Which of the following happens to be the key organism in the nitrogen cycle

Options

A. All green plants
B. Leguminous plants
C. Bacteria
D. Heterotrophic plants

Answer

**Step-by-Step Solution:**

1. **Understanding the Nitrogen Cycle**: The nitrogen cycle is a biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. It involves several processes, including nitrogen fixation, nitrification, denitrification, and ammonification.

2. **Identifying Key Organisms**: In the nitrogen cycle, various organisms play roles, but we need to identify the key organism that is crucial for the cycle to function effectively.

3. **Evaluating Options**:
- **Option 1: All Green Plants**: While green plants utilize nitrogen, they are not the primary agents of nitrogen conversion. Therefore, this option is incorrect.
- **Option 2: Leguminous Plants**: These plants have symbiotic relationships with nitrogen-fixing bacteria in their root nodules. However, they are not the key organisms themselves; they serve as hosts for the bacteria. Thus, this option is also incorrect.
- **Option 3: Bacteria**: Bacteria, particularly nitrogen-fixing bacteria, play a crucial role in converting atmospheric nitrogen (N2) into ammonia (NH3), which can be utilized by plants. This makes bacteria the key organisms in the nitrogen cycle.
- **Option 4: Heterotropic Plants**: These plants depend on other organisms for nitrogen and do not contribute to the nitrogen cycle in the same way as bacteria. Therefore, this option is incorrect.

4. **Conclusion**: Based on the evaluation of the options, the correct answer is **Option 3: Bacteria**. They are essential for nitrogen fixation and maintaining the nitrogen cycle.

---