| Land reclamation and grazing are the major anthropogenic activities in the Inner Mogolia steppes. This study was conducted to address the effects of cultivation, grazing, and sheep excrement spreading on the fluxes of CH4, N2O and CO2 and soil organic carbon fractions in Xilin River Basin, Inner Mongolia.1. After cultivated for nearly 30 years, there was no significant decline for soil organic carbon and soil microbial biomass (SMB-C), but a significant decrease (24%) of soil labile C(Lab-C) was found in the 0～20 cm soil depth. Therefore, Lab-C seems to be better than Org-C and SMB-C to explain the early changes of soil carbon dynamics. SMB-C and Lab-C showed strong seasonal changes and decresed with soil depths. Grassland reclamation increased the rates of CH4 consumption, 48.9 and 29.0μg C m-2 h-1 in native meadow steppe and cropland, respectively. Similarly reclamation increased the rates of N2O emission, 56.6 and 38.6 ugN m-2 h-1 in native meadow steppe and cropland, respectively.2. The application of C4 dung to a C3 grass pasture allowed us to successfully trace dung-derived C and to distinguish the resource of CO2 emission in the soil. Sheep dung incorporated into the soil strongly increased soil CO2 respiration compared with the unamended soil, 60% came from dung and 40% from soil itself. Both soils with high C content (Leymus chinensis community) and low C content (Artemisia frigida community) showed obvious net C lost, 7.0% and 2.6% of total organic C were lossed in the higher C content and lower C content soils, respectively. Our results suggested that more net soil C would be lost in the degraded grassland than in the climax grassland after cultivated with manure.
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