Nitrogen Cycle Summary

Nitrogen is an important structural component of many necessary compounds, particularly proteins. Atmosphere is the reservoir of free gaseous nitrogen and nitrogen compounds are found in bodies of organisms and in the soil.

Living organisms cannot pickup elemental gaseous nitrogen directly from the atmosphere (except for nitrogen fixing bacteria). It has to be converted into nitrates to be utilised by plants.

Nitrogen cycling involves several stages:

Nitrogen Fixation

Conversion of nitrogen into nitrates is called nitrogen fixation. It occurs in 3 ways.

1) Atmospheric nitrogen fixation

2) Biological nitrogen fixation

3) Industrial nitrogen fixation

Atmospheric Nitrogen Fixation

Thunderstorms and lightning are the common photochemical and electrochemical reactions in nature, which convert atmospheric gaseous nitrogen to oxides of nitrogen. They get dissolved in water forming nitrous acid and nitric acid, which inturn combine with other salts to produce 'nitrates'.

An average amount of 7.6 x 10⁶ metric tonnes per year of nitrogen is estimated to be produced in nature.

Chemical reactions are represented as:

Biological Nitrogen Fixation

The process is the transformation of gaseous nitrogen into nitrates by living organisms. Biological nitrogen fixation occurs by:

a) Symbiotic nitrogen fixation, which is brought about by certain bacteria such as:

i) Rhizobium species in the root nodules of legumes (pea family)

ii) Nostoc and Anabaena (cyanobacetria) in the coralloid roots of cycas

iii) Actinomycetes in the root nodules of Alnus, Casuarina, etc

An average of about 54 x 10⁶ metric tonnes / year of nitrates is estimated to be produced by biological fixation.

b) Free living (asymbiotic) nitrogen fixing organisms, are primitive nitrogen fixers, which operate under poor aeration conditions by reductional process. These include -

i) Obligatory aerobes such as Azotobacter.

ii) facultative aerobes such as Escherichia, Bascillus, etc.

iii) anaerobic bacteria like Clostridium.

iv) photosynthetic bacteria like Rhodospirillum (purple bacteria) etc.

Industrial Nitrogen Fixation

Ammonia is produced in industry by combining nitrogen and hydrogen under high pressure of 200 atmospheres and extreme high temperature of 400^oC (Haber's process).

Ammonification

Involves the decomposition of protein of dead plants and animals, and nitrogenous wastes like urea, uric acid, etc to ammonia, in the presence of ammonifying bacteria or putrifying bacteria. Common ammonifying bacteria are Bascillus ramosus, Bascillus vulgaris and Bascillus mycoides.

Part of the plant proteins are broken down into nitrogenous animal proteins. In animal body, the plant proteins are consumed and are broken down into nitrogenous wastes like urea, uric acid and ammonia. Nitrogenous wastes are excreted out which are acted upon by decomposing microorganisms such as Actinomycetes and fungi.

Proteins Amino acids Ammonia

Nitrification

Involves the oxidation of ammonia to nitrates through nitrites in the presence of nitrifying bacteria, which are also chaemosynthetic autotrophs.

First, ammonia is converted into nitrites by Nitrosomonas and Nitrococcus bacteria. The nitrites are then converted into nitrates by Nitrobacter and Nitrocystis, which are now available for plant absorption.

Denitrification

It is a biological process where in the ammonium compounds, nitrates and nitrites are reduced to molecular nitrogen in the presence of denitrifying bacteria such as Bascillus subtilis, Micrococcus denitrificans, Pseudomonas stutzeri, Pseudomonas aeruginosa, etc.

Denitrification reduces soil fertility and is stimulated by water logging, poor drainage, lack of aeration and accumulation of organic matter in the soil.