Abstract

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 δ2H) 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 δ2H and 0.6 to 0.8 ‰ for δ18O for natural-abundance samples. Although 2H-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 δ18O but observed higher variation in δ2H 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.

Highlights

  • Since the introduction of isotope-ratio infrared spectrometers (IRIS), the analysis of water stable isotope samples has become much more popular in many fields, e.g., in hydrogeologic, watershed, oceanographic or eco(hydro)logical studies (Tweed et al, 2019; Oerter and Bowen, 2017; Oerter et al, 2019)

  • 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

  • We observed the same trend for the samples obtained in the field trial for δ18O but observed higher variation in δ2H compared to the lab trial

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Summary

Introduction

Since the introduction of isotope-ratio infrared spectrometers (IRIS), the analysis of water stable isotope samples has become much more popular in many fields, e.g., in hydrogeologic, watershed, oceanographic or eco(hydro)logical studies (Tweed et al, 2019; Oerter and Bowen, 2017; Oerter et al, 2019). 60 the in-situ setup in practice is limited in spatial resolution, as it requires tubing at the length of the distance from the sampling place to the IRIS, which is advisably kept short as increased tubing length increases the possibility of condensation (Beyer et al, 2020) These factors limit the utility of in-situ measurement systems to field sites in vicinity to civil infrastructure, which potentially leads to research sites chosen because of proximity to power rather than suitability as research location, and location biases (e.g. monitoring wildlife in vicinity to universities (Piccolo et al, 2020), or 65 the location of protected areas worldwide (Joppa and Pfaff, 2009)). We include a section “preceding work” in the Results section to give the reader a chance to avoid repeating our failures if attempting to improve this methodology

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