Nitrogen Cycle

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Nitrogen is an essential constituent of animal and plant matter as it forms proteins, which are the building blocks of life. The ultimate source of nitrogen is atmospheric nitrogen but neither plants nor animals are capable of assimilating free nitrogen. Thus the process of converting atmospheric nitrogen to useful nitrogenous compounds by plants, passing it to animals and then the decomposition of these compounds to give back free nitrogen in the atmospheres is called Nitrogen Cycle (Fig.12.12). It is because of the nitrogen cycle that the percentage of nitrogen remains constant in the air.

A) How does nitrogen enter the soil?

The process of conversion of free nitrogen of the air to useful nitrates is termed as nitrogen fixation.

• Biological nitrogen fixation

When nitrogen fixation is brought about by micro-organisms the process is called biological nitrogen fixation.

Examples: Rhizobium, a nitrogen fixing bacteria present in the roots of leguminous plants, fixes atmospheric nitrogen to the soil. Azotobacter and Clostridium are other free-living nitrogen fixing bacteria.

• Physical fixation

When nitrogen fixation is brought about by physical process it is called physical fixation. For example, during lightening and thunder, atmospheric nitrogen and oxygen combine to form nitrogen dioxide.

Nitrogen dioxide so formed dissolves in rain water to form nitric acid, which goes into the soil. This nitric acid reacts with minerals of the soil to form soluble nitrates.

• By the addition of compost and fertilizers.

B) How does nitrogen enter the body of plants and animals?

• The nitrates in the soil are absorbed by plants as mineral salts. Plants convert the inorganic nitrates to organic proteins.

• The proteins from the plants enter the body of animals in form of food.

C) How is nitrogen returned to the soil?

• When plants and animals die their bodies decompose. The organic proteins undergo a series of chemical changes, brought about by micro-organisms e.g. ammonifying bacteria, nitrosomonas, nitrosococcus. These changes convert the proteins back to inorganic nitrates.

• In humans and animals some proteins are broken down to ammonia and carbon dioxide, which forms urea and is excreted out as urine. Their waste matter is also treated by micro-organisms to convert it into inorganic nitrates.

D) How does nitrogen enter the atmosphere?

Certain bacteria found in the soil called denitrifying bacteria convert nitrates of the soil to free nitrogen. Although this decreases the fertility of the soil it is inevitable.

Micro-organisms involved in Nitrogen Cycle