The nitrogen cycle describes the transformation of nitrogen between its various chemical forms through biological and physical processes. Nitrogen is essential for plant growth but must be fixed from atmospheric nitrogen by bacteria, lightning, or symbiotic bacteria in legume root nodules before plants can use it. As plants and animals use and recycle nitrogen compounds, bacteria in the soil ultimately convert some back to atmospheric nitrogen to complete the cycle.

1. NITROGEN CYCLE
BY
LEKSHMY RAJAN

2. • The nitrogen cycle is the process by which nitrogen is converted between its
various chemical forms. This transformation can be carried out through both
biological and physical processes.

3. Nitrogen is the most important element for plant growth.
Major constituent of the atmosphere.
Ultimate source of nitrogen to the ecosystem is molecular nitrogen present in
atmosphere.
Essential constituent of all living organisms as part of proteins, Chlorophyll,
nucleic acids and vitamins.

4. Organisms cannot use atmospheric nitrogen directly.
Molecular nitrogen enters the nitrogen cycle due to nitrogen fixation by free
living ( azotobacter, blue green algae, clostridium )and symbiotic nitrogen
fixing microbes.
They convert nitrogen molecules into simpler forms such as nitrates and
nitrites.

5. Symbiotic Nitrogen fixing bacteria like rhizobium are seen in the roots of the
dicot plants such as legumes.
These are present in special structures called root nodules.

6. Nitrogen fixation also takes place during lightning.
The energy from lightning causes nitrogen (N2) and water (H2O) to combine
to form ammonia (NH3) and nitrates (NO3). Precipitation carries the
ammonia and nitrates to the ground, where they can be assimilated by
plants.
Only a small amount of nitrogen is fixed in this way.

7. Plants take up these nitrates and nitrites and converts them into amino acids
which are used to make proteins.
Animals feeding on plants get these proteins and other nitrogen compounds.
When plants and animals die, bacteria and fungi present in soil convert
nitrogenous wastes into nitrogenous compounds which is to be used by
plants again.

8. Certain denitrifying bacteria convert some part of nitrogenous compounds in
soil back to nitrogen gas which goes back into the atmosphere.
E.g. Thiobacillus denitrificans