A simple greenhouse experiment to explore the effect of cryogenic water extraction for tracing plant source water

Abstract

Stable isotopes of water (2H and 18O) are useful tracers for determining root water uptake depths. In such studies, plant and soil water are extracted most commonly by cryogenic vacuum distillation. However, recent studies have suggested that cryogenic extraction conditions (extraction time, temperature, and vacuum) and soil physicochemical properties affect the isotopic composition of extracted soil water. Here, we perform a simple greenhouse trial with 2 plant species (Taraxacum officinale and Pelargonium spp.) in 2 soil types (clayey loam and sand) to test our ability to match plant water to its putative soil water source(s) by using different extraction conditions (30–240 min, 80–200 °C, 0.1 Pa). We irrigated plants with water of known isotopic composition, sampled root crowns and soils at 2 depths, and varied the cryogenic water extraction conditions. Our isotope results from the sandy soils were unaffected by cryogenic extraction conditions. In contrast, extraction parameters affected the isotope composition of waters recovered from clayey soil. This influenced the estimates of plant water sourcing, where δ2H and δ18O returned different results from each other. With higher extraction temperatures and longer extraction times, we gradually extracted more enriched soil water, which reflected the source water of both plant species. Our results imply that longer extraction times and temperatures for clayey soils are needed to reduce fractionation effects during the extraction procedure. Future studies should explore how these effects apply to natural clay‐rich soils as well as plant tissue isotope composition.

The Demographics of Water: A Review of Water Ages in the Critical Zone

“…One pitfall for the application of water stable isotopes in ecohydrological and unsaturated zone studies is the lack of standard protocols for soil (and plant) water extraction for isotope analysis (Orlowski, Breuer, et al, ; Orlowski, Winkler, et al, ; Penna et al, ). Several laboratory‐ and field‐based water extraction methods for isotope analysis have been developed (see review by Sprenger, Herbstritt, & Weiler, ).…”

Ideas and perspectives: Tracing terrestrial ecosystem water fluxes using hydrogen and oxygen stable isotopes – challenges and opportunities from an interdisciplinary perspective

“…Obtaining representative samples in soils is also challenging because soil water content, soil texture, mineral composition, and the content of organic matter are spatially heterogeneous and strongly influence how soils interact with water molecules (e.g. Barnes and Allison, 1983;Oerter et al, 2014;Oshun et al, 2016;Gaj et al, 2017a). Hence, the interaction with soil particle surfaces (Lin et al, 2018), soil organic matter (Orlowski et al, 2016a), local soil properties (Yang et al, 2016), microorganisms (Blake et al, 1997;Kool et al, 2007), and plants (Vargas et al, 2017) may introduce additional isotopic heterogeneity.…”

Bound and mobile soil water isotope ratios are affected by soil texture and mineralogy, whereas extraction method influences their measurement

“…Recently, there has been increased interest in the impact that soil properties, namely, clay minerals (Gaj et al, ; Oerter et al, ), organic matter (Meißner, Köhler, Schwendenmann, Hölscher, & Dyckmans, ; Orlowski, Breuer, & McDonnell, ), and water content (Araguás‐Araguás, Rozanski, Gonfiantini, & Louvat, ; Meißner et al, ; Newberry, Prechsl, Pace, & Kahmen, ) appear to have on measured soil water isotope ratios. In addition, lab studies under controlled conditions have revealed that sample preparation, extraction methods, and extraction conditions have the potential to alter the δ 2 H and δ 18 O of soil water (Araguás‐Araguás et al, ; Gaj et al, ; Orlowski et al, ; Orlowski, Winkler, McDonnell, & Breuer, ). To determine the appropriate extraction method, interlaboratory comparisons across techniques and extraction conditions have been conducted using isotopically labelled water (Orlowski et al, ; Walker et al, ).…”