Nitrogen Cycle

It is the cycling of nitrogen and its compounds in nature.

Involve 3 main processes; nitrogen fixation, nitrification and denitrification.

Free atmospheric nitrogen is converted into nitrates through a process known as nitrogen fixation.

(I) Nitrogen fixation

Nitrogen fixation is the conversion of the free atmospheric nitrogen into forms that are utilisable by the plant.

It occurs in 2 forms; biological and non-biological fixation.

Biological fixation of nitrogen is done by nitrogen-fixing bacteria; which are either symbiotic or free-living;

Symbiotic bacteria are of the genus Rhizobium and are found in the root nodules of the leguminous plants.

The bacteria convert free atmospheric nitrogen into ammonia that is used directly by the leguminous plants to form nitrogen-containing organic compounds (amino acids, nucleic acids, proteins).

Nitrogen fixation is also done by free-living bacteria e.g. Clostridium and some algae e.g.  Anabaena, Chlorella and Nostoc.

Non-biological nitrogen fixation is achieved by lightning. During thunderstorms, the lightning energy combines atmospheric nitrogen with oxygen to form nitrous acid and nitric acid.

These are then chemically converted into nitrates.

Plants absorb nitrates from the soil and then assimilate them into plant proteins.

Thus animals obtain the nitrogen in form of proteins directly or indirectly from plants. The proteins are digested into amino acids and assimilated into animal proteins.

When organisms die (or their waste and droppings), saprophytic bacteria and fungi break down the protein material in their bodies into ammonia.

Ammonia is eventually converted into nitrates in a process called nitrification.