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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/hess-2019-687
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2019-687
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 28 Jan 2020

Submitted as: research article | 28 Jan 2020

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This preprint is currently under review for the journal HESS.

Combination of soil water extraction methods quantifies isotopic mixing of waters held at separate tensions in soil

William H. Bowers1, Jason J. Mercer1, Mark S. Pleasants2, and David G. Williams1,2 William H. Bowers et al.
  • 1Department of Botany, University of Wyoming, Laramie, 82070, USA
  • 2Department of Ecosystem Science and Management, University of Wyoming, Laramie, 82070, USA

Abstract. Measurements of the isotopic composition of water recovered from soil at different tensions provide a powerful means to identify potential plant water sources and quantify heterogeneity in residence time and connectivity among soil water regions. Yet incomplete understanding of mechanisms affecting isotopic composition of different soil water pools and the interactions between antecedent and new event water hinders interpretation of the isotope composition of extracted soil and plant waters. Here we present an approach for quantifying the time-dependent isotopic mixing of water held at separate tensions in soil. We wetted oven-dried, homogenized sandy loam soil first with isotopically “light” water (𝛿2H = −130 ‰; 𝛿18O = −17.6 ‰) using a sufficient volume to fill only the smallest soil pores, and then with “heavy” water (𝛿2H = −44 ‰; 𝛿18O = −7.8 ‰) to fully saturate the remaining soil regions. Soil water effluents were then sequentially extracted at three tensions (low centrifugation = 0.016 MPa; medium centrifugation = 1.14 MPa; and high cryogenic vacuum distillation at an estimated tension greater than 100 MPa) starting after variable equilibration periods of 0 h, 8 h, 1 d, 3 d and 7 d. We assessed differences in the isotopic composition of extracted effluents over the 7 d equilibration period with a MANOVA and a mixing model describing the time-dependent effects of isotope self-diffusion and exchange. The saturated moisture conditions used in our experiment likely facilitated rapid isotope exchange and equilibration among different pools. Despite this, the isotope composition of waters extracted at medium compared to high tension remained significantly different (MANOVA) for up to 1 day, and that for waters extracted at low compared to high tension remained significantly different for greater than 3 days after soil wetting. Equilibration (assuming no fractionation) predicted from the time-dependent mixing model for water held at high tension occurred after approximately 4.33 days. Our approach will be useful for assessing how soil texture and other physical and chemical properties influence isotope exchange and mixing times for studies aiming to properly characterize and interpret the isotopic composition of extracted soil and plant waters, especially under variably unsaturated conditions.

William H. Bowers et al.

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William H. Bowers et al.

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