Abstract:The nitrogenase activities of endophytic diazotrophs in Neyraudia reynaudiana and Cynodon dactylon, which are the pioneer grasses for soil and water conservation, were studied by acetylene reduction assay (ARA). The results showed that the communities of the endophytic diazotrophs isolated from the stems of Neyraudia reynaudiana and the leaves of Cynodon dactylon showed high nitrogenase activities. Forty strains were isolated and purified on nitrogen-free medium, in which twenty-six strains showed weak nitrogenase activities, and fourteen had no nitrogenase activities. The natural communities of endophytic diazotrophs showed high nitrogenase activity under facultative anaerobic conditions or covered by paraffine. The communities of endophytic diazotrophs under laboratory-cultured showed lower nitrogenase activity than that of the natural communities.