Article

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

Global Biogeochemical Cycles (Impact Factor: 3.97). 03/2009; 23(1). DOI: 10.1029/2008GB003331

ABSTRACT

1] The carbon and oxygen isotopes of CO 2 and the oxygen isotopes of H 2 O 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 18 O-CO 2 and transpiration enrichment of leaf water in 18 O-H 2 O in field conditions. Our results suggest that variations in surface roughness across the landscape can contribute to spatial variations in the composition of atmospheric 18 O-CO 2 and that temporal trends in wind circulation on land can play a role in the interannual variability of atmospheric 18 O-CO 2 . In comparison, air turbulence has a limited effect on the isoforcing of 13 C-CO 2 .

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    • "Similar results are found in the literature. In an experiment in a soybean field,Lee et al. (2009)reported larger prediction errors (up to 308 W m −2 ) for the early part of the growing season (L AI < 2) than in the middle part of the growing season (error < 15 W m −2 ) when the LAI is large (L AI = 7.6). Chen and Zhang (2009) compared the observed heat transfer coefficient C h from Ameriflux sites and the modeled C h according to the Noah LSM, and reported large model overestimations for short vegetation. "
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