中文摘要：

正确评估新疆草地生态系统碳源/汇效应，对区域尺度碳循环研究具有重要意义。放牧是新疆草地生态系统中主要的人类活动，但放牧对草地碳平衡与碳动态的影响还具有很大的不确定性。利用生态系统放牧模型Biome-BGC grazing，通过情景模拟综合评价新疆草地生态系统碳源/汇的动态。结果表明：1） 1979-2007年新疆草地生态系统的碳源总量为0.38PgC，其中由放牧导致的碳释放为0.37PgC；2） 当平均放牧率小于0.24头标准羊/hm^2时，放牧能够促进草地碳固定。研究实现了Biome-BGC grazing模型在区域尺度的应用，研究结果将有助于理解气候变化及放牧对干旱区草地生态系统碳动态变化的驱动机理，对探明干旱区草原生态系统的源/汇特征具有重要意义。

英文摘要：

Identifying the carbon source/sink strength of grassland ecosystems in Xinjiang is of great importance for the regional carbon cycle. Using the modified Biome-BGC grazing model, we modeled carbon dynamics in grasslands in Xinjiang, northwest China with varying grazing intensities. In general, the regional simulation estimated that the grassland ecosystems in Xinjiang acted as a net carbon source, with a value of 0.38 Pg during the period 1979-2007, of which 0.37 Pg was caused by grazing. In general, the strength of carbon sequestration improved when grazing intensity was less than 0.24 head/hm^2. However, the over-compensation effect may also be the result of the growth of poisonous grass. Therefore, in the future, by adding the ＂vegetation succession＂ module, we should improve the Biome-BGC grazing model to study the compensation effect more intensively. Our findings have implications for grassland ecosystem management as they relate to carbon sequestration and climate change mitigation, e.g., removal of grazing should be considered in strategies that aim to increase terrestrial carbon sequestrations at local and regional scales. We anticipate that our study will emphasize the need for large-scale assessments of how grazing affects carbon cycling.