Abstract: Two strains of bacteria E-25 and K-12 were isolated from rice rootsgrowing in Beijing area.Both E-25 and K-12 possessed significant capability of nitrogen fixation.The maximum nitrogenase activity measured by acetylene reduction assay of E-25 was 552 nmol C2H4/hr/mg protein,and K-12 was676 nmol C2H4/hr/mg protein.During the exponential growth,the total cell protein as well as nitrogenase activity increased rapidly.After the exponential growth,nitrogenase activity declined as the increase of cell protein stopped.The nitrogenase activity of E-25 and K-12,which are faculative aerobic bacteria,were higher under anaerobic or low oxygen level than under high oxygen level,while E-25 was more sensitive to oxygen than K-12.The E-25 associated with N2-fixing bacteria,isolated from rice root growing in alkaline soil in Beijing area,significantly differed from that in southern China acidic soil in the utilization of carbon source.The former uses fructose,glucose and sucrose as energy sources for nitrogen fixation,but does not use succinate and citrate.It seems that habitat difference causes the diversity of these bacteria.It issuggested that the energy for nitrogen fixation of E-25 may come from the glycalytic pathway.

Key words: Rice, Rhzosphere microorganism, Nitrogen fixation