Nitrogen Cycle Stage Identification

Steps:

• Recall the nitrogen cycle: atmospheric N2 is converted to usable forms by bacteria, plants, animals, and back to N2.
• Locate stage X: it depicts conversion of gaseous N2 into ammonia (NH3) by symbiotic or free-living bacteria.
• Match to processes: this initial step is nitrogen fixation, not later conversions or breakdowns.
• Confirm: fixation enables plants to absorb nitrogen for growth.

Why C is correct:

• Nitrogen fixation is the biological process where prokaryotes (e.g., Rhizobium) reduce N2 to NH3 using nitrogenase enzyme, per the reaction N2 + 8H+ + 8e- → 2NH3 + H2.

Why the others are wrong:

• A: Denitrification converts nitrates back to N2 gas by bacteria in anaerobic conditions, a final stage.
• B: Decomposition breaks down organic matter, releasing ammonia, but not directly fixing atmospheric N2.
• D: Nitrification oxidizes ammonia to nitrites/nitrates by soil bacteria like Nitrosomonas, occurring after fixation.

Final answer: C