In situ measurements of soil and plant water isotopes: a review of approaches, practical considerations and a vision for the future

Abstract

Abstract. The number of ecohydrological studies involving water stable isotope measurements has been increasing steadily due to technological (e.g., field-deployable laser spectroscopy and cheaper instruments) and methodological (i.e., tracer approaches or improvements in root water uptake models) advances in recent years. This enables researchers from a broad scientific background to incorporate water-isotope-based methods into their studies. Several isotope effects are currently not fully understood but might be essential when investigating root water uptake depths of vegetation and separating isotope processes in the soil–vegetation–atmosphere continuum. Different viewpoints exist on (i) extraction methods for soil and plant water and methodological artifacts potentially introduced by them, (ii) the pools of water (mobile vs. immobile) measured with those methods, and (iii) spatial variability and temporal dynamics of the water isotope composition of different compartments in terrestrial ecosystems. In situ methods have been proposed as an innovative and necessary way to address these issues and are required in order to disentangle isotope effects and take them into account when studying root water uptake depths of plants and for studying soil–plant–atmosphere interaction based on water stable isotopes. Herein, we review the current status of in situ measurements of water stable isotopes in soils and plants, point out current issues and highlight the potential for future research. Moreover, we put a strong focus and incorporate practical aspects into this review in order to provide a guideline for researchers with limited previous experience with in situ methods. We also include a section on opportunities for incorporating data obtained with described in situ methods into existing isotope-enabled ecohydrological models and provide examples illustrating potential benefits of doing so. Finally, we propose an integrated methodology for measuring both soil and plant water isotopes in situ when carrying out studies at the soil–vegetation–atmosphere continuum. Several authors have shown that reliable data can be generated in the field using in situ methods for measuring the soil water isotope composition. For transpiration, reliable methods also exist but are not common in ecohydrological field studies due to the required effort. Little attention has been paid to in situ xylem water isotope measurements. Research needs to focus on improving and further developing those methods. There is a need for a consistent and combined (soils and plants) methodology for ecohydrological studies. Such systems should be designed and adapted to the environment to be studied. We further conclude that many studies currently might not rely on in situ methods extensively because of the technical difficulty and existing methodological uncertainties. Future research needs to aim on developing a simplified approach that provides a reasonable trade-off between practicability and precision and accuracy.

Reviews and syntheses: Gaining insights into evapotranspiration partitioning with novel isotopic monitoring methods

“…Note that it is not the present work's intention to give a thorough review of the physically-based and isotope-enabled soilvegetation-atmosphere numerical models used by Haverd et al (2011), Sutanto et al (2012), and Rothfuss et al (2012) for simulation of T/ET. For this, the readers may refer also to Haverd and Cuntz (2010) and Braud et al (2005). Likewise, the authors choose not to describe one particular ensemble of methods in detail (used in seven different studies, see Table 2 and referred to as "water balance" in Fig.…”

“…T/ET was determined to increase from 6% (16 days after sowing) to 95% (43 days after sowing). Rothfuss et al (2012) further confronted the isotopic data with simulations with the SiSPAT-Isotope model (Braud et al, 2005). One year earlier, Haverd et al (2011) used another isotopically enabled soil-vegetation-atmosphere transfer (SVAT) model, Soil-Litter-Iso (Haverd and Cuntz, 2010), using data from a field experiment (Eucalyptus forest, south eastern Australia) in a similar framework, i.e., by running a multi-objective calibration to estimate a given set of model parameters.…”

“…Later this family of models was extended to unsaturated soil water conditions, non-isothermal conditions, and time-variable evaporation flux (e.g., Barnes and Allison, 1988; Barnes and Walker, 1989). More recently, Braud et al (2005) and Haverd and Cuntz (2010) implemented isotopic transport in both liquid and vapour phases of the soil, with a coupling to temperature dynamics, in numerically solved SVAT models (SiSPAT-Isotope and Soil-Litter-Iso). All the above-mentioned studies underline the localized character of the EF and the strong isotopic gradient in liquid water at its location.…”

“…Part of these limitations were overcome with the availability of field-deployable laser-based spectrometers. These instruments allow for long-term monitoring of soil water vapour and plant transpiration isotopic compositions when combined with gas-permeable membrane or tubing technology (Beyer et al, 2020).…”

Ecohydrological travel times derived from in situ stable water isotope measurements in trees during a semi–controlled pot experiment

“…Such limitations can be overcome by in situ measuring methods for stable water isotopes which are more and more used in the ecohydrological community. An extensive review on in situ measurement methods for stable water isotopes can be found in Beyer et al (2020). So far, many in situ measurement methods are based on the vapour equilibrium principal (Wassenaar et al, 2008) and consist of gas permeable membranes (tubes or probes) through which water vapour is directed to a isotope analyser for real-time stable water isotope measurements (Gaj et al, 2016;Marshall et al, 2020;Oerter et al, 2016;Rothfuss et al, 2013;Volkmann et al, 2016a;Volkmann and Weiler, 2014).…”