中文摘要：

碳、水循环是沙质草地生态系统物质和能量循环的两个关键生态过程,认识碳、水循环的变化对了解沙质草地生态系统结构与功能对区域气候变化和人类活动的响应具有重要作用。2013年利用箱式法对科尔沁围封和放牧的沙质草地进行了一个生长季的观测研究,结果表明：（1）在观测周期内,沙质草地生态系统生产力（GEP）、生态系统呼吸（ER）、蒸散量（ET）在围封和放牧样地之间存在显著差异（p〈0.05）。围封17年样地的GEP、ER、ET均最大,其次为围封22样地的,放牧样地的最小,且最大值分别为最小值的2.23倍、1.65倍、1.94倍。（2）碳水（GEP和ET）之间存在显著的线性正相关关系（p〈0.01）,ET可解释GEP58%–60%的变异,水分利用效率（WUE）从大到小依次为：围封22年（2.85μmol·nmol–1）〉围封17年（2.75μmol·nmol–1）〉放牧（2.10μmol·nmol–1）。（3）GEP、ER和土壤含水率之间有显著的线性正相关关系（p〈0.01、p〈0.05）,指数模型能够较好地模拟ER对土壤温度变化的响应,ER的温度敏感系数（Q10值）从大到小依次为：围封17年（1.878）〉围封22年（1.733）〉放牧（1.477）。因此,围封能够使退化沙质草地生态系统的碳水循环速率提高,但围封时间不宜过久。

英文摘要：

Aims Carbon and water cycling are two key processes in circulation of materials and energy flow of sandy grassland ecosystem;recognition of changes in carbon and water cycling is important for understanding the responses of sandy grassland ecosystem structure and function to regional climate change and human activities.Our objectives were to determine the regulations of carbon sequestration and emission,and to examine the relationships of carbon and water fluxes with environmental factors.Methods Chamber method was used to measure carbon and water fluxes in grazed plots and fencing enclosures of different durations in a sandy grassland over a growing season in 2013.Important findings（1） Variance analysis revealed that,during the observational period,the gross ecosystem productivity（GEP）,ecosystem respiration（ER）,and evapotranspiration（ET） were all in an order of：17-year enclosure 22-year enclosure grazing,with significant differences（p〈0.05） among the treatments;the maximum values were 2.23,1.65,and 1.94 times of the minimum values,respectively.（2） There was a significant linear and positive correlation between GEP and ET（p〈0.01）;ET explained 58% to 60% of the variations in GEP.Water use efficiency（WUE） decreased from 2.85 μmol·nmol–1 in the 22-year enclosure to 2.75 μmol·nmol–1 in the 17-year enclosure,and to 2.10 μmol·nmol–1 in grazed plots.（3） There were significant linear and positive correlations between GEP,ER and soil water content.Exponential model better fitted the responses of ecosystem respiration to soil temperature.The temperature sensibility coefficient Q10 values of ER ranked in the order of 17-year enclosure（1.878）〉22-year enclosure（1.733）〉grazing（1.477）.In conclusion,enclosure enhanced the carbon and water cycling in degraded sandy grassland ecosystem.However,a 22-year enclosure is not beneficial to this process than 17-year enclosure,suggesting that proper enclosure is conductive to the recovery of degraded sandy grassland and