The bacterium which reduces the fertility of soil is

Options

A. Nitrosomonas
B. Bacillus denitrificans
C. Azotobacter sp.
D. Nitrobacter.

Answer

**Step-by-Step Solution:**

1. **Understanding Soil Fertility**: Soil fertility is primarily influenced by the availability of essential nutrients, particularly nitrogen, phosphorus, and potassium. Nitrogen is crucial for plant growth and is often a limiting factor in soil fertility.

2. **Identifying Beneficial Bacteria**: Certain bacteria, such as Nitrosomonas and Azotobacter, are known to enhance soil fertility. Nitrosomonas converts ammonia (a product of nitrogen fixation) into nitrite, while Azotobacter is a free-living nitrogen fixer that increases nitrogen content in the soil.

3. **Recognizing Harmful Bacteria**: Conversely, some bacteria can reduce soil fertility by converting nitrogen compounds back into nitrogen gas (N2), which is released into the atmosphere. This process is known as denitrification.

4. **Focusing on Bacillus denitrificans**: Among the options provided, Bacillus denitrificans is a bacterium that is specifically involved in the denitrification process. It converts nitrates and nitrites in the soil back into nitrogen gas, thereby depleting the nitrogen content and reducing soil fertility.

5. **Conclusion**: Based on the role of Bacillus denitrificans in denitrification, it can be concluded that this bacterium reduces the fertility of the soil.

**Final Answer**: The bacterium which reduces the fertility of soil is **Bacillus denitrificans**.

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