Abstract:The variation of organic carbon content and its distribution in different particle-size fractions in oasis farmland soils with different cultivation history in Xinjiang was studied，and the influence of longterm cultivation on soil organic carbon characteristics in the northwest arid area was discussed in this paper. Soil samples were collected from native land (without cultivation，used as a control) and three typical farming land with different cultivation times，including oasis soil in Lanzhouwan，NO31. Farm and Puhui Farm in southern and northern Xinjiang. A physical fractionation method was applied to study the content of organic carbon and its distribution in different particle-size fractions of the soil.The results were summarized as follows. (1) Cultivation practices enhanced to the accumulation of total organic carbon in soils，increased rapidly in the initial period (1～5 a)with an annual average increase about 0.5 g kg-1, but the increase was diminished after 10 years of cultivation. (2) Cultivation practices increased the content of organic carbon in different particle-size fractions. Organic carbon in the sand fraction reached its maximum in 10 years after cultivation and tended to be stable afterwards. Organic carbon in the silt and clay increased continuously with cultivation time. (3) Ratio of particulate organic carbon (POC) and mobile organic carbon (MOC) increased rapidly with cultivation in the initial period (1～5 a)， reached its maximum in 10 years after cultivation，and started to decline afterwards. These results indicate that cultivation practices in the oasis farmland area of Xinjiang facilitates the accumulation of organic carbon in soils，particularly in the silt and clay size fractions and tends to improve the quality of organic matter as evidenced by the increased ratio of POC over MOC in the first 10 years after cultivation. However，extended cultivation for more than 10 years may cause decline of organic matter content in soil，thus affecting soil quality sustainability. Therefore，management practices need to take consideration of increasing organic matter inputs.