Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ<sup>17</sup>O of atmospheric CO<sub>2</sub>
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Published:2020-07-31
Issue:14
Volume:17
Page:3903-3922
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ISSN:1726-4189
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Container-title:Biogeosciences
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language:en
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Short-container-title:Biogeosciences
Author:
Adnew Getachew AgmuasORCID, Pons Thijs L.ORCID, Koren GerbrandORCID, Peters WouterORCID, Röckmann ThomasORCID
Abstract
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 Δ17O. We directly
quantify the effect of photosynthesis on Δ17O of atmospheric CO2 for the first time. Our results demonstrate the established
theory for δ18O is applicable to Δ17O(CO2) at
leaf level, and we confirm that the following two key factors determine the effect of
photosynthetic 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 and cm is the one at the site of oxygen
isotope exchange between CO2 and H2O. At low cm∕ca ratios
the discrimination is governed mainly by diffusion into the leaf, and at
high cm∕ca ratios it is governed 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 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.
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
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