Enhancing the indigenous ability of biological N 2 fixation (BNF) in paddy soils to increase N fertility is important to develop sustainable rice cultivation systems. This study aimed at evaluating the effect of light conditions and long-term soil management on the seasonal changes in the BNF activity of paddy soils associated with the decomposition of rice straw and cellulose, by determining the acetylene reduction activity (ARA) and N enrichment in the Tohoku region of Japan under cool temperate conditions. ARA of the straw applied on the soil surface showed a peak in the mid-cropping season in the summer. The ARA of the straw applied in autumn was 30% of that in spring application. Carbon use efficiency for N enrichment with cellulose was about 3 times higher under light than dark conditions. Among the long-term soil management practices, continuous application of fresh organic materials such as rice straw and manure considerably enhanced N enrichment and ARA. In the soil without P, the inhibition of N enrichment by the low P availability appeared in the early cropping season. Soils from paddy fields converted to upland fields (hereafter referred to as "converted upland soils") showed the highest level of N enrichment when the soils were amended with cellulose. The highest level of N enrichment with and without cellulose amendment was estimated to be 1.6-2.0 g m -2in one cropping season. The results indicated the important contribution of BNF even under cool temperate conditions and the significant effect of soil management related to organic matter application and the water regime on the magnitude of BNF and, therefore, possibly on the N cycling of the paddy soil systems.

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

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