The black soil region (122° ~ 132°E, 43° ~ 50°N) in Northeast China is the most important grain production region of the country. The paddy soil derived from black soil after long-term rice cultivation has become an important paddy soil resource in Northeast China. As the region is cold in weather with the soil kept frozen for a long time period each year, and the paddy soil has only a short period of time in a year remaining waterlogged, it is essential to elucidate nitrogen-mineralization capability and N supply capacity of the black soil derived paddy soil. For that end, soil investigations were carried out and soil samples collected in paddy fields in Qing’an County of Suihua, Central Heilongjiang in 2011. The soil samples were treated with the Bremner method and long-term waterlogged incubation method and analyzed for composition of soil organic nitrogen and mineralizable N in the paddy soil and relationship between the two relative to organic carbon content. Results showed that acidolyzable N and non-acidolyzable N in the soil varied in the range of 1 014 ~ 1 834 mg kg^-1 and 378 ~ 851 mg kg^-1, and averaged 383 mg kg^-1 and 633 mg kg^-1, respectively. The former accounted for 64.9% ~ 72.9% or 68.9% on average of the total N; and the latter did for 27.1% ~ 35.1% or 31.1% on average, Obviously, the former was much higher than the latter in both content and proportion to the total. Soil acidolysable N was significantly and positively related to organic carbon and total N (p<0.01) in content, with correlation coefficient being 0.985 and 0.985, respectively, and soil non-acidolyzable N (p<0.01) was too, with correlation coefficient being 0.981 and 0.977, respectively. In terms of content and the proportion of total N, components of the soil acidolyzable organic nitrogen followed an order of unknown N > amino acid N > ammonia N > amino sugar N. The content of acidolzsable ammonia N was significantly and positively related to organic carbon and total N (p < 0.01), with correlation coefficient being 0.987 and 0.973, respectively, and acidolyzable amino acid N was too, with correlation coefficient being 0.991 and 0.989, respectively, but the fraction of unknown acidolyzable N was significantly positive related to total N (p< 0.05) only, with correlation coefficient being 0.880. After 161 days of waterlogging incubation, cumulative NH₄ -N in the soil reached close to the maximum, ranging between 38.7 and 176.1 mg kg^-1, with an average being 99.3 mg kg^-1 and cumulative mineralized nitrogen content was in an extremely significant positive relationship with soil organic carbon (r = 0.975, p< 0.01), and a significantly positive relationship with total nitrogen (r = 0.957, p< 0.05), too. Nitrogen mineralization potential (N ₀) varied in the range of 38 ~175.3 mg kg^-1, and the mineralization rate constant (k ₀) in the range of 0.022~0.041 d^-1. Nitrogen mineralization potential (N0) was ultra-significantly and positively related to the cumulative NH₄ -N content , soil organic carbon and total N after 161 days of incubation, with correlation coefficient being 0.999, 0.978 and 0.962, respectively. Both soil C/N and pH were significantly and positively related to nitrogen mineralization potential (N ₀) (p < 0.01), but negatively to mineralization rate constant (k ₀) (p < 0.05 or p< 0.01). Obviously, soil organic carbon (N), C/N and pH are important factors that affect soil organic nitrogen mineralization. Among the components of soil organic nitrogen, acidolyzable ammonia N and acidolyzable amino N and non-acidolyzable N were significantly and positively related to N mineralization potential (N ₀) (p < 0.01), with correlation coefficient being 0.999, 0.986 and 0.999，respectively, but stepwise multiple regressions of the three with N mineralization potential shows that acidolyzable ammonia N was the largest contributor of mineralizable nitrogen. Path analysis further indicates that acidolyzable ammonia N and mineralizable nitrogen has a very high path coefficient (1.142), but acidolyzable amino acid N and non-acidolyzable-N a very low direct path coefficient, being 0.024 and 0.172, respectively. The findings suggest that acidolyzable ammonia N is a component that has important direct contribution to mineralizable nitrogen and hence the major source of soil mineralizable nitrogen.