Carbon sequestration, as an important ecological function of terrestrial ecosystems is receiving unprecedented attention during the past 30 years. Grassland is the largest terrestrial biome in the world, being utilized primarily by the means of mowing and grazing, and confronted with nitrogen deposition, which is one of the global change factors. However, there is no consensus on the effects of these factors on carbon sequestration of grassland ecosystems so far. Using static chamber method, we observed and compared the changes of carbon flux of Leymus chinensis grassland ecosystem under mowing and simulated nitrogen deposition in Songnen Plain, and found that net ecosystem exchange, total ecosystem respiration, and gross ecosystem productivity under nitrogen addition are higher than under the control, while those under mowing conditions are lower than under the control. No difference was found in soil respiration among the three treatments. During the whole growing season, the cumulative CO₂ emissions of control, mowing and nitrogen addition treatments were estimated as 107.8, 285.2 and 102.9 g·m^-2·a^-1, respectively. Therefore, we conclude that the investigated grassland ecosystem was carbon source, the strength of which was weakened by nitrogen addition, but strengthened by mowing with high frequency and intensity. It is suggested that the longlasting fertilization may covert the grassland from carbon source to carbon sink. These findings implied that grassland ecosystem should be managed by rational grazing and proper fertilization, so that the functions of production and carbon sequestration can be guaranteed simultaneously, and thus a sustainable use of grassland ecosystems can be realized.