Isotopic approaches to quantifying root water uptake and redistribution: a review
and comparison of methods

Rothfuss, Youri; Javaux, Mathieu

doi:10.5194/bg-2016-410

Cited by 18 publications

(16 citation statements)

References 84 publications

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“…This means that diffusive transport of vapour out of and within a soil column appear to be driven by soil tension gradients and cause fractionation during water vapour transport. If our results are supported by others in future experiments, it would have significant implications for recent work on plant water source analysis (a good summary can be found in Rothfuss & Mathieu, ). This will be especially relevant under dry conditions where plant root water uptake itself may induce high soil water tensions and therefore cause tension mediated fractionation of the soil water left behind.…”

Section: Discussionsupporting

confidence: 75%

Possible soil tension controls on the isotopic equilibrium fractionation factor for evaporation from soil

Gaj

McDonnell

2019

Hydrological Processes

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The stable isotopes of hydrogen and oxygen (δ 2 H and δ 18 O) are useful conservative tracers for tracking the movement of water in soil. But although the tracking of water infiltrating through the soil profile and its movement as run-off and groundwater recharge are well developed, water movement through the soil can also include evaporative fractionation. Soil water fractionation factors have, until now, been largely empirical. Unlike open water evaporation where temperature, humidity, and vapour pressure gradient define fractionation, soil water evaporation includes fractionation by soil matrix effects. These effects are still poorly characterized. Here, we present preliminary results from a simple laboratory experiment with four soil admixtures with grain sizes that range from sand to silt and clay. Our results show that soil tension seems to control the isotope fractionation of resident soil water. The relationship between soil tension and equilibrium fractionation appears to be independent of soil texture and appears well supported by thermodynamic theory. Although these results are preliminary, they suggest that future work should go after soil tension effects as a possible explanatory factor of soil water and water vapour fractionation.

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Section: Discussionsupporting

confidence: 75%

Possible soil tension controls on the isotopic equilibrium fractionation factor for evaporation from soil

Gaj

McDonnell

2019

Hydrological Processes

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“…8). However, Bayesian mixing-models were shown to perform best in a recent comparison of approaches to quantify root water uptake (Rothfuss and Javaux, 2016). Although it is worth 425 noting that these comparisons are based on the assumption that no fractionation occurs.…”

Section: Estimation Of Source Contribution To Xylem Watermentioning

confidence: 99%

Hydrogen isotope fractionation affects the identification and quantification of tree water sources in a riparian forest

Barbeta

Jones

Clavé

et al. 2018

Preprint

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Abstract. We investigated plant-water sources of an emblematic refugial population of Fagus sylvatica (L.) in the Ciron river gorges in South-Western France using stable isotopes. The stable isotopes of water are a powerful tracer of water fluxes in the soil-plant-atmosphere continuum. It is generally assumed that no isotopic fractionation occurs during root water uptake, and that xylem water isotopes effectively reflect source water isotopes. However, recent studies showed that under certain conditions the isotopes in plant water do not reflect any of the potential sources considered. Highly resolved datasets covering a range of environmental conditions could shed light on possible plant-soil fractionations processes. In this study, the hydrogen (δ2H) and oxygen (δ18O) isotope compositions of all potential tree water sources and xylem water were measured bi-weekly over an entire growing season. Using Bayesian isotope mixing models (MixSIAR), we then quantified the contribution of the considered sources to xylem water of F. sylvatica and Quercus robur (L.) trees. Based on δ18O data alone, both species used a mix of top and deep soil water over the season, with Q. robur using soil water relatively deeper than F. sylvatica. The contribution of stream water appeared to be marginal despite the proximity of the trees to the stream, as already reported for other riparian forests. Xylem water δ18O could always be interpreted as a mixture of deep and shallow soil waters, but the δ2H of xylem water was often more depleted than any other possible water source. We argue that an isotopic fractionation in the unsaturated zone and/or within the plant tissues could underlie this unexpected δ2H depletion of xylem water, as already observed in halophytic and xerophytic species. By means of a sensitivity analysis, we found that the estimation of plant-water sources using isotope mixing models was largely affected by this isotopic δ2H depletion. A better understanding of what causes this isotopic separation between xylem and source water is urgently needed.

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“…Moreover, many trees have access to groundwater, which may originate elsewhere (Busch et al, 1992;Beyer et al, 2018). Hence, xylem water isotopic composition is of particular interest for root water uptake studies (Schulze et al, 1996;Moreira et al, 2000;Kulmatiski et al, 2010;Rothfuss and Javaux, 2016) and when investigating hydraulic redistribution (Priyadarshini et al, 2016;Rothfuss and Javaux, 2016;Sprenger et al, 2016). Partitioning of evapotranspiration into its evaporation and transpiration components requires knowledge of the isotopic composition of the two components.…”

Section: Introductionmentioning

confidence: 99%

Borehole Equilibration: Testing a New Method to Monitor the Isotopic Composition of Tree Xylem Water in situ

Marshall¹,

Cuntz²,

Beyer³

et al. 2020

Front. Plant Sci.

104

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Forest water use has been difficult to quantify. One promising approach is to measure the isotopic composition of plant water, e.g., the transpired water vapor or xylem water. Because different water sources, e.g., groundwater versus shallow soil water, often show different isotopic signatures, isotopes can be used to investigate the depths from which plants take up their water and how this changes over time. Traditionally such measurements have relied on the extraction of wood samples, which provide limited time resolution at great expense, and risk possible artifacts. Utilizing a borehole drilled through a tree's stem, we propose a new method based on the notion that water vapor in a slow-moving airstream approaches isotopic equilibration with the much greater mass of liquid water in the xylem. We present two empirical data sets showing that the method can work in practice. We then present a theoretical model estimating equilibration times and exploring the limits at which the approach will fail. The method provides a simple, cheap, and accurate means of continuously estimating the isotopic composition of the source water for transpiration.

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Isotopic approaches to quantifying root water uptake and redistribution: a review and comparison of methods

Cited by 18 publications

References 84 publications

Possible soil tension controls on the isotopic equilibrium fractionation factor for evaporation from soil

Possible soil tension controls on the isotopic equilibrium fractionation factor for evaporation from soil

Hydrogen isotope fractionation affects the identification and quantification of tree water sources in a riparian forest

Borehole Equilibration: Testing a New Method to Monitor the Isotopic Composition of Tree Xylem Water in situ

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