Which statement best describes the nitrogen cycle?

Which statement best describes the nitrogen cycle?

• A. The movement of nitrogen through rocks
• B. The cycle of nitrogen in soils
• C. The movement of nitrogen through rivers
• D. The series of processes by which nitrogen and its compounds are converted in the environment and in living organisms, including nitrogen fixation and decomposition

Answer: (D)

The nitrogen cycle describes how nitrogen is transformed through different chemical forms as it moves between air, water, soil, and living organisms. Microbes drive most of these changes: nitrogen fixation turns atmospheric N2 into ammonia that plants can use; in the soil, nitrifying bacteria convert ammonia to nitrite and nitrate; plants take up these forms and incorporate them into organic compounds; when organisms die or waste, decomposition and ammonification release ammonium back into the soil; denitrifying microbes can convert nitrate back to nitrogen gas, returning it to the atmosphere. Human activities also add reactive nitrogen to ecosystems, altering the balance. This explanation captures the full set of interconnected processes, not just a single pathway like movement through rocks, rivers, or soils, which only represent parts of the cycle.

The nitrogen cycle describes how nitrogen is transformed through different chemical forms as it moves between air, water, soil, and living organisms. Microbes drive most of these changes: nitrogen fixation turns atmospheric N2 into ammonia that plants can use; in the soil, nitrifying bacteria convert ammonia to nitrite and nitrate; plants take up these forms and incorporate them into organic compounds; when organisms die or waste, decomposition and ammonification release ammonium back into the soil; denitrifying microbes can convert nitrate back to nitrogen gas, returning it to the atmosphere. Human activities also add reactive nitrogen to ecosystems, altering the balance. This explanation captures the full set of interconnected processes, not just a single pathway like movement through rocks, rivers, or soils, which only represent parts of the cycle.