Hey folks, I wrote down this little methodological post on identifying deep water uptake using isotope techniques, hope you like it!

The methodological aspect of identifying deep water uptake through plants’ roots requires further attention. We focus here on methods based on water isotopes but refer to (Maeght, Rewald, & Pierret, 2013) and (missing reference)

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Conceptual illustration of the soil vegetation atmosphere continuum of a typical semi-arid environment from a stable isotope perspective.

Under well-defined preconditions, techniques based on stable water isotopes can be used to identify the contribution of each water source (e.g. soil and groundwater) to a mixture (the water within the stem of plant). These prerequisites are that the isotopic composition of all potential sources is captured and the isotopic compositions of these sources differ from each other (i.e. isotopic ratios from groundwater are different from soil water isotopes and the isotopic composition of the soil water profile is not constant). Under these conditions, the contribution of each water source to the mixture of water in the stem can be determined using isotopic mixing models (for a recent review on mixing models, we refer to (Rothfuss & Javaux, 2016).

Many studies applying these principles have been carried out in the last decades (Cook & O’Grady, 2006; Dawson & Ehleringer, 1991; Dawson & Pate, 1996; Grossiord et al., 2016; Rings et al., 2013; Zhao et al., 2016) . (Chimner & Cooper, 2004) used stable isotopes to examine the water sources of native shrubs in the desert environment of San Luis Valley, Colorado (USA). The authors found that different desert shrubs are able to tap groundwater deeper than 2 m depth, even though their isotopic analysis is based on an incomplete soil profile. In addition, their deep soil and groundwater isotopic compositions are almost identical, which makes an interpretation only based on water isotopes non-conclusive. The latter is a crucial point limiting the applicability of natural abundances and/or mixing models for inferring deep water uptake of water-limited sites: the before mentioned prerequisites are often not fulfilled because the isotopic signatures of the potential water sources for plants do not differ substantially. We thoroughly elaborate on the underlying theories in this section.

The crux – similar isotopic signatures

In thick unsaturated zones of many water-limited environments the characteristic profile of soil water isotopes at depth approaches a constant value (Allison, Barnes, Hughes, & Leaney, 1984; Cook et al., 1994; Clark & Fritz, 1997) . In the Figure above , the soil-vegetation-atmosphere continuum of a typical semi-arid environment - from a stable isotope perspective - is illustrated for the wet and dry season, respectively.
In temperate, continental regions, this constant value at depth often resembles the isotopic signature of weighted mean annual precipitation (missing reference). In drier environments, however, it more likely reflects the weighted mean of heavier rain events that infiltrate deeper. Different explanations were also given earlier, but rectified later; see (Allison, Barnes, Hughes, & Leaney, 1984; Allison, 1988). In addition, deep soil water and groundwater often have a similar isotopic composition, if groundwater is not of fossil origin (missing reference). Hence, three potential water sources with a near-similar isotopic signature exist and would lead to ambiguous results when applying any mixing model. As a consequence, it is not possible (with stable water isotopes) to evaluate uptake depths from soil layers deeper than the critical depth, if the soil water isotope profile becomes constant. In fact, only within the shallow soil (we define shallow soil as soil with depth less than 1m) - which is affected by direct evaporation and diffusion/mixing - the isotopic signatures (enriched in heavy isotopes) can be clearly distinguished from precipitation input, deep soil water and groundwater.

Lots of research to be done here - how can we identify deep water uptake in such environments more precise? Looking forward to discuss and work on this topic!


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  2. Rothfuss, Y., & Javaux, M. (2016). Isotopic approaches to quantifying root water uptake and redistribution: a review and comparison of methods. Biogeosciences Discussions, 1–47. https://doi.org/10.5194/bg-2016-410
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