MERRYLAND INTERNATIONAL SCHOOL

DEPARTMENT OF BIOLOGY GRADE-7

NITROGEN CYCLE

What is Nitrogen Cycle?

“Nitrogen Cycle is a biogeochemical process which transforms the inert nitrogen
present in the atmosphere to a more usable form for living organisms.”

Furthermore, nitrogen is a key nutrient element for plants. However, the abundant
nitrogen in the atmosphere cannot be used directly by plants or animals.

[Nitrogen Cycle is a biogeochemical process through which nitrogen is converted into

many forms, consecutively passing from the atmosphere to the soil to organism and
back into the atmosphere.]

It involves several processes such as nitrogen fixation, nitrification, denitrification,

decay and putrefaction.
The nitrogen gas exists in both organic and inorganic forms. Organic nitrogen
exists in living organisms, and they get passed through the food chain by the
consumption of other living organisms.

Inorganic forms of nitrogen are found in abundance in the atmosphere. This nitrogen
is made available to plants by symbiotic bacteria which can convert the inert nitrogen
into a usable form – such as nitrites and nitrates.
Nitrogen undergoes various types of transformation to maintain a balance in the
ecosystem. Furthermore, this process extends to various biomes, with the marine
nitrogen cycle being one of the most complicated biogeochemical cycles.

Stages of Nitrogen Cycle

Process of Nitrogen Cycle consists of the following steps –
Nitrogen fixation, Nitrification, Assimilation, Ammonification, and Denitrification.
These processes take place in several stages and are explained below:

Nitrogen fixation
It is the initial step of the nitrogen cycle. Here, Atmospheric nitrogen (N2) which is
primarily available in an inert form is converted into the usable form -ammonia
(NH3).

During the process of Nitrogen fixation, the inert form of nitrogen gas is deposited
into soils from the atmosphere and surface waters, mainly through precipitation.
Later, the nitrogen undergoes a set of changes, in which two nitrogen atoms get
separated and combines with hydrogen to form ammonia (NH4+).

The entire process of Nitrogen fixation is completed by symbiotic bacteria which are
known as Diazotrophs. Azotobacter and Rhizobium also have a major role in this
process. These bacteria consist of a nitrogenase enzyme which has the capability
to combine gaseous nitrogen with hydrogen to form ammonia.

Nitrogen fixation can occur either by the atmospheric fixation- which involves
lightening or industrial fixation by manufacturing ammonia under high temperature
and pressure condition. This can also be fixed through man-made processes,
primarily industrial processes that create ammonia and nitrogen-rich fertilizers.

Types of Nitrogen Fixation

i) Atmospheric fixation: A natural phenomenon where the energy of

lightning breaks the nitrogen into nitrogen oxides and is then used plants.

ii) Industrial nitrogen fixation: 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, and later, it is converted into
various fertilizers such as urea.

iii) Biological nitrogen fixation: We already know that nitrogen is not usable
directly from the air for plants and animals. Bacteria like Rhizobium and
blue-green algae transform the unusable form of nitrogen into other
compounds that are more readily usable. These nitrogen compounds get
fixed in the soil by these microbes.
2. Nitrification

In this process, the ammonia is converted into nitrate by the presence of bacteria in
the soil. Nitrites are formed by the oxidation of Ammonia with the help of
Nitrosomonas bacterium species. Later, the produced nitrites are converted into
nitrates by Nitrobacter. This conversion is very important as ammonia gas is toxic
for plants.

3. Assimilation
Primary producers – plants take in the nitrogen compounds from the soil with the help of
their roots, which are available in the form of ammonia, nitrite ions, nitrate ions or
ammonium ions and are used in the formation of the plant and animal proteins. This
way, it enters the food web when the primary consumers eat the plants.

4. Ammonification
When plants or animal die, the nitrogen present in the organic matter is released back into
the soil. The decomposers, namely bacteria or fungi present in the soil, convert the
organic matter back into ammonium. This process of decomposition produces
ammonia which is further used for other biological processes.

5. Denitrification

`Denitrification is the process in which the nitrogen compounds makes its way back into
the atmosphere by converting nitrate (NO3-) into gaseous nitrogen (N). This process of the
nitrogen cycle is the final stage and occurs in the absence of oxygen. Denitrification is
carried out by the denitrifying bacterial species- Clostridium and Pseudomonas, which will
process nitrate to gain oxygen and gives out free nitrogen gas as a byproduct.

Nitrogen Cycle in Marine Ecosystem

The process of the nitrogen cycle occurs in the same manner in the marine ecosystem
as in the terrestrial ecosystem. The only difference is that it is carried out by marine
bacteria.

The nitrogen-containing compounds that fall into the ocean as sediments get compressed
over long periods and form sedimentary rock. Due to the geological uplift, these
sedimentary rocks move to land. Initially, it was not known that these nitrogen-containing
sedimentary rocks are an essential source of nitrogen. But, recent researches have
proved that the nitrogen from these rocks is released into the plants due to the weathering
of rocks.
Importance of Nitrogen Cycle

1. Helps plants to synthesize chlorophyll from the nitrogen compounds.

2. Helps in converting inert nitrogen gas into a usable form for the plants through
the biochemical process.
3. In the process of ammonification, the bacteria help in decomposing the animal
and plant matter, which indirectly helps to clean up the environment.
4. Nitrates and nitrites are released into the soil, which helps in enriching the soil
with necessary nutrients required for cultivation.
5. Nitrogen is an integral component of the cell, and it forms many crucial
compounds and important biomolecules.

Nitrogen is also cycled by human activities such as combustion of

fuels and the use of nitrogen fertilizers. These processes increase the levels of nitrogen-
containing compounds in the atmosphere. The fertilizers containing nitrogen are washed
away in lakes and rivers and results in eutrophication.

Conclusion

 Nitrogen is abundant in the atmosphere, but it is unusable to plants or animals

unless it is converted into nitrogen compounds.

 Nitrogen-fixing bacteria play a crucial role in fixing the atmospheric nitrogen into
nitrogen compounds that can be used by the plants.

 The plants absorb the usable nitrogen compounds from the soil through their
roots. Then, these nitrogen compounds are used for the production of proteins
and other compounds in the cell.

 Animals assimilate nitrogen by consuming these plants or other animals that

contain nitrogen. Humans consume proteins from these plants and animals, and
then, the nitrogen assimilates into our system.

 During the final stages of the nitrogen cycle, bacteria and fungi help decompose
organic matter, where the nitrogenous compounds get dissolved into the soil
which is again used by the plants.

 Some bacteria then convert these nitrogenous compounds in the soil and turn it
into nitrogen gas. Eventually, it goes back to the atmosphere.

These set of processes repeat continuously and thus maintain the percentage of nitrogen in
the atmosphere.