Abstract
Algal nitrogen fixation in grassland was investigated by using acetylene reduction method. Nostoc sp. crusts grown in the mixed grassland of shiba (Zoysia japonica) and common lespedeza (Kummerowia striata) showed the acetylene reducing activity (ARA), and its activity was about the same level as that of common lespedeza on the basis of unit area of the grassland. However, the activity in units per gram of fresh weight was about 1/10 in compare to that of common lespedeza root and was only 1/800 to that of the nodules, respectively. Besides, it was observed that the ARA of Nostoc sp. crusts was higher under the light condition than the dark one. Although Nostoc sp. crusts in grassland were in the wide ranges of water contents (98-31%) and the ARA appeared to highly correspond to their water content, the highest activity was recognized at about 25% of water content under laboratory condition. The rewetting treatment to the dried Nostoc sp. crusts made their ARA increase rapidly beyond expectation. It was proved from this investigation that algal nitrogen flxation such as Nostoc sp. crusts was one of the important factors affecting nitrogen input into grassland and their activity was mainly restricted by water supply to their growing sites.