The conversion of ammonium salts into nitrates is called_______

Step 1: Analyze the question The question asks for the name of the process that converts ammonium salts (NH₄⁺) into nitrates (NO₃⁻) in the nitrogen cycle.

Step 2: Explain the concept The nitrogen cycle has several key processes:

• Putrefaction (ammonification): Decomposers break down dead plants and animals, and release ammonia or ammonium compounds.
• Nitrification: Nitrifying bacteria convert ammonium compounds into nitrites and then nitrates.
• Nitrogen fixation: Atmospheric nitrogen (N₂) is converted into nitrogen compounds, such as ammonia or nitrates. It is done by bacteria such as Rhizobium in legume root nodules, Azotobacter, and by lightning.
• Denitrification: Denitrifying bacteria such as Pseudomonas convert nitrates back into free nitrogen gas, which returns to the atmosphere.

Step 3: Solve step by step The conversion happens in two stages, each carried out by a different group of bacteria:

1. Ammonium salts → nitrites: by Nitrosomonas 2NH₄⁺ + 3O₂ → 2NO₂⁻ + 4H⁺ + 2H₂O
2. Nitrites → nitrates: by Nitrobacter 2NO₂⁻ + O₂ → 2NO₃⁻

The whole process is nitrification.

Step 4: Check the options

• A. Putrefaction: this produces ammonia from dead organisms, so it is the wrong direction.
• B. Denitrification: it converts nitrates into nitrogen gas, so it is the reverse of what is asked.
• C. Fixation: it starts from atmospheric N₂, not ammonium salts.
• D. Nitrification: it converts ammonium into nitrates, which is correct.

Answer: D. Nitrification

Exam tip: Look at the prefix and the direction of the change. Nitrification makes nitrates (think "nitrate-ification"), while denitrification removes nitrates. A quick sequence to remember is: Putrefaction → Ammonium → Nitrification → Nitrates → Denitrification → N₂ gas.