Long Term and High Frequency Non-Destructive Monitoring of Soil Water Stable Isotope Compositions in the Laboratory

The fate and dynamics of water stable isotopologues (¹H²H¹⁶O and ¹H₂¹⁸O) are currently well implemented into physically based Soil–Vegetation–Atmosphere Transfer (SVAT) models (e.g. Hydrus 1D, SiSPAT-I, Soil–Litter iso, TOUGHREACT). However, 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), water stable isotope compositions (δ²H and δ¹⁸O) are analyzed following destructive sampling, and thus are available only at a given time. Thus, there are important discrepancies in time resolution between soil water and stable isotope information which greatly limit the insight potential of the latter.

Recently however, a technique based on direct infrared laser absorption spectroscopy was developed that allows simultaneous and direct measurements of δ²H and δ¹⁸O in water vapor.

Here, we present a non-destructive method for monitoring soil liquid δ²H and δ¹⁸O by sampling and measuring water vapor equilibrated with soil water using gas-permeable polypropylene tubing and a Cavity Ring-Down laser Spectrometer (CRDS).

An acrylic glass column (d=11 cm, h=60 cm) was (i) equipped with temperature and soil water probes in addition to gas-permeable tubing sections at eight different depths, (ii) filled with pure quartz sand, (iii) saturated from the bottom, and (iv) installed on weighing balances and let dry for 250 days.

Each day, soil vapor δ²H and δ¹⁸O were measured for each depth by purging the soil water vapor sampled in the tubing sections with dry air and analyzing it with a CRDS. Soil liquid water δ²H and δ¹⁸O were then inferred from the values measured in the vapor.

The experimental setup allowed following the evolution of the soil water δ²H and δ¹⁸O profile, which developed as a result of isotope convective capillary rise and back-diffusion of the stable isotope excess at the soil surface due to fractionating soil evaporation, with unprecedentedly high temporal resolution. As the soil dried out, we could show for the first time the increasing influence of the depleted ambient water vapor on the enriched liquid water close to the soil surface. Finally, the obtained data was simulated using the fully coupled 1D isotope-enabled SVAT model SiSPAT-Isotope.