Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ<sup>17</sup>O of atmospheric CO<sub>2</sub>

Abstract

<p><strong>Abstract.</strong> Understanding the processes that affect the triple oxygen isotope composition of atmospheric CO<sub>2</sub> 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 CO<sub>2</sub> with different <sup>17</sup>O-excess. The oxygen isotope composition of CO<sub>2</sub> was used to estimate cm, the mole fraction of CO<sub>2</sub> at the CO<sub>2</sub>-H<sub>2</sub>O exchange site. Our results demonstrate that two key factors determine the effect of gas exchange on the Δ<sup>17</sup>O of atmospheric CO<sub>2</sub>. The relative difference between Δ<sup>17</sup>O of the CO<sub>2</sub> entering the leaf and the CO<sub>2</sub> in equilibrium with leaf water, and the back-diffusion flux of CO<sub>2</sub> from the leaf to the atmosphere, which can be quantified by the cm/ca ratio where ca is the CO<sub>2</sub> 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 CO<sub>2</sub> that has equilibrated with the leaf water. Plants with a higher cm/ca ratio modify the Δ<sup>17</sup>O of atmospheric CO<sub>2</sub> more strongly than plants with a lower cm/ca ratio. Based on the leaf cuvette experiments, the global value for discrimination against Δ<sup>17</sup>O of atmospheric CO<sub>2</sub> during the photosynthetic gas exchange is estimated to be −0.57+/−0.14 ‰ using cm/ca values of 0.3 and 0.7 for C<sub>4</sub> and C<sub>3</sub> plants, respectively. The main uncertainties in this global estimate arise from variation in cm/ca ratios among plants and growth conditions.</p>