Molybdenum and Vanadium - Inorganic Nitrogen and Sulphur Metabolism
Molybdenum must be available in nature in order to meet the demand of Mo for the formation of nitrogenase necessary for N2-fixation. Now, it is known that MO inhibits formation of Va containing nitrogenase.
As you may know the relative distribution of Mo and Va in nature is in favour of Va. Therefore it is quite likely that some ecological habitats could be deficient in Mo not in Va and N2-fixers of such habitat are expected to produce Va containing nitrogenase for N2-fixation. Accordingly, there could be two nitrogen cycles functioning in nature one mediated by Va system and the other mediated by Mo system.