Meaning of nitrogen fixation: Which statement best defines the process and its outcome for plant nutrition?

Introduction / Context:
Precise terminology matters in nutrient cycles. “Nitrogen fixation” is often confused with transformations of already fixed nitrogen. This question clarifies the definition relevant to agronomy and ecology.

Given Data / Assumptions:

• Nitrogen fixation concerns atmospheric N2.
• Usable forms include ammonia/ammonium, which plants can assimilate.
• Statements (b) and (d) describe other processes or incorrect ideas.

Concept / Approach:
Nitrogen fixation specifically means reducing N2 to ammonia via nitrogenase (biological route) or by industrial/abiotic means (e.g., Haber–Bosch). Transformations of organic amines or nitrate are not fixation; they are ammonification, assimilation, or reduction steps within the fixed nitrogen pool.

Step-by-Step Solution:
Identify the substrate in true fixation: atmospheric N2. Identify the usable product: ammonia/ammonium for plant uptake. Reject statements that deal with organic N (b) or that make nitrogen unusable (d). Choose the statement that matches the accepted definition: (a).

Verification / Alternative check:
Any standard nitrogen cycle diagram shows fixation as N2 → NH3, distinct from nitrification, ammonification, and assimilation.

Why Other Options Are Wrong:

• (b): Describes changes in existing organic nitrogen (ammonification), not fixation.
• (c): Incorrect because (b) is wrong for fixation.
• (d): Opposite of reality; fixation makes nitrogen usable, not unusable.
• (e): Plants convert nitrate to amino acids via assimilation inside cells, not by external “fixation.”

Common Pitfalls:
Equating any nitrogen transformation with fixation; only N2 → NH3 qualifies.

Final Answer:
Nitrogen fixation changes free N2 into a plant-usable form (ammonia/ammonium).