Describe on Steps of Nitrogen Cycle - QS Study
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Nitrogen cycle: The process by which the balance of nitrogen is always maintained by the exchange of nitrogen between living world and the environment is known as nitrogen cycle.

Steps of nitrogen cycle: Nitrogen cycle has five steps, such as:

  1. Nitrogen fixation
  2. Nitrification
  3. Assimilation
  4. Ammonification
  5. De-Nitrification

Except assimilation all the four other types are completed by bacteria.

First step: Nitrogen fixation:

The process by which nitrogen gas is converted to ammonia by various nitrogen fixing bacteria is known as nitrogen fixation. In this process the strong bond nitrogen gas is fixed in such a condition, that it can be received by the living would.

Most nitrogen fixation is happened by biological process. Most nitrogen fixing bacteria live in soil and cyano bacteria live in aquatic environment. They use nitrogen enzymes to break down nitrogen molecule and mix with hydrogen. Nitrogen enzyme works in absence of oxygen.

Some nitrogen fixation can occur by physical process. Atmospheric free nitrogen evoked due to combustion, volcanic eruption, lightning etc. fixes up with oxygen, hydrogen etc.

Second step: Nitrification: The process by which ammonia modified to form nitrate is said to be Nitrification.

Nitrification usually, occurs under two sub-stepts.

  • First sub step: In this sub step, root bacteria such as, Nitrosomonas, Nitrococcus convert ammonia to Nitrite (NO2)
  • Second sub step: In that sub step, Nitrobacter, converts nitrite (NO2) nitrate (NO3)


NH3 ……….→ NO2

NO2 ………→ NO3


Third step: Assimilation: Plants, usually, meow NO3 of NH3 by roots which they use to manufactures protein and nucleic acid in their bodies.

Forth step: Ammonification

  • The process by which different organic nitrogen are convened to ammonia (NH3) is known as ammonification
  • Nitrogen is present in plant and animal bodies as different organic compounds, such as; protein, ammo acid, nucleic acid etc. After their death, nitrogen is released as ammonia (NH3) to the inorganic environments from the dead bodies with the action of amonilying bacteria. This NH3 either can be assimilated by plants or can be converted Nitrate (NO3) by nitrification.

Fifth step: De-nitrification: Nitrate NO3 is formed in the soil through nitrification this nitrate again being converted to nitrogen comes back to the atmosphere. The process by which nitrate (NO3) of the soil is converted to nitrogen gas is known as de-nitrification.

The bacteria that convert nitrate (NO3) to free nitrogen arc known as de-nitrifying bacteria. These are the anaerobic bacteria that live to such environment where there is no oxygen.