A modeling investigation of canopy-air oxygen isotopic exchange of water vapor and carbon dioxide in a soybean field

The oxygen isotopes of CO₂ and H₂O ( ¹⁸O-CO₂ and ¹⁸O-H₂O) provide unique information regarding the contribution of terrestrial vegetation to the global CO₂ and H₂O cycles. In this paper, a simple isotopic land surface model was used to investigate processes controlling the isotopic exchange of ¹⁸O-H₂O and ¹⁸O-CO₂ between a soybean ecosystem and the atmosphere. We included in a standard land surface model a nonsteady state theory of leaf water isotopic composition, a canopy kinetic fractionation factor, and a big-leaf parameterization of the ¹⁸O-CO₂ isoforcing on the atmosphere. Our model simulations showed that the Pclet effect was less important than the nonsteady state effect on the temporal dynamics of the water isotopic exchange. The model reproduced the highly significant and negative correlation between relative humidity and the ecosystem-scale ¹⁸O-CO₂ isoforcing measured with eddy covariance. But the model-predicted isoforcing was biased high in comparison to the observations. Model sensitivity analysis suggested that the CO₂ hydration efficiency must have been much lower in the leaves of soybean in field conditions than previously reported. Understanding environmental controls on the hydration efficiency and the scaling from the leaf to the canopy represents an area in need of more research.