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

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.

Temperature dependence of isotopic fractionation in the CO2‐O2 isotope exchange reaction

“…A detailed description of the CO 2 ‐O 2 exchange method established at Utrecht University is given elsewhere 1,16 . In brief, equal amounts of CO 2 and O 2 were allowed to equilibrate in a quartz reactor in the presence of platinum sponge as a catalyst (purity ≥ 99.9 %, Sigma Aldrich, USA) at the bottom of the reactor.…”

“…The CO 2 ‐O 2 exchange technique has been used in different laboratories to study the carbon cycle 1–4,6,22,23 and precise measurement of Δ′ 17 O is used to correct for biases in clumped isotope measurements of CO 2. 24–27 However, the fractionation factor in CO 2 ‐O 2 isotope exchange is not well established, and different values have been reported by different laboratories.…”

“…Measurements of Δ′ 17 O (defined in Equation ) of oxygen‐containing molecules can provide insight into, for example, the carbon cycle and its link to the hydrological cycle 1–10 and are also used to reconstruct paleoclimate 10–14 . The Δ′ 17 O of CO 2 is a promising tracer to study the CO 2 exchange flux between the biosphere/hydrosphere and the atmosphere 3,5,6,15 .…”

Relevance of isotope exchange between soil water and CO2 to the budget of Δ17O in CO2

“…Printer-friendly version Discussion paper ∆ 17 O in CO 2 was first proposed as a tracer of GPP by Hoag et al (2005). More recently, laboratory studies confirmed the effect of photosynthesis on ∆ 17 O in CO 2 (Adnew et al, 2020), and we simulated large-scale variations of ∆ 17 O in atmospheric CO 2 (Koren et al, 2019). We struggled with representing the soil exchange in that model, and for follow-up studies we can possibly improve our representation of soil exchange using Eq.…”

Oxygen isotope exchange between water and carbon dioxide in soils is controlled by pH, nitrate and microbial biomass through links to carbonic anhydrase activity

“…This is possible because the δ 18 O of leaf-atmosphere CO 2 exchange is relatively enriched in 18 O compared to that of atmospheric CO 2 and the δ 18 O of soil-atmosphere CO 2 exchange (Francey and Tans, 1987;Wingate et al, 2009;Welp et al, 2011). Similarly, photochemical processes in the stratosphere cause anomalies between the δ 17 O and δ 18 O of atmospheric CO 2 that are subsequently reset during leafatmosphere CO 2 exchange (Hoag et al, 2005;Koren et al, 2019;Adnew et al, 2020). However, the routine use of these tracers to constrain the photosynthetic term of the atmospheric mass budget for the δ 18 O and δ 17 O of CO 2 has been hampered by an incomplete understanding of how the influence of soil-atmosphere CO 2 exchange varies across different soil types and environmental conditions.…”