Physical Cycle Sites
Henderson's Dictionary of Biological Terms, by Eleanor Lawrence. 10th Ed.
Proteins, nucleic acids, and other organic chemicals contain nitrogen, so nitrogen is a very important atom in biological organisms. Nitrogen makes up 79% of Earth's atmosphere, but most organisms can not use nitrogen gas (N2). N2 enters the trophic system through a process called nitrogen fixation. Bacteria found on the roots of some plants can fix N2 to organic molecules, making proteins. Again, animals get their nitrogen by eating plants. But after this point, the nitrogen cycle gets far more complicated than the carbon cycle.
Animals releases nitrogen in their urine. Fish releases NH3, but NH3 when concentrated, is poisonous to living organisms. So organisms must dilute NH3 with a lot of water. Living in water, fish have no problem with this requirements, but terrestrial animals have problems. They convert NH3 into urine, or another chemical that is not as poisonous as NH3. The process of releases NH3 is called ammonification.
Because NH3 is poisonous, most of the NH3 which is released is untouchable. But soil bacteria have the ability to assimilate NH3 into proteins. These bacteria effectively eats the NH3, and make proteins from it. This process is called assimilation.
Some soil bacteria does not convert NH3 into proteins, but they make nitrate NO3- instead. This process is called nitrification. Some plants can use NO3-, consuming nitrate and making proteins. Some soil bacteria, however, takes NO3-, and converts it into N2, returning nitrogen gas back into the atmosphere. This last process is called denitrification, because it breaks nitrate apart.