Relative to dry land, paddy field originally referred to artificially irrigated fields, where either paddy rice or upland crops were planted. Now only the field where paddy rice or some other aquatic plants are planted is called paddy field, or rice field. Different from the orginal paddy field, it has ridges to store water. In South China, paddy fields are not only long in cultivation history but also be vast in area and studies on such paddy fields started quite earlier and are great in volume both at home and abroad, but only a few has been reported on paddy soil in North China. Rice is the main grain crop in China. A large proportion of the total grain output in this country comes from Northeast China, where the paddy fields reach over 66.7 billion hm² (including 53.3 billion hm² in Heilongjiang, and 6.6 billion hm² each in Jilin and Liaoning) and occupy an important position in the rice cultivation area of the country. However, the rice fields are mostly short in cultivation history and most of the paddy fields were turned from upland fields, and what’s more, some of the paddy fields were turned back into upland fields again due to incomplete water conservancy facilities. During this kind of irregular changes, the soils in the fields would certain change in carbon fixation capacity, soil fertility and CO₂ emission. But little has been done at home and abroad on how they evolve. Therefore, the study on paddy soils in Northeast China is of great significance. Rice yield depends mainly on soil fertility, of which soil organic matter is an important component. Soil humus, being an important component of the soil solid phase material, plays a huge role in the genesis and development of soil fertility, and hence is also an important indicator for soil fertility. It does not only influence productivity of the farmland ecosystem, but also have a great impact on CO₂ concentration in the atmosphere through decomposition of soil organic matter. The study on how soil organic matter in the paddy field varies in content and composition is very important to making full use of land resources, realizing quality crop production, increasing grain output and being friendly to the environment. Paddy fields different in cultivation history (5 a，20 a and 50 a) in Qingshan Village, Chaoyangshan Town of Panshi City in Jilin Province, were selected as subjects for the study on changes in content of soil organic carbon (SOC), composition of water soluble substance (WSS), fulvic acid (FA), humic acid (HA) and humin (Hu), structure of HA and content of soil nutrients as a function of cultivation histroy. Results show that with rice cultivation going on from 5a to 50a, changes occurred in the surface (0~15 cm) and the subsurface (15~30 cm) soil layers. (1) The electric conductivity increased by 74.8% and by 113.7% respectively and soil available nitrogen, available phosphorus and available potassium by 0.24%, 15.8% and 29.7% and by 0.24%, 6.92% and 14.7%, respectively , while soil pH remained almost unchanged; (2) Total soil organic carbon increased by 42.1% and by 47.9%, respectively, and WSS, HA and Hu increased by 28.4%, 51.9% and 76.2% and by 28.4%, 84.9% and 70.1%, respectively, while FA decreased by 9.2% and by 1.74%, respectively, which indicates that rice cultivation in paddy field favors accumulation of organic carbon in the soil; (3) HA/(HA+FA) ratio (PQ) increased, indicating that humification of the soil is intensified; and . (4) H/C, ∆LgK, E2920/1720 and E2920/1620 of HA all decreased, indicating that HA in the soil is enhanced in aromatization and condensation and becomes more complicated in structure.