Δ18o of Water Vapour, Evapotranspiration and the Sites of Leaf Water Evaporation in a Soybean Canopy
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Plant, Cell and Environment (2008) doi: 10.1111/j.1365-3040.2008.01826.x d18O of water vapour, evapotranspiration and the sites of leaf water evaporation in a soybean canopy LISA R. WELP1, XUHUI LEE1, KYOUNGHEE KIM1, TIMOTHY J. GRIFFIS2, KAYCIE A. BILLMARK2 & JOHN M. BAKER2,3 1School of Forestry and Environmental Studies, Yale University, New Haven, CT 06511, USA, 2Department of Soil, Water, and Climate, University of Minnesota, St. Paul, MN 55108, USA and 3Agricultural Research Service, United States Department of Agriculture, St. Paul, MN 55108, USA ABSTRACT understand what controls leaf transpiration in the present climate, and also how these environmental factors and feed- Stable isotopes in water have the potential to diagnose backs may change in the future. changes in the earth’s hydrological budget in response to The heavy isotope of oxygen, 18O, is a valuable tracer climate change and land use change. However, there have of plant–atmosphere water interactions (e.g.Yakir & Stern- been few measurements in the vapour phase. Here, we berg 2000). During evaporation from leaves and the soil, present high-frequency measurements of oxygen isotopic 16 the lighter H2 O molecules evaporate more readily, leaving ) and evapotranspiration compositions of water vapour (dv surface water pools more enriched in the heavy isotopo- (dET) above a soybean canopy using the tunable diode laser 18 logues (H2 O). The enrichment is linked in a predictable (TDL) technique for the entire 2006 growing season in way to environmental conditions. Isotopic mass balance Minnesota, USA. We observed a large variability in surface dictates that at the time scale of days to weeks, the isotopic from the daily to the seasonal timescales,largely explained dv composition of water transpired by leaves must be the same by Rayleigh processes, but also influenced by vertical atmo- as water entering the plant from the soil at the rooting spheric mixing, local evapotranspiration (ET) and dew for- depth. This is referred to as the ‘steady state’. At subdaily mation. We used measurements to calculate the isotopic dET time scales, it is the difference between leaf water isotopic composition at the sites of evaporative enrichment in leaves enrichment and dv that determines the diurnal change in the (dL,e) and compared that with the commonly used steady- isotopic signature of transpiration (dT), reflecting short- ).There was generally a good agreement state prediction (dL,s term non-steady state (Dongmann et al. 1974; Harwood averaged over the season, but larger differences on indi- et al. 1998; Lee, Kim & Smith 2007). vidual days.We also found that vertical variability in relative The isotopic composition of precipitation has been humidity and temperature associated with canopy structure studied globally for many years (IAEA/WMO 2004). In must be addressed in canopy-scale leaf water models. comparison, ecosystem water pools, such as soil, plant Finally, we explored this data set for direct evidence of the xylem and leaf water, are much more spatially variable and Péclet effect. difficult to measure because traditional methods require separation of the water from the soil or organic matter Key-words: canopy scale; dew; evaporative site; non-steady (Ehleringer, Roden & Dawson 2000). For example, obser- state; oxygen isotopes and tunable diode laser; Péclet effect. vations of bulk leaf water variability across seasons or among seasons are rare (Ometto et al. 2005; Pendall, Williams & Leavitt 2005). Leaf water enrichment is highly INTRODUCTION variable and may be estimated from measurements of envi- ronmental conditions using the Craig–Gordon model, Leaf transpiration transfers a significant portion of water assuming steady state where the isotopic composition of from the soil to the atmosphere with consequences for transpiration equals that of xylem water (Craig & Gordon regional humidity and precipitation patterns. In the central 1965; Yakir & Sternberg 2000). However, a considerable USA, the relative humidity has increased by 0.5–2.0% number of investigators have found that the Craig–Gordon -1 decade from 1976 to 2004 (Dai 2006), and this may be steady-state equilibrium prediction is approached only partially attributable to increased agricultural productivity during midday in field conditions (e.g. Dongmann et al. and crop density delivering more water to the atmosphere 1974; Harwood et al. 1998; Cernusak, Pate & Farquhar (Changnon, Sandstrom & Schaffer 2003). Transpiration is 2002). regulated by complicated interactions of plant physiology An increasing body of literature shows that in field con- and environmental conditions; therefore, it is important to ditions, the Craig–Gordon model predicts dL,s greater than Correspondence: X. Lee. Fax: 203 432 5023; e-mail: xuhui.lee@ dL,b (Dongmann et al. 1974; Bariac et al. 1989; Walker et al. yale.edu 1989; Flanagan & Ehleringer 1991; Flanagan, Comstock & © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Ltd 1 2 L. R. Welp et al. Ehleringer 1991). There are two confounding issues at play steady-state model; and (3) test the Péclet effect at the here. Firstly, there is evidence that either discrete leaf water canopy scale and under non-steady-state field conditions. compartments containing unenriched source water or an isotopic gradient within the leaf caused by convection of LIST OF VARIABLES source water by transpiration opposing the back-diffusion 18 of water enriched in H2 O from the site of evaporative Variable Description enrichment leads to average leaf water more negative than C molar density of water (mol m-3) d . The later process, known as the Péclet effect, is most L,s D diffusivity of H 18O in water (m2 s-1) important during midday when transpiration rates are large 2 E soil evaporation (mol m-2 s-1) (Farquhar & Lloyd 1993). Secondly, d may not equal d S T x E transpiration (mol m-2 s-1), assumed equal to ET especially during the transitional periods of the day as envi- T ET evapotranspiration (mol m-2 s-1) ronmental conditions are changing rapidly (around sunrise f fractional enrichment of leaf water and sunset). Lee et al. (2007) have shown that 24 h is too h humidity relative to the saturation vapour short to assume isotopic steady-state transpiration. Both pressure at leaf temperature the Péclet effect and the d = d steady-state assumption will T x I ET isoforcing (mmol m-2 s-1 ‰) be explored in this paper. ET L effective diffusion length (m) Leaf water isotopic composition at the site of enrichment eff R 18O/16O ratio of dew (d ) not only controls d , but also has consequences for d L,e T R 18O/16O ratio of ET other isotope tracers. For example, it plays a critical role in ET T temperature (°C or K) determining the oxygen isotopic composition of atmo- v water mixing ratio in the atmosphere [mmol spheric CO (Farquhar et al. 1993), and using this to parti- 2 (mol dry air)-1] tion net CO fluxes into gross photosynthesis and 2 x molar mixing ratio of water isotopologue [e.g. respiration (Yakir & Wang 1996; Ogée et al. 2004), recon- mol H 18O (mol dry air)-1] structing past climate conditions from cellulose records, 2 d d18O of dew (‰) through d ’s effect on d , (Epstein & Yapp 1977; Roden, d L,e L,b d d18O of evapotranspiration (‰) Lin & Ehleringer 2000), and in paleostudies to estimate the ET d d18O of ground water (‰) ratio of marine to terrestrial biospheric productivity calcu- g d d18O of bulk leaf water (‰) lated from the Dole effect (Bender, Sowers & Labeyrie L,b d d18O of leaf water at the site of evaporative 1994; Hoffmann et al. 2004). Unfortunately, d is not a L,e L,e enrichment calculated using d measurements directly measurable quantity. Measurements of d , d , accu- T T v (‰) rate relative humidity with respect to the leaf temperature, d d18O of leaf water at the site of evaporation and estimates of kinetic isotope fractionation allow us to L,s calculated assuming steady-state conditions, infer the values of d without invoking the steady- L,e d = d (‰) state assumption (Craig & Gordon 1965; Harwood et al. T x d d18O of precipitation (‰) 1998). P d d18O of soil water (‰) The vapour phase d and d vary on even shorter time s v T d d18O of canopy storage of water vapour (‰) scales than bulk water pools. Traditional measurements storage d d18O of transpiration, assumed equal to d (‰) used to resolve such variability are time-consuming because T ET d d18O of water vapour (‰) of the need to first condense the vapour to the liquid phase v d d18O of liquid water in equilibrium with d (‰) and then to perform mass spectrometer analysis (Helliker v,e v d d18O of xylem water (‰) et al. 2002). Recent advances in mid-infrared spectroscopy x e equilibrium fractionation (‰) have now made it possible to make high temporal resolu- eq e kinetic fractionation (‰) tion measurements of d and transpiration d in the surface k v T ℘ Péclet number layer (Lee et al. 2005), and also lower resolution measure- ments of dv throughout the upper troposphere and strato- sphere on a global scale from satellites (Worden et al. 2006; THEORY Herbin et al. 2007). Departure from steady state In this study, we made near-continuous ecosystem-scale measurements of dv and dET above a soybean canopy in The isotopic composition of transpiration (dT) can be Minnesota using tunable diode laser (TDL) technology. understood using theory developed for evaporating pools These observations also included periods of high humidity of water by Craig & Gordon (1965) (herein C&G), which that allowed us to examine theoretical predictions in describes the d18O of evaporation as a function of humidity, extreme environmental conditions. Our objectives were to: kinetic and equilibrium isotope fractionation, the isotopic (1) quantify the temporal dynamics of the d18O composition composition of water of the evaporating surface, and atmo- of atmospheric water vapour and ecosystem water pools and spheric vapour.