Nitrifying bacteria convert ammonium to nitrates in soil

Steps:

• Recall the nitrogen cycle: atmospheric nitrogen fixed into compounds, then transformed by soil bacteria.
• Identify nitrifying bacteria's role: they oxidize ammonium (NH4+) to nitrites (NO2-) and then nitrates (NO3-).
• Match to choices: process involves ammonium in soil becoming nitrates in soil.
• Confirm: this nitrification stage enables plant uptake of nitrates.

Why B is correct:

• Nitrifying bacteria (Nitrosomonas and Nitrobacter) perform oxidation of ammonium to nitrates, as per the biochemical process: NH4+ → NO2- → NO3-.

Why the others are wrong:

• A: Ammonium compounds are the starting substrate, not the stage carried out by nitrifying bacteria.
• C: Nitrogen in air is fixed by lightning or bacteria like Rhizobium, unrelated to nitrifiers.
• D: Nitrogen compounds in plants result from nitrate uptake and assimilation, not bacterial nitrification.

Final answer: B