We investigated the relationship between nitrogen enrichment by biological nitrogen fixation(BNF) and several soil managements in ways to pack soil into inverted polyethylene bottles, with opened bottoms and mouths set on a net in a paddy field in the Tohoku district of Japan. The polyethylene bottles were removed about 4 months later and the packed soils were divided into two layers; surface (1-2cm in depth) and next (1-2cm<). The amount of nitrogen enrichment was calculated by multiplying the difference in nitrogen contents between the two layers by dry weight of the surface layer soil. The results are as follows. 1) The amount of nitrogen enrichment by BNF was measured mostly within 1-3 g m^<-2> and that in common paddy soils without application of organic matter was 2.0gm^<-2>. The effects of soil management on nitrogen enrichment was shown in the following order: surface-spreading rice straw>puddling>mixed-rice straw>non-treatment. Especially, the effect of surface-spreading rice straw was remarkably high on drained paddy soils. 2) Nitrogen enrichment under surface-spreading rice straw was inhibited by the application of nitrogen fertilizer, and this inhibition in creased along with the movement to a higher concentration of ammonium nitrogen in the soil solution measured late in July. 3) The amount of nitrogen enrichment was correlated negatively with both total nitrogen content of soil and the concentration of ammonium nitrogen by soil incubation under submerged conditions. 4) The surface and next layer of soil were incubated under submerged conditions and a rate of mineralization of nitrogen enrichment was calculated from the difference in concentration of ammonium nitrogen between the two layers. The rate of mineralization was negatively correlated with the amount of nitrogen enrichment.