NITROGENASE ACTIVITY OF RHIZOSPHERE NITROGEN-FIXING BACTERIA IN PURE AND MIXED CULTURES

Five nitrogen-fixing mixed bacterial cultures with high nitrogenase ac tivity were isolated from samples of rhizosphere soil. The taxonomic c omposition, relative content of individual populations in the bacteria l associations, and growth and functional characteristics of these mix ed cultures remained constant after eight successive culture transfers on solid and liquid media devoid of nitrogen. Pure bacterial cultures isolated from these associations had a nitrogenase activity 10 - 25 t imes lower than the original mixed cultures. In addition, the developm ent of bacteria in the associations prolonged the period of their acti vity, while pure cultures died off soon after growth cessation. It was found in laboratory experiments with rape seedlings that mixed cultur es of nitrogen-fixing bacteria acclimate well in the rape rhizosphere and show high nitrogenase activity (220 - 380 nmole C2H4 per h).