The Nitrogen Cycle Free nitrogen gas (N2) (N2)in atmosphere Decomposition Nonsymbiotic 7 Protein Leguminous Azotobacter Industrial from plants Beljerinckia. fixation as dead cells 风 Cyanobacteria. fertilizer Clostridium 0 Fixation (N2O) Decay organisms (aerobic and anaerobic bacteria Symbiotic Denitrifying bacteria and fungi) Rhizobium (Pseudomonas, Bradyrhizobium Bacillus licheniformis. Assimilation Nitrites Ammonia Paracoccus (NO) Ammonification (NH3) denitrificans. Nitrosomonas and others) Key Denitrification Nitrates (NO3) Nitrites Nitrification Ammonification (NOS) Fixation Nitrification Nitrobacter Denitrification Figure 27.4The Nitrogen Cycle Figure 27.4