| Qilian Mountains are rich in grassland resources with extensive types, which are the dominant bases of animal husbandry in Northwest China. During the past 50 years, the grassland management in the Qilian Mountains is undergoing great changes, especially in the agro-pastoral ecotone of northern slopes, which are mainly for the grassland enclosing, cultivation, abandonment and returning cultivation to grassland. In this study, the main changes of grassland management in the eastern, middle and western areas of Qilian Mountains were chosen to analysis the changes in soil organic carbon (SOC), total nitrogen (TN) pools.The target is to provide a reference for the grassland management in the Qilian Mountains. Through the study, we obtained the results as follows.(1) Fencing management of degradated grassland not only improved the community structure of grassland vegetation, but also increased SOC and TN pools to a different degree. After Alpine Shrub Meadow which distributed in the eastern of Qilian Mountain was fenced for nearly 10 years, the community structure of grassland vegetation was changed. SOC and TN contents significantly increased by 13-48% and 13-37%, respectively. SOC and TN stocks (0-30cm) significantly increased by 17% and 15%. Fencing management implemented for 8 years in Alpine Meadow, which located in the eastern of Qilian Mountain, made the grassland vegetation cover significantly increase and the aboveground biomass double, and formed a high-quality pasture where Stipa capillata and Elymus nutans were the dominant and sub-dominant species. SOC and TN contents significantly increased by 16-31% and 15-21%, and the stocks of SOC as well as TN (0-30cm) significantly increased by 17% and 17%. After Desert Grassland distributing in the western of Qilian Mountain was fenced for 6 years, the grassland community cover and vegetation biomass increased. SOC and TN stocks (0-30cm) increased by 7% and 16%. From the results above, fencing management had a different effect on differnent grassland types distributing in Qilian Mountains. The effect on Alpine Shrub Meadow and Alpine Meadow was more significant than that on Desert Grassland.(2) There was a significant effect of cultivation on SOC and TN pools of grassland. After Alpine Meadow distributing in the eastern of Qilian Mountain was cultivated for 10 years, SOC and TN contents decreased by 8-19% and 4-17%. SOC and TN stocks (0-30cm) significantly decreased by 5.7% and 4.9%. Barley cultivation for 30 years, SOC and TN contents significantly decreased by 43-51% and 37-46%, and the stocks of them (0-30cm) significantly decreased 49.5% and 41.7%, respectively. Temperate Grassland which distributed in the middle of Qilian Mountain was cultivated for 30 years, SOC and TN contents significantly decreased by 30-66% and 42-58%. Difference to the two grassland types above, Desert Grassland converted to cultivated pasture would increase SOC and TN contents significantly. Comparison with Desert Grassland, the SOC and TN stocks (0-30cm) of the pasture cultivation for 2 years significantly increased by 44% and 26%,and they significantly increased by 93% and 73% in grassland cultivation for 4 years. Therefore, Desert Grassland cultivation was an effective measure to improve the soil quality.(3) After Alpine Grassland cultivation which distributed in the middle of Qilian Mountain was abandoned for 3 years, SOC and TN contents increased by 17-23% and 4-9%. Abandonment for 10 years, they significantly increased by 23-34% and 9-20%. The increasing rate of SOC and TN stocks reached at 5.21 Mg ha-1 a-1 and 0.28 Mg ha-1 a-1 after oats cultivation was abandoned for 10years. Therefore, the natural recovery of grassland vegetation by abandonment measure was able to achieve SOC and TN pools accumulation.(4) After Temperate Grassland cultivation distributing in the middle of Qilian Mountain plant perennial grass-alfalfa for 7 years, SOC and TN contents significantly increased by 63-118% and 23-49%. The increasing rate of SOC and TN stocks was 4.31 Mg ha-1 a-1 and 0.19 Mg ha-1 a-1. Thus, in the the middle of Qilian Mountain, returning the cultivation to grassland was an effective measure to improve the soil quality.(5) The active indicators such as soil microbial biomass carbon (MBC), soil microbial biomass nitrogen (MBN) and light fraction organic carbon (LFOC) showed a significant positive correlation with SOC and TN, the change rate of which was higher than that of SOC and TN content changes. Therefore, the active indicators can be used as an early indicator to grassland management and use changes on soil carbon and nitrogen.
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