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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-512
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-512
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 23 Jan 2020

Submitted as: research article | 23 Jan 2020

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This preprint is currently under review for the journal BG.

Diurnal variation in the isotope composition of plant xylem water biases the depth of root-water uptake estimates

Hannes P. T. De Deurwaerder1,2, Marco D. Visser2, Matteo Detto2, Pascal Boeckx3, Félicien Meunier1,4, Liangju Zhao5,6, Lixin Wang7, and Hans Verbeeck1 Hannes P. T. De Deurwaerder et al.
  • 1CAVElab – Computational & Applied Vegetation Ecology, Faculty of Bioscience Engineering, Ghent University, Ghent, Belgium
  • 2Department of Ecology and EvolutionaryBiology, Princeton University, Princeton, NJ, USA
  • 3ISOFYS – Isotope Bioscience Laboratory, Faculty of Bioscience Engineering,Ghent University, Ghent, Belgium
  • 4Ecological Forecasting Lab, Department of Earth and Environment, Boston University, Boston, Massachusetts, USA
  • 5Shaanxi Key Laboratory of Earth Surface System and Environmental Carrying Capacity, College of Urban and Environmental Sciences, Northwest University, Xi’an 710127, China
  • 6Key Laboratory of Ecohydrology and Integrated River Basin Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China
  • 7Department of Earth Sciences, Indiana University-Purdue University Indianapolis (IUPUI), Indianapolis, IN 46202, USA

Abstract. 1. Stable isotopologues of water are a widely used tool to derive the depth of root water uptake (RWU) in lignified plants. Uniform isotope composition of plant xylem water (i-H2O-xyl) along the stem length is a central assumption, which has never been properly evaluated.

2. We studied the effects of diurnal variation in RWU, sap flux density and various other soil and plant parameters on i-H2O-xyl within a plant using a mechanistic plant hydraulic model and empirical field observations from French Guiana and northwestern China.

3. Our model predicts significant i-H2O-xyl variation arising from diurnal RWU fluctuations and vertical soil water heterogeneity. Moreover, significant differences in i-H2O-xyl emerge between individuals with different sap flux densities. In line with model predictions, field data show excessive i-H2O-xyl variation during the day or along stem length ranging up to 25.2 ‰ in δ2H and 6.8 ‰ in δ18O, largely exceeding the measurement error range.

4. Our work show that the fundamental assumption of uniform i-H2O-xyl is violated both theoretically and empirically and therefore a real danger exists of significant biases when using stable water isotopologues to assess RWU. We propose to include monitoring of sap flow and soil water potential for more robust RWU depth estimates.

Hannes P. T. De Deurwaerder et al.

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Hannes P. T. De Deurwaerder et al.

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Short summary
A popular technique in the plant ecohydrological community is to use xylem water isotopic composition to assess the depth of root water uptake of plants. This technique, however, relies on a poorly corroborated assumption of uniform xylem water isotopic composition along a plants' stem. We show that this fundamental assumption is easily falsified, both empirically and theoretically. Our work provides a timely warning and a way forward to correctly assess root water uptake depth.
A popular technique in the plant ecohydrological community is to use xylem water isotopic...
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