EFFECT OF AMMONIUM ON THE NITROGEN-FIXING ACTIVITY OF MIXED CULTURES OF RHIZOSPHERIC DIAZOTROPHIC BACTERIA

Five nitrogen-fixing mixed bacteria cultures (associations) exhibiting high and stable nitrogenase activity during laboratory cultivation we re isolated from rhizospheric soil samples. Under microaerophilic cond itions, complete inhibition of nitrogen fixation in diazotrophs occurr ed in the preseuce of 10(-3) M of NH4+. Under aerobic conditions, ther e was a 1.2 - 3-fold increase in nitrogen fixation at a low (10(-6) M) concentration of ammonium in the medium. After a short-term incubatio n with ammonium, pure cultures showed a lesser decrease in activity th an mixed cultures. After prolonged exposure to ammonium, mixed culture s retained higher nitrogenase activity than pure cultures. It was foun d that the doses of ammonium fertilizer used in modern agricultural pr actice yield an ammonium concentration in the soil solution sufficient to inhibit nitrogenase activity in diazotrophic associations.