Isotope Fractionation of Soil Water in Arid Environments

包气带土壤水的同位素组成已被广泛用于干旱区陆面系统的时空尺度研究。然而，缺乏基本的理解与土壤水在干燥的非饱和带的相变同位素分馏。长期以来的假设，即同位素分馏与相变的'紧'土壤沃茨（毛细管，颗粒内孔，粘土夹层，表面膜）是相同的散装液态水从来没有受到严重的挑战和检查之前。如果这一假设是不正确的，我们目前对干旱地区土壤水和蒸汽通量的同位素组成的解释和建模可能存在严重缺陷。该建议的目的是测试的假设，吸附/孔隙凝结水在土壤和水蒸气之间的平衡同位素分馏显着不同的散装液体水-水蒸气系统在相同的温度下，由于复杂的亲水性相互作用之间的土壤表面和水分子。将进行一系列实验室实验，以系统和准确地确定水的吸附-解吸等温线以及吸附/冷凝水和水蒸气之间的相关氧和氢同位素分馏。简单的工程介孔材料和两种层状结构的粘土矿物将被研究。该提案的目标是：（a）确定控制土壤水和水蒸气之间的同位素分馏的土壤-水系统的物理化学参数，以及（B）建立一个简单的，预测更复杂矿物及其混合物（土壤）同位素分馏的经验模型。陆地环境干旱区的陆表系统位于陆-气-水界面，植被连续体该项目的成功完成将改变我们目前对干旱环境中土壤水和蒸汽通量同位素行为的认识。该项目预期成果的其他直接、更广泛的影响包括改进目前对土壤水同位素组成的分析技术，以及更好地了解干旱地区的植物-水系统。对沙漠土壤系统的新的和更好的看法预计也将导致更好地了解干旱地区的能源-水系统，从地方到区域到全球范围，包括全球模型的陆地表面参数化。

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.