Causes and Factors of Cryogenic Extraction Biases on Isotopes of Xylem Water

Abstract

Plant transpiration is the largest contributor to continental water flux (Jasechko, 2018;Jasechko et al., 2013). Understanding the sources of transpiration water is important for predicting the effects of global change on water security and ecosystem services. Stable isotope tracing is one of the most useful tools for the aforementioned purpose (Evaristo et al., 2015;Sprenger et al., 2016). A central assumption in the use of stable isotopes to investigate plant water sources is that, excluding leaf-evaporative effects, the stable isotopes 2 H and 18 O act as conservative tracers. Therefore, the isotopic composition of xylem water effectively reflects that of source water (Ehleringer & Dawson, 1992). This requires that there is no occurrence of significant isotopic fractionation during root water uptake and the subsequent water transport in plant organs (Ehleringer & Dawson, 1992;Washburn & Smith, 1934;Zimmermann et al., 1968); and sampling and isotopic analysis of plant water do not introduce errors (Brunel et al., 1995).However, recent studies have shown that isotopic signatures of the extracted xylem water do not always match those of the water source(s), thus creating isotopic offsets between stem water and the water source. Isotopic offsets, usually depleted in 2 H, of stem water relative to the water source, have now been reported in ecologically diverse plant species (de la Casa et al., 2021). Lin and Sternberg (1993) and Ellsworth and Williams (2007) first reported that fractionation, associated with transmembrane water transport at the root-soil interface, results in the depletion of stem water for drought-tolerant and salinity-tolerant plants. Zhao et al. (2016) did not find evidence for deuterium fractionation during water uptake; they attributed the observed soil-stem water isotopic offsets as a putative discrimination during water transport and redistribution within the plant. Other alternative explanations for the isotopic offsets of stem water have also been hypothesized, such as soil pore-scale isotopic heterogeneity (Lin & Horita, 2016;Oerter & Bowen, 2019;Oerter et al., 2014), vapor transport and condensation on the root tips (Vargas et al., 2017), and a mixture of sap and tissue water (Barbeta et al., 2022). However, a recent study by Chen et al. (2020) proved that little isotopic fractionation occurs during water uptake and transport among diverse habitats/species, and the offsets in δ 2 H are artifacts of cryogenic vacuum extraction (CVE).

Dry season plant water sourcing in contrasting tropical ecosystems of Costa Rica

“…The cryogenic vacuum distillation extraction method has been the most widely used analytical technique for tracing plant water sources (e.g., Amin et al, 2020; Barbeta et al, 2020; Brooks et al, 2010; Hervé‐Fernández et al, 2016). From a physicochemical perspective, cryogenic vacuum distillation is an invasive water extraction technique that certainly results in high extraction efficiencies (Wen et al, 2022). However, in soil and plant samples, the cryo‐extracted water inexorably reflects a combination (of unknown proportions) between chemically bounded water (i.e., immobile mineralogical or biological water) and soil/xylem mobile water (i.e., the analyte of interest involved in the evapotranspiration process) (von Freyberg et al, 2020, and references therein).…”

“…The latter may concatenate or propagate errors in mixing calculations or modelling applications. However, the concern has been raised that this technique collects bulk stem water (sap + symplastic + capillary + fibre water), not only sap water (Allen & Kirchner, 2022; Barbeta et al, 2022; Penna et al, 2018; Song et al, 2021; Wen et al, 2022), thus hindering water source uptake through the SPAC. The centrifuge method has been previously used for extracting water for isotopic analysis (Bowers et al, 2020; Li et al, 2007; Millar et al, 2018; Orlowski, Pratt, & McDonnell, 2016; Sprenger et al, 2015; Tsuruta et al, 2019).…”

A process‐based water stable isotope mixing model for plant water sourcing

“…The mean difference or offset in isotopic composition between the source and extracted plant water was then determined and added to the original sample values in order to correct for the apparent CVD‐induced fractionation. Because previous studies have observed a lack of CVD‐induced fractionation in sandy soils (Chen et al, 2020; Newberry et al, 2017; Orlowski et al, 2016; Wen et al, 2022), the soils of the current study were omitted from the rewetting experiment.…”

“…Preliminary results from the isotope analyses revealed that the plant waters were depleted in hydrogen in comparison with the respective soil waters. Some researchers have suggested that the CVD method introduces considerable depletion of the hydrogen in plant water (Barbeta et al, 2019(Barbeta et al, , 2020Chen et al, 2020;Newberry et al, 2017;Wen et al, 2022;Zhao et al, 2016). In order to address any offset and correct for CVD-induced hydrogen fractionation, a rewetting experiment was performed on previously extracted pine tree samples following the method outlined in Chen et al (2020).…”

Cavitron extraction of xylem water suggests cryogenic extraction biases vary across species but are independent of tree water stress

“…for a range of conifer species Wen et al (2022). found large variations in the δD of xylem water between apple Distribution of mean δD biases per tree species.…”