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Nitrogen assimilation:
Nitrogen assimilation is the process through which plants and animals incorporate nitrate ions and ammonia.
Plants absorb nitrate ions and use them to make organic matter like nucleic acids, amino acids and proteinsAnimals then utilize the nitrogen from plant tissues. Herbivorous animals convert plant protein to animal protein. Carnivorous animals synthesize proteins from their food. Through this process, nitrogen enters the food chain in our ecosystem.
The process of decomposition of nitrogenous waste by putrefying bacteria and fungi into ammonium compounds is called ammonification.
It involves the conversion of organic nitrogen into ammonia. Animal proteins are excreted in the form of urea, uric acid or ammoniaFungi and putrefying bacteria decompose the animal proteins, dead animals and plants into ammonium compounds. The ammonia produced by the process is excreted into the environment.
The ammonium compounds that are formed by the ammonification process is oxidised to soluble nitrates.
The process of formation of nitrate is called nitrification. The bacteria that carry out the process is called nitrifying bacteria.
It is a two-step process where ammonia is converted to nitrite (\(NO_2^-\)) by Nitrosomonas. Then Nitrobacter oxidizes nitrite (\(NO_2^-\)) to nitrate (\(NO_3^-\)).
Step 1: 2NH4++3O2Nitrosomonas2NO2+4H++2H2O(Ammonia)(Nitrite)
Step 2: 2NO2+O2Nitrobacter2NO3(Nitrite)(Nitrate)
Denitrification is the reduction of nitrate ions (\(NO_3^-\)) to the gaseous nitrogen.
Free-living, aerobic bacteria present in the soil like Pseudomonas sp. reduce nitrate ions of the soil into the gaseous nitrogen that enters the atmosphere.

Nitrogen cycle
A table displaying the microorganisms involved in the nitrogen cycle:
Role played in the nitrogen cycle
Azotobacter (in soil), Rhizobium (root nodules), 
Nostoc - Blue-green algae
Nitrogen fixation
Putrefying bacteria, FungiAmmonification
Nitrosomonas, and Nitrobacter
Denitrifying bacteria, PseudomonasDenitrification
Human impacts on nitrogen cycle:
  • Burning fossil fuels
  • Application of nitrogen-based fertilisers
  • Other activities increase the amount of biologically available nitrogen in an ecosystem. Nitrogen that is applied to the agricultural fields enters aquatic systems - rivers and marine ecosystem. This alters the biodiversity, changes the structure of the food web and destroys the general habitat.