Monitoring water stable isotope composition in soils using gas-permeable tubing and infrared laser absorption spectroscopy

Abstract

Abstract: The water stable isotopologues 1H2H16O and 1H218O are powerful tracers of processes occurring in nature. Their slightly different masses as compared to the most abundant water isotopologue (1H216O) affect their thermodynamic (e.g. during chemical equilibrium reactions or physical phase transitions with equilibration) and kinetic (liquid and vapor phases transport processes and chemical reactions without equilibration) properties. This results in measurable differences of the isotopic composition of water within or between the different terrestrial ecosystem compartments (i.e. sub-soil, soil, surface waters, plant, and atmosphere). These differences can help addressing a number of issues, among them water balance closure and flux partitioning from the soil-plant-atmosphere continuum at the field to regional scales. In soils particularly, the isotopic composition of water (δ2H and δ18O) provides qualitative information about whether water has only infiltrated or already been re-evaporated since the last rainfall event or about the location of the evaporation front. From water stable isotope composition profiles measured in soils, it is also possible, under certain hypotheses, to derive quantitative information such as soil evaporation flux and the identification of root water uptake depths. In addition, water stable isotopologues have been well implemented into physically based Soil–Vegetation–Atmosphere Transfer models (e.g. SiSPAT-Isotope; Soil–Litter iso; TOUGHREACT) and have demonstrated their potential. However, the main disadvantage of the use of stable isotopes in soil water studies is that, contrary to other state variables (e.g. water content and tension) that can be monitored over long periods (e.g. by time-domain reflectometry, capacitive sensing, tensiometry or micro-psychrometry), stable isotope compositions are analyzed following destructive sampling, and thus are available only at a given time. As a consequence, there are important time discrepancies between soil water and stable isotope information which greatly limit the insight potential of the latter. Recently, a novel technique based on direct infrared laser absorption spectroscopy was developed that allows simultaneous and direct measurements of 1H216O, 1H2H16O and 1H218O composition in water vapour, which constitutes a major breakthrough in stable isotope analysis. Many applications can be found in the literature for varying temporal and spatial scales. In this study, we present a simple methodology for monitoring soil liquid water stable isotope composition (δ2H, δ18O) in a non-destructive manner by sampling and measuring water vapour equilibrated with soil water, using gas-permeable polypropylene tubing and a cavity ring-down laser absorption spectrometer. We will first give a detailed presentation of our laboratory controlled experimental setup and water vapour sampling protocol. We will then show that, following a preliminary calibration in a saturated fine sand, it is possible to follow the changes of isotopic composition of soil water over a wide range of water availability conditions. Limits of this technique as well as advices to potential field users will be given