Abstract
The clumped isotope composition (Δ47, the anomaly of the mass 47 isotopologue relative to the abundance expected from a random isotope distribution) of CO2 has been suggested as an additional tracer for gross CO2 fluxes. However, the effect of photosynthetic gas exchange on Δ47 has not been directly determined and two indirect/conceptual studies reported contradicting results. In this study, we quantify the effect of photosynthetic gas exchange on Δ47 of CO2 using leaf cuvette experiments with one C4 and two C3 plants. The experimental results are supported by calculations with a leaf cuvette model. Our results demonstrate the important roles of the Δ47 value of CO2 entering the leaf, kinetic fractionation as CO2 diffuses into, and out of the leaf and CO2–H2O isotope exchange with leaf water. We experimentally confirm the previously suggested dependence of Δ47 of CO2 in the air surrounding a leaf on the stomatal conductance and back-diffusion flux. Gas exchange can enrich or deplete the Δ47 of CO2 depending on the Δ47 of CO2 entering the leaf and the fraction of CO2 exchanged with leaf water and diffused back to the atmosphere, but under typical ambient conditions, it will lead to a decrease in Δ47.
Highlights
The clumped isotope composition (Δ47, the anomaly of the mass 47 isotopologue relative to the abundance expected from a random isotope distribution) of CO2 has been suggested as an additional tracer for gross CO2 fluxes
The Δ47 value of C O2 has been suggested as a possible tracer for gross primary production, two previous studies presented contradicting conclusions on the effect of photosynthesis on the Δ47 value of C O2 19,22
In this study, using a leaf cuvette experiment under controlled conditions and a leaf cuvette model, we showed that photosynthetic gas exchange can in principle increase or decrease the Δ47 value of C O2 depending on the Δ47 value of the C O2 entering the leaf, the C O2–H2O exchange temperature and the back-diffusion flux
Summary
The clumped isotope composition (Δ47, the anomaly of the mass 47 isotopologue relative to the abundance expected from a random isotope distribution) of CO2 has been suggested as an additional tracer for gross CO2 fluxes. Our results demonstrate the important roles of the Δ47 value of CO2 entering the leaf, kinetic fractionation as CO2 diffuses into, and out of the leaf and CO2–H2O isotope exchange with leaf water. Laboratory studies have shown that the rate of exchange between C O2 and water is the same for δ18O and Δ4732,33 This finding suggests that the equilibration between CO2 and leaf water, soil water or open surface water should affect Δ47 with similar kinetics as δ18O, but in contrast to δ18O, Δ47 does not depend on the isotopic composition of the different water pools. The rapid isotope exchange of C O2 with leaf and surface waters is thought to drive the Δ47 of atmospheric C O2 towards the thermodynamic equilibrium v alue[1,19]
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