Fractionation of clumped isotopes of CO2 during photosynthesis

Stable isotope (δ13C and δ18O) and mole fraction measurements of CO2 are used to constrain the carbon cycle. However, the gross fluxes of the carbon cycle, especially photosynthesis and respiration, remain uncertain due to the challenging task of distinguishing individual flux terms from each other. The clumped isotope composition (Δ47) of CO2 has been suggested as an additional tracer for gross CO2 fluxes since it depends mainly on temperature but not on the bulk isotopic composition of leaf, soil and surface water, unlike δ18O of CO2.

In this study, we quantify the effect of photosynthetic gas exchange on Δ47 of CO2 using leaf cuvette experiments with two C3 and one C4 plants and discuss challenges and possible applications of clumped isotope measurements. The experimental results are supported by calculations with a leaf cuvette model. Our results demonstrate how the effect of gas exchange on Δ47 is controlled by CO2-H2O isotope exchange (using plants with different carbonic anhydrase activity), and kinetic fractionation as CO2 diffuses into and out of the leaf (using plants with different stomatal and mesophyll conductance). We experimentally confirm the previously suggested dependence of Δ47­­ on the stomatal conductance and back-diffusion flux.