中文摘要：

用静态箱法于2012年3月~2013年2月对黄土高原自然放牧及其围封6年的草地土壤呼吸和净生态系统CO2交换速率进行为期1年的实地观测.结果表明：（1）围封和自然放牧草地全年的碳交换总量均表现为碳汇,即CO2净吸收;围封草地全年净生态系统碳交换量为（-58.0±14.5）g·m^-2,其中生长季和非生长季交换量分别为（-70.3±11.4）g·m^-2和（12.4±3.1） g·m^-2;自然放牧草地生态系统全年碳交换量为（-48.7±14.0）g·m^-2,其中生长季和非生长季交换量分别为（-56.9±10.6）g·m^-2和（8.1±3.3）g·m^-2.（2）草地净生态系统CO2交换速率生长季主要受土壤含水量的控制,而在非生长季则主要受土壤温度的控制;草地土壤呼吸速率主要受土壤温度和土壤含水量共同影响,其中在生长季受土壤含水量的影响作用更大,而在非生长季则受土壤温度的影响作用更大.（3）土壤有机质含量的差异是造成围封草地土壤呼吸速率大于自然放牧草地的主要原因.（4）研究区草地存在土壤呼吸负通量现象,样地灰钙土偏高的碱性条件（pH＞8.2）促进了土壤次生碳酸盐的淀积可能是造成这一现象的主要原因.

英文摘要：

Terrestrial ecosystems can act as both ＂carbon source＂ and ＂carbon sink＂ in the global carbon cycle and play an important role in regulating the global carbon balance.The carbon exchange between grassland ecosystem and atmosphere is an important component of terrestrial ecosystem carbon cycle.The Loess plateau is known for its thick loess and wide distribution,which brings a great impact on the agricultural production of China,as well as the global climate change.However,the carbon sequestration potential and the capacity and distribution of carbon source/sink in the grassland ecosystem on the Loess plateau is still not clear.Based on the continuous measurement of soil respiration and net ecosystem CO2 exchange rate between fenced and natural grazed grassland in Loess Plateau by using closed chamber method from March 2012 to February 2013,we studied the temporal variations of soil respiration and net ecosystems CO2 exchange with the effects of soil temperature and soil moisture.The results are as follows：（1）The annual total carbon exchange in each sample plot appears as carbon sink.The annual total carbon exchange in fenced grassland is （-58.0±14.5） g · m 2,of which growing season and non-growing season make up （-70.3± 11.4） g · m 2 and （12.4±3.1） g · m 2,respectively.The annual total carbon exchange in natural grazed grassland is （-48.7±14.0） g · m^-2,of which growing season and non-growing season make up （-56.9± 10.6） g · m 2 and （8.1±3.3） g · m^-2,respectively.（2）The net ecosystem CO2 exchange rate in growing season is mainly controlled by soil moisture,while in non-growing season is mainly controlled by soil temperature.The soil respiration rate is mainly affected by soil temperature and soil moisture,but the situation is different in different seasons,the effect of soil moisture is greater in growing season,while the effect of soil temperature is greater in non-growing season.（3）The difference of soil organic carbon content between two sample areas is