Which of the following is free living nitrogen fixing bacteria present in soil?

Options

A. Azotobacter
B. Nitrosomonas
C. Rhizobium
D. Pseudomonas.

Answer

**Step-by-Step Solution:**

1. **Understanding Nitrogen-Fixing Bacteria**: Nitrogen-fixing bacteria are microorganisms that convert atmospheric nitrogen (N₂) into a form that plants can use, such as ammonia (NH₃). This process is essential for soil fertility and plant growth.

2. **Identifying Free-Living Nitrogen-Fixing Bacteria**: Free-living nitrogen-fixing bacteria are those that do not form symbiotic relationships with plants. They can live independently in the soil and still perform nitrogen fixation.

3. **Analyzing the Options**: The question provides several options for nitrogen-fixing bacteria:
- **Azotobacter**
- **Nitrosomonas**
- **Rhizobium**
- **Pseudomonas**

4. **Evaluating Each Option**:
- **Azotobacter**: This is a free-living nitrogen-fixing bacterium found in soil. It does not depend on plants for its nitrogen-fixing capabilities.
- **Nitrosomonas**: This bacterium is involved in the nitrification process and does not fix nitrogen.
- **Rhizobium**: This bacterium forms nodules on the roots of legume plants and is a symbiotic nitrogen fixer, not free-living.
- **Pseudomonas**: While some species can fix nitrogen, they are not primarily known as free-living nitrogen fixers.

5. **Conclusion**: Based on the evaluation, Azotobacter is the correct answer as it is a free-living nitrogen-fixing bacterium present in the soil.

**Final Answer**: Azotobacter

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