In paddy fields, biological nitrogen fixation is chiefly brought about by

Options

A. Cyanobacteria
B. Green algae
C. Mycorrhiza
D. Rhizobium

Answer

**Step-by-Step Solution:**

1. **Understanding Biological Nitrogen Fixation**: Biological nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted into ammonia (NH3) by certain microorganisms. This process is crucial for enriching soil fertility.

2. **Identifying Key Organisms**: In agricultural practices, several organisms are known for their role in nitrogen fixation. These include cyanobacteria, Rhizobium, and other microorganisms.

3. **Focus on Paddy Fields**: The question specifically asks about paddy fields, which are flooded rice fields. The conditions in paddy fields favor certain types of microorganisms over others.

4. **Role of Cyanobacteria**: Cyanobacteria, also known as blue-green algae, are particularly effective in nitrogen fixation in paddy fields. They thrive in the wet, anaerobic conditions typical of these fields and can convert atmospheric nitrogen into a form that plants can use.

5. **Comparison with Other Organisms**: While Rhizobium is a well-known nitrogen-fixing bacteria, it primarily associates with leguminous plants and is not the chief contributor in paddy fields. Mycorrhiza, which is a symbiotic association between fungi and plant roots, aids in nutrient absorption but does not directly fix nitrogen.

6. **Conclusion**: Therefore, the organism chiefly responsible for biological nitrogen fixation in paddy fields is cyanobacteria.

**Final Answer**: Cyanobacteria.

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