中文摘要：

本文选取甘肃省正宁县1995，2000和2010年土地利用数据和相关耕地统计数据为分析依据，以ENVI，ArcGIS和Fragstats主要工作平台，对研究区的土地利用生态风险时空变化进行了分析。为了避免了评价指标选取的主观性，在景观水平上针对土地利用斑块数量、形状和蔓延度等特点选取了相应的景观指数，结合生态系统服务价值，构建了区域土地利用的生态风险评价指数。为了研究生态风险的空间变化和验证生态退耕政策实施的效果，本文引入了生态风险质心迁移模型和土地利用的空间分布模型，定量描述了甘肃正宁县实施生态退耕工程前后生态风险的相对大小和时空变化规律。结果表明：（1）生态退耕工程的实施使得正宁县东、中、西部的生态风险均减小，减小幅度在空间分布上表现为：东部〉西部〉中部。东部为生态风险防范管理的最重要地区；（2）正宁县生态退耕前后生态风险质心向西南方向迁移了4288m，东部和西部景观破碎度的大幅度变化对县域生态风险质心的迁移起到了主要的作用。研究结果提示：为了进一步加强正宁县生态安全管理，东部地区应以林地和草地生态保护为主，中部地区应保证耕地的质量并强化对新增建设用地的规划和控制，而西部地区则需构建水土保持型农业生态系统。

英文摘要：

The Grain-for-Green Policy aims to convert cropland to grassland and forest across western China, and evaluating ecological risk is essential to its implementation. Because few recent studies have focused on eco-risk changes of land use in the areas affected by significant policies, this paper took Zhengning County in Gansu Province as our focal area, and studied spatial-temporal changes in ecological risk before and after policy implementation. Based on indices of landscape fragmentation and ecosystem service value, an ecological risk assessment method using ArcGIS and Fragstats was done. The regional gravity center model and land spatial distribution model were also used to enrich the quantitative description of divisional eco-risk and its spatial-temporal variation in the county. Results showed that the implementation of the policy has contributed to an overall reduction in ecological risk in Zhengning County, with a divisional degree order reduction following the pattern： eastern Zhengning 〉 western Zhengning 〉 central Zhengning. The gravity center for eco-risk shifted 4288 m southwest from 1995 to 2010 due to landscape fragmentation. The study implies that greater attention should be paid to forest and grassland restoration in eastern Zhengning, cropland protection in central Zhengning, and soil and water conservation in western Zhengning.