【Objective】 Soil organic carbon (SOC), which is the largest storage of organic carbon in the terrestrial ecosystem, is subject to influences of a number of factors, such as climate, geography, human activities, etc. Irrigation is an effective measure to ensure crop production as well as to increase SOC content, particularly, in arid and semiarid areas. Northwest China is an area that has a long history of irrigation with sediment laden river water, where a special layer of anthropogenic soil or irrigation-silt has formed. The layer is quite uniform in soil color, composition, texture, calcium carbonate content, and organic carbon content. When the layer of irrigation-silted is > 50 cm in thickness, it is termed as irrigation-silted soil. The soil is > 4 g kg-1 in SOC content, even at the bottom of the irrigation-silted layer. Composition of the soil organic matter (SOM) in fraction, heavy or light is the major factor affecting stability of the SOC storage in the layer.【Method】An irrigation zone in Ningxia Province, Northwest China, was selected in the study to evaluate effects of cultivation and irrigation with sediment laden Yellow River water on content and fractionation of SOC. The Yellow River flows through northern part of Ningxia from south to north. Irrigated-alluvial soil, Light sierozem soil, Aeolien sandy soil, Fluvi-aquic soil and Fluvent soil are the types of soils commonly distributed in the Zone. Based on that, a total of 45 soil profiles were specified, including 6 in non-cultivated and non-irrigated natural fields as control, and 39 in irrigated fields different in irrigation history. Each profile was divided into four layers (0~20, 20~30, 30~60 and 60~100 cm). Soil samples were collected from the layers for analysis of SOC content and for fractionations of SOC, light and heavy by density using 1.7 g m3 NaI solution, so as to illustrate effects of the irrigation with Yellow River water on content and fractionation of SOC. 【Result】Both light and heavy SOMs were found to have increased in content after years of irrigation, but the increment varied with the duration of irrigation and the fraction. The longer the history of irrigation, the higher the content of both light and heavy OMs in the soil regardless of type. As a result of farming cultivation, including fertilizer or manure application, the contents of OM increased the most significantly (p <0.001) in the plow or surface layer (0~20 cm), and the content and the increment declined along the profile and varied with the type of soil. Irrigated-alluvial soil with a long history of irrigation was found to be highest in OM content, which implies that soil type is another important factor influencing accumulation of SOM. Compared to non-irrigated and non-cultivated soils, irrigated soils exhibited a close relationship between the fraction of light OM and the total SOM in the 0~60 cm soil layer, and the relationship weakened with increasing soil depth, but a very close relationship was found between the fraction of heavy SOM and the total SOM in all the soil layers of both irrigated and non-irrigated fields, which indicates that heavy organic matter is the major component of SOM and accumulates more rapidly than light organic matter. 【Conclusion】 Irrigation with sediment laden Yellow River water helps increase SOC storage, either light or heavy in the Ningxia Irrigation Zone. Heavy OM is the major component of SOM, while light OM is more sensitive to cultivation and irrigation. And the former plays a better role than the latter does in sequestrating soil carbon.