NITROGEN CYCLE

NITROGEN CYCLE

• Nitrogen is the most abundant element in the atmosphere. Although, 78 % volume of the atmosphere is Nitrogen gas.
• Nitrogen is essential to life because it is a key component of proteins and nucleic acids.
• The Nitrogen cycle refers to the cycle of nitrogen atoms through the living and non-living systems of Earth.

STEPS INVOLVED IN NITROGEN CYCLE

Nitrogen cycle

• In general, Nitrogen cycle has Five steps. They are

1)    Nitrogen fixation

2)    Nitrification

3)    Assimilation

4)    Ammonification

5)    Denitrification

1) Nitrogen fixation

• Nitrogen fixation refers to the conversion of atmospheric Nitrogen gas (N₂) into Ammonia (NH₄) or related nitrogenous compounds, a form usable by plants and other organisms.
• In Nitrogen fixation, the N₂ is split up into free N₂ atoms by breaking the triple bond (N≡N) with the help of an enzyme Nitrogenase.
• Nitrogen fixation can takes place by two methods. They are

1)    Non – Biological or Physical Nitrogen fixation or Natural Nitrogen fixation

2)    Biological Nitrogen fixation

a)     Non – symbiotic or Asymbiotic or Free living Nitrogen fixation

b)    Symbiotic Nitrogen fixation

c)     Associative Symbiotic Nitrogen fixation

d)    Endophytic Nitrogen fixation

• Microorganisms (particularly Prokaryotes) fix 70 % Nitrogen for requirement of plants.
• Microorganisms that can fix N₂ are called Diazotrophs (both bacteria and archaea).
• Biological nitrogen fixation occurs when atmospheric Nitrogen is converted to Ammonia by an enzyme called a Nitrogenase.
• The overall reaction for Biological nitrogen fixation (BNF) is

N₂ + 16 ATP + + 8 e⁻ + 8 H⁺ → 2 NH₃ + H₂ + 16 ADP + 16Pi

• The whole Biological Nitrogen fixation process requires 8 electrons and atleast 16 ATPs.
• The ability to reduce atmosphere Nitrogen is restricted only to bacteria, which are belonging to the diverse groups.
• The root nodule associations were the first to be recognized for their ability to fix atmosphere N₂. Rhizobia are the first group of organism realized for its potential of nitrogen fixation.

2) Nitrification

• In Nitrification, a host of soil bacteria participate in turning Ammonia into Nitrate – the form of nitrogen that can be used by plants and animals.
• Nitrification is the 2 Steps process.

ü  In Step 1, Ammonia is oxidized into Nitrite with the help of bacteria Nitrosomonas sp. (Nitrosomonas eurospora) or Nitrococcus sp. or Nitrosolobus sp. The Ammonia oxidizing bacteria are Chemoautotrophs in nature.

ü  In Step 2, Nitrite is oxidized into Nitrate with the help of Nitrobacter sp. or Nitrospira sp.

 3) Assimilation

• In Nitrogen assimilation, plants consume the Nitrates made by soil bacteria through its roots and use them to make Nucleotides, amino acids, plant proteins and other vital chemicals for life.
• Animals that eat the plants are then able to use these amino acids and nucleic acids in their own cells.

4) Ammonification

• Ammonification is performed by Soil bacteria which decompose dead plants and animals.
• During the Ammonification process, these decomposers break down Amino acids and Nucleic acids into Nitrates and Ammonia and release those compounds back into the soil.
• The Ammonia produced may be taken up again by Plants and Nitrifying bacteria.
• The bacteria helps in Ammonification process are Pseudomonas sp., Bacillus sp., Clostridium sp. and Serratia sp.
• The fungi helps in Ammonification process are Alternaria sp., Mucor sp., Aspergillus sp. and Penicillium sp.

5) Denitrification

• Denitrification is the final step of Nitrogen cycle.
• Denitrification is the process that converts Nitrate to Nitrogen gas by Denitrifying bacteria.

• 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.
• Some of the Denitrifying bacteria involved in Denitrification process are Pseudomonas sp., Bacillus sp., Paracoccus denitrificans and Thiobacillus denitrificans.