Evolution spatio-temporelle de la composition isotopique de l'eau dans le continuum sol-plante-atmosphere

Abstract

Intensive daily sampling of the soil-plant-atmosphere continuum was carried out to determine the vertical evolution of diurnal variation of the isotopic composition of 1. i) water in leaves from a maize canopy at the experimental site of Villeau, Eure et Loir (France). 2. ii) water vapour in the atmosphere above and within the canopy. Applying some reasonable assumptions, the isotopic model of the transpiration process (I.M.T.) fits well the daily enrichment cycle of 18O and 2H in the leaf water sampled at different levels of the plants. The most important factors influencing the variations of 18O and 2H content are the relative humidity of the air and the kinetic enrichment factor occurring during transpiration. The discrepancies between the measured and the calculated values seem to be closely related to the differences between the isotopic composition of the water pools in the leaf. Without experimental data on the transpiration flux and the root water uptake, the I.M.T. allows a relatively precise determination of the nature (transient or stationary) of the isotopic state of the water in the leaf. When δ 18 O and δ 2 H present steady-state values, the water fluxes are always conservative in the transpiring pools of the leaf. When the heterogeneity of the pools of the leaf is taken into account, it appears that the leaf does not present systematically an instantaneous isotopic steady-state. The kinetic fractionation factor ( ϵ k ) can be estimated at the leaf level by comparison between measured and calculated values of the isotopic composition of water in the leaves. The results reported here indicate that the assumption of the constancy of ϵ k values is invalid: the discrepancy in ϵ k values between the leaves at the top of the canopy and the other ones can be related to the increase of the turbulence of the atmosphere with height in the canopy. When an isotopic steady-state is attained in the leaf water, transpiration flux induces an isotopic enrichment in the surrounding water vapour by returning to the atmosphere a specific vapour. The isotopic composition of this vapour is the same as that in the soil water of the layers supplying water to the roots. There is no fractionation during water uptake by the roots. The isotopic composition of the vapour generated by the canopy is different from those of the regional one. At different levels, analysis of the isotopic composition of water vapour by sampling of atmospheric layers shows that the isotopic enrichment in water vapour is largest near the ground and decreases upward. The fractions of the vapour originating from transpiration and the oceanic area are components of the mixing observed at each level. In order to confirm such a conclusion, additional observations of the evolution of the isotopic composition of water vapour during the growing season are required to estimate the influence of the canopy on the atmosphere.