Nitrogen Cycle

Nitrogen is required for the manufacturing
of all amino acids and nucleic acids; however,
the average organism cannot use atmospheric
nitrogen for these tasks and as a result is
dependent on the nitrogen cycle as a source for
its usable nitrogen. The nitrogen cycle begins with
nitrogen stored in the atmosphere as N2 or
nitrogen stored in the soil as ammonium (NH4+),
ammonia (NH3), nitrite (NO2-), or nitrate (NO3).
Nitrogen is assimilated into living organisms
through three stages: nitrogen fixation,
nitrification, and plant metabolism. Nitrogen
fixation is a process which occurs in prokaryotes
in which N2 is converted to (NH4+). Atmospheric
nitrogen can also undergo nitrogen fixation by
lighting and UV radiation and become NO3-.
Following nitrogen fixation, nitrification occurs.
During nitrification, ammonia is converted into
nitrite, and nitrite is converted into nitrate.
Nitrification occurs in various bacteria. In the
final stage, plants absorb ammonia and nitrate
and incorporate it into their metabolic pathways.
Once the nitrogen has entered the plant
metabolic pathway, it may be transferred to
animals when the plant is eaten. Nitrogen is
released back into the cycle when denitrifying
bacteria convert NO3- into N2 in the process of
denitrification, when detritivorousbacteria
convert organic compounds back into ammonia
in the process of ammonification, or when
animals excrete ammonia, urea, or uric acid.
A lot of environmental problems are caused
by the disruption of the nitrogen cycle by human
activity, some of the problems caused range from
the production of troposphere (lower
atmospheric) smog to the perturbation of
stratospheric ozone and contamination of ground
water. An example of one of the problems caused
is the formation of greenhouse gas. Like carbon
dioxide and water vapor greenhouse gas traps
heat near the earth’s surface and destroys the
stratospheric ozone. Once that occurs nitrous
oxide in the earth’s atmosphere is broken down
by UV light into nitrogen dioxide and nitric oxide.
These two products can reduce the ozone.
Nitrogen oxides can be changed back into nitrates
and nitrite compounds and recycled back into the
earth’s surface.