Question

The fixation of free nitrogen by bacteria in the soil is done by
A. Azotobacter
B. Nitrosomonas
C. Nitrobacter
D. Thiobacillus

Answer 1

Nitrogen fixation is the process of fixation of free atmospheric nitrogen, it is a nitrogen in the air that is converted into ammonia or related nitrogenous compounds in soil. In nature, most nitrogen is harvested from the atmosphere by microorganisms to form ammonia, nitrites and nitrates.

Complete answer:
Rhizobium bacteria that is present in the roots of plants convert the atmospheric nitrogen so that it can be used by plants. This bacteria enters the roots of the plant where it forms the nodules. In the plants this bacteria converts nitrogen gas into nitrogenous compounds. Plants and bacteria are in a symbiotic relationship with each other. This plant benefits the rhizobium bacteria by supplying sugars and in return bacteria benefits the plants by providing soluble nitrogen. Azotobacter is another bacteria that does the same function as that of rhizobium but it is not present in a symbiotic relationship with the plant.

Option A- Azotobacter: It is a free living bacteria that helps in nitrogen fixation in the soil. It helps in seed germination and has a positive response on Crop Growth Rate. It also helps in maintaining soil fertility.

Option B- Nitrosomonas: This is a bacteria that helps in the oxidation of ammonia into nitrite, this process is known as nitritation. Nitrosomonas are useful in a polluted water and waste treatment.

Option C- Nitrobacter: This bacteria helps in oxidizing nitrite into nitrate in soil and plays an important role in nitrogen cycle.

Option D- Thiobacillus: This bacteria is a denitrifying bacteria which helps in de nitrification. Denitrification is a process of reduction of nitrates to nitrites.

Hence, the correct answer is option (A).

Note: Azotobacter is usually found in neutral and alkaline soils, in aquatic environments and some plants. These are free living, nitrogen fixing bacteria in contrast to rhizobium species. These bacteria do not have symbiotic relationships with plants as that of rhizobium.