Nitrogen fixation

1. Biological Nitrogen
fixation
Ajit Kumar Singh
B. Sc. (H) Biotechnology

2. Introduction
The biological availability of N,P & K are
considerable an economic importance because
they are the major plant nutrients derived from
th e soil.
Of the three nitrogen is the most important one
this element is a key building block of protei n
molecule upon which life is based.
So the conversion of molecular nitrogen into
ammonia is known as Nitrogen Fixation.

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4. Plant incapable to
Use this N₂ gas
Nitrogen is major
nutrient for plants
Atomospheri
c
N₂
Triple bond makes
Highly inert
N₂
Utilization of atmospheric nitrogen gas as a source of cell nitrogen
by way of its reduction to amonia is called nitrogen fixation.

5. Sources of nitrogen
• Atmospheric Nitrogen
-78% of atmosphere
-Plants cannot utilize this form
-Some Bacteria, Blue Green Algae &
leguminous plants
• Nitrates, Nitrites and Ammonia
• Amino acids in the soil
• Organic Nitrogenous compounds in

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Generally, physical nitrogen fixation takes place in the rainy season during
lightning, thunderstorm and atmospheric pollution.
This type of fixation can be discussed in further two divisions mentioned below:
Non-biological or Physical
Nitrogen Fixation
Natural Nitrogen Fixation
The reactions involved are:
N2+O2(Lightning)→2NO(NitricOxide)
2NO+O2→2NO2Oxidation(Nitrogenperoxide)
Following the chemical reaction involved in the
process:
2NO2+H2O→HNO2+HNO3
HNO3+CaorKsalts→CaorKnitrates

8. Industrial Nitrogen Fixation:
Fig: Production of ammonia by
HABER PROCESS

9. Biological Nitrogen Fixation
The process of conversion of atmospheric nitrogen into
nitrogenous compounds by microorganisms such as bacteria, fungi
and algae is known as Biological Nitrogen Fixation (BNF) or
diazotrophy.
Some prokaryotes such as bacteria and cyanobacteria that can fix
atmospheric nitrogen are called nitrogen fixers or diazotrophs.
The atmospheric nitrogen is reduced to ammonia in the presence
of a catalyst known as nitrogenase. This enzyme is found naturally
in certain microorganisms like symbiotic (Rhizobium and Frankia)
and non-symbiotic or free-living (Azospirillum, Azotobacter and

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Symbiotic Nitrogen Fixation
Symbiotic nitrogen fixation is an example of a mutualistic
relationship between plants and microbes.
The various types of biological symbiotic nitrogen fixation
1. Nitrogen Fixation through Nodule formation
2. Nitrogen Fixation through Non-nodulation

11. B. Nitrogen Fixation through Nodule Formation in Non-Leguminous Plants:
1. Many plant species from families other than Leguminosae are also
known to produce root nodules.
2. Some of the important examples of non-leguminous plants that
produce root nodules and fix nitrogen include:
a. Genus Frankia forms root nodules in association with Alnus sp.,
Casuarina equisetifolia, Myrica gale, etc.
A. Nitrogen Fixation through Nodule formation in leguminous plants:
Symbiotic nitrogen fixers are found in a large number of legume plants,
mainly the genus Rhizobium.

12. 1 2
3 4
Nitrogen Fixation through Nodule formation

13. 2. Nitrogen Fixation through Non-
nodulation
There are few plants in which symbiotic nitrogen fixation takes
place, but nodules are not formed. This type of association is
known as Pseudo symbiotic or Pseudo symbiosis.
Following are some examples:
a. Lichens, an association with fungi and algae (e.g., cyanobacteria
or green algae)
b. Anthoceros (Bryophyte) associated with Nostoc.
c. Azolla (fern) in association with Anabaena.

14. Non-symbiotic Nitrogen Fixation
Biological nitrogen fixation by microorganisms living freely or
staying without direct contact with plant cells is called non-
symbiotic biological nitrogen fixation.
Soil contains several free-living nitrogen-fixing
microorganisms.
These involve many aerobic and anaerobic bacteria and
blue-green algae.

16. T H A N K
Y O U