Kyounghee Kim

δ18O of water vapour, evapotranspiration and the sites of leaf water evaporation in a soybean canopy

Stable isotopes in water have the potential to diagnose changes in the earths hydrological budget in response to climate change and land use change. However, there have been few measurements in the vapour phase. Here, we present high-frequency measurements of oxygen isotopic compositions of water vapour (delta(v)) and evapotranspiration (delta(ET)) above a soybean canopy using the tunable diode laser (TDL) technique for the entire 2006 growing season in Minnesota, USA. We observed a large variability in surface delta(v) from the daily to the seasonal timescales, largely explained by Rayleigh processes, but also influenced by vertical atmospheric mixing, local evapotranspiration (ET) and dew formation. We used delta(ET) measurements to calculate the isotopic composition at the sites of evaporative enrichment in leaves (delta(L,e)) and compared that with the commonly used steady-state prediction (delta(L,s)). There was generally a good agreement averaged over the season, but larger differences on individual days. We also found that vertical variability in relative humidity and temperature associated with canopy structure must be addressed in canopy-scale leaf water models. Finally, we explored this data set for direct evidence of the Péclet effect.

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

Received 18 August 2008; revised 16 October 2008; accepted 21 October 2008; published 4 February 2009. [1] The carbon and oxygen isotopes of CO2 and the oxygen isotopes of H2 Oa re 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-CO2 and transpiration enrichment of leaf water in 18 O-H2O 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-CO2 and that temporal trends in wind circulation on land can play a role in the interannual variability of atmospheric 18 O-CO2. In comparison, air turbulence has a limited effect on the isoforcing of 13 C-CO2.

A modeling investigation of canopy-air oxygen isotopic exchange of water vapor and carbon dioxide in a soybean field

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 Peclet 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.

Transition of stable isotope ratios of leaf water under simulated dew formation

Dew formation, a common meteorological phenomenon, is expected to intensify in the future. Dew can influence the H₂¹⁸O and HDO isotopic compositions of leaf water (δ(L) ), but the phenomenon has been neglected in many experimental and modelling studies. In this study, the dew effect on δ(L) was investigated with a dark plant chamber in which dew formation was introduced. The H₂¹⁸O and HDO compositions of water vapour, dew water and leaf water of five species were measured for up to 48 h of dew exposure. Our results show that the exchanges of H₂¹⁸O and HDO in leaf water with the air continued in the darkness when the net H₂¹⁶O flux was zero. Our estimates of the leaf conductance using the isotopic mass balance method ranged from 0.035 to 0.087 mol m⁻² s⁻¹, in broad agreement of the night-time stomatal conductance reported in the literature. In our experiments, the conductance of the C₃ species was 0.04 ± 0.01 mol m⁻² s⁻¹ and that of the C₃ plants was 0.10 ± 0.04 mol m⁻² s⁻¹. At the end of 16 h dew exposure, 72 (±17) and 94 (±11)% of the leaf water came from dew according to the ¹⁸O and D tracer, respectively.

Temporal dynamics of oxygen isotope compositions of soil and canopy CO2 fluxes in a temperate deciduous forest

Partitioning of CO2 exchange into canopy (FA) and soil (FR) flux components is essential to improve our understanding of 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., the 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 nonsteady 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 midday in this study, so the nonsteady 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.