Nitrogen Cycle

Nitrogen Cycle

  • All organisms need nitrogen to synthesize protein, nucleic acids, and other nitrogen containing compounds.
  • Molecular nitrogen (N2) makes up almost 80% of the Earth’s atmosphere.
  • For plants to assimilate and use nitrogen, it must be fixed, that is, taken up and combined into organic compounds.
  • The activities of specific microorganisms are important to the conversion of nitrogen to usable forms.

Nitrogen Cycle


  • All the nitrogen in the soil exists in organic molecules.
  • When an organism dies, the process of microbial decomposition results in the hydrolytic breakdown of proteins into amino acids.
  • In a process called deamination, the amino groups of amino acids are removed and converted into ammonia (NH3).
  • The release of ammonia is called ammonification.

Ammonification, brought about by numerous bacteria and fungi, can be represented as follows:


  • Microbial growth releases extracellular proteolytic enzymes that decompose proteins.
  • The amino acids are transported into the microbial cells, where ammonification occurs.
  • The ammonia produced by ammonification depends on soil conditions because ammonia is a gas, it rapidly disappears from
    dry soil, but in moist soil it becomes solubil ized in water, and ammonium ions (NH4 +) are formed:

Ammonium ions

  • Ammonium ions from this sequence of reactions are used by bacteria and plants for amino acid synthesis.


  • The next sequence of reactions in the nitrogen cycle involves the oxidation of the nitrogen in the ammonium ion to produce nitrate, a process called nitrification.
  • Nitrifying bacteria, such as those of the genera Nitrosomonas and Nitrobacter.
  • These microbes obtain energy by oxidizing ammonia or nitrite.

In the first stage, Nitrosomonas oxidizes ammonium to nitrites:


In the second stage, such organisms as Nitrobacter oxidize nitrites to nitrates:


  • Plants use nitrate as their source of nitrogen for protein synthesis because nitrate is highly mobile in soil and is more likely to encounter a plant root than ammonium.
  • Ammonium ions would actually make a more efficient source of nitrogen because they require less energy to incorporate into protein.


  • The form of nitrogen resulting from nitrification is fully oxidized and no longer contains any biologically usable energy.
  • However, it can be used as an electron acceptor by microbes metabolizing other organic energy sources in the absence of atmospheric oxygen.
  • This process is called denitrification, can lead to a loss of nitrogen to the atmosphere, especially as nitrogen gas.

Denitrification can be represented as follows:


  • Denitrification occurs in waterlogged soils, where little oxygen is available.
  • In the absence of oxygen as an electron acceptor, denitrifying bacteria substitute the nitrates of agricultural fertilizer.
  • This converts much of the valuable nitrate into gaseous nitrogen that enters the atmosphere and represents a considerable economic loss.

Nitrogen Fixation

  • The air we breathe is about 75% nitrogen.
  • Some creatures on earth that can use it directly as a nitrogen source are a few species of bacteria, including cyanobacteria.
  • The process by which they convert nitrogen gas to ammonia is known as nitrogen fixation.
  • Bacteria that arc responsible for nitrogen fixation all depend on the same enzyme, nitrogenase.
  • Nitrogen fixation is brought about by two types of microorganisms: Free-living and Symbiotic.
  • (Agricultural fertilizers arc made up of nitrogen that has been fixed by industrial physical chemical processes.)

Free-living Nitrogen-Fixing Bacteria:

  • Free-living nitrogen-fixing bacteria arc found in particularly high concentrations in the rhizosphere, a region of 2 millimeters from the plant root.
  • Among the free-living bacteria that can fix nitrogen are aerobic species such as Azotobacter.
  • Some anaerobic bacteria, such as certain species of Clostridium, also fix nitrogen .
  • Most of the free-living nitrogen -fixing bacteria are capable of fixing large amounts of nitrogen under laboratory conditions.

Symbiotic Nitrogen-Fixing Bacteria:

  • Symbiotic nitrogen-fixing bacteria play an important role in plant growth for crop production .
  • Members of the genera Rilizobium, Bradyrhizobium, and others infect the roots of leguminous plants, such as soybeans, beans, peas, peanuts, alfalfa, and clover.
  • Rhizobia, as these bacteria, specially adapted to particular leguminous plant species, on which they form root nodules.
  • Nitrogen is then fixed by a symbiotic process of the plant and the bacteria.
  • The plant provides anaerobic conditions and growth nutrients for the bacteria, and the bacteria fix nitrogen that can be incorporated into plant protein.


  1. Another important contribution to the nitrogen economy of forests is made by lichens, which are a combination of fungus and an alga or a cyanobacterium in a mutualistic relationship.
  2. When one symbiont is a nitrogen-fixing cyanobacterium, the product is fixed nitrogen that eventually enriches the forest soil.

Nitrogen Cycle

ALSO SEE:  The Carbon Cycle


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