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

(1)。98个白三叶生态型主要采自日本东北地区。在采集的白三叶中发现了一些性状的变异。(2)从追施氮肥后固氮酶活性的变化规律来看，白三叶可分为5种类型，即0型：活性不降低，AA型：下降快，恢复快，AB型，下降快，恢复慢，BB型，下降慢，恢复慢。因此，不同白三叶的固氮酶活性对施氮量的反应是不同的。白三叶的过氧化物酶同工酶有6条谱带。采集的白三叶按带型不同分为a、b、c、d四种类型。同工酶的D型与0型固氮酶活性密切相关。在混播草地、白三叶和果园草中，施氮对固氮酶活性的抑制作用比白三叶草地有所减轻。这可能是由于草种对土壤中硝态氮的吸收所致。大田条件下，白三叶的固氮酶活性在5、7、9~10月份出现三个高峰。活动季节变化与平均气温、日照时数、降雨量及土壤中N%无显著相关性。由此可见，有14个白三叶生态型在氮素充足的条件下表现出较高的固氮酶活性，通过混种栽培改善土壤氮素环境，白三叶的固氮酶活性将保持较高水平。

(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.