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 explore 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. We calculated 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 soil water (SW-excess < 0) by 3.02 ± 0.65 ‰. In 95 % of the cases where SW-excess was negative, LC-excess was negative, indicating that the uptake of water from mobile pools was unlikely to explain the observed soil-plant water isotopic offsets. SW-excess was more negative in cold and wet sites, whereas it was more positive in warm sites. Soil class and plant traits did not have any significant effect on SW-excess. The climatic effects on SW-excess suggest that methodological artefacts are unlikely to be the sole cause of observed isotopic offsets. Instead, our results support the idea that these offsets are caused by isotopic heterogeneity within plant stems whose relative importance will depend on soil and plant water status and evaporative demand. 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.

Highlights

  • We calculated 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

  • It has been suggested that the stable isotope composition of water in plant stems could be used to identify the origin of root water uptake and plant transpiration (Ehleringer and Dawson, 1992)

  • We found that for 95% of these campaigns (175 out of 184), LC-excess was negative, (Figure 1). In these 175 cases, there would be a mismatch in isotopic composition between stem and source water, regardless of whether plants were taking up mobile water or water stored in the soil that had been subject to evaporation enrichment

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Summary

Introduction

It has been suggested that the stable isotope composition of water (i.e. its 2H/1H and 18O/16O ratios, usually reported in ‰ VSMOW as δ2H and δ18O, respectively) in plant stems could be used to identify the origin of root water uptake and plant transpiration (Ehleringer and Dawson, 1992). W., & Smith, 1934; Zimmermann U, Ehhalt D, 1967) and ever since, numerous published field studies have reported plant water isotope compositions that correspond well to a mixture of ecologically relevant potential water sources (e.g. Brunel et al, 1997; Liu et al, 2019; Rong et al, 2011; Schwendenmann et al, 2015) It was 55 shown that isotopic offsets between plant and soil water could be found in some plants adapted to growing in xeric and saline environments (Ellsworth and Williams, 2007; Lin and Sternberg, 1993). In the past decade, many studies have reported similar isotopic offsets between plant and source water in various biomes, including plants typical of temperate and humid ecosystems in the field (Barbeta et al, 2019; Brooks et al, 2010; Brum et al, 2019; Carrière et al, 2020; De Deurwaerder et al, 2018; Evaristo 65 et al, 2016; Geris et al, 2015; Tetzlaff et al, 2021), in addition to controlled experiments (Barbeta et al, 2020a; Vargas et al, 2017). Failing to identify the cause of these plant-source isotopic offsets can lead to biased estimates of plant water use 70 from potential sources, including an over-estimation of groundwater use by vegetation (Barbeta et al, 2019; Zuecco et al, 2020)

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