Nitrogen fixation in paddy soils

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Abstract
The 4 long-term experimental plots (Umbric haplaquept) with different fertilizer treatment at Cent. Agric. Exp. Sta. in Konosu City, Saitama Prefecture, were used for the sites of investigation. The 4 plots were NF (applied with no fertilizer), IF (applied with inorganic fertilizers), GM (applied with green manure and CaCO2), and OM (applied with manure and inorganic fertilizers). Flooded water, floating weed, upper (0-2cm) and lower (2-10cm) parts of Apg horizon and rhizosphere were collected from each plot before flooding, during flooding, and after drainage. These samples were analyzed for N2-fixing activity by acetylene reduction method, pH, Eh, and contents of Fe2+, NH4 +, chlorophyll-type compounds, and water-soluble carbohydrates. The N2-fixing activity of all samples showed almost the same pattern of change with time: very low before flooding, rapidly increased after flooding, the maximum value at the maximum tillering stage of rice plant, declined afterwards and reached a very low value after drainage. Rough estimation of the “N2-fixing capacity” of each part of the paddy field revealed that the most important site of the N2 fixation was the reduced Apg horizon, that the importance of flooded water and/or the oxidized layer in the N2 fixation was rather low except in infertile soil, and that the role of rhizosphere in the N2 fixation could not be neglected also in Japan. Reduced condition and content of easily decomposable organic substances were judged to be main factors which control the N2-fixing activity in the flooded soil on the basis of correlations between the Nt-fixing activity and several analytical data of the paddy soils.

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