CBSE Class 8 Answered

The main steps in the nitrogen cycle are:

(i) The atmosphere (or air) contains nitrogen gas. The nitrogen-fixing bacteria (present in the soil and in the root nodules of leguminous plants), blue-green algae and lightning in the sky fix nitrogen gas from the atmosphere and convert it into compounds of nitrogen which go into soil.

(ii) The plants take compounds of nitrogen from the soil for their growth. The plants absorb the nitrogen compounds from the soil through their roots and convert them into plant proteins and other organic compounds which make up the body of plants.

(iii) The plants are eaten up by animals as food. Animals convert plant proteins into animal proteins and other organic compounds which make up their body. Some animals also eat other animals to obtain nitrogen compounds. Thus, animals obtain nitrogen compounds by eating plants as well as other animals.

(iv) When plants and animals die, the complex nitrogen compounds (like proteins, etc.) present in their dead bodies are decomposed and converted into simple compounds of nitrogen by certain bacteria and fungi present in the soil. Animal excretions (urine, etc.) are also converted into simple compounds of nitrogen. All the simple compounds of nitrogen formed in this way go into the soil.

(v) Some of the compounds of nitrogen (formed from the decay of dead plants and animals) are decomposed by denitrifying bacteria present in the soil to form nitrogen gas. This nitrogen gas goes back into the atmosphere.

In this way, the nitrogen gas which was removed from the atmosphere during fixation, is put back into the atmosphere.