The Role of Matric Potential, Solid Interfacial Chemistry, and Wettability on Isotopic Equilibrium Fractionation

Abstract

Core Ideas Matric potential controls the equilibrium fractionation factor between soil water and water vapor. Surface chemistry determined by X‐ray spectroscopy affects the equilibrium fractionation factor. A conceptual isotope retention characteristic approach is presented. Soil water stable isotopes are widely used for geo‐ and ecohydrological applications. However, the signature of the soil water isotopic composition in the environment depends on various factors. While recent work has shown matric potential effects on equilibrium fractionation, little work has examined other soil parameters concerning soil water energy status like the surface wettability, usually quantified in terms of contact angle. We simultaneously explored the role of matric potential, contact angle, and soil surface chemistry effects on the equilibrium fractionation factor during soil water evaporation. We present a simple laboratory experiment with four different soils of various textures. Subsamples of each texture class were treated with dichlorodimethylsilane to modify surface wetting properties. Additionally, we tested two natural soil samples to explore wettability effects. Samples were dried at temperatures between 40 and 550°C to produce chemically modified surface properties. All samples were spiked with water of known isotopic composition at different water contents. The isotopic signature was determined using the vapor‐bag equilibration method. The matric potential of each sample was measured with a soil water potential meter, the contact angle was determined with the sessile drop method, and the surface chemistry by X‐ray photoelectron spectroscopy. In addition to temperature and soil matric potential, the elemental composition has apparently some control on the equilibrium fractionation factor. Based on findings, we introduce a new soil water isotope retention characteristic approach to summarize how these factors (matric potential, contact angle, and soil surface chemistry) each control the equilibrium fractionation factor for 18O/16O and 2H/H. Corresponding retention curve approach parameters are promising to be applied in the future to predict soil water fractionation effects under natural and non‐stationary conditions.

Calibration method affects the measured δ2H and δ18O in soil water by direct H2Oliquid–H2Ovapour equilibration with laser spectroscopy

“…Therefore, the bulk soil water (added reference water) is partitioned into two parts: water in smaller pores with depleted isotopic composition and water in relatively large pores with enriched isotopic composition. Moreover, this effect increases with increasing soil matric suction (Gaj et al, ), which responds to increasing clay content at the same soil water content or decreasing water content for the same soil (Assouline et al, ). Furthermore, similar effect of clay and water contents on the measured δ 18 O was also reported by Oerter et al ().…”

“…Moreover, this effect increases with increasing soil matric suction (Gaj et al, 2019), which responds to increasing clay content at the same soil water content or decreasing water content for the same soil (Assouline et al, 1998). Furthermore, similar effect of clay and water contents on the measured δ 18 O was also reported by Oerter et al (2014).…”

Development of a universal microinfiltrometer to estimate extent and persistence of soil water repellency as a function of capillary pressure and interface chemical composition

“…In this study, soil materials also received thermal treatment at 80°C. This produced samples with the same texture and carbon content, but increased SWR (i.e., increased CA; Gaj et al, 2019). Such conditions may also be found in natural soils, e.g., in the contact zone of wildfires.…”

“…To provide homogeneous samples to allow for repetitive tests, samples were air-dried, sieved < 2-mm and divided into two portions. One portion of the airdried materials was treated at 25°C in a climate-controlled chamber and the other portion was treated at 80°C for 24 hours in an oven to emulate a higher level of SWR in conjunction with stronger persistence of SWR (Gaj et al, 2019). The 25°Ctreated material served as reference.…”

Intercomparison of soil pore water extraction methods for stable isotope analysis and interpretation of hillslope runoff sources

“…This capillary fringe of effectively saturated soil water may increase mixing processes throughout the hillslope and negate any evaporative enriched shallow soil water as the quantity of water in the near surface is quantitatively smaller than in the subsurface. Gaj et al () showed that the degree of transport, mixing and fractionation of water stable isotopes within the unsaturated zone under drought is controlled by the composition of the biogeochemical interface, soil tension, and wettability. Especially the use of stable isotope signatures of soil water and water vapour at the dry end of the water retention curve (around wilting point) is largely unexplored, but nevertheless important, because these dry conditions occur preferentially at the shallow soil depths, where evaporation into the atmosphere occurs (Gaj & McDonnell, ).…”