ARS | Publication request: Canopy-scale kinetic fractionation of atmospheric carbon dioxide and water vapour isotopes

Submitted to: Global Biogeochemical Cycles
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 7, 2008
Publication Date: N/A

Interpretive Summary: A major challenge in understanding climate change is deciphering the mechanisms responsible for the temporal changes in the two major greenhouse gases, carbon dioxide (CO2) and water vapor (H2O). Each gas has isotopologues, or variants of higher molecular weight, a difference that affects their movement through the water and carbon cycles. There are both biological and non-biological factors that differentially affect transport. A thorough understanding of these factors, coupled with careful measurement of the atmospheric distribution of each isotopologue will be a powerful tool for testing the atmospheric models that are crucial for predicting changes in weather and climate. To date, most research has focused on biological discrimination in exchange processes, under the assumption that non-biological discrimination was confined to simple diffusion. We have shown that, contrary to previous thought, turbulence plays an important role and its proper representation is critical for accurate prediction of both the isotopic flux of 18-O CO2 and the leaf water enrichment in 18-O. Incorporation of this mechanism in global isotopic budgets may help to explain the large difference between observations and model estimates, providing a tool for improving atmospheric models.

Technical Abstract: The isotopic fluxes of carbon dioxide (CO2) and water vapour (H2O) between the atmosphere and terrestrial plants provide powerful constraints on carbon sequestration on land 1-2, changes in vegetation cover 3 and the Earth¿s Dole effect 4. Past studies, relying mainly on leaf-scale observations, have clarified the role of biological discrimination in the exchange process. The non-biological, kinetic fractionation is thought to be dominated by molecular diffusion and has received little attention. Here we report the first direct measurements of canopy isotopic fluxes that reveal a high sensitivity of the kinetic effect to turbulent diffusion. We show that, counter-intuitively, air turbulence enhances the kinetic effect rather than suppressing it as in the marine environment 5, and that accounting for turbulence is essential for accurate predictions of the isotopic flux of 18O-CO2 and leaf water enrichment in 18O in field conditions. Inclusion of the canopy fractionation mechanism in global 18O-CO2 budget calculations may reconcile the large difference between the observed 6 and model-inferred kinetic factor 7-8 for soil respiration. In comparison, air turbulence has a negligible effect on the isotopic flux of 13C-CO2.