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

In soils, the isotopic composition of water (delta H-2 and delta O-18) provides qualitative (e.g., location of the evaporation front) and quantitative (e.g., evaporation flux and root water uptake depths) information. However, the main disadvantage of the isotope methodology is that contrary to other soil state variables that can be monitored over long time periods, delta H-2 and delta O-18 are typically analyzed following destructive sampling. Here we present a nondestructive method for monitoring soil liquid water delta H-2 and delta O-18 over a wide range of water availability conditions and temperatures by sampling water vapor equilibrated with soil water using gas-permeable polypropylene tubing and a cavity ring-down laser absorption spectrometer. By analyzing water vapor delta H-2 and delta O-18 sampled with the tubing from a fine sand for temperatures ranging between 8 degrees C and 24 degrees C, we demonstrate that our new method is capable of monitoring delta H-2 and delta O-18 in soils online with high precision and after calibration, also with high accuracy. Our sampling protocol enabled detecting changes of delta H-2 and delta O-18 following nonfractionating addition and removal of liquid water and water vapor of different isotopic compositions. Finally, the time needed for the tubing to monitor these changes is compatible with the observed variations of delta H-2 and delta O-18 in soils under natural conditions.