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Nitrogen Fixation Definition, Meaning and Nitrogen Cycle in Nature

Biology Topics encompass a wide range of subjects, from cell biology to ecology.

Nitrogen Fixation and Nitrogen Metabolism

Our atmosphere contains a lot of nitrogen gas. In fact, our atmosphere has 78 per cent nitrogen gas. The atmospheric nitrogen gas cannot be utilised directly by the plants or animals. In order that the nitrogen gas of atmosphere can be utilised by plants for their growth, it has first to be converted into nitrogen compounds (which can be absorbed by the roots of the plants).
Nitrogen Fixation Definition & Meaning 1
The process of converting nitrogen gas of atmosphere (or air) into compounds of nitrogen (which can be used by the plants) is called nitrogen fixation. The ‘nitrogen gas’ is the ‘free nitrogen’ whereas ‘nitrogen compounds’ (like nitrates) are said to be fixed nitrogen. The nitrogen gas of atmosphere (or air) can be ‘fixed’ (converted into nitrogen compounds)

  1. by certain nitrogen-fixing bacteria present in the soil,
  2. by Rhizobium bacteria present in the root nodules of leguminous plants,
  3. by blue-green algae, and
  4. by lightning. The nitrogen fixing Rhizobium bacteria live in the root nodules of leguminous plants (such as peas, beans, etc.) (see Figure).

Rhizobium bacteria have symbiotic relationship with leguminous plants. Thus, some nitrogen-fixing bacteria live freely in the soil whereas other nitrogen-fixing bacteria (Rhizobium bacteria) live in the root nodules of leguminous plants.

Nitrogen gas of atmosphere also gets fixed through the action of lightning in the sky. This happens as follows : When lightning takes place in the sky during thunderstorm, a high temperature is produced in the atmosphere. At this high temperature, nitrogen gas of air combines with oxygen gas of air to form nitrogen compounds.

These nitrogen compounds dissolve in rain water, fall to earth with rain water and go into the soil. Nitrogen of atmosphere can also be fixed by artificial methods. We will study the artificial fixation of nitrogen in higher classes.

The Nitrogen Cycle

Nitrogen is required by both, plants and animals for their growth and development. Nitrogen is an essential component of proteins which make up the bodies of plants and animals. Nitrogen is also present in chlorophyll, nucleic acids and vitamins.

The same nitrogen element is circulated again and again through living things (like plants and animals) and non-living things (like air, soil and water). The circulation of nitrogen element through living things (plants and animals) and non-living environment (air, soil and water) is called nitrogen cycle in nature. A labelled diagram of nitrogen cycle in nature is given in Figure.
Nitrogen Fixation Definition & Meaning 2
We will now describe the nitrogen cycle in nature. The main steps in the nitrogen cycle in nature are as follows :
(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. The plants convert the compounds of riitrogen 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. In this way, the compounds of nitrogen which were taken by the plants from the soil during their growth are returned to the soil. From the soil, these nitrogen compounds are again absorbed by the new plants for their growth and this part of nitrogen cycle is repeated endlessly.

(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 (from where it initially came) (This process is the reverse of fixation of nitrogen). In this way, the nitrogen gas which was removed from the atmosphere during fixation is put back into the atmosphere.

From the atmosphere, nitrogen gas is used again during nitrogen fixation and the nitrogen cycle is repeated in nature again and again. As a result of nitrogen cycle in nature, the percentage of nitrogen gas in the atmosphere (or air) remains constant.

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