The fixation of free nitrogen by bacteria in the soil is done by A. AzotobacterB. NitrosomonasC. NitrobacterD. Thiobacillus

The correct answer is A. Azotobacter
Explanation:
1. Fixation of Free Nitrogen:
o Nitrogen fixation is the process by which nitrogen gas (N₂) from the atmosphere is converted into a form that can be used by plants, typically ammonia (NH₃). This is an essential process because nitrogen is a crucial element for plant growth, but most plants cannot directly use atmospheric nitrogen.
o Certain bacteria, known as nitrogen-fixing bacteria, play a key role in converting atmospheric nitrogen into a usable form for plants.
2. Azotobacter:
o Azotobacter is a genus of free-living nitrogen-fixing bacteria found in the soil. These bacteria can fix nitrogen directly from the atmosphere and convert it into ammonia, which is then available to plants.
o Azotobacter bacteria do not require a host plant to carry out nitrogen fixation, making them important in improving soil fertility by enriching it with usable nitrogen.
3. Other Bacteria:
o B. Nitrosomonas: Nitrosomonas bacteria are involved in the nitrification process, which converts ammonia into nitrites, not nitrogen fixation.
o C. Nitrobacter: Nitrobacter bacteria also play a role in the nitrification process. They convert nitrites into nitrates, which are then used by plants but do not fix atmospheric nitrogen.
o D. Thiobacillus: Thiobacillus is a genus of bacteria that are involved in sulfur oxidation and the cycling of sulfur in the environment, not nitrogen fixation.
Azotobacter is the correct bacterium that fixes atmospheric nitrogen in the soil, converting it into a usable form for plants. Therefore, the correct answer is A. Azotobacter.