Certain bacteria living in the soil poor in oxygen convert nitrates into nitrites and then to free nitrogen and such bacteria are termed as

Options

A. nitrogen fixing bacteria
B. denitrifying bacteria
C. ammonifying bacteria
D. saprophytic bacteria.

Answer

To solve the question regarding the type of bacteria that convert nitrates into nitrites and then to free nitrogen in oxygen-poor soil, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Process**: The question describes a process where certain bacteria convert nitrates (NO3-) into nitrites (NO2-) and then into free nitrogen (N2). This process is known as denitrification.

**Hint**: Look for the key terms "convert nitrates into nitrites and then to free nitrogen" to identify the process involved.

2. **Identifying the Type of Bacteria**: The bacteria that perform denitrification are specifically known as denitrifying bacteria. They thrive in anaerobic (low oxygen) conditions and play a crucial role in the nitrogen cycle by reducing nitrates to nitrogen gas.

**Hint**: Focus on the bacteria that are involved in the conversion of nitrates to nitrogen gas, which is a key aspect of the nitrogen cycle.

3. **Evaluating the Options**: The options provided are:
- Nitrogen fixing bacteria
- Denitrifying bacteria
- Ammonifying bacteria
- Saprophytic bacteria

Among these, we need to identify which one is responsible for the conversion of nitrates to nitrogen gas.

**Hint**: Recall the definitions of each type of bacteria to see which one matches the process described.

4. **Eliminating Incorrect Options**:
- **Nitrogen fixing bacteria**: These bacteria convert atmospheric nitrogen (N2) into ammonia (NH3), which is not the process described.
- **Ammonifying bacteria**: These bacteria convert organic nitrogen into ammonia, which is also not the process described.
- **Saprophytic bacteria**: These are decomposers that break down dead organic matter and do not specifically convert nitrates to nitrogen gas.

**Hint**: Compare each option with the process described to eliminate those that do not fit.

5. **Conclusion**: The only bacteria that fit the description of converting nitrates into nitrites and then into free nitrogen are the denitrifying bacteria.

**Hint**: The answer should directly relate to the specific process of denitrification mentioned in the question.

### Final Answer:
The bacteria that convert nitrates into nitrites and then to free nitrogen are termed as **denitrifying bacteria**.