| 1. | Aerobic nitrate oxidation, and nitrite reduction | 
| 2. | Decomposition of organic nitrogen, and conversion of dinitrogen to ammonium compounds | 
| 3. | Enteric fermentation in cattle, and nitrogen fixation by Rhizobium in root nodules of legumes | 
| 4. | Anaerobic ammonium oxidation, and denitrification |