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

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.

Oak Transpiration Drawn From the Weathered Bedrock Vadose Zone in the Summer Dry Season

“…As diffusion is slow relative to advection, this represents a conservative estimate of the time scale for isotopic homogenization, and suggests that in a seasonally saturated subsurface, isotopic heterogeneity in bulk pore water will not persist at measurable levels beyond a few days. This reasoning is generally consistent with recent experimental observations of isotope diffusion dynamics in sandy loams, in which equilibration was observed over a matter of days (Bowers et al, 2020), which is still much shorter than the months-long saturation experienced at our site in the winter. At our field site it is therefore likely that pore fluid does not preserve the signature of a single early wet season storm's rainout effect through to the following dry season.…”

“…At our field site it is therefore likely that pore fluid does not preserve the signature of a single early wet season storm's rainout effect through to the following dry season. The duration of saturation and soil texture are likely important factors in determining the extent of equilibration (Bowers et al, 2020;Sprenger et al, 2019).…”

Testing the ‘two water worlds’ hypothesis under variable preferential flow conditions

“…The notion of fast versus slow hydraulic domains in soils has been conceptualized by hydrologists for decades (De Smedt & Wierenga, 1979;Edwards et al, 1979;Gaudet et al, 1977;Gerke & Van Genuchten, 1993;Jarvis et al, 1997;Šimů nek et al, 2003;Van Genuchten & Wierenga, 1976), yet wellmixed storage and inter-domain exchange are considered implicitly in the theoretical development of many models (Beven & Germann, 1981;Chen & Wagenet, 1992;Hoogmoed & Bouma, 1980) or explicitly considered as a fitting parameter (Jarvis, 2007;Jarvis et al, 2009;Köhne et al, 2009a;Maciejewski et al, 2006;Wu, Gao, et al, 2014). Hydrodynamic dispersion and molecular diffusion are known to drive exchange from large (higher hydraulic conductivity) to fine (lower hydraulic conductivity) soil pores Biggar & Nielsen, 1962;Griffioen et al, 1998;Van Genuchten, 1980), with complete mixing of pore water expected to occur within days (Bowers et al, 2020;Vargas et al, 2017;Wassenaar et al, 2008).…”

Ecohydrological travel times derived from in situ stable water isotope measurements in trees during a semi-controlled pot experiment

“…In a second step, the continuous measurements of the standards were used to transform measured in situ stable isotope measurements from vapor to liquid isotopic signals. Isotope data were further corrected for changes in the humidity using the approach by Brand et al (2009). Finally, the entire isotope dataset was further checked for outliers.…”