Definition: Nitrogen cycle is
the cyclic movement of nitrogen between atmosphere, organisms and soil.
or
Nitrogen cycle is a
continuous series of natural processes by which nitrogen passes successively
from air to soil to organisms and back to air or soil involving principally
nitrogen fixation, nitrification, decay, and denitrification.
Nitrogen
cycle is a gaseous cycle as
atmosphere is the major reservoir of nitrogen with nearly 79%.
Why nitrogen is important?
Nitrogen is
an essential element required for the synthesis of bio-molecules like proteins,
DNA, vitamins, chlorophyll, alkaloids etc. It is a critical limiting element
for plant growth and reproduction. Atmospheric N2 is extremely
stable, unreactive and inert with triple covalent bond between nitrogen atoms.
Therefore, plants and animals cannot use atmospheric N2. So it should
be converted to some other usable forms such as nitrates (NO3-).
This conversion process is primarily carried out by microorganisms. N2
The Major steps involved in nitrogen cycle
1. Nitrogen
fixation
2.
Nitrification
3. Nitrate
assimilation
4. Ammonification
5. Denitrification
and Anammox
Step 1:
Nitrogen fixation
Nitrogen fixation is the conversion of atmospheric N2
to ammonia (NH3) which can be readily utilized by the plants for
synthesis of bio-molecules.
3 Major Methods of Nitrogen fixation
a) Biological
Nitrogen fixation (BNF): carried out by prokaryotes called as
nitrogen fixers or diazotrophs. It accounts nearly 70% of natural nitrogen
fixation.
Nitrogen fixers include some bacteria like Rhizobium, blue green algae like Anabaena and lichens like Collema.
Nitrogen fixers are either symbiotic or free living.
Symbiotic N2 fixers include the bacteria Rhizobium and Bradyrhizobium found in the root nodules of leguminous plants (pea,
beans etc).
Other
examples: The water fern Azolla’s symbiosis
with a cyanobacterium Anabaena azollae.
Anabaena colonizes cavities formed at the base of Azolla fronds. The cyanobacteria fix significant amounts of
nitrogen in specialized cells called heterocysts.
Examples of free
living Nitrogen fixers include species in the genera Azotobacter, Bacillus, Clostridium, and Klebsiella.
N2
+ 8 H+ + 8 e−+ 16 ATP → 2 NH3 + H2
+16 ADP +16 Pi
N2
fixation is a high energy requiring process and N2 fixers uses 16
moles of ATP to fix each molecule of Nitrogen (N2)
b)
Non-biological N2 fixation by lightning, volcanic eruptions etc.
c) Industrial
nitrogen fixation (fertilizers) by the Haber-Bosch process
N2
+ 3H2 à2NH3
in the presence of catalyst like Ni at 5000C and 300 atm pressure.
Today, nearly
80% of the nitrogen found in human tissues originated from the Haber-Bosch
process.
This is the summarised video for better understanding
2. Nitrification
It is the
process that converts ammonia (NH3) to nitrite (NO2-)
and then to nitrate (NO3-) by nirtrifying bacteria.
Step 1: Oxidation of ammonia (NH3) to nitrite
(NO2-) or NH3 àNO2-
Microbes
involved: bacteria in the genera Nitrosomonas, Nitrosospira, and Nitrosococcus
(Nitrite bacteria)
Step 2: Oxidation of nitrite (NO2-)
to nitrate (NO3-) or NO2-à NO3-
Microbes
involved: bacteria in the genera Nitrobacter (Nitrate oxidizing bacteria)
3. Nitrate assimilation
Now soil nitrates (NO3-)
formed by nitrification can be taken up by the plants for the synthesis of
amino acids, DNA, pigments etc. This is known as nitrate assimilation.
From plants
(producers) nitrogen as bio-molecules like amino acids enters food chain and
moves to animals (consumers) and decomposers.
4. Ammonification
It is the
conversion of organic nitrogen (amino acids, DNA etc) in plant and animal
tissues to ammonia (NH3). After the death of plants and animals, various fungi,
actinomycetes and some bacteria (ammonifying bacteria) then decompose the
tissue and convert organic nitrogen (e.g. amino acids, DNA) back into the
ecosystem as ammonia. This ammonia is available for uptake by plants and other
microorganisms for growth.
5. Denitrification
It is the
conversion of soil nitrate (NO3-) to N2. It is
the process that removes fixed nitrogen (i.e., nitrate) from the ecosystem and returns it
to the atmosphere in inert form (N2).
NO3-à NO2-à NOàN2
Microbes
involved: Denitrifying bacteria include species in the genera Bacillus,
Paracoccus, and Pseudomonas.
Anammox
(anaerobic ammonia oxidation)
It is the conversion of ammonia (NH3) to
N2 using nitrite as
the electron acceptor under anoxic condition.
NH4+
+ NO2-
àN2 + 2H2O
Microbes
involved: prokaryotes
belonging to the Planctomycetes phylum of Bacteria, Brocadia anammoxidans
In some areas of the ocean, the
anammox process is considered to be responsible for a significant loss of
nitrogen
Soil nitrogen
is replenished by excretion of animals (as ammonia, urea and uric acid),
ammonification and nitrification.
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