Technical note: Conservative storage of water vapour: a key to practical measurements of water stable isotopes in tree stems and soils

• Main Authors: Magh, Ruth, Gralher, Benjamin, Herbstritt, Barbara, Kübert, Angelika, Lim, Hyungwoo, Lundmark, Tomas, Marshall, John
• Format: Text
• Language: English
• Published: 2022
• Subjects: Northern Sweden
• Online Access: https://doi.org/10.5194/hess-2022-37; https://hess.copernicus.org/preprints/hess-2022-37/

Using water stable isotopes to track plant water uptake or soil water processes has become an invaluable tool in ecohydrology and physiological ecology. Recent studies have shown that laser absorption spectroscopy can measure equilibrated water vapour well enough to support inference of liquid stable isotope composition of plant or soil water, on-site and in real-time. However, current in-situ systems require the presence of an instrument in the field. Here we tested, first in the lab and then in the field, a method for equilibrating, collecting, storing, and finally analysing water vapour for its isotopic composition that does not require an instrument in the field. We developed a vapour storage vial system (VSVS) that relies on in-situ sampling into crimp neck vials with a double-coated cap using a pump and a flow meter powered through a small battery and measuring the samples in a laboratory. All components are inexpensive and commercially available. We tested the system’s ability to store the isotopic composition of its contents by sampling a range of water vapour of known isotopic compositions (from −95 to +1700 ‰ for δ 2 H) and measuring the isotopic composition after different storage periods. Samples for the field trial were taken in a boreal forest in northern Sweden. The isotopic composition was maintained to within 0.6 to 4.4 ‰ for δ 2 H and 0.6 to 0.8 ‰ for δ 18 O for natural-abundance samples. Although 2 H-enriched samples showed higher uncertainty, they were sufficient to quantify label amounts. We detected a small change in the isotopic composition of the sample after long storage period, but it was correctable by linear regression models. We observed the same trend for the samples obtained in the field trial for δ 18 O but observed higher variation in δ 2 H compared to the lab trial. Our method combines the best of two worlds, sampling many trees in-situ while measuring at high precision in the laboratory. This provides the ecohydrology community a tool that is not only cost-efficient but also easy to use.