Isotope Fractionation of Soil Water in Arid Environments

包气带土壤水的同位素组成已被广泛地用于研究干旱地区广泛的时空尺度上的陆面系统。然而，对于与干旱非饱和带土壤水的相变相关的同位素分馏缺乏基本的了解。长期以来的假设是，与致密的土壤水(毛细管、颗粒内孔隙、粘土夹层和表面膜)的相变相关的同位素分馏与大部分液态水的相变是相同的，这一假设以前从未受到过认真的挑战和检验。如果这一假设是错误的，我们目前对干旱地区土壤水分和水汽通量的同位素组成的解释和建模可能会出现严重缺陷。由于土壤表面与水分子之间复杂的亲水作用，这一假设的目的是检验土壤和水蒸气中吸附/孔隙凝结水之间的平衡同位素分馏在相同温度下显著不同于整体液态水-水蒸气系统的平衡同位素分馏。将进行一系列实验室实验，以系统和准确地测定水的吸附-解吸等温线以及吸附/凝结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。该建议的目标是：(A)确定控制土壤水分和水汽之间同位素分馏的土壤-水系统的物理化学参数；(B)建立一个简单的经验模型来预测更复杂的矿物及其混合物(土壤)的同位素分馏。陆地环境干旱地区的陆面系统位于陆地-大气-植被连续体中非常关键的界面上。该项目的成功完成将改变我们目前对干旱环境中土壤水分和水汽通量同位素行为的理解。该项目预期结果的其他直接和更广泛的影响包括改进目前土壤水的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改进的看法预计还将有助于更好地了解干旱地区的能源-水系统，从地方到区域再到全球范围，包括全球模型的陆地-地表参数。

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.