Investigation of white clover ecotypes showing high nitrogenase activity under an abundant nitrogen and applying them to mixed grassland.

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(1). Ninety eight of white clover ecotypes were mainly collected from Tohoku district in Japan. The variations of some characteristics were identified among the collected white clovers.(2). From variation pattern of nitrogenase activity after an application of nitrogen fertilizer, white clovers were grouped into 5 types, i.e., 0 type; no decrease of activity, AA type: quick decrease and quick recovery, AB type: quick decrease and slow recovery, BA type: slow decrease and quick recovery, and BB type: slow decrease and slow recovery. Thus, the response of nitrogenase activity to the nitrogen application differs among white clovers.(3). The peroxidase isozyme of white clover appears in 6 bands. White clovers collected were grouped into a,b,c,d type by the different pattern of bands. The d type of isozyme pattern was closely related to 0 type of nitrogenase activity.(4). In the mixed grassland, white clover and orchard grass, the inhibition of nitrogenase activity after nitrogen application was reduced compared with white clover grassland. This is assumed as the result of the absorption of nitrate nitrogen in soil by grass species.(5). Under field condition, white clover shows the three peaks of nitrogenase activity in the year, May, July and September to October. The pattern of the seasonal variation of activity did not show significant correlation with mean temperature, sunshine hours and precipitation and N% in soil.(6). From the mentioned above, there are white clover ecotypes showing high nitrogenase activity under an abundant nitrogen and 14 of the ecotypes like that were collected in this investigation, and the nitrogenase activity of white clover will be kept higher by the improvement of nitrogen circumstance in soil with mixing culture with grass species.