Abstract. 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 (δ2H =-130 ‰; δ18O =-17.6 ‰) to represent antecedent moisture held at high matric tension. We then
brought the soil to near saturation with “heavy” water (δ2H =-44 ‰; δ18O =-7.8 ‰) that represented new input water. Soil water samples
were subsequently sequentially extracted at three tensions (“low-tension”
centrifugation ≈0.016 MPa; “mid-tension” centrifugation ≈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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