1. Stable isotope ratios of water (δ¹⁸O, δ²H) have long been used to study a core question in plant ecology and ecohydrology: ‘From where do plants take up water?’ Indeed, decades of research has involved sampling potential plant water sources in the subsurface, classifying those sources as distinct endmembers (e.g. deep vs. shallow soil waters) and then evaluating their contributions to a xylem water sample through mixing‐model analysis to identify the depths of root water uptake.
2. However, more detailed interrogations of the subsurface and plant domains have revealed under‐considered transport and isotopic fractionation phenomena. These now apparent complexities raise new questions and challenge the many past assumptions inherent in endmember‐mixing models that now seem overly simple.
3. Here, we introduce discussions of these recent insights and provide an overview of isotope effects that occur naturally in the root zone and in the plant, as well as artificially during sample handling. Better accounting for these complexities and their associated uncertainties can lead to more accurate and robust study designs, analytical frameworks and, ultimately, inferences.
4. Finally, to more robustly characterize plant water sources using δ¹⁸O and δ²H, we provide some practical recommendations that aim at maximizing the isotopic contrast between endmembers and/or minimizing potential uncertainties.

中文翻译：

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植物和根区水同位素很难测量，解释和预测：确定植物水源的一些实用建议

1. 水的稳定同位素比值（δ ¹⁸ O，δ ² 2H）一直被用来研究植物生态学和生态水文学一个核心问题：“从哪里植物吸收水分？确实，数十年来的研究涉及对地下潜在的植物水源进行采样，将这些水源分类为不同的最终成员（例如深层土壤水与浅层土壤水），然后通过混合模型分析评估其对木质部水样的贡献，从而确定深度根水吸收量。
2. 但是，对地下和植物域的更详细的调查显示，人们对运输和同位素分馏现象的考虑不足。这些明显的复杂性提出了新的问题，并挑战了过去似乎过于简单的端构件混合模型固有的许多假设。
3. 在这里，我们介绍了对这些最新见解的讨论，并概述了在根部区域和植物中以及样品处理过程中自然发生的同位素效应。更好地解释这些复杂性及其相关的不确定性可以导致更准确，更可靠的研究设计，分析框架，并最终得出推论。
4. 最后，为了更有力地使用δ特征分析植物水源¹⁸ O和δ ² H，我们提供了旨在最大化端元和/或最小化潜在的不确定性之间的同位素对比度一些实际的建议。