Toward a Common Methodological Framework for the Sampling, Extraction, and Isotopic Analysis of Water in the Critical Zone to Study Vegetation Water Use

Analyzing the stable isotopic composition of hydrogen and oxygen in water samples from
soils and plants can help identify sources of vegetation water uptake. This approach requires
that the heterogeneous nature of plant and soil matrices is carefully accounted for during
experimental design, sample collection, water extraction, and analyses. The comparability and
shortcomings of the different methods for extracting water and analyzing isotopic composition
have been discussed in specialized literature. Yet, despite insightful comparisons of extraction
methods and benchmarking methodologies of laboratories worldwide, the community still lacks
a roadmap to guide sample collection, extraction, and isotopic analyses, and many practical
issues for potential users remain unresolved: for example, which (soil or plant) water. pool(s)
does the extracted water represent? These constitute a hurdle for the implementation of the
approach by newcomers. Here, we summarize discussions led in the framework of the COST
Action WATSON (“WATer isotopeS in the critical zONe: from groundwater recharge to plant
transpiration” CA19120). We provide guidelines for (1) sampling soil and plant material for
isotopic analysis, (2) methods for laboratory or in situ water extraction, and (3) measurements
of isotopic composition. We highlight the importance of considering the process chain as a
whole, from experimental design to isotopic analysis, to minimize biased estimates of the
relative contribution of different water sources to plant water uptake. We conclude by
acknowledging some of the limitations of this methodology and advice on collecting key
environmental parameters before sample collection for isotopic analyses.