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Fri, Feb 14, 2025 at 3:24
Prince Pratap <pratapprince402@gmail.com>
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To: Prince Pratap <pratapprince402@gmail.com>
Nitrogen Fixation:-
A. Biological Nitrogen Fixation:-
This is a complex biological process in which atmospheric nitrogen is first converted
into ammonia and then into amino acids, which are the building blocks of proteins.
Various components such as prime, cytochrome, and ferredoxin are involved in this
process. After the death of the cell, the protein is decomposed by microorganisms,
and nitrifying bacteria convert it into nitrate and nitrite.
Nitrogen is the largest component of the atmosphere (approximately 78%). Generally,
the ability to fix atmospheric nitrogen is limited to only a few nitrogen-fixing bacteria
and blue-green algae, which convert atmospheric nitrogen into organic nitrogen in
their protoplasm.
The main nitrogen-fixing bacteria are as follows:
(a) Clostridium: Anaerobic, free-living, and soil-borne
(b) Azotobacter: Aerobic, free-living, saprophytic, and soil-borne
(c) Rhizobium: Symbiotic, saprophytic, and soil-borne
(d) Klebsiella: Symbiotic, saprophytic, and water-borne
(e) Chlorobium: Anaerobic, autotrophic, and free-living
(f) Rhodospirillum: Anaerobic, autotrophic, and free-living
Nitrogen-fixing blue-green algae are as follows:
(i) Nostoc
(ii) Anabaena
(iii) Aulosira
(iv) Gloeotrichia
(v) Calothrix
(vi) Rivularia
Root nodules are found in the roots of leguminous plants, such as peas, chickpeas,
soybeans, peanuts, pigeon peas, mung beans, lentils, etc. The bacterium Rhizobium,
capable of nitrogen fixation, is found in these nodules as an endosymbiont. It is a
non-spore-forming, non-motile, and saprophytic bacterium. It enters the roots through
root hairs, and under the influence of its infection, the cells of the cortex begin to
divide rapidly and form nodules. The newly formed cells are tetraploid (4n), while
other cells remain diploid (2n). The nodules contain a pink pigment called
leghemoglobin, which helps in nitrogen fixation.
It appears red or pink. It fixes atmospheric nitrogen for itself and its host. Nodules
lacking leghemoglobin provide protection and food to Rhizobium. In return,
Rhizobium provides useful nitrogen compounds to leguminous plants (pulses), which
aid in their growth and the formation of protein-rich seeds (Figure 42.8).
Figure 42.8
A).Nodules formed by bacteria in the roots of leguminous plants. (B).Entry of bacteria
into a root hair. (C).Bacterial colony in a nodule.
Nitrogen fixation is also found in non-leguminous plants like Alnus, Casuarina, and
Myrica. These bacteria are mainly related to Actinomycetes. Klebsiella bacteria in the
leaf nodules of Psychotria establish a significant symbiotic relationship for nitrogen
fixation.
B. Non-biological Nitrogen Fixation
(By Electric Discharge and Rains): During the rainy season, some atmospheric
nitrogen and oxygen combine to form nitric oxide (NO) due to lightning. This later
becomes nitrogen peroxide, dissolves in rainwater, and reaches the earth as nitric
acid. It combines with calcium and potassium in the soil to form nitrates.
3. Nitration or Nitrification :-
After the death of organisms, the protein is decomposed into ammonia by
microorganisms, which is then oxidized into nitrite and nitrate by nitrifying bacteria.
The oxidation of ammonia to nitrite (NO₂) and subsequently to nitrate (NO₃) is called
nitration or nitrification. This process is carried out in two specific stages by
chemosynthetic bacteria.
In the first stage, Nitrosomonas, Nitrosococcus, Nitrosospira, and Nitrosocystus (all
bacteria of the Nitrosomonas group) convert ammonia into nitrite:
2NH3 + 3O2 → 2HNO2 + 2H2O + energy
In the second stage, bacteria of the Nitrobacter group, such as Streptomyces and
Nocardia, convert nitrite to nitrate:
2HNO2 + O2 → 2HNO3 + energy
1. Ammonification :-
Many bacteria, such as Clostridia and Actinomycetes, decompose the protein
substances present in the dead organic matter of plants and animals in the soil,
resulting in the formation of amino acids, and ultimately ammonia is produced by
oxidative denitrification. If the protein is decomposed (rotted) in an anaerobic
environment, amides are formed, which upon oxidation produce ammonia. Other
major bacteria that perform ammonification are Bacillus mycoides, Bacillus ramosus,
and Bacillus vulgaris. Free ammonia is toxic and its accumulation in the soil is
harmful. Therefore, it is converted to nitrate (NO3) by nitrification.
5. Denitrification :-
Certain species of bacteria, such as Thiobacillus denitrificans and Pseudomonas
denitrificans [sometimes Achromobacter and Clostridium as well], have the ability to
release all types of (organic or inorganic) bound nitrogen back into the gaseous state
for recycling. In this way, they reduce the fertility of the soil, but play an important role
in the nitrogen cycle. The loss of nitrogen from the soil by denitrification is also
caused by seasonal flooding or excessive irrigation.
The unusable proteins of plants and animals are decomposed into ammonia by
aerobic and anaerobic bacteria after their death. This ammonia is converted into
nitrite (NO₂) and then nitrate (NO₃) by nitrifying bacteria, which becomes available
again as raw material for plants. If there is an excess of nitrate or ammonia in the
soil, denitrifying bacteria convert them back into nitrogen gas for recycling. Biological
nitrogen fixation is an additional source for increasing soil fertility.
Occasionally, humans also interfere with the natural nitrogen cycle by burning dead
organic matter on the surface of the soil or by mixing nitrogenous fertilizers to
increase soil fertility and yield.
A high amount of nitrate in drinking water causes a disease called "blue-baby
syndrome" or "methemoglobinemia" in children.
Thank you!
Generative AI is experimental.
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