What is the Nitrogen Cycle?

          Our atmosphere contains about 78% of nitrogen. A certain amount of this nitrogen is constantly being removed, and an approximately equal amount is being returned. This continuous circulation of nitrogen among the soil, water, air and living organisms is known as the Nitrogen cycle. Let us see how the percentage of nitrogen in the air remains constant.

          All living things need nitrogen. It is part of proteins and nucleic acids, both of which are vital for life. How nitrogen is removed from atmosphere and again returned to the atmosphere is given below.

          A part of the atmospheric nitrogen is removed from the air by lightning. The sudden discharge of electricity causes some of the nitrogen and oxygen components in the air to combine, forming the oxides of nitrogen. When these nitrogen oxides are dissolved in water, they combine with other elements to form nitrogenous compounds.

          Some nitrogen is removed, from the air by certain bacteria and algae in a process called nitrogen fixation. Symbiotic bacteria present in the nodules of roots of some plants, such as peas, beans, gram etc. take up atmospheric nitrogen directly, and pass it on to the plants. Plants take up nitrogen compounds and convert them into proteins. These proteins are assimilated by animals. Some other plants, like rice, have symbiotic blue-green algae which fix atmospheric nitrogen.

          As a result of death, decay and excretion by plants and animals, the organic matter is converted into ammonium salts in the soil. Special nitrifying bacteria convert ammonia into nitrogenous compounds that are used up by plants. Animals get their nitrogenous compounds by eating plants, or other animals that eat plants.

          Thus an approximately equal amount of nitrogen is also being constantly returned to the atmosphere. Denitrifying bacteria change some of the nitrogenous compounds in the soil, back into gaseous form of nitrogen. These gases then return to the air.

          Thus nitrogen from the atmosphere passes into the soil, plants and animals and finally returns to the air. It may take thousands or millions of years, but every molecule of nitrogen eventually returns to the air.