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Natural Resources — Science STD 9 — Question

Gujarat BoardEnglish MediumSTD 9ScienceNatural Resources1 Mark
Question
Describe nitrogen cycle.
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Answer

The continuous process by which nitrogen is exchanged between organisms and the environment is called nitrogen cycle. Nitrogen is an essential nutrient, needed to make amino acids and other important organic compounds, but most organisms cannot use free nitrogen, which is abundant as a gas in the atmosphere.
Nitrogen cycle involves the following steps:
  1. Nitrogen fixation: This process involves the conversion of atmospheric nitrogen into nitrates, which are soluble in water. This is done by various nitrogen fixing bacteria.
Example: Rhizobium, blue green algae and bacterium Azotobacter.
  1. Ammonification: It is the process of decomposing complex, dead organic matter into ammonia. This is done by microorganisms living in the soil.
  2. Nitrification: It is the process of conversion of ammonia into nitrites and then into nitrates. This is done by nitrifying bacteria.
Example: Nitrosomonas and Nitrobacter.
  1. Denitrification: It is the process of reducing nitrates present in the soil to release nitrogen back into the atmosphere.
Example: Pseudomonas.
The decomposers help in decomposing the dead bodies of plants and animals, and hence act as cleansing agents of environment. The decomposers also help in putting back the various elements of which the dead plants and animals were made, back into the soil, air and water for reuse by the producers like crop$-$plants, e.g. the decomposers like purifying bacteria and fungi decompose the dead plants and animal bodies into ammonia. This ammonia is converted into nitrates by nitrifying bacteria present in soil. These nitrates act as fertilizer in the soil and are again absorbed by the plants for their growth. Thus, the nitrates act as fertilizer in the biosphere so that the process of life may go on and on like an unending chain.

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