Nitrogen fixation converts atmospheric N2 to usable forms

Steps:

• Recall nitrogen cycle stages: fixation, ammonification, nitrification, denitrification.
• Identify fixation as converting N2 gas to ammonia (NH3) by bacteria or lightning.
• Match labelled arrows to cycle processes based on direction and organisms involved.
• Select arrow showing N2 to NH3 as fixation.

Why A is correct:

• Nitrogen fixation is defined as the biological or abiotic conversion of inert N2 to reactive NH3, matching arrow A in standard diagrams.

Why the others are wrong:

• B represents ammonification, decomposing organic matter to NH3.
• C shows nitrification, oxidizing NH3 to nitrates (NO3-).
• D indicates denitrification, reducing nitrates back to N2 gas.

Final answer: A