Major nitrogen fixation is carried out by

Options

A. lightning
B. chemical industries
C. symbiotic bacteria
D. leaching

Answer

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process of converting atmospheric nitrogen (N₂) into a form that plants can absorb and utilize, such as nitrates, nitrites, or ammonia.

2. **Methods of Nitrogen Fixation**:
- There are several methods through which nitrogen fixation occurs:
- **Natural Method**: This includes processes like lightning, which can convert atmospheric nitrogen into usable forms, but it is not a reliable or consistent method.
- **Biological Nitrogen Fixation**: This is the primary method of nitrogen fixation and involves certain bacteria that can convert atmospheric nitrogen into forms that plants can use.
- **Industrial Nitrogen Fixation**: This involves the production of nitrogen-rich fertilizers through chemical processes.
- **Leaching**: This refers to the process where nitrogen seeps into water from soil, but it is not a significant method of nitrogen fixation.

3. **Identifying Major Contributors**:
- Among the methods mentioned, biological nitrogen fixation is the most significant contributor to nitrogen fixation in nature. This process is primarily carried out by specific bacteria.

4. **Examples of Nitrogen-Fixing Bacteria**:
- Some well-known nitrogen-fixing bacteria include:
- **Rhizobium**: These are symbiotic bacteria that live in the root nodules of leguminous plants.
- **Anabaena**: A genus of cyanobacteria that can fix nitrogen.
- **Nostoc**: Another genus of cyanobacteria that is capable of nitrogen fixation.

5. **Conclusion**:
- The major nitrogen fixation is predominantly carried out by symbiotic bacteria, particularly those associated with leguminous plants.

### Final Answer:
Major nitrogen fixation is carried out by **symbiotic bacteria**.

---