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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 21 Oct 2019

Submitted as: research article | 21 Oct 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Disentangling temporal and population variability in plant root water uptake from stable isotopic analysis: a labeling study

Valentin Couvreur1,*, Youri Rothfuss2,*, Félicien Meunier3, Thierry Bariac4, Philippe Biron4, Jean-Louis Durand5, Patricia Richard4, and Mathieu Javaux1,2 Valentin Couvreur et al.
  • 1Earth and Life Institute (ELI), Université catholique de Louvain (UCL), Louvain-la-Neuve, 1348, Belgium
  • 2Institute of Bio-and Geosciences, IBG-3 Agrosphere, Forschungszentrum Jülich GmbH, Jülich, 52425, Germany
  • 3CAVElab – Computational & Applied Vegetation Ecology, Faculty of Bioscience Engineering, Ghent University, Campus Coupure links 653, Gent, 9000, Belgium
  • 4Institute of Ecology and Environmental Sciences (IEES) – Paris, UMR 7618, CNRS-Sorbonne Université, Campus AgroParisTech, Thiverval-Grignon, 78850, France
  • 5UR P3F (INRA), Lusignan, 86600, France
  • *These authors contributed equally to this work.

Abstract. Isotopic labeling techniques have the potential to minimize the uncertainty of plant root water uptake (RWU) profiles estimated through multi-source (statistical) modeling, by artificially enhancing soil water isotopic gradient. Furthermore, physical models can account for hydrodynamic constraints to RWU if simultaneous soil and plant water status data is available.

In this study, a population of tall fescue (Festuca arundinacae cv Soni) was grown in a macro-rhizotron setup under semi-controlled conditions to monitor such variables for a 34-hours long period following the oxygen stable isotopic (18O) labeling of deep soil water. Aboveground variables included tiller and leaf water oxygen isotopic compositions as well as leaf water potential (ψleaf), relative humidity, and transpiration rate. Belowground profiles of root length density (RLD), soil water content and isotopic composition were also sampled. While there were strong correlations between hydraulic variables as well as between isotopic variables, the experimental results underlined the discrepancy between variations of hydraulic and isotopic variables.

In order to dissect the problem, we reproduced both types of observations with a one-dimensional physical model of water flow in the soil-plant domain, for 60 different realistic RLD profiles. While simulated ψleaf followed clear temporal variations with little differences across plants as if they were “on board of the same rollercoaster”, simulated δtiller values within the plant population were rather heterogeneous (“swarm-like”) with relatively little temporal variation and a strong sensitivity to rooting depth. The physical model thus suggested that the discrepancy between isotopic and hydraulic observations was logical, as the variability captured by the former was spatial and may not correlate with the temporal dynamics of the latter.

For comparison purposes a Bayesian statistical model was also used to simulate RWU. While they predicted relatively similar cumulative RWU profiles, the physical model could differentiate spatial from temporal dynamics of the isotopic signature, and supported that the local increase of soil water content and formation of a peak of labelled water observed overnight were due to hydraulic lift.

Valentin Couvreur et al.
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Valentin Couvreur et al.
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Short summary
Isotopic labeling of soil water is a broadly used tool for tracing the origin of water extracted by plants and compute root water uptake (RWU) profiles with multi-source mixing models. In this study we show how such method may misconstrue time series of xylem water isotopic composition as temporal dynamics of RWU by simulating data collected during a tall-fescue rhizotron experiment with an isotope-enabled physical soil-root model accounting for variability in root traits.
Isotopic labeling of soil water is a broadly used tool for tracing the origin of water extracted...