Nitrogen fixation in paddy soils

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Abstract
The soil samples were collected from the Apg horizon of the four long-term experimental plots with different fertilizer treatments before flooding. They were incubated in beakers under submerged condition in a growth chamber at 30°C with 12 hr illumination per day. This submerged soil was regarded as a simple model system for the actual paddy field. After appropriate period of incubation, the surface water, the upper and the lower parts of the soil were analyzed for Eh, pH, contents of Fe2+, NH4 +, chlorophyll-type compounds, N2-fixing activity with C2H3 reduction method and so forth. The results obtained by using this model system coincided fairly well with the results observed in the actual paddy field. The best coincidence was found between the lower part of the soil in the model system and the lower part of the Apg horizon in the actual paddy field. However, the surface water and/or the upper part of the model system showed much higher N2-fixing activity than the flooded water and/or the upper part of the Apg horizon in the paddy field. These results were considered to be mainly caused by the much larger contribution of the N2-fixing algae to the model system than to the paddy field. In addition, the correlation between the Na-fixing activity and several chemical properties of soil such as Eh and contents of ferrous iron and easily-decomposable organic substances were much better in this model system than in the actual field. Consequently, the simple model system used in this experiment was judged to be only suitable for the detailed investigation of the N2-fixing activity brought about by heterotrophic microorganisms of non-rhizosphere soil. From another point of view, the results obtained in this experiment gave strong support to the results found by surveying the actual paddy field.