Modeling biophysical controls on canopy foliage water 18O enrichment in wheat and corn

Key Laboratory of Meteorological Disaster of Ministry of Education & Yale-NUIST Center on Atmospheric Environment, Nanjing University of Information Science & Technology, Nanjing, China; School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut, USA; Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China; Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun, China
Global Change Biology (Impact Factor: 8.22). 04/2012; 18(5):1769 - 1780. DOI: 10.1111/j.1365-2486.2012.02648.x

ABSTRACT Leaf water 18O enrichment is an important factor controlling the H218O, C18OO, and O18O exchanges between the biosphere and the atmosphere. At present, there is limited capacity to explain the enrichment mechanisms in field conditions. In this study, three models of varying complexity were used to simulate the leaf water 18O enrichment at the canopy scale. Comparisons were made among the models and with high-frequency isotopic measurements of ecosystem water pools in wheat and corn. The results show that the steady state assumption was a better approximation for ecosystems with lower canopy resistance, that it is important to consider the effect of leaf water turnover in modeling the enrichment and not necessary to deal with time changes in leaf water content, and that the leaf-scale Péclet effect was incompatible with the big-leaf modeling framework for canopy-air interactions. After turbulent diffusion has been accounted for in an apparent kinetic factor parameterization, the mean 18O composition of the canopy foliage water was a well-behaved property predictable according to the principles established by leaf-scale studies, despite substantial variations in the leaf water enrichment with leaf and canopy positions. In the online supplement we provided a discussion on the observed variability of leaf water 18O composition with leaf and canopy positions and on the procedure for correcting isotopic measurements for organic contamination.

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    ABSTRACT: The high temporal resolution measurements of δD, δ18O and deuterium excess (d) of atmospheric water vapor provide an improved understanding of atmospheric and eco-hydrological processes at ecosystem to global scales. In this study, δD, δ18O and d of water vapor and their flux ratios were continuously measured from May to September 2012 using an in situ technique above an arid artificial oasis in the Heihe River Basin, which has a typical continental arid climate. The monthly δD and δ18O increased slowly and then decreased, whereas the monthly d showed a steady decrease. δD, δ18O and d exhibited a marked diurnal cycle, indicating the influence of the entrainment, local evapotranspiration (ET) and dewfall. The departures of δD, δ18O and d from equilibrium prediction were significantly correlated with rain amount, relative humidity (RH) and air temperature (T). The “amount effect” was observed during one precipitation event. δD and δ18O were log linear-dependent on water vapor mixing ratio with respective R2 of 17% and 14% whereas d was significantly correlated with local RH and T, suggesting the less influence of air mass advection and more important contribution of the local source of moisture to atmospheric water vapor. Throughout the experiment, the local ET acted to increase δD and δ18O, with isofluxes of 102.5 and 23.50 mmol m−2 s−1 ‰, respectively. However, the dominated effect of entrainment still decreased δD and δ18O by 10.1 and 2.24‰, respectively. Both of the local ET and entrainment exerted a positive forcing on the diurnal variability in d.
    Journal of Geophysical Research: Atmospheres. 09/2014;

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