Possible soil tension controls on the isotopic equilibrium fractionation factor for evaporation from soil

Abstract

The stable isotopes of hydrogen and oxygen (δ 2 H and δ 18 O) are useful conservative tracers for tracking the movement of water in soil. But although the tracking of water infiltrating through the soil profile and its movement as run-off and groundwater recharge are well developed, water movement through the soil can also include evaporative fractionation. Soil water fractionation factors have, until now, been largely empirical. Unlike open water evaporation where temperature, humidity, and vapour pressure gradient define fractionation, soil water evaporation includes fractionation by soil matrix effects. These effects are still poorly characterized. Here, we present preliminary results from a simple laboratory experiment with four soil admixtures with grain sizes that range from sand to silt and clay. Our results show that soil tension seems to control the isotope fractionation of resident soil water. The relationship between soil tension and equilibrium fractionation appears to be independent of soil texture and appears well supported by thermodynamic theory. Although these results are preliminary, they suggest that future work should go after soil tension effects as a possible explanatory factor of soil water and water vapour fractionation.

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

“…3). Samples dried at 105°C plotted along the same regression line as found by Gaj and McDonnell, (2019), considering all values below pF 5. Values above pF 5 showed a decrease in α x − y only for the samples dried at 105°C.…”

“…All values plot within the calculated α values using the Kelvin equation. A regression line found by Gaj and McDonnell (2019) is also indicated (dashed gray). Heat treatment of the field samples at 105°C resulted in higher slopes of α 18 O.…”

“…Adsorption experiments showed that the equilibrium fractionation factor is controlled by the vapor pressure (Lin and Horita, 2016; Lin et al, 2018). Other recent work has shown that the equilibrium fractionation factor of natural soil samples is controlled by the matric potential, which, in turn, controls the water vapor pressure (Gaj and McDonnell, 2019).…”

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

“…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, ). However, the strength of the relationship between mobility and isotopic composition remains poorly constrained (Shuler, ).…”

“…McDonnell () suggested discrete isotopic sampling along the soil moisture release curve to tackle this phenomenon. Some have started the evaluation of this phenomenon suggesting that soil water retention characteristics in the future may be a critical component influencing the soil water isotopic composition (Gaj & McDonnell, ), and others are attempting to model bulk water processes versus mobile water processes to further explain fractionation effects and delineate the water pools (Sprenger et al, ).…”