The common nitrogen-fixer in paddy fields is

Options

A. Frankia
B. Rhizobium
C. Azospirillum (B.G.A)
D. Oscillatoria

Answer

To solve the question "The common nitrogen-fixer in paddy fields is," we will follow these steps:

### Step-by-Step Solution:

1. **Understand the Role of Nitrogen Fixation**:
- Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into a form that plants can use, such as ammonia (NH₃). This is crucial for plant growth, particularly in paddy fields where nitrogen is a vital nutrient.

2. **Identify the Organisms Involved**:
- There are various organisms that can fix nitrogen, including certain bacteria and algae. The options provided are:
- Frankia
- Rhizobium
- Agiospirellum
- BZA (which refers to Blue-Green Algae)
- Oscillatorio

3. **Analyze Each Option**:
- **Frankia**: This is a genus of nitrogen-fixing bacteria, but it is more commonly associated with actinorhizal plants rather than paddy fields.
- **Rhizobium**: This genus is known for forming symbiotic relationships with legumes, not typically found in paddy fields.
- **Agiospirellum**: This is a genus of gram-negative bacteria that can fix nitrogen and is known to be present in aquatic environments, including paddy fields.
- **BZA (Blue-Green Algae)**: These are also known as cyanobacteria and are well-known for their ability to fix nitrogen in aquatic ecosystems, including rice paddies.
- **Oscillatorio**: This is another type of cyanobacteria, but it is less commonly referenced as a nitrogen fixer compared to other blue-green algae.

4. **Determine the Most Common Nitrogen Fixer**:
- Among the options, both Agiospirellum and Blue-Green Algae (BZA) are capable of fixing nitrogen in paddy fields. However, Agiospirellum is specifically noted for its role in paddy fields in the context of the question.

5. **Conclude the Answer**:
- Therefore, the common nitrogen-fixer in paddy fields is **Agiospirellum**.

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