The common nitrogen-fixer in paddy fields is

Options

A. Frankia
B. Rhizobium
C. Azospirillum
D. Oscillatoria

Answer

**Step-by-Step Solution:**

1. **Understanding the Role of Nitrogen in Agriculture:**
- Nitrogen is an essential nutrient for plant growth and is a key component of amino acids, proteins, and nucleic acids. In paddy fields, nitrogen is crucial for enhancing rice productivity.

2. **Identifying Nitrogen Fixers:**
- Nitrogen-fixing organisms convert atmospheric nitrogen (N2) into a form that plants can use. This process is vital in agriculture, especially in rice cultivation, where nitrogen is often a limiting nutrient.

3. **Recognizing Common Nitrogen Fixers in Paddy Fields:**
- In paddy fields, various nitrogen-fixing organisms can be found, but one of the most common is **Azospirillum**. This genus of bacteria is known for its ability to fix atmospheric nitrogen and promote plant growth.

4. **Mechanisms of Azospirillum:**
- Azospirillum not only fixes nitrogen but also produces growth-promoting substances that enhance root development. This leads to improved nutrient uptake from the soil and helps in inhibiting pathogenic fungi and bacteria in the rhizosphere, which is the region of soil around the roots.

5. **Conclusion:**
- Therefore, the common nitrogen-fixer in paddy fields is **Azospirillum**.

**Final Answer:**
The common nitrogen-fixer in paddy fields is Azospirillum.

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