Isotope-based plant–soil water tracing assumes that the isotope signal in the plant xylem does not fractionate (1). However, this assumption has been questioned increasingly in recent years with numerous reports for different plants of deuterium depletion in xylem water, particularly for halophyte and xerophytes (2). Chen et al. (3) go beyond what others have done to date and show how organics linked to the water cryogenic extraction from stem material result in a dynamic exchange between organically bound deuterium and liquid water. They further establish a correction method to challenge previous conclusions of ecohydrological separation, e.g., the “two water worlds” hypothesis (4).
Here, we show three problems with their corrected method of which readers of Chen et al.’s study should be aware:
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1)
It is misleading to regenerate the regression curve by combining nine species' data (i.e., one data per species). Different habitats/species will behave differently. In addition, individual species’ water conditions, from low to high, need to be taken into account. If we separate the different habitats and reanalyze the “raw” data of figure 4 from Chen et al., their regression relation (and hence correction method) goes away, as shown in Fig. 1. While their previously positive correction between stem relative water content and the deuterium offset remains for the saline habitat, there is no correlation for xeric habitat and even a negative correlation for mesic habitat.
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2)
Without a strict test of deuterium offset under a wide range of conditions for stem relative water content by individual species, the species-averaged value cannot be extrapolated to correct deuterium offsets. Intriguingly, although Chen et al. (their figure 4) did not confirm the wetness-based correction, it may imply plant hydration–dehydration effects on deuterium depletion as indicated by the large differences of deuterium offsets between rehydration and transpiration experiments.
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3)
The Chen et al.’s correction method does not directly impact ecohydrological separation due to uncertainties inherited from the “raw” data conducted in different plant/soil water sampling status and also the various locations, seasons, and biomes (4). Their direct finding is also limited to solely sandy soil and under greenhouse conditions. A detailed soil wetting–drying and plant hydration–dehydration experiment would be necessary to deliver the underlying process of extraction-generated deuterium bias.
Fig. 1.
Relationships between stem relative water content and δ2H offset for the transpiration experiment in Chen et al. (raw data of figure 4). The black, red, and blue colors indicate xeric, saline, and mesic habitats, respectively, and the symbols are labeled with the species name.
Overall, the results of Chen et al. are intriguing, but the use of their correction method is incorrect and does not hold up to closer scrutiny.
Footnotes
The author declares no competing interest.
References
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