Denitrification by Rhizobium fredii

Denitrification and anaerobic growth were investigated in fast‐growing isolates of Rhizobium fredii and in slow‐growing isolates of Bradyrhizobium japonicum. All isolates of R. fredii grew anaerobically, reducing NO−3 (5.5 mM) to N2O in the absence and presence of C2H2. The stationary phase of growth was reached by most fast‐growing isolates after 50 h incubation, at which time NO−3 was no longer detectable in the culture filtrate. Most slow‐growing isolates of B. japonicum, incubated under anaerobic conditions with NO−3, took twice as long to reach the stationary phase of growth as R. fredii. The terminal product of NO−3 reduction in the slow‐growing isolates was either N2O or N2.