The conversion of nitrates to nitrogen gas by bacteria is called

Options

A. denitrification
B. nitrification
C. nitrogen fixation
D. excretion.

Explanation

To answer the question "The conversion of nitrates to nitrogen gas by bacteria is called," we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Processes Involved**:
- We need to identify the processes related to nitrogen compounds in the soil and the role of bacteria in these processes.

2. **Identify the Options**:
- The options provided are:
- Denitrification
- Nitrification
- Nitrogen fixation
- Excretion

3. **Analyze Each Option**:
- **Denitrification**: This is the process where nitrates (NO3-) are converted into nitrogen gas (N2) by bacteria. This process helps in returning nitrogen to the atmosphere.
- **Nitrification**: This process involves the conversion of ammonia (NH3) to nitrites (NO2-) and then to nitrates (NO3-). It is not the conversion of nitrates to nitrogen gas.
- **Nitrogen Fixation**: This is the process of converting atmospheric nitrogen (N2) into ammonia (NH3), making it available for plants. It does not involve the conversion of nitrates to nitrogen gas.
- **Excretion**: This refers to the removal of waste products from an organism's body and is not related to the nitrogen cycle.

4. **Conclusion**:
- Based on the analysis, the correct answer is **Denitrification**, as it specifically refers to the conversion of nitrates to nitrogen gas by bacteria.

### Final Answer:
The conversion of nitrates to nitrogen gas by bacteria is called **Denitrification**.

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