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 that converts nitrates back to nitrogen gas. This process involves the reduction of nitrates by bacteria in low oxygen environments, such as wetlands and ocean sediments. A. Nitrification is the process of converting ammonia into nitrates by bacteria, which is an earlier step in the nitrogen cycle. B. Nitrogen fixation is the conversion of atmospheric nitrogen into a form that can be used by plants and other organisms. This occurs primarily through a symbiotic relationship with bacteria and is not a step in the nitrogen cycle that converts nitrates back to nitrogen gas. C. Assimilation is the process of plants and animals incorporating nitrogen into their tissues. This is not a step in the nitrogen cycle that converts nitrates back to nitrogen gas. Hence, the correct answer is D because denitrification is the