Which of the following is not part of the nitrogen cycle?

• A. Photosynthesis
• B. N fixation
• C. Nitrification
• D. Denitrification

Answer: (A)

The nitrogen cycle tracks how nitrogen moves between different chemical forms in the environment, transforming N2 from the atmosphere into usable forms like ammonia, nitrite, and nitrate, and then back to gases. Key steps include nitrogen fixation, which makes ammonia from atmospheric nitrogen; nitrification, which converts ammonia to nitrate; and denitrification, which reduces nitrate back to N2 gas. Photosynthesis, by contrast, is the process that plants and other organisms use to capture light energy to convert carbon dioxide and water into organic carbon compounds like glucose, releasing oxygen in the process. It relies on nitrogen-containing molecules for enzymes and chlorophyll, but the act of photosynthesis itself does not alter nitrogen’s form or move nitrogen through its cycle. So photosynthesis is not part of the nitrogen cycle.