Nitrogen Fixation in the Nitrogen Cycle

Steps:

• Recall that nitrogen-fixing bacteria in leguminous plants convert atmospheric N2 gas into ammonia (NH3) usable by plants.
• Identify arrows in the diagram: A shows N2 entering plant roots via nodules, B depicts ammonia to nitrates, C shows denitrification, D represents plant uptake or decomposition.
• Match the process: Arrow A aligns with fixation, as it points from atmosphere to plant-available nitrogen.
• Confirm by excluding other arrows that represent different cycle stages.

Why A is correct:

• Nitrogen fixation by Rhizobium bacteria in legume root nodules directly transforms N2 into NH3, per the biological nitrogen fixation definition.

Why the others are wrong:

• B: Represents nitrification, converting NH3 to NO3- by soil bacteria.
• C: Shows denitrification, releasing N2 back to atmosphere from nitrates.
• D: Indicates ammonification, breaking down organic nitrogen from dead matter.

Final answer: A