Which sentence best summarizes the central claim about nitrogen fixation in clover?

Which sentence best summarizes the central claim about nitrogen fixation in clover?

• A. Clover increases external nitrogen inputs.
• B. Clover has no effect on soil nitrogen.
• C. Clover fixes nitrogen but requires fertilizer.
• D. Clover's nitrogen-fixing ability reduces the need for external nitrogen inputs.

Answer: (D)

Nitrogen fixation in clover comes from a partnership with nitrogen-fixing bacteria inside the plant’s root nodules. These bacteria convert atmospheric nitrogen into forms the plant can use, enriching both the clover and the surrounding soil. Because of this natural supply, clover can cut down the amount of nitrogen fertilizer needed for itself and for subsequent crops in rotation. So the statement that best captures the idea is that clover's nitrogen-fixing ability reduces the need for external nitrogen inputs. The other options misstate the impact: clover does not add external nitrogen fertilizers, it does affect soil nitrogen, and its fixation doesn’t require fertilizer to occur (fertilizer might be used in farming, but the fixation process itself provides nitrogen).

Nitrogen fixation in clover comes from a partnership with nitrogen-fixing bacteria inside the plant’s root nodules. These bacteria convert atmospheric nitrogen into forms the plant can use, enriching both the clover and the surrounding soil. Because of this natural supply, clover can cut down the amount of nitrogen fertilizer needed for itself and for subsequent crops in rotation. So the statement that best captures the idea is that clover's nitrogen-fixing ability reduces the need for external nitrogen inputs. The other options misstate the impact: clover does not add external nitrogen fertilizers, it does affect soil nitrogen, and its fixation doesn’t require fertilizer to occur (fertilizer might be used in farming, but the fixation process itself provides nitrogen).