Non–symbiotic nitrogen fixing bacteria :

Options

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

Answer

To solve the question regarding non-symbiotic nitrogen-fixing bacteria, we will analyze the options provided and identify which one fits the criteria of being non-symbiotic.

### Step-by-Step Solution:

1. **Understand the Definition**:
- Non-symbiotic nitrogen-fixing bacteria are those that can fix atmospheric nitrogen into ammonia without forming a symbiotic relationship with plants. They live freely in the soil.

2. **Evaluate the Options**:
- **Option 1: Rhizobium**:
- Rhizobium is a symbiotic nitrogen-fixing bacteria. It forms a mutualistic relationship with leguminous plants (like peas and beans) where it resides in root nodules and provides nitrogen in exchange for carbohydrates and shelter.
- **Conclusion**: This is not a non-symbiotic bacteria.

- **Option 2: Azospirillum**:
- Azospirillum is known for its association with plant roots but does not form a symbiotic relationship like Rhizobium. However, it is primarily known for promoting plant growth rather than fixing nitrogen directly.
- **Conclusion**: This is not primarily recognized as a nitrogen-fixing bacteria.

- **Option 3: Azotobacter**:
- Azotobacter is a free-living nitrogen-fixing bacteria that does not require a plant host to fix nitrogen. It can convert atmospheric nitrogen into ammonia independently.
- **Conclusion**: This is a non-symbiotic nitrogen-fixing bacteria.

- **Option 4: Nitrosomonas**:
- Nitrosomonas is involved in nitrification, converting ammonia into nitrites, but it does not fix atmospheric nitrogen.
- **Conclusion**: This is not a nitrogen-fixing bacteria.

3. **Final Decision**:
- Based on the evaluations, the only non-symbiotic nitrogen-fixing bacteria among the options is **Azotobacter** (Option 3).

### Answer:
The correct answer is **Option 3: Azotobacter**.

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