The main component of the nitrogen cycle starts with the element nitrogen
in the air. Two nitrogen oxides are found in the air as a result of interactions
with oxygen. Nitrogen will only react with oxygen in the presence of high
temperatures and pressures found near lightning bolts and in combustion
reactions in power plants or internal combustion engines. Nitric oxide,
NO, and nitrogen dioxide, NO2, are formed under these conditions. Eventually
nitrogen dioxide may react with water in rain to form nitric acid, HNO3.
The nitrates thus formed may be utilized by plants as a nutrient.
Nitrogen in the air becomes a part of biological matter mostly through
the actions of bacteria and algae in a process known as nitrogen fixation.
Legume plants such as clover, alfalfa, and soybeans form nodules on the
roots where nitrogen fixing bacteria take nitrogen from the air and convert
it into ammonia, NH3. The ammonia is further converted by other bacteria
first into nitrite ions, NO2-, and then into nitrate ions, NO3-. Plants
utilize the nitrate ions as a nutrient or fertilizer for growth. Nitrogen
is incorporate in many amino acids which are further reacted to make proteins.
Ammonia is also made through a synthetic process called the Haber Process.
Nitrogen and hydrogen are reacted under great pressure and temperature
in the presence of a catalyst to make ammonia. Ammonia may be directly
applied to farm fields as fertilizer. Ammonia may be further processed
with oxygen to make nitric acid. The reaction of ammonia and nitric acid
produces ammonium nitrate which may then be used as a fertilizer. Animal
wastes when decomposed also return to the earth as nitrates.
To complete the cycle other bacteria in the soil carry out a process
known as denitrification which converts nitrates back to nitrogen gas.
A side product of this reaction is the production of a gas known as nitrous
oxide, N2O. Nitrous oxide, also known as "laughing gas" - mild
anesthetic, is also a greenhouse gas which contributes to global warming.