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. Understanding the processes that affect the triple oxygen isotopecomposition of atmospheric CO2 during gas exchange can help constrainthe interaction and fluxes between the atmosphere and the biosphere. Weconducted leaf cuvette experiments under controlled conditions using threeplant species. The experiments were conducted at two different lightintensities and using CO2 with different Δ17O. We directlyquantify the effect of photosynthesis on Δ17O of atmospheric CO2 for the first time. Our results demonstrate the establishedtheory for δ18O is applicable to Δ17O(CO2) atleaf level, and we confirm that the following two key factors determine the effect ofphotosynthetic gas exchange on the Δ17O of atmosphericCO2. The relative difference between Δ17O of the CO2entering the leaf and the CO2 in equilibrium with leaf water and theback-diffusion flux of CO2 from the leaf to the atmosphere, which canbe quantified by the cm∕ca ratio, where ca is the CO2 molefraction in the surrounding air and cm is the one at the site of oxygenisotope exchange between CO2 and H2O. At low cm∕ca ratiosthe discrimination is governed mainly by diffusion into the leaf, and athigh cm∕ca ratios it is governed by back-diffusion of CO2 that has equilibratedwith 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 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.

biogeosciences

Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ&amp;lt;sup&amp;gt;17&amp;lt;/sup&amp;gt;O of atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;