Ontario Grade 9 Science Class Practice Exam

What is the final step in the nitrogen cycle that converts nitrates back to nitrogen gas?

1. Nitrification

2. Nitrogen fixation

3. Assimilation

4. Denitrification

The correct answer is: Denitrification

Denitrification is the final step in the nitrogen cycle, where nitrates in the soil are converted back into nitrogen gas (N₂) and released into the atmosphere. This process is crucial for maintaining the balance of nitrogen in the environment. Denitrifying bacteria, which thrive in anaerobic (low oxygen) conditions, play a key role in this process by using nitrates as an electron acceptor in their metabolic processes. As a result, nitrogen gas is produced and returned to the atmosphere, completing the nitrogen cycle. In contrast, nitrification refers to the conversion of ammonia to nitrates, which is an earlier step in the cycle. Nitrogen fixation involves the conversion of atmospheric nitrogen into forms that plants can utilize, while assimilation is the uptake of ammonium or nitrate by plants to form organic nitrogen compounds. All these processes contribute to the nitrogen cycle, but denitrification is specifically responsible for returning nitrogen gas to the atmosphere, making it the final step in the cycle.