自然農法におけるイネ品種の生長と根面および根内の窒素固定菌の動態

This study was conducted to evaluate the suitability of rice varieties for nature farming by analyzing the relationship between the function of nitrogen-fixing rhizobacteria and the growth of rice plants cultivate in rice fields under nature and conventional (control) farming conditions.<BR>In 1994, four rice varieties (Koshihikari, Nihonmasari, J195 and J235) and two rice varieties (Koshihikari and J195) were grown in fields under nature and conventional farming conditions in Shimada City and Yaizu City, respectively, Shizuoka Prefecture. The number of nitrogen-fixing rhizobacteria was found to change with the growth stages of rice varieties in relation to the changes in dry matter weight and nitrogen content in the plants both under nature and conventional farming conditions. The varieties with a higher number of nitrogenfixing rhizobacteria at the early growth stages (from planting to the maximum tiller stage) showed a higher increase of the ratio of dry matter weight and nitrogen content in plants at the early growth stages. On the other hand, the varieties with a higher number of nitrogenfixing rhizobacteria at the late growth stages (from heading to the ripening stage) showed a higher increase of the ratio of dry matter weight and nitrogen content in plants of the late growth stages. The number of nitrogen-fixing rhizobacteria under nature farming was found to be higher than that in the conventional farming. It is noted that, under nature farming, the varieties with a higher number of nitrogen-fixing rhizobacteria at the late growth stages showed high rough rice yields compared with those under conventional farming. It is suggested that selection of the varieties with higher number of nitrogen-fixing rhizobacteria at the late growth stages might be effective for the breeding of high-yielding varieties under nature farming. Japanese native J195 line used in this experiment may be a suitable material for the breeding of high-yielding varieties under nature farming or for sustainable agriculture, because this line showed a higher number of nitrogen-fixing rhizobacteria at the late growth stages and higher yield under nature farming.<BR>The growth stage when the number of nitrogen-fixing rhizobacteria was high under nature farming differed depending on the locations (Yaizu and Shimada) where the experiments were conducted. The stage was early in Yaizu, whereas late in Shimada. The Differences in the experimental results with the locations might be attributed to the kind of organic manure used. Namely, rice bran applied in Shimada had a high C/N ratio compared to Chinese milk vetch applied in Yaizu, resulting in a slower effect of organic manure on the growth of the plants. The type of organic manure applied should be considered in the breeding process for developing rice varieties suitable for the nature farming or sustainable agriculture.