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


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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    • "The leaf-scale kinetic fractionation factor is linked to diffusion through the stomatal opening and the leaf boundary layer (Farquhar et al. 1993). The canopy kinetic factor is more appropriate at the ecosystem scale in field conditions where diffusion consists of both turbulent and molecular contributions (Lee et al. 2009). The presence of dew water on the leaf surface can affect d L (Kim and Lee 2011; Welp et al. 2008). "
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    ABSTRACT: Dew formation has the potential to modulate the spatial and temporal variations of isotopic contents of atmospheric water vapor, oxygen and carbon dioxide. The goal of this paper is to improve our understanding of the isotopic interactions between dew water and ecosystem water pools and fluxes through two field experiments in a wheat/maize cropland and in a short steppe grassland in China. Measurements were made during 94 dew events of the D and (18)O compositions of dew, atmospheric vapor, leaf, xylem and soil water, and the whole ecosystem water flux. Our results demonstrate that the equilibrium fractionation played a dominant role over the kinetic fractionation in controlling the dew water isotopic compositions. A significant correlation between the isotopic compositions of leaf water and dew water suggests a large role of top-down exchange with atmospheric vapor controlling the leaf water turnover at night. According to the isotopic labeling, dew water consisted of a downward flux of water vapor from above the canopy (98%) and upward fluxes originated from soil evaporation and transpiration of the leaves in the lower canopy (2%).
    Oecologia 08/2011; 168(2):549-61. DOI:10.1007/s00442-011-2091-0 · 3.09 Impact Factor
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    • "Finally, the impact on the isotopic composition of surface-layer air can be determined from the isotopic forcing (I F ) principle (Lee et al. 2009 "
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    ABSTRACT: The oxygen isotope composition of evapotranspiration (δ F ) represents an important tracer in the study of biosphere-atmosphere interactions, hydrology, paleoclimate, and carbon cycling. Here, we demonstrate direct measurement of δ F based on the eddy-covariance and tunable diode laser spectroscopy (EC-TDL) techniques. Results are presented from laboratory experiments and field measurements in agricultural ecosystems. The field measurements were obtained during the growing seasons of 2008 and 2009. Water vapour mixing ratios (χ w ) and fluxes (F) were compared using EC-TDL and traditional eddy-covariance and infrared gas analyser techniques over a soybean canopy in 2008. The results indicate that χ w and F agreed to within 1 and 6%, respectively. Measurements of δ F above a corn canopy in 2009 revealed a diurnal pattern with an expected progressive ¹⁸O enrichment through the day ranging from about -20 per thousand before sunrise to about 5 per thousand in late afternoon. The isotopic composition of evapotranspiration was similar to the xylem water isotope composition (δ x = -7.2 per thousand) for short periods of time during 1400-1800 LST, indicating near steady-state conditions. Finally, the isotopic forcing values (I F ) revealed a diurnal pattern with mean maximum values of 0.09ms⁻ per thousand at midday. The I F values could be described as an exponential relation of relative humidity confirming previous model calculations and measurements over a soybean canopy in 2006. These patterns and comparisons indicate that long-term continuous isotopic water vapour flux measurements based on the eddy-covariance technique are feasible and can provide new insights related to the oxygen isotope fractionation processes at the canopy scale.
    Boundary-Layer Meteorology 11/2010; 137(2). DOI:10.1007/s10546-010-9529-5 · 2.47 Impact Factor
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    ABSTRACT: 1] The oxygen isotopes of CO 2 and H 2 O (18 O-CO 2 and 18 O-H 2 O) provide unique information regarding the contribution of terrestrial vegetation to the global CO 2 and H 2 O cycles. In this paper, a simple isotopic land surface model was used to investigate processes controlling the isotopic exchange of 18 O-H 2 O and 18 O-CO 2 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 18 O-CO 2 isoforcing on the atmosphere. Our model simulations showed that the Péclet 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 18 O-CO 2 isoforcing measured with eddy covariance. But the model-predicted isoforcing was biased high in comparison to the observations. Model sensitivity analysis suggested that the CO 2 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., A modeling investigation of canopy-air oxygen isotopic exchange of water vapor and carbon dioxide in a soybean field, J. Geophys. Res., 115, G01004, doi:10.1029/2009JG001163.
    01/2010; 115(G1). DOI:10.1029/2009JG001163
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