EFFECTS OF DIFFERENT NITROGEN-SOURCES ON NITROGEN-FIXATION AND BACTERIAL-GROWTH OF PANTOEA AGGLOMERANS AND AZOSPIRILLUM SP IN BACTERIAL PURE CULTURE - AN INVESTIGATION USING N-15(2) INCORPORATION AND ACETYLENE-REDUCTION MEASURES

Due to energetic limitations the microbiological associative nitrogen fixation from the air is solely not able to cover the nitrogen demand of the plant. It is therefore necessary to find diazotrophic strains w hich are able to fur atmospheric nitrogen in presence of additional ni trogen sources and to excrete nitrogen into the surrounding medium. Th e diazotrophic strains Azospirillum sp. and Pantoea agglomerans were p roved to fix Nz in presence of ammonium, nitrate, glutamine and glutam ate in nutrient broth as sole nitrogen sources. Azospirillum sp. was c ompletely inhibited in its nitrogenase activity after addition of 1 mM glutamine, 1 mM KNO3 and 2 mM NH4Cl and higher concentrations of thes e nitrogen sources. The total inhibition of nitrogenase activity of Az ospirillum sp. induced by 50 mg NH4+ - N1(-1) was derepressed by simu ltaneous addition of L-Methionine-DL-sulfoximine (MSX) the inhibitor o f glutamine synthetase (GS). Pantoea agglomerans, however, showed a di fferent response to ammonium, nitrate, glutamine and glutamic acid app lication than Azospirillum sp. The ammonium induced partial repression of nitrogenase activity was not derepressed by addition of MSX. Nitro genase activity of P. agglomerans was stronger inhibited by glutamic a cid than by ammonium or glutamine and was uneffected by nitrate. The n itrogenase activity analysed by acetylene reduction was compared to th e amount of N-15(2) incorporated into the bacterial biomass following incubation with N-15(2) labelled air. The good correlation (r = 0.81) of acetylene reduction activity and the amount of biologically fixed nitrogen (estimated by N-15(2) incorporation measures with Azospirillu m spp.) remains stable after the addition of 50 mg NO3- - N1(-1) or 5 0 mg NH4+ - N1(-1). These results allowed us to use the acetylene red uction activity as a measure of biological nitrogen fixation by associ ative bacterial strains independent on the nitrogen source applied.