A combination of soil water extraction methods quantifies the isotopic mixing of waters held at separate tensions in soil

Bowers, William H.; Mercer, Jason J.; Pleasants, Mark S.; Williams, David G.

Measurements of the isotopic composition of separate and potentially interacting pools of soil water provide a powerful means to precisely resolve plant water sources and quantify water residence time and connectivity among soil water regions during recharge events. Here we present an approach for quantifying the time-dependent isotopic mixing of water recovered at separate suction pressures or tensions in soil over an entire moisture release curve. We wetted oven-dried, homogenized sandy loam soil first with isotopically “light” water (inline-formulaδ2Hinline-formula M2inlinescrollmathml = - normal 130 38pt10ptsvg-formulamathimg6fff7f8a80fff388ca4b4de7a6949b12 hess-24-4045-2020-ie00001.svg38pt10pthess-24-4045-2020-ie00001.png  ‰; inline-formulaδ18Oinline-formula M4inlinescrollmathml = - normal 17.6 41pt10ptsvg-formulamathimg38dcaf1973326b25acb4437495b95563 hess-24-4045-2020-ie00002.svg41pt10pthess-24-4045-2020-ie00002.png  ‰) to represent antecedent moisture held at high matric tension. We then brought the soil to near saturation with “heavy” water (inline-formulaδ2Hinline-formula M6inlinescrollmathml = - normal 44 32pt10ptsvg-formulamathimg42a2c3f868672fca600c65276274bad1 hess-24-4045-2020-ie00003.svg32pt10pthess-24-4045-2020-ie00003.png  ‰; inline-formulaδ18Oinline-formula M8inlinescrollmathml = - normal 7.8 35pt10ptsvg-formulamathimgd29ba8836ae65dc23d4cd9118d83d6e0 hess-24-4045-2020-ie00004.svg35pt10pthess-24-4045-2020-ie00004.png  ‰) that represented new input water. Soil water samples were subsequently sequentially extracted at three tensions (“low-tension” centrifugation inline-formula≈0.016 MPa; “mid-tension” centrifugation inline-formula≈1.14 MPa; and “high-tension” cryogenic vacuum distillation at an estimated tension greater than 100 MPa) after variable equilibration periods of 0 h, 8 h, 1 d, 3 d, and 7 d. We assessed the differences in the isotopic composition of extracted water over the 7 d equilibration period with a MANOVA and a model quantifying the time-dependent isotopic mixing of water towards equilibrium via self-diffusion. The simplified and homogenous soil structure and nearly saturated moisture conditions used in our experiment likely facilitated rapid isotope mixing and equilibration among antecedent and new input water. Despite this, the isotope composition of waters extracted at mid compared with high tension remained significantly different for up to 1 d, and waters extracted at low compared with high tension remained significantly different for longer than 3 d. Complete mixing (assuming no fractionation) for the pool of water extracted at high tension occurred after approximately 4.33 d. Our combination approach involving the extraction of water over different domains of the moisture release curve 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.

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Bowers, William H. / Mercer, Jason J. / Pleasants, Mark S. / et al: A combination of soil water extraction methods quantifies the isotopic mixing of waters held at separate tensions in soil. 2020. Copernicus Publications.

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