Question

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

Answer 1

The bacteria have basically two categories which help in nitrogen fixation, one being the free living and the other being the symbiotically living. These free-living bacteria remain in the soil unattached to the plant and they only fix nitrogen in the soil.

Complete answer:
We all know that the plants require nitrogen for their metabolic processes and this requirement of nitrogen is taken care by the bacteria which are present in the soil. Now the bacteria can be present in two ways into the soil, one being that they can be free living inside the soil and have no dependency upon the plants on the other hand the other type of bacteria are present in the soil relying upon the plants for their nutrition or we can say in a mutualistic manner.
The other type of association is also known as symbiotic association. These bacteria remain attached to the root nodules of e plants and directly transfer nitrogen to the root nodules. This is present in the plants which cannot take up nitrogen on their own. The other types which we have discussed are free living bacteria which submit the nitrogen in the soil and the root nodules directly pick up the nitrogen from the soil.
So, the free-living bacteria are of the family of Cyanobacteria which are also known as blue-green algae. The genera which belong to this family are Nostoc, Anabaena, and Azotobacter.

Therefore, from the given options we can that option A is correct as rest all are symbiotically living bacteria.

Note: The symbiotic bacteria are mostly known to associate with the leguminous plants like pulses, peas and they invade their root nodules and convert the nitrogen to ammonia which is taken up by the plant and is used to perform metabolism.