Write a short note on nitrogen cycle with the help of a diagram.

Nitrogen is an essential constituent of protein which is a building block of all living tissue. It constitutes nearly 16% by weight of all the proteins. There is an inexhaustible supply of nitrogen in the atmosphere but the elemental form cannot be used directly by most of the living organisms. Nitrogen needs to be ‘fixed’, that is, converted to ammonia, nitrites or nitrates, before it can be taken up by plants. Nitrogen fixation on earth is accomplished in three different ways: (i) by certain free-living bacteria and bluegreen algae (e.g. Anabaena, Spirulina), and symbiotic bacteria (e.g. Rhizobium); (ii) by human being using industrial processes (fertilizer factories) and (iii) to a limited extent by atmospheric phenomena such as thunder and lighting.

As you can see from Fig, nitrogen at any time is tied up in different ‘compartments’ or ‘pools’ ¾ the atmosphere, soil and water, and living organisms. The periodic thunderstorms convert the gaseous nitrogen in the atmosphere to ammonia and nitrates which eventually reach the earth’s surface through precipitation and then into the soil to be utilized by plants. More important, however, are certain microorganisms capable of fixing atmospheric nitrogen into ammonium ions (NH4 + ). These include free living nitrifying bacteria (e.g. aerobic Azotobacter and anaerobic Clostridium) and symbiotic nitrifying bacteria living in association with root nodules present in leguminous plants (e.g. Rhizobium) as well as blue green algae (eg. Anabaena, Spirulina). Ammonium ions can be directly taken up as a source of nitrogen by some plants, or are oxidized to nitrites or nitrates by two groups of specialised bacteria: Nitrosomonas bacteria which promotes transformation of ammonia into nitrite. Nitrite is then further transformed into nitrate by the bacteria Nitrobacter.

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