深层细根是植被的重要组成部分，其结构与功能蕴含了植被对环境的适应能力。黄土高原退耕植被普遍出现土壤干层，造成生态水文环境恶化。降水是土壤水分补给的主要来源，根系吸水是深层土壤水分消耗的主要途径。理清降水、深层细根、土壤水分这三者之间的关系，有助于深入认识植被的环境适应机制。本项目选择主要退耕植被（油松、刺槐、柠条、苜蓿），系统地查明不同类型植被的深层细根分布和土壤水分状况，分析同种植被深层细根分布和土壤水分随降水量变化的规律，阐明降水对它们的影响；分析主要退耕植被深层土壤水分与细根分布、细根最大深度之间的耦合量化关系，进而揭示不同类型植被深层细根对降水的长期适应机制。为黄土高原退耕植被环境效应及适应性评价、探索深层地下生态方面的指标体系，填补国内外在这方面的数据空白，并为建立一个完整的深层细根环境适应机制的理论提供实质性数据，以及降水变化对黄土区深层土壤水分的影响预测提供数据基础。

Deep fine root is an important part of the vegetation, its structure and function reflect the adaptive ability to the environment. Soil dry layer are widespread in the plantation where farmland was converted back to forestland in loess region, which leads to ecological and hydrological environment deterioration. Precipitation is the main source of soil water supply in this region, and root uptake is the principal pathway of deep soil water depletion. Therefore, clarifying the relations among precipitation, deep fine root and soil water will help us to better understand the adaptive mechanism of the plantation to water environment. This study will investigate deep fine root distribution and soil water characteristics in the four vegetations such as Chinese pine (Pinus tabulaeformis carr.), Locust (Robinia pseudoacacia L.), Caragana (Caragana intermedia.), and alfalfa (Medicago sativa.), analyze variation pattern of deep fine root and soil water with precipitation respectively, illustrate the precipitation regulation on fine root and soil water, evaluate the coupling quantitative relationship among deep soil water, fine root distribution and the maximum fine rooting depth, reveal the long-term fine root adaptive mechanism to precipitation. This study will provide data which could establish a complete theory how deep fine root is adapt to environment, evaluate plantation which converted from farmland back to forestland adaptation to environment, explore new belowground ecological evaluation index system, fill the blank data at home and abroad, and forecast the response of deep soil water to precipitation variation in loess region.