Canopy-scale kinetic fractionation of atmospheric carbon dioxide and water vapor isotopes

The carbon and oxygen isotopes of CO2 and the oxygen isotopes of H2O are powerful tracers for constraining the dynamics of carbon uptake and water flux on land. The role of land biota in the atmospheric budgets of these isotopes has been extensively explored through the lens of leaf-scale observations. At the ecosystem scale, kinetic fractionation is associated with molecular and turbulent diffusion. Intuitively, air turbulence, being nondiscriminative in diffusing materials, should act to erase the kinetic effect. Using the first canopy-scale isotopic flux measurements, we show just the opposite: that in the terrestrial environment, air turbulence enhances the effect, rather than suppressing it. The sensitivity of kinetic fractionation to turbulence is striking in situations where the canopy resistance is comparable to or lower than the aerodynamic resistance. Accounting for turbulent diffusion greatly improves land surface model predictions of the isoforcing of O-18-CO2 and transpiration enrichment of leaf water in O-18-H2O in field conditions. Our results suggest that variations in surface roughness across the landscape can contribute to spatial variations in the composition of atmospheric O-18-CO2 and that temporal trends in wind circulation on land can play a role in the interannual variability of atmospheric O-18-CO2. In comparison, air turbulence has a limited effect on the isoforcing of C-13-CO2.