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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Available from: Lisa R Welp
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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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    ABSTRACT: Sensible heat flux is an important component of the surface energy balance. Land surface models often use the radiative surface temperature instead of the aerodynamic temperature to predict the surface sensible flux, because the former is much easier to observe by remote sensing or to compute from the surface energy balance equation. Here, measurements from 44 FLUXNET sites are used to examine the stability and leaf area index (LAI) dependence of the radiometric resistance, a resistance that should be included in the bulk transfer method if the radiometric temperature is used for the flux calculation. Results show that the radiometric resistance is much higher under stable conditions than under unstable conditions. In unstable conditions, the radiometric resistance is highly sensitive to LAI, decreasing exponentially as LAI increases. Omission of the radiometric resistance from the bulk transfer method causes a large overestimation in the sensible heat flux, especially for low-LAI surfaces and under unstable conditions.
    Full-text · Article · Sep 2015 · Boundary-Layer Meteorology
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    • "图6 选择0–5或15–20 cm土壤作为蒸发前缘对栾城冬小麦(A)和夏玉米(B)生态系统土壤蒸发δ 18 O (δ E,5cm 和δ E,20cm )日变化的影 响。 Fig. 6Effect of soil evaporating front at 0–5 cm depth or 15–20 cm depth on daily variations of δ 18 O of soil evaporation (δ E,5cm and δ E,20cm ) in a winter wheat (A) and summer maize (B) ecosystem at Luancheng. 水汽通量必须经过湍流扩散才能到达观测高度(Lee et al., 2009;Xiao et al., 2010)。夜间的湍流扩散较白 天弱(r a 较大), 此时的湍流扩散对动力分馏系数的 影响相应地会加强(Xiao et al., 2010)。因此, 冠层动 力分馏系数会使夜间的δ E 富集程度更大。Lee et al., 2005;Wen et al., 2008)、离轴积分腔输出 光谱(off-axis integrated cavity output spectroscopy, OA-ICOS) 技 术 和 波 长 扫 描 光 腔 衰 荡 光 谱 (wavelength-scanned cavity ring down spectroscopy, WS- CRDS)技术(Wen et al., 2012b "

    Full-text · Article · Jun 2013
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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%).
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