Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ 17 O of atmospheric CO 2

Abstract. 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 17O-excess. The oxygen isotope composition of CO2 was used to estimate cm, the mole fraction of CO2 at the CO2-H2O exchange site. Our results demonstrate that two key factors determine the effect of gas exchange on the Δ17O of atmospheric CO2. The relative difference between Δ17O 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 cm/ca ratio where ca is the CO2 mole fraction in the surrounding air. At low cm/ca ratio the discrimination is governed mainly by diffusion into the leaf, and at high cm/ca ratio by back-diffusion of CO2 that has equilibrated with the leaf water. Plants with a higher cm/ca ratio modify the Δ17O of atmospheric CO2 more strongly than plants with a lower cm/ca ratio. Based on the leaf cuvette experiments, the global value for discrimination against Δ17O of atmospheric CO2 during the photosynthetic gas exchange is estimated to be −0.57+/−0.14 ‰ using cm/ca values of 0.3 and 0.7 for C4 and C3 plants, respectively. The main uncertainties in this global estimate arise from variation in cm/ca ratios among plants and growth conditions.