Nitrogen Cycle: Nitrification & Nitrate Reduction

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Nitrogen Cycle: Nitrification & Nitrate Reduction

Several biochemical steps involved in the nitrogen cycle are:
1. Proteolysis
2. Ammonification
3. Nitrification
4. Nitrate reduction and
5. Denitrification.

3. Nitrification:
Ammonical nitrogen / ammonia released during ammonification are oxidized to nitrates and the process is called “nitrification”. Soil conditions such as well aerated soils rich in calcium carbonate, a temperature below 30 ° C, neutral PH and less organic matter are favorable for nitrification in soil.

Nitrification is a two stage process and each stage is performed by a different group of bacteria as follows.

Stage I: Oxidation of ammonia of nitrite is brought about by ammonia oxidizing bacteria viz. Nitrosomnonas europaea, Nitrosococcus nitrosus, Nitrosospira briensis, Nitrosovibrio and Nitrocystis and the process is known as nitrosification. The reaction is presented as follows.

2 NH3 + 1/2O2 ——————-> NO2 + 2 H + H2 O
 Ammonia                                         Nitrite    

Stage II: In the second step nitrite is oxidized to nitrate by nitrite-oxidizing bacteria such as Nitrobacter winogradsky .Nitrospira gracilis, Nirosococcus mobiiis etc, and several fungi (eg. Penicillium, Aspergillus) and actinomycetes (eg. Streptomyces, Nocardia).

NO2 (-)   +   ½ O2 ———————->   NO3
Nitrite ions                                             Nitrate ions 

The nitrate thus, formed may be utilized by the microorganisms, assimilated by plants, reduced to nitrite and ammonia or nitrogen gas or lost through leaching depending on soil conditions. The nitrifying bacteria (ammonia oxidizer and nitrite oxidizer) are aerobic gram-negative and chemoautotrophic and are the common inhabitants of soil, sewage and aquatic environment.

4. Nitrate Reduction:
Several heterotrophic bacteria (E. coli, Azospirillum) are capable of converting nitrates to nitrites and nitrites to ammonia. Thus, the process of nitrification is reversed completely which is known as nitrate reduction. Nitrate reduction normally occurs under anaerobic soil conditions (water logged soils) and the overall process is as follows:
HNO3 + 4 H2 ——————–> NH4 + 3 H20
 Nitrate                  Reductase         ammonium

Nitrate reduction leading to production of ammonia is called "dissimilatory nitrate reduction" as some of the microorganisms assimilate ammonium for synthesis of proteins and amino acid.

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