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 the main cause that the soil respiration rate of fenced land is greater than natural grazed land.(4)Negative CO₂ flux was observed in the sample areas,which might be attributed to the precipitation of soil inorganic carbon,which can be accelerated in the alkaline soil (pH>8.2).