Question

By which of the following process do bacteria change free nitrogen in the atmosphere into usable nitrogen compounds for plants?

• A. Denitrification
• B. Ammonification
• C. Decomposition
• D. Nitrogen fixation
• E. Nitrification

Answer 1

The correct option is D Nitrogen fixation

A. Nitrate present in soil is reduced to nitrogen by the process of denitrification. This process is carried out by the bacteria Pseudomonas and Thiobacillus.

B. Proteins and nucleic acids of the dead remains of plants, animals, and excretory products of animals are degraded by microorganisms with the liberation of ammonia and this process is called ammonification.

C. Decomposition is the breakdown of complex organic material into simpler ones by the action of detritivores.

D. Nitrogen fixation is the conversion of atmospheric molecular nitrogen into usable forms of nitrogen like nitrate, ammonia, amino acids etc. Nitrogen is present in the atmosphere as dinitrogen (N${}_2$) which cannot be utilized directly by plants. So, it is converted or fixed into compounds prior to utilization by the action of microbes.

E. The conversion of ammonia to nitrate by certain microorganisms is called nitrification. It is performed in two steps, nitrite formation, and nitrate formation.

So, the correct answer is 'Nitrogen fixation'.