The Role of Traces of Molybdenum in the Physiology and Agrobiology of Azotobacter

A study has been made of the interrelationships existing between the influences of molybdenum, vanadium, natural soil humate, and synthetic sugar humates upon the growth of strains of Azotobacter cultivated in free nitrogen and in a variety of fixed nitrogen compounds at different temperatures over the physiological range 20° 40°C. A. chroococcum Strain B8, obtained from a garden soil in Moscow, U.S.S.R., gave more response to addition of molybdenum, and fixed nitrogen faster under optimum conditions, . than did A_. vinelandii, as indicated in the following table giving the growth velocities (0.69/ generation times) in different forms of nitrogen, at about optimum temperature (S3 36°). It is evident that, contrary to a long-established opinion, it is quite possible for Azotobacter to grow faster in nitrogen gas as a source of nitrogen than in probably most fixed nitrogen compounds except ammonia and possibly urea, providing adequate Mo is present. When Mo is partially deficient, as indicated in the table, many fixed nitrogen compounds may be superior sources of nitrogen for nutrition. According to our recent results, Mo has no influence on the growth of Azotobacter in fixed nitrogen, providing N2 gas is not present, as was the case in the experiments reported in Table I. These findings have an obvious bearing on the question of nitrogen fixation by Azotobacter in soils, especially in relation to their molybdenum content. Van Niel has shown that molybdenum can be sufficiently deficient in soils to prevent the growth of Azotobacter under conditions otherwise entirely favorable for its multiplication. As is evident from Table II, the concentrations of Mo in such soils are of the same order as those we have found to be limiting in solution cultures, due allowance being made for the heterogeneous nature of the soil.