The bulk of nitrogen in nature is fixed by-

Options

A. Lightening
B. Chemical industries
C. Denitrifying bacteria
D. Symbiotic bacteria

Answer

**Step-by-Step Solution:**

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process through which atmospheric nitrogen (N₂) is converted into a form that is usable by living organisms, primarily plants. This conversion is essential for the growth of plants as they require nitrogen for the synthesis of proteins and nucleic acids.

2. **Types of Nitrogen Fixation**:
- Nitrogen fixation can occur through various processes:
- **Physical processes**: Such as lightning, which can fix a small amount of nitrogen.
- **Chemical processes**: Such as those occurring in industrial settings (e.g., Haber process).
- **Biological processes**: This is the primary method through which the bulk of nitrogen is fixed in nature.

3. **Identifying the Main Contributors**:
- Among the biological processes, nitrogen fixation is predominantly carried out by certain bacteria. These bacteria can be classified into:
- **Symbiotic bacteria**: These bacteria form a mutualistic relationship with plants, particularly legumes. They reside in the root nodules of these plants and are responsible for fixing nitrogen.
- **Free-living bacteria**: These bacteria exist in the soil and can also fix nitrogen but to a lesser extent compared to symbiotic bacteria.

4. **Key Bacteria Involved**:
- The main types of symbiotic bacteria involved in nitrogen fixation include:
- **Rhizobium**: Associated with leguminous plants.
- **Frankia**: Associated with non-leguminous plants.
- Additionally, nitrifying bacteria convert ammonia into nitrites and nitrates, further facilitating nitrogen availability in the ecosystem.

5. **Conclusion**:
- Given the options and the understanding of nitrogen fixation, the bulk of nitrogen in nature is fixed by **symbiotic bacteria**.

**Final Answer**: The bulk of nitrogen in nature is fixed by **symbiotic bacteria**.

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