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Recently available isotope ratio infrared spectroscopy can directly measure the isotopic composition of atmospheric water vapour (δ(18) O, δ(2) H), overcoming one of the main limitations of isotope ratio mass spectrometry (IRMS) methods. Calibrating these gas-phase instruments requires the vapourisation of liquid standards since primary standards in principle are liquids. Here we test the viability of calibrating a wavelength-scanned cavity ring-down spectroscopy (CRDS) instrument with vapourised liquid standards. We also quantify the dependency of the measured isotope values on the water concentration for a range of isotopic compositions. In both liquid and vapour samples, we found an increase in δ(18) O and δ(2) H with water vapour concentration. For δ(18) O, the slope of this increase was similar for liquid and vapour, with a slight positive relationship with sample δ-value. For δ(2) H, we found diverging patterns for liquid and vapour samples, with no dependence on δ-value for vapour, but a decreasing slope for liquid samples. We also quantified tubing memory effects to step changes in isotopic composition, avoiding concurrent changes in the water vapour concentration. Dekabon tubing exhibited much stronger, concentration-dependent, memory effects for δ(2) H than stainless steel or perfluoroalkoxy (PFA) tubing. Direct vapour measurements with CRDS in a controlled experimental chamber agreed well with results obtained from vapour simultaneously collected in cold traps analysed by CRDS and IRMS. We conclude that vapour measurements can be calibrated reliably with liquid standards. We demonstrate how to take the concentration dependencies of the δ-values into account. Copyright © 2010 John Wiley & Sons, Ltd.

International audienceIn this study, we performed a partitioning of evapotranspiration (ET) under fully controlled conditions (climatic chamber) along growth of a tall fescue cover (Festuca arundinacea) into soil evaporation (Ev) and plant transpiration (Tr) by measuring their stable oxygen isotopic compositions (δET, δEv and δTr). We showed that it was possible, under the chamber's particular conditions, to realize the partition without (1) making the hypothesis of steady state transpiration usually done in the field, nor (2) calculating δEv as a function of air relative humidity, soil water and atmospheric vapour isotopic compositions. The contribution of Ev to total ET decreased over the experiment from 100% (bare soil) to 94% [16 days after the seeding (DAS), 83% (28 DAS), 70% (36 DAS) and 5% (43 DAS)]. Soil isotopic profiles calculated using a typical exponential-type expression and measured Ev flux were compared with the bare soil steady state measured profiles. Agreement between modelled and measured values was sensitive to soil tortuosity and kinetic fractionation values. Another significant result was highly enriched isotopic values estimated for soil water at the evaporation front [the surface under our experimental conditions (δsurf)]. The theoretical estimate of δsurf was about 1-6‰ enriched as compared to the values measured in the top 1 cm of the soil, raising important implications for the determination of δEv as a function of δsurf under field conditions. Our work also points out uncertainties related to the determination of partition values and isotopic composition measured in the field, a point that is often ignored in many papers on isotopic biogeochemistry applied to geochemical fluxes, although it can be important. Copyright © 2010 John Wiley & Sons, Ltd

We show that sapflow is a useful tool for studies of water fluxes in forest ecosystems, because (i) it gives access to the spatial variability within a forest stand, (ii) it can be used even on steep slopes, and (iii) when combined with eddy correlation measurements over forests, it allows separation of individual tree transpiration from the total water loss of the stand. Moreover, sapflow techniques are quite easy to implement.Four sapflow techniques currently coexist, all based on heat diffusion in the xylem. We found a good agreement between three of these techniques. Most results presented here were obtained using the radial flow meter (Granier 1985).Tree sapflow is computed as sap flux density times sapwood area. To scale up from trees to a stand, measurements have to be made on a representative sample of trees. Thus, a number of trees in each circumference class is selected according to the fraction of sapwood they represent in the total sapwood area of the stand. The variability of sap flux density among trees is usually low (CV. 10-15%) in close stands of temperate coniferous or deciduous forests, but is much higher (35-50%) in a tropical rain forest. It also increases after thinning or during a dry spell. A set of 5-10 sapflow sensors usually provides an accurate estimate of stand transpiration. Transpiration measured on two dense spruce stands in the Vosges mountains (France) and one Scot's pine plantation in the Rhine valley (Germany)showed that maximum rate was related to stand LAI and to local climate. Preliminary results comparing the sapflow of a stand of Pinus banksiana to the transpiration of large branches, as part of the BOREAS programme in Saskachewan, Canada showed a similar trend.For modelling purposes, tree canopy conductance (gc) was calculated from PenmanMonteith equation. In most experiments, calculated canopy conductance was dependent on global radiation (positive effect) and on vapour pressure deficit (negative effect) in the absence of other limiting factors. A comparison of the vapour pressure deficit response curves of g^ for several tree species and sites showed only small differences among spruce, oak and pine forests when including understorey. Tropical rainforests exhibited a similar behaviour.

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Stable water isotopes such as oxygen 18, are natural tracers of water movement within the soil-vegetation-atmosphere continuum. They provide useful information for a better understanding of evaporation and water vapor transport within soils. This paper presents a novel controlled experimental set up. It is dedicated to detailed measurements of the evaporation fluxes from bare soil columns, as well as to the corresponding isotopic composition of the water vapor, under non steady state conditions. The experiment allowed an accurate determination of these quantities. The formulae encountered in the literature were used to estimate the isotopic composition of the evaporated water vapor. None of them was able to correctly reproduce the measured isotopic composition of water. The data were then used to estimate the value of the isotopic composition of the soil liquid water, which should be used to get the right results for the isotopic composition of the evaporated water vapor. Results suggest that, when liquid transfer is dominant within the soil, the isotopic composition of evaporation was controlled by the isotopic composition of the liquid water within very thin soil surface layers. When there is a peak in the isotopic profile, i.e. when water vapor is dominant close to the surface, the isotopic composition of the evaporated water seems to be governed by the isotopic composition of the soil liquid water at the peak. The data were also used to estimate the kinetic fractionation factor. The results suggest that the latter is not constant in time. The values seem to depend on the shape of the isotopic profile. In both cases, the uncertainty on the results is very large. The estimation of the kinetic fractionation factor is studied more in details using the modeling results presented in Part II of a companion paper where the data set is modeled using the SiSPAT_Isotope model.

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