Effective utilization of the biological nitrogen fixation (BNF) activity is important for enhancing the N fertility of paddy soils and for developing sustainable rice cultivation systems. To analyze the soil factors that affect BNF in paddy soils, in this study, the effects of the temperature, water regime, and longterm soil management on ¹⁵N₂ fixation were examined in relation to the decomposition of organic matter in incubation experiments. Within the range of 15-30℃, heterotrophic ¹⁵N₂ fixation under dark conditions changed almost proportionally to the formation of CO₂+CH₄ with glucose and straw as C sources by increasing the temperature, while the C use efficiency (N₂ fixed/(CO₂+CH₄)) was relatively higher at low temperatures in the presence of cellulose. The examination of the effect of the water regime on heterotrophic ¹⁵N₂ fixation indicated that flooding after aerobic conditions promoted heterotrophic ¹⁵N₂ fixation as well as the decomposition of cellulose. Among the soils with different types of management, the soils amended with manure and rice straw showed the largest photodependent ¹⁵N₂ fixation. On the other hand, the soils with a lower content of mineralizable-N tended to depend more on heterotrophic ¹⁵N₂ fixation. Soils from paddy fields converted into upland fields (hereafter referred to as “converted upland soils”), particularly showed a high heterotrophic ¹⁵N₂ fixation. Overall, it appeared that the management of organic matter application and the water regime may result in significant variations in BNF in paddy soils. The results obtained suggested that further studies should be conducted on the C and N metabolism involved in BNF during the decomposition processes of organic matter along with the changes in the soil redox status to identify methods for efficient soil management to promote BNF.