Nitrogen cycle, process, steps and stages of nitrogen cycle i.e., nitrogen fixation, assimilation ammonification, nitrification, denitrification

1. NITROGEN CYCLE
Presented by Shanza Maryam

2. Introduction
Nitrogen is absolutely necessary for the
existence of life, as it is essential
component of proteins and nucleic
acids, which are vital to living
organisms.
 Luckily the earth’s atmosphere has
78.9% of nitrogen by volume

3. Although nitrogen is so important for
organisms and present in huge quantity
in atmosphere, it cannot be used by
plants and animals in gaseous and
elemental form.
Plants can only use nitrogen is present in
the form of nitrate(NO3), nitrite (NO2)
or ammonium ions(NH4).

4. The plants absorb nitrate compounds
and ammonium ions from the soil and
make proteins and nucleic acids.
whereas animals get their nitrogen
already fixed in plant tissues they eat.
The major reservoir of nitrogen is
atmosphere where nitrogen exist

5. Process of nitrogen cycle
Nitrogen cycle consists of following steps
namely:
1.Nitrogen Fixation
2.Assimilation
3.Ammonification/ Decay
4.Nitrification
5.De-nitrification
It is important to note that microorganisms play
an important role in each of these steps.

6. 1.Nitrogen Fixation
 The conversion of free or gaseous
nitrogen into nitrate compounds or
ammonia is called Nitrogen
Fixation.
There are three principal ways in
which nitrogen fixation can occur.

7. (a)Atmospheric fixation:
When lightning or thunderstorms occur,
greater quantities of atmospheric
nitrogen are turned into nitrates or nitric
acid
 These are dissolved in rain and carried
to earth.
 Plants acquire these when they absorb
water and other minerals through their
roots.

8. (b)Biological fixation
Microorganisms capable of transforming
atmospheric nitrogen into fixed nitrogen
(inorganic compounds usable by plants).
More than 90 percent of all nitrogen
fixation is effected by these organisms,
which thus play an important role in the
nitrogen cycle.

9.  Two kinds of nitrogen-fixing bacteria
are recognized. The first kind, the
free-living (nonsymbiotic) bacteria,
includes the cyanobacteria Anabaena
and Nostoc.
 The second kind comprises the
mutualistic (symbiotic) bacteria;
examples include Rhizobium,

10. The symbiotic nitrogen-
fixing bacteria invade the
root hairs of host plants,
where they multiply and
stimulate formation of root
nodules,Within the nodules
the bacteria convert free
nitrogen to ammonia, which
the host plant utilizes for its
development.

11. Nitrogenase is an
enzyme responsible for
catalyzing nitrogen
fixation, Nitrogenase
catalyzes the breaking
of triple bonds between
nitrogen atoms and the
addition of three
hydrogen atoms to each
nitrogen atom.

12. (c)Industrial fixation
It is a man-made alternative that
aids in nitrogen fixation by the use
of ammonia. Ammonia is produced
by the direct combination of
nitrogen and hydrogen. Later, it is
converted into various fertilizers
such as urea.

13. 2.Assimilation
Once the nitrogen has been fixed in
the soil, plants can absorb nitrogen
through their roots. This process of
absorption is known as assimilation.

14. 3.Ammonification
This is another process by which ammonia
can be generated. Organic remains of
plants and animals are broken down in the
soil by some bacteria to release ammonia
into the soil. These dead and waste matter
is used by these microorganisms as food
and they release ammonia into the soil

16. 4. Nitrification
Nitrification is a two-step process in
which NH3/ NH4+ is converted to NO3– .
First, the soil bacteria Nitrosomonas and
Nitrococcus convert NH3 to NO2–
(nitrites), and then another soil
bacterium, Nitrobacter, oxidizes NO2– to
NO3–(nitrates).

17. The reaction involved in the process of Nitrification is as
follows:
2NH3 + 3O2 → 2NO2
– + 2H+ + 2H2O
2NO2
– + O2 → 2NO3
–

18. 6.Denitrification
Most nitrate or ammonia compounds in
the soil are reduced again to free
molecular nitrogen (N2) which is released
back to the atmosphere. Reduction of
nitrates into free molecular nitrogen is
called De-nitrification.