Water vapor sampling - a novel method for semi-in situ soil and plant water isotope analysis

In the last decade, in situ soil and plant water isotope sampling has made substantial progress, enabling researchers to study water transport processes in the soil-plant-atmosphere continuum in great detail and improve process understanding. However, in situ soil and plant water isotope measurements have been criticized as methodologically challenging, laborious and expensive (due to the need for an isotope analyzer in the field). We present a novel way of collecting equilibrated soil and plant xylem water vapor for a subsequent isotope analysis in the laboratory. The method is based on directing dry air through the desired soil depth (via a gas permeable membrane) or tree xylem (via stem boreholes, Marshall et al., 2020), allowing for isotopic equilibration within the media to be measured, and collecting the sample in tighly sealed 250ml headspace vials. The proposed method allows for a control of flow rates and test for isotopic equilibrium. We present a comparison of liquid isotope standards, soil and xylem water isotope values with established methods and find good agreement. This novel way of sample collection and analysis will enable researchers to collect soil and plant water isotope data in a high temporal resolution without the need for extensive destructive sampling and be of great value for ecohydrological studies.