Water Stable Isotopes in Ecohydrological Field Research: Comparison Between In Situ and Destructive Monitoring Methods to Determine Soil Water Isotopic Signatures

Abstract

Ecohydrological isotope based field research is often constrained by a lack of temporally explicit soil water data, usually related to the choice of destructive sampling in the field and subsequent analysis in the laboratory. New techniques based on gas permeable membranes allow to sample soil water vapor in situ and infer soil liquid water isotopic signatures. Here, a membrane-based in situ soil water vapor sampling method was tested at a grassland site in Freiburg, Germany. It was further compared with two commonly used destructive sampling approaches for determination of soil liquid water isotopic signatures: cryogenic vacuum extraction and centrifugation. All methods were tested under semi-controlled field conditions, conducting an experiment with dry-wet cycling and two isotopically different labeling irrigation waters. We found mean absolute differences between cryogenic vacuum extraction and in situ vapor measurements of 0.3-14.2 (δ 18 O) and 0.4-152.2 (δ 2 H) for soil liquid water. The smallest differences were found under natural abundance conditions of 2 H and 18 O, the strongest differences were observed after irrigation with labeled waters. Labeling strongly increased the isotopic variation in soil water: Mean soil water isotopic signatures derived by cryogenic vacuum extraction were-11.6 ± 10.9 (δ 18 O) and +61.9 ± 266.3 (δ 2 H). The in situ soil water vapor method showed isotopic signatures of-12.5 ± 9.4 (δ 18 O) and +169.3 ± 261.5 (δ 2 H). Centrifugation was unsuccessful for soil samples due to low water recovery rates. It is therefore not recommended. Our study highlights that the in situ soil water vapor method captures the temporal dynamics in the isotopic signature of soil water well while the destructive approach also includes the natural lateral isotopic heterogeneity. The different advantages and limitations of the three methods regarding

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

“…For monitoring the isotope composition of xylem water in situ under field conditions, on the other hand, there is only one existing study applying isotopic labeling (Volkmann et al, 2016a, b). Future efforts should be directed towards testing and improving the methods suggested and developing novel approaches with the ultimate goal of measuring natural abundances of plant water isotopes in situ (Beyer et al, 2019;Kühnhammer et al, 2020;Marshall et al, 2020). Subsequently, continuous soil and plant water isotope measurements should be combined (for a recent example, see Orlowski et al, 2020).…”

“…Direct in situ estimates of the isotope composition of transpiration have also been used to derive root water uptake proportions (Kühnhammer et al, 2020;Volkmann et al, 2016b) by assuming isotopic steady state and substituting δ 18 O X (xylem) with δ 18 O T . Recently, Kühnhammer et al (2020) used a classical isotope mass balance approach (SIAR; Stable Isotope Analysis in R) and added physiological restraints by combining soil and plant water potentials to derive more physiologically accurate root water uptake proportions and plant reactions to water availability changes in different depths. However, given the often very likely violation of steady-state assumptions under natural field conditions, this can be recommended only under very tightly controlled laboratory conditions and knowing the leaf water turnover time.…”

“…-Dry air is diluted directly in the membrane system (Volkmann et al, 2016a;Volkmann and Weiler, 2014) or shortly after (Oerter et al, 2017;Bowen, 2017, 2019;Rothfuss et al, 2013, 2015, Kühnhammer et al, 2020. This way, the water vapor concentration of the system is lowered, and condensation is less likely.…”

“…Flushing the system with dry air prior to the measurement removes water that condensated before the current measurement (Kühnhammer et al, 2020;Volkmann and Weiler, 2014). An ideal system would include different measures to automatically ensure the prevention of condensation both during and in between measurements.…”

“…Most commonly, purely statistical approaches (i.e., mixing models) are used. While these can also benefit from a better representation of the temporal variability enabled by in situ measurements (Kühnhammer et al, 2020), efforts should be directed at using physically based models. Those models, only accounting for 4 % of reviewed studies , enable a better mechanistic understanding of root water uptake and help to improve its representation in land surface models.…”