Nitrogen Fixation (by bacteria)
Nitrogen gas converted to Ammonia or Ammonium by bacterial metabolism
Nitrogen Fixation (by lightning)
Atmospheric Nitrogen split by being hit by lighting can combine with H20 in the atmosphere to from HNO2
Ammonia is oxidized to form NO2- ions and NO3- later. These are food for plants which become food for animals
When an animal forms ammonia in its system after eating plant proteins, or after it dies and decomposes.