- 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.