EFFECTS OF MN2- AND NH4+ BY SOIL-MICROORGANISMS( AND MG2+ ON ASSIMILATION OF NO3)

Although it has been demonstrated that Mn2+ and Mg2+ can influence the activity of glutamine synthetase in various organisms, there is littl e information concerning the effects of these cations on the activity of this enzyme in soil microorganisms or on ability of these microorga nisms to assimilate NO3- and NH4+. We studied the effects of different concentrations of Mn2+ and Mg2+ on assimilatory NO3- reduction and NH 4+ assimilation in cultures of two microorganisms commonly found in so il [Pseudomonas fluorescens (ATCC 13525) and Azotobacter chroococcum ( ATCC 9043)] and in an enrichment culture of soil microorganisms. We fo und that Mn2+ strongly inhibited NH4+ assimilation by soil microorgani sms and blocked the inhibitory effect of NH4+ on assimilatory NO3- red uctase (ANR) activity, thereby uncoupling ANR activity from nitrogen a ssimilation and causing the NH4+ formed by ANR activity to be released to the environment. Mg2+ counteracted the effect of Mn2+ on microbial metabolism of nitrogen, which suggests that the overall effect of the se cations on nitrogen assimilation by soil microorganisms will depend on the ratio of their concentrations in soil.