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Nitrogen Cycle

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Nitrogen Cycle - Lesson Summary

Nitrogen is an important element for living organisms as it is an essential constituent of amino acids, nucleic acids, alkaloids, chlorophylls, vitamins and hormones. It is one of the major constituents in the atmosphere and comprises 78% of all gases by volume. Atmospheric nitrogen cannot be used by plants. Only a small amount of nitrogen is present in soil, and plants compete with microorganisms for it.
Atmospheric nitrogen is in the form of elemental nitrogen which requires a large amount of energy to break it into forms suitable for living organisms to absorb. In nature, elemental nitrogen is converted into oxides with the help of electrical nitrogen fixers such as lightning, thunder and ultraviolet radiation. Some of these nitrogen oxides are brought to earth by precipitation as rainfall. But most of it is biochemically fixed by microorganisms. This is called biological nitrogen fixation. Plants absorb nitrogen in the form of nitrates through their roots, which then transport it to the leaves. Nitrogen in plants reaches animals through the food they eat. When plants and animals die, the organic material in them returns to the soil. The nitrogen in dead plants and animals is decomposed by ammonifying bacteria to form ammonia. This process is called ammonification. Ammonia formed by ammonification is converted into nitrites and nitrates by nitrifying bacteria in the soil. This is called nitrification. During the nitrifying process, chemoautotrophic bacteria such as nitrosomonas and nitrococcus act on ammonia and oxidise it into nitrite. Nitrites are further oxidised into nitrates by the bacterium nitrobacter.  
Nitrates in soil are either taken up by plants or is processed by denitrifying bacteria such as pseudomonas, thiobacillus and micrococcus, which release it as elemental nitrogen into the atmosphere. This process is called denitrification. Industries, forest fires, automobile exhaust and power-generating stations also release oxides of nitrogen into the atmosphere through combustion. Ammonia in soil is converted into nitrites and nitrates by nitrifying bacteria, thereby enabling plants to absorb nitrates from the soil. Nitrogen compounds from plants are passed along the food chain to animals, which ultimately return it to the soil when they die. Nitrogen from the dead and decaying organic matter is converted into ammonia by ammonification. Denitrifying bacteria then convert the nitrates back into nitrogen.  


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