheric CO2 for the first time. Our results demonstrate the established theory for delta O-18 is applicable to Delta O-17 (CO2) at leaf level, and we confirm that the following two key factors determine the effect of photosynthetic gas exchange on the Delta O-17 of atmospheric CO2. The relative difference between Delta O-17 of the CO2 entering the leaf and the CO2 in equilibrium with leaf water and the back-diffusion flux of CO2 from the leaf to the atmosphere, which can be quantified by the c(m)/c(a) ratio, where c(a) is the CO2 mole fraction in the surrounding air and c m is the one at the site of oxygen isotope exchange between CO2 and H2O. At low c(m)/c(a) ratios the discrimination is governed mainly by diffusion into the leaf, and at high c(m)/c(a) ratios it is governed by back-diffusion of CO2 that has equilibrated with the leaf water. Plants with a higher c(m)/c(a) ratio modify the Delta O-17 of atmospheric CO2 more strongly than plants with a lower c(m)/c(a) ratio. Based on the leaf cuvette experiments, the global value for discrimination against Delta O-17 of atmospheric CO2 during photosynthetic gas exchange is estimated to be -0.57 +/- 0.14 parts per thousand using c(m)/c(a) values of 0.3 and 0.7 for C-4 and C-3 plants, respectively. The main uncertainties in this global estimate arise from variation in c(m)/c(a) ratios among plants and growth conditions.