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The Nitrogen Cycle

The Nitrogen Cycle

The process of conversion of atmospheric nitrogen into essential nitrogenous compounds and decomposition of these nitrogenous compounds back to gaseous nitrogen constitutes the nitrogen cycle.

Plants need nitrogen for making proteins. However, most plants cannot directly use the nitrogen gas present in the air. Therefore, the nitrogen has to be fixed, i.e., it has to be * combined with other elements to form soluble compounds like nitrites and nitrates so that it can be used by the plants.

This fixing of nitrogen is done by special bacteria present in the soil. There are also certain types of bacteria present in the soil that help reverse the process and help break down the proteins of dead organic matter and release free nitrogen into the atmosphere.

This fixing and releasing of atmospheric nitrogen helps maintain the proportion of nitrogen in the air. This is the nitrogen cycle.

The process of converting atmospheric nitrogen into useful nitrogenous compounds by natural or artificial means is known as fixation of atmospheric nitrogen.

The Nitrogen Cycle

The Nitrogen Cycle

The main steps of the nitrogen cycle are :

1. Fixation of atmospheric nitrogen.

2. Assimilation of the fixed nitrogen by plants.

3. Transfer of the fixed nitrogen from the . plant to the soil.

4. Nitrogen is set free from the nitrogenous compounds and returned to the, atmo- . sphere.

Nitrogen Fixation

The process of converting atmospheric nitrogen into useful nitrogenous compounds by natural or artificial means is known as fixation of atmospheric nitrogen.

Nitrogen fixation may be by natural processes or artificial processes.

Natural Methods of Nitrogen Fixation

1. During rain and thunder storms :

During thunder and lightning, the high temperatures produced (3000°C-5000°C) cause the nitrogen and oxygen of the air to combine and form unstable nitric oxide.

N2 + 02 2N0

This further reacts with the oxygen of the air to form nitrogen dioxide which dissolves in rain water to form nitric acid.

2N0 + 02 2N02

Nitric acid enters the soil and dissolves the minerals to form soluble nitrates. Certain bacteria present in the soil, called nitrifying bacteria, make the nitrogen in the form of soluble nitrates available to the plant.

MgC03 + 2HN03 Mg(N03)2 + H20 + C02T

2. Symbiotic bacteria

Roots of leguminous plants have symbiotic bacteria which help convert atmospheric nitrogen into soluble nitrates and make it available to the plant.

The Nitrogen Cycle

Artificial Fixation of Nitrogen

In addition to the conversion of atmospheric nitrogen into assimilable nitrogenous compounds by natural methods, artificial processes are also used to form nitrogenous compounds which can be converted into fertilizers for use by the plants and for the production of ammonia and nitric acid.

Nowadays, all the ammonia is manufactured by the Haber's process. Earlier, it was also made by the Birkeland and Eyde process and the Cyanamide process. Both these processes have now become obsolete and are no longer used.

The ammonia produced is directly converted into fertilizers like ammonium sulphate, ammonium nitrate and urea.

Ammonia is converted to nitric acid by the Ostwald's Process.

Nitric acid is then used in the production of fertilizers.

Points to Note

Nitrogen enters the soil in the following ways :

1. During lightning discharge when it is converted into soluble nitrates.

2. With the help of nitrogen fixing bacteria in leguminous plants.

3. By the addition of natural (compost) and artificial fertilizers into the soil.

Nitrogen from the soil enters plants and animals

1. The root hair of plants absorb the nitrates from the soil. Enzymes present in the plant body convert them into amino acids which are the building blocks of proteins.

2. The plants are eaten by the animals. Thus, nitrogen in the form of plant proteins reaches the body of animals.

Nitrogen from plant and animal, bodies is returned to the soil and air :

1. In the animal body, the plant proteins are broken down into urea which is excreted in urine. Hydrolysis of urea produces ammonia.

2. Action of soil bacteria.