The Nitrogen Cycle
Nitrogen is the most common element in the atmosphere at about 78%. Nitrogen is also vital for all life on Earth because amino acids (constituent of proteins) and nucleic acids (constituent of DNA) would not exist without nitrogen. In the atmosphere nitrogen exists as a very stable molecule (N2) which is unusable by plants and animals. The process of “fixing” nitrogen so that it can be used by plants and animals is carried out by bacteria. Nitrogen fixing bacteria are specialized in that they can use atmospheric nitrogen (N2) and as a byproduct release ammonia (NH4). Next, nitrite-forming bacteria combine the ammonia with oxygen, forming nitrites (NO2–). Another group of bacteria then converts the nitrites to nitrates (NO3–). Nitrites can be absorbed and used by green plants. In plants the nitrates are reduced to ammonium (NH4+) which is used to build amino acids. Animals receive their needed nitrogen from plants. Nitrogen reenters the atmosphere primarily by the actions of decomposers which break down; dead organisms, leaves that fell off in the winter, skin, hair, urine etc. Decomposers use the nitrates (ammonia and ammonium) and produce a byproduct of nitrogen gas, either N2 or N2O.