The isotopic compositions of soil water in vadose zones have been extensively utilized to study the land surface system in arid regions over a wide range of spatial and temporal scales. However, fundamental understanding is lacking for isotope fractionation associated with the phase transitions of soil water in dry unsaturated zones. The long-held assumption that isotope fractionation associated with the phase transition of 'tight' soil waters (capillary, intraparticle pores, clay interlayers, and surface films) is identical to that for bulk liquid water has never been seriously challenged and examined before. If this assumption is incorrect, our current interpretation and modeling of the isotopic compositions of soil water and vapor flux in arid regions could be seriously flawed. The objective of this proposal is to test the hypothesis that equilibrium isotope fractionation between adsorbed/pore condensed water within soils and water vapor differs significantly from that for bulk liquid water-water vapor system at the same temperature, due to complex hydrophilic interactions between the soil surface and water molecules. A series of laboratory experiments will be conducted to systematically and accurately determine the adsorption-desorption isotherm of water and associated oxygen and hydrogen isotope fractionation between adsorbed/condensed water and water vapor. Simple engineered mesoporous materials and two clay minerals of layered structures will be investigated. The goals of this proposal are to: (a) identify physicochemical parameters of the soil-water system that control the isotope fractionation between soil water and water vapor and (b) build a simple, empirical model for predicting the isotope fractionation for more complex minerals and their mixtures (soils).
The land surface system in arid regions of terrestrial environments is located at a very critical interface within land-atmosphere-vegetation continuum. Successful completion of this project will change our current understanding of the isotopic behaviors of soil water and vapor flux in arid environments. Other immediate, broader impacts of expected outcomes from this project include refinements of the current analytical techniques for the isotopic composition of soil water and better understanding of the plant-water system in arid regions. New and improved views of the desert soil system are also expected to lead to better understanding of the energy-water system in arid regions, ranging from local to regional to global scales, including land-surface parameterization of global models.
