Nitrogen Cycle (With Diagram)

Nitrogen Cycle (With Diagram)!

Introduction to Nitrogen Cycle:

Nitrogen cycle, one of the nature’s most vital dynamic cycles, operates through continuous exchange of N₂ within the ecosystem.

The atmospheric nitrogen is converted into its compounds and the combined nitrogen again passes back to the atmosphere.

Nitrogen and its compounds (e.g., proteins, amino acids, vitamins, nucleic acids, enzymes and alkaloids etc.) are essential for the maintenance of life processes in the biosphere. Today, N atom may be throbbing in the cells of meadow grass, tomorrow it may be pulsating through the tissues of a living animal.

The N atom may arise from decaying animal refuse and stream to the atmosphere where it may be yoked with oxygen in a flash of lightning and return as plant food (NO^–₃) to the soil by rain. Or it may be directly absorbed from the atmosphere by the soil and there rendered available for plant food by the action of symbiotic bacteria. Thus, each nitrogen atom has undergone a never ceasing cycle of reactions through countless aeons of time.

Nitrogen Fixation:

Nitrogen is the main constituent (79%) of the atmosphere but it cannot be absorbed directly by most forms of life. It must be fixed before it can be utilised by plants and animals. Nitrogen fixation is of two types.

1. Nitrogen Fixa ion by Physico-chemical Reactions:

Due to thunder storm in rainy season, N₂ and O₂ of air combine with each other:

Nitric acid (acid rain) comes to the earth and combine with basic components in soil to form nitrates which are the basic food of the plants in the soil.

2. Nitrogen Fixation or Nitrogen Transformations by Bacteria:

Nitrogen cycle, the micro-organisms mediated cycle, is based on four major chemical transformations.

(i) Nitrogen fixation where molecular N₂ is fixed as organic nitrogen by Rhizobium bacteria.

(ii) Nitrification is the process of oxidising NH₃ to NO ^–₃ by Nitrosomonas and Nitrobacter.

(iii) Nitrate reduction to nitrite ion.

(iv) Denitrification, involves reduction of NO₃ and NO₃ to NO₂, followed by recycling of N₂ into the atmosphere.

Some soil micro-organisms transform nitrogen into soluble nitrogen compounds at a rate of 0.1 tonne per hectare per year. In the total cycle about 4-7 tonne of nitrogen is added to the soil each year.

Man has interfered with nitrogen cycle by fixing nitrogen industrially. Excessive nitrates from nitrogenous fertilizers and agricultural run-off have contributed to eutrophication.