Isotopic offsets between bulk plant water and its sources are larger in cool and wet environments
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Published:2022-08-08
Issue:15
Volume:26
Page:4125-4146
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ISSN:1607-7938
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Container-title:Hydrology and Earth System Sciences
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language:en
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Short-container-title:Hydrol. Earth Syst. Sci.
Author:
de la Casa JavierORCID, Barbeta AdriàORCID, Rodríguez-Uña Asun, Wingate LisaORCID, Ogée Jérôme, Gimeno Teresa E.ORCID
Abstract
Abstract. Isotope-based approaches to study plant water sources rely on the
assumption that root water uptake and within-plant water transport are
non-fractionating processes. However, a growing number of studies have
reported offsets between plant and source water stable isotope composition
for a wide range of ecosystems. These isotopic offsets can result in the
erroneous attribution of source water used by plants and potential
overestimations of groundwater uptake by the vegetation. We conducted a
global meta-analysis to quantify the magnitude of these plant source water
isotopic offsets and explored whether their variability could be explained
by either biotic or abiotic factors. Our database compiled 112 studies
spanning arctic to tropical biomes that reported the dual water isotope
composition (δ2H and δ18O) of plant (stem) and
source water, including soil water (sampled following various methodologies
and along a variable range of depths). We calculated plant source 2H
offsets in two ways: a line conditioned excess (LC-excess) that describes
the 2H deviation from the local meteoric water line and a soil water
line conditioned excess (SW-excess) that describes the deviation from the
soil water line, for each sampling campaign within each study. We tested for the effects of climate (air temperature and soil water content), soil class, and plant traits (growth form, leaf habit, wood density, and parenchyma fraction and mycorrhizal habit) on LC-excess and SW-excess. Globally, stem water was more depleted in 2H than in soil water (SW-excess < 0) by 3.02±0.65 ‰ (P < 0.05 according to estimates of our linear mixed model and weighted by sample size within studies). In 95 % of the cases where SW-excess was negative, LC-excess was negative, indicating that the uptake of water that had not undergone evaporative enrichment (such as groundwater) was unlikely to explain the observed soil–plant water isotopic offsets. Soil class and plant traits did not have any significant effect on SW-excess. SW-excess was more negative in cold and wet sites, whereas it was more positive in warm sites. The climatic effects on SW-excess suggest that methodological artefacts are unlikely to be the sole cause of observed isotopic offsets. Our results would imply that plant source water isotopic offsets may lead to inaccuracies when using the isotopic composition of bulk stem water as a proxy to infer plant water
sources.
Funder
Ministerio de Ciencia e Innovación Eusko Jaurlaritza
Publisher
Copernicus GmbH
Subject
General Earth and Planetary Sciences,General Engineering,General Environmental Science
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