Which of the following bacteria reduce nitrate in soil into nitrogen?

Options

A. Nitrobacter
B. Nitrococcus
C. Thiobacillus
D. Nitrosomonas

Answer

To solve the question of which bacteria reduce nitrate in soil into nitrogen, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Process**: The question is about the reduction of nitrate (NO3-) in the soil to nitrogen gas (N2). This process is known as denitrification.

2. **Identifying Denitrifying Bacteria**: Denitrification is carried out by specific types of bacteria known as denitrifying bacteria. These bacteria convert nitrates into nitrogen gas, which is then released into the atmosphere.

3. **Analyzing the Options**: The options provided are:
- Nitrobacter
- Nitrococcus
- Thiobacillus
- Nitrosomonas

4. **Eliminating Incorrect Options**:
- **Nitrobacter**: This bacteria is primarily involved in the oxidation of nitrite to nitrate, not in denitrification.
- **Nitrococcus**: Similar to Nitrobacter, it is involved in the nitrification process, converting ammonia to nitrite and then to nitrate.
- **Nitrosomonas**: This bacterium is also involved in nitrification, converting ammonia to nitrite.

5. **Identifying the Correct Answer**: The only option left that is known to perform denitrification is **Thiobacillus**. This bacterium is involved in the reduction of nitrates to nitrogen gas.

6. **Conclusion**: Therefore, the correct answer to the question is **Thiobacillus**.

### Final Answer:
**Thiobacillus**