In nitrogen cycle, atmospheric nitrogen is fixed by bacteria and converted into ammonia. Ammonia is further converted into other forms of nitrogen. At the end of the cycle it returns to the atmosphere by the process of :

Options

A. Ammonification
B. Nitrification
C. Denitrification
D. Assimilation

Explanation

To solve the question regarding the nitrogen cycle and the process through which nitrogen returns to the atmosphere, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Atmospheric nitrogen (N₂) is fixed by nitrogen-fixing bacteria, converting it into ammonia (NH₃). This is the first step in the nitrogen cycle.

**Hint**: Remember that nitrogen-fixing bacteria play a crucial role in converting atmospheric nitrogen into a usable form.

2. **Conversion of Ammonia**:
- Ammonia can be further converted into nitrites (NO₂⁻) and nitrates (NO₃⁻) through a process called nitrification. These forms of nitrogen are usable by plants.

**Hint**: Nitrification is essential for converting ammonia into forms that plants can absorb.

3. **Utilization by Plants**:
- Plants absorb nitrates and nitrites from the soil and use them to synthesize proteins and other nitrogen-containing compounds.

**Hint**: Plants are the primary consumers of nitrates and nitrites, which are critical for their growth.

4. **Return to the Atmosphere**:
- At the end of the nitrogen cycle, the nitrates and nitrites in the soil are converted back to atmospheric nitrogen (N₂) through a process known as denitrification.

**Hint**: Denitrification is the process that completes the nitrogen cycle by returning nitrogen to the atmosphere.

5. **Role of Denitrifying Bacteria**:
- Specific bacteria, such as Pseudomonas, are responsible for denitrification. They convert nitrates and nitrites back into nitrogen gas, which is released into the atmosphere.

**Hint**: Remember that denitrifying bacteria are key players in the process of returning nitrogen to the atmosphere.

### Final Answer:
The process through which nitrogen returns to the atmosphere at the end of the nitrogen cycle is called **denitrification**.