The nitrogen cycle consists of 5 main stages: (1) Ammonification is the conversion of organic nitrogen into ammonium by microbes. (2) Nitrification is the oxidation of ammonium to nitrite and then nitrate by bacteria. (3) Plants uptake nitrate and convert it into amino acids and other nitrogen compounds. (4) Nitrogen fixation converts atmospheric nitrogen to ammonium via bacteria and cyanobacteria. (5) Denitrification reduces nitrates back to nitrogen gas which re-enters the atmosphere. The nitrogen cycle involves the circulation of nitrogen through air, soil, plants and animals via specialized microbes.

1. Nitrogen cycle

2. Biology Discussion
5 Stages of Nitrogen Cycle (With Diagram)
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Nitrogen being 79 per cent of the atmosphere, the atmospheric phase is predominant in the
global nitrogen cycle. It is required by organisms in the synthesis of proteins, nucleic acids,
and other nitrogenous compounds.
Atmospheric nitrogen serves as the ultimate source. But aerial nitrogen, the most abundant
component of air, is rather inert chemically and actually cannot be used as such by the
majority of organisms.
It must first be “fixed” by specialized organisms or by industrial processes.
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From an ecological perspective, the nitrogen cycle consists of the following
stages:
(i) Ammonification
(ii) Nitrification,
(iii) Nitrogen uptake by plants,
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(iv) Fixation of Nitrogen, and
(v) Denitrification
Ammonification:
Ammonification is a process in which the organic nitrogen of plants and animals after their
death is converted to ammonium ions (NH4) by the action of saprotrophic fungi and
bacteria. The saprotrophs use the ammonia (NH3) to synthesize their own proteins and
other nitrogen-containing organic compounds.
Nitrification:

3. Ammonium ions added to the soil by ammonification, are soon oxidized by a process known
as nitrification. It takes place in two stages. In the first stage, ammonium (NH4
+) is
converted to nitrite (NO2-). This reaction involves the addition of oxygen to ammonia, giving
rise to hydroxylamine (NH2OH), which is further oxidized to nitrite. This reaction is
completed by the bacteria such as Nitrosomonas, Nitrosospira, Nitrosolobus and
Nitrosococcus (Hamilton, 1988).
The second stage of nitrification involves the oxidation of nitrite (NO2) to nitrate (NO3) by
bacteria of the genera Nitrobacter, Nitrospira and Nitrococcus. The reaction proceeds by the
addition of water followed by the removal of hydrogen (Hamilton, 1988). The bacteria
responsible for these reactions occur in the same soil. The association between
Nitwsomonus and Nitrobacter has been described as one of commensalism (Gooday, 1988).
Nitrogen uptake by Plants:
Nitrate (NO3
–) formed in the process of nitrification is used by most plants as a mineral
metabolite and may be converted by them into amino groups and other nitrogen- containing
compounds. Nitrates are also added to the soil through rock dissolution and combination of
atmospheric nitrogen with oxygen by lightning (nitrates so formed reach the soil by rain).
However, many plants also absorb ammonium from the soil.
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Fixation of Nitrogen:
The reduction of atmospheric nitrogen (N2) to the ammonium ion (NH+
4) is called nitrogen
fixation. This process can only be carried out by certain species of bacteria and
cyanobacteria (Postgate, 1988). Some of these bacteria are free-living, occurring in soil or
water (saprophytic bacteria like Azotobacter and Clostridium ; others exist in symbiotic
relationship with plants of the Family Legxuninosae, e.g., nodule bacteria Rhizobium
leguminosarum).
Species of the genus Rhizobium occur in the soil until they infect a damaged epidermal cell
or root hair. The plant responds to this infection by producing root nodules, about 1 to 3
mm in diameter. These root nodules contain leg hemoglobin, which functioning like
hemoglobin, transports oxygen.
As these bacteria are aerobic, some oxygen is required for the bacteria to survive, but too
much oxygen inactivates the enzyme nitrogenase that catalyses nitrogen fixation. The
nitrogen molecule (N2) is very stable and 16 molecules of ATP are needed for each molecule
of nitrogen that is fixed. In the soil microorganism Klebsiella pneumoniae a total of 17
genes, called ‘nif’ genes are known to be responsible in nitrogen fixation.

4. Researches in biotechnology are now attempting transfer of ‘nif’ genes from prokaryotes to
crop plants so that yield of crops like rice and wheat may be increased. However, scientists
have not yet succeeded in this attempt.
Denitrification:
It is a process in which the nitrate ion (NO3) is reduced to nitrogen dioxide (NO2), di-
nitrogen oxide (N2O), nitrogen monoxide (NO) or nitrogen (N2) by certain soil bacteria like
Pseudomonas denitrificans. Thus, nitrogen is liberated into the atmosphere. Plants also lose
small amounts of nitrogen to the atmosphere as gaseous ammonia, N2O, NO2 and NO
especially when well fertilized with nitrogen (Wetselaar and Farquhar, 1980).
Thus, nitrogen cycle (Fig. 5.8) depends upon at least four different kinds of bacteria known
as the decay causers, the nitrifiers, the denitrifiers, and the nitrogen-fixers and there is a
regular circulation of nitrogen through the air, soil, plants and animals.
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