Stem water cryogenic extraction biases estimation in deuterium isotope composition of plant source water

Abstract

The hydrogen isotope ratio of water cryogenically extracted from plant stem samples (δ2Hstem_CVD) is routinely used to aid isotope applications that span hydrological, ecological, and paleoclimatological research. However, an increasing number of studies have shown that a key assumption of these applications—that δ2Hstem_CVD is equal to the δ2H of plant source water (δ2Hsource)—is not necessarily met in plants from various habitats. To examine this assumption, we purposedly designed an experimental system to allow independent measurements of δ2Hstem_CVD, δ2Hsource, and δ2H of water transported in xylem conduits (δ2Hxylem) under controlled conditions. Our measurements performed on nine woody plant species from diverse habitats revealed a consistent and significant depletion in δ2Hstem_CVD compared with both δ2Hsource and δ2Hxylem. Meanwhile, no significant discrepancy was observed between δ2Hsource and δ2Hxylem in any of the plants investigated. These results cast significant doubt on the long-standing view that deuterium fractionation occurs during root water uptake and, alternatively, suggest that measurement bias inherent in the cryogenic extraction method is the root cause of δ2Hstem_CVD depletion. We used a rehydration experiment to show that the stem water cryogenic extraction error could originate from a dynamic exchange between organically bound deuterium and liquid water during water extraction. In light of our finding, we suggest caution when partitioning plant water sources and reconstructing past climates using hydrogen isotopes, and carefully propose that the paradigm-shifting phenomenon of ecohydrological separation (“two water worlds”) is underpinned by an extraction artifact.

Stable isotopes of water reveal differences in plant – soil water relationships across northern environments

“…This is potentially a factor in northern xylem water. Very recent experimental work by Chen et al (2020) showed that cryogenic extraction can enhance deuterium exchange with organically bound water and contribute to the deuterium depletion. Moreover, they showed the effect can be greatest under more F I G U R E 7 Potential explanations for the deuterium-offset observed between soil and xylem water stable isotopes moisture-limited conditions which may explain the tendency for more negative sw-excess values as sites become drier.…”

On the Spatio-Temporal Under-Representation of Isotopic Data in Ecohydrological Studies

“…(2) Artificial effects (ART in Figure 1) mainly include soil and xylem water field sampling and extraction methods as well as instrumental measurement uncertainty in laboratory analysis, as follows: a) Different methods for soil water sampling and extraction, such as tension lysimeters and cryogenic vacuum distillation returning different isotopic composition from the same sample (Geris et al, 2015;Orlowski et al, 2016Orlowski et al, , 2018Orlowski et al, , 2019. b) Different methods for plant water sampling and extraction such as cryogenic vacuum distillation, Scholander-type pressure chamber, and other destructive methods possibly producing artifacts and returning different isotope values (Thoma et al, 2018;Fischer et al, 2019;Zuecco et al, 2020), preferentially affecting 2 H rather than 18 O values (Chen et al, 2020). Moreover, these destructive methods might not be able to sample the accessible (or mobile) water as in-situ methods do (e.g., .…”

On the use of leaf water to determine plant water source: A proof of concept

“…Ellsworth and Williams (2007) showed early on that where one samples in the transpiration stream may impact one's ability to map tree water sources (Barbeta et al, 2020;Vargas, Schaffer, Yuhong, & Sternberg, 2017). Of course, how one samples the soil and/or plant water source-that is, the extraction technique used-can also impact the source calculation (Chen et al, 2020;Orlowski, Pratt, & McDonnell, 2016;Zhao et al, 2016).…”