nitrogen in the atmosphere is absorbed into the ground to create nitrogen fixating bacteria. Also, plant matter is eaten by animals and becomes crud. In the ground the crud becomes decaying organic matter that either goes back into the air or becomes free nitrates, nitrates, or ammonia. If it becomes ammonia or nitrates/free nitrates it becomes denitrifying bacteria. the denitrifying bacteria goes back into the atmosphere. the nitrogen in atmosphere can become green and blue age in the water that fish eat. When the fish die they become decaying organic matter that turns into free nitrates, nitrates, or ammonia witch can become denitrifying bacteria and the cycle goes on.