Article

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

School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut, USA
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.

0 Followers
 · 
126 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Seasonal drought will become more intense and frequent in some regions due to global climate change, which may lead to significant changes in the competitive interactions and uptake depths of plant water sources. Based on δ18O and δD in xylem and soil water, we investigated the impacts of seasonal drought on plant water sources from July 2011 to October 2013 for a subtropical coniferous plantation in southeastern China. Our results indicated that the studied tree species of Pinus massoniana, Pinus elliottii and Cunninghamia lanceolata appeared to have inter-specific competition for water resources from similar depths. There was a switch of the major water source from shallow soil during the non-drought periods (July to October) to deep soil during the drought periods (November–June). Similar seasonal trend of water uptake was estimated by δ18O and δD. However, relative to the results of δD, water sources predicted by δ18O showed that trees seemed to derive more deep soil water during the drought periods and more shallow soil water during the non-drought periods. The differences of plant water source partitioning based on δ18O and δD were significant during both the drought and non-drought periods, which probably resulted from the artifacts of the cryogenic vacuum distillation and spectral contamination correction. These findings will have important implications for further studies when just one of the dual stable isotopes of δ18O and δD is applied.
    Agricultural and Forest Meteorology 02/2015; 201. DOI:10.1016/j.agrformet.2014.11.020 · 3.89 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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.
    10/2014; 119(19). DOI:10.1002/2014JD021891
  • [Show abstract] [Hide abstract]
    ABSTRACT: Partitioning of CO2 exchange into canopy (FA) and soil (FR) flux components is essential to improve our understanding ecosystem processes. The stable isotope C18OO can be used for flux partitioning, but this approach depends on the magnitude and consistency of the isotope disequilibrium (Deq), i.e. difference between the isotope compositions of FR (δA) and FA (δR). In this study high temporal resolution isotopic data were used: 1) to test the suitability of existing steady state and non-steady models to estimate H218O enrichment in a mixed forest canopy, 2) to investigate the temporal dynamics of δA using a big-leaf parameterization, and 3) to quantify the magnitude of the C18OO disequilibrium (Deq) in a temperate deciduous forest throughout the growing season and to determine the sensitivity of this variable to the CO2 hydration efficiency (θeq). A departure from steady state conditions was observed even at mid-day in this study, so the non-steady state formulation provided better estimates of leaf water isotope composition. The dynamics of δR was mainly driven by changes in soil water isotope composition, caused by precipitation events. Large Deq values (up to 11‰) were predicted; however the magnitude of the disequilibrium was variable throughout the season. The magnitude of Deq was also very sensitive to the hydration efficiencies in the canopy. For this temperate forest during most of the growing season, the magnitude of Deq was inversely proportional to θeq, due to the very negative δR signal, which is contrary to observations for other ecosystems investigated in previous studies.
    Journal of Geophysical Research: Biogeosciences 05/2014; 119(5). DOI:10.1002/2013JG002525 · 3.44 Impact Factor

Full-text (2 Sources)

Download
91 Downloads
Available from
Jun 6, 2014