Utilization of Fixed Nitrogen by Azotobacter and Influence on Nitrogen Fixation

Abstract

Out of 35 organic N compounds tested as sources of N for Azotobacter, grown in the absence of N gas, only urea, asparagine, aspartic acid, adenine and glutamic acid were definitely assimilated. Urea was the only one of these that was as readily utilized as ammonium salts, nitrites and nitrates. In addition, guanine, cytosine, allantoin, and uramil produced some growth after a long incubation period. With adequate molybdenum in the medium, gaseous N was utilized more readily by Azotobacter than was adenine, but with asparagine and nitrate both free and fixed N were used about equally. Little or no N fixation occurs in a medium containing ammonium salts. The extent of decrease in N fixation, due to the addition of N compounds, was usually determined by the amount of fixed N utilized. The unavailable compounds, except those that were toxic, had little effect on N fixation. These studies indicate that in most soils N fixation by Azotobacter is not markedly retarded by the nitrogenous materials present.