Which process in the nutrient cycle converts atmospheric nitrogen into a form that plants can utilize?

Step 1: Analyze the question The question asks which process takes atmospheric nitrogen (N₂) and converts it into a form plants can use. The key phrase is "atmospheric nitrogen," meaning the starting material is N₂ gas from the air.

Step 2: Explain the concept Although nitrogen makes up about 78% of the air, plants cannot absorb N₂ gas directly. It must first be changed into compounds like ammonia (NH₃), ammonium (NH₄⁺) or nitrates (NO₃⁻). Here is what each process does:

• Nitrogen fixation: Converts N₂ from the air into ammonia/ammonium. It is done by nitrogen-fixing bacteria such as Rhizobium (in root nodules of legumes like beans, groundnut and cowpea), Azotobacter and Clostridium in the soil, and by blue-green algae. Lightning also fixes small amounts of nitrogen.
• Ammonification: Decomposers break down dead plants and animals and animal waste into ammonia. The nitrogen is already in the soil or organic matter, not from the atmosphere.
• Nitrification: Ammonia is converted to nitrites (by Nitrosomonas) and then to nitrates (by Nitrobacter). The starting material is ammonia, not atmospheric N₂.
• Denitrification: Does the opposite. Bacteria like Pseudomonas denitrificans convert nitrates back into N₂ gas, returning nitrogen to the atmosphere.

Step 3: Solve Only one process begins with atmospheric N₂ and ends with a plant-usable form: nitrogen fixation.

• A. Denitrification: returns nitrogen to the air, so it is wrong.
• B. Nitrogen fixation: correct.
• C. Ammonification: starts from organic matter, so it is wrong.
• D. Nitrification: starts from ammonia, so it is wrong.

Step 4: Answer The correct option is B. Nitrogen fixation.

Exam tip: Match the process with its starting material:

• Fix = from the air (N₂) → ammonia
• Ammonification = from dead matter → ammonia
• Nitrification = ammonia → nitrates
• Denitrification = nitrates → back to air ("De" means taking nitrogen away from the soil)

Also remember that Rhizobium lives in the root nodules of legumes. JAMB loves this link!