中文摘要：

采用涡度相关法,分析了2004年（平水年）和2005-2006年（干旱年）生长季内蒙古锡林河流域羊草（Leymus chinensis）草原的净生态系统碳交换（net ecosystem exchange,NEE）、总初级生产力（gross primary productivity,GPP）和生态系统呼吸（ecosystem respiration,Re）的季节和年度变化。结果表明：平水年羊草草原的日最大GPP和Re分别为4.89和1.99g C·m^-2·d^-1,而干旱年GPP和Re分别为1.53-3.01和1.38-1.77g C·m^-2·d^-1。与平水年相比,干旱年日最大GPP、Re分别下降了38%-68%和11%-12%。平水年羊草草原累积的GPP、Re分别为294和180g C·m^-2,而在干旱年分别为102-123g C·m^-2和132-158g C·m^-2。和平水年相比,干旱年的GPP、Re分别下降了58%-65%和12%-27%。用Van＇t Hoff模型模拟的8个窄土壤含水量（θ）跨度生态系统呼吸（Re）对土壤温度（Ts）的敏感程度表明：曲线斜率在θ=0.16-0.17m^3·m^-3范围内达到最大,高于或者低于这个阈值,Re对Ts的敏感度降低。干旱胁迫降低了生态系统生产力和生态系统呼吸量。与平水年相比,干旱年的GPP比Re下降的幅度更大,干旱胁迫降低了内蒙古羊草草原的碳累积,使生态系统由碳汇变为碳源。

英文摘要：

Aims Droughts are common in arid and semiarid regions and affect the capacity of carbon sequestration of grassland ecosystems by influencing the process of ecosystem carbon cycling. We analyzed the continuous measurements of ecosystem CO2 fluxes during three growing seasons （May-September） over a Leymus chinensis steppe in Inner Mongolia in order to examine the effect of drought stress on carbon accumulation of this grassland ecosystem. Methods We used the eddy covariance technique to measure CO2 fluxes during the 2004-2006 growing seasons. Only 126 and 215 mm precipitation fell during the 2005 and 2006 growing seasons,respectively,far less than normal （in 2004,364 mm）; therefore,the steppe was in an extreme drought condition. Important findings Maxima for gross primary productivity （GPP） and ecosystem respiration （Re） were 4.89 and 1.99 g C·m^-2·d^-1,respectively,in the 2004 growing season （normal year）. However,in drought years,GPP and Re were 1.53-3.01 and 1.38-1.77 g C·m^-2·d^-1,respectively. GPP and Re in the drought years decreased by 68% and 11%,respectively,compared with the normal year. Accumulated GPP and Re were 294 and 180 g C.m-2,respectively,during the growing season in 2004 and 102-123 and 132-158 g C.m-2,respectively,in drought years. Accumulated GPP and Re decreased 58%-65% and 12%-27%,respectively,in drought years compared with those of the normal year. The slope of the curve in the sensitivity for Re to Ts （Vant＇Hoff type） reached its maximum at θ = 0.16-0.17 m^3·m^-3; below or above this value of θ,the sensitivity of Re to Ts decreases. GPP and Re decline under drought stress conditions,with GPP having a larger decline. Long-term and continuous drought reduced C-accumulation and resulted in the steppe ecosystem switching from a carbon sink in typical years to a carbon source in drought years.