Abstract
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.
Highlights
Quantifying the residence time and connectivity of soil water requires methods that differentiate the isotopic signature of water pools held across different-sized soil pores and ranges of matric tensions or suction pressures
We used a novel combination of centrifuge extraction and cryogenic vacuum distillation to recover pools of soil water held at discrete ranges of tension, spanning gravitationally drained, capillary, and hygroscopic water pools
The isotope ratio of water recovered using cryogenic vacuum distillation (CVD) of BSElight samples indicates that the water in the sample at this step was potentially altered slightly by evaporative enrichment of heavy isotopes mixed into the oven-dried soil, which had a high amount of surface area exposed to the dry local atmosphere
Summary
Quantifying the residence time and connectivity of soil water requires methods that differentiate the isotopic signature of water pools held across different-sized soil pores and ranges of matric tensions or suction pressures. A variety of fieldand lab-based methods are typically employed for such analyses, and each separately assesses different pools of water recovered at discrete ranges of tension (Oerter and Bowen, 2017; Orlowski et al, 2016b; Sprenger et al, 2015). These methods effectively recover and analyze water from different soil-pore size ranges, and only a few methods are capable of sampling hygroscopic water, i.e., the water that forms thin films around soil particles held at matric tensions greater than plants are able to extract.
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