Question

What are the three different ways of nitrogen fixation?

Answer 1

Nitrogen fixation is a process in which the molecular nitrogen is converted into fixed nitrogen and nitrogen products which can be used by plants or for other purposes.

Complete answer:
Nitrogen is one of the most prevalent micronutrients present in the environment. It is a building materials for plant and plants absorb nitrogen in form of nitrate or ammonium ions. Nitrate is found more in oxygenated non-acidic soils while ammonium is more predominant in acidic soils. Since the molecular oxygen is highly inert and cannot be directly used by the plants, it needs to be fixed so that plants can easily take it.

There can be three methods by which nitrogen can be fixed:

Industrial nitrogen fixation: In this nitrogen is combined with hydrogen at high temperature and pressure to form ammonia. This is then converted into fertilisers for the growth of plants.

Atmospheric nitrogen fixation: During lightning, the nitrogen and oxygen present in air react to form nitric oxide (NO) which is further oxidised to form nitrogen peroxide $(NO_2)$. This $NO_2$ combines with water during rain forming nitric acid $(HNO_3)$ and nitrous acid $(HNO_2)$ which forms water soluble nitrates and nitrites when they fall with water. These soluble forms of nitrogen can easily be absorbed by the plants.

Biological nitrogen fixation: When a biological organism helps in the conversion of atmospheric nitrogen to fixed nitrogen it is called biological nitrogen fixation. Some organisms live in symbiotic relationships with such plants while others are free living nitrogen fixers. In this process enzyme nitrogenase catalyses the reaction for the conversion of atmospheric nitrogen to ammonia and related products.

Note: Azotobacter, anabaena, and clostridium are examples of free living nitrogen fixing organisms while rhizobium, cycad roots and nostoc are examples of symbiotic nitrogen fixing organisms.