Concept:The nitrogen cycle requires nitrifying bacteria to convert ammonia into forms usable by plants.
Explanation:Ammonium compounds (
NH4+) present in soil are oxidised by bacteria through nitrification.
Nitrosomonas carries out the first and specific step of converting ammonium compounds into nitrites (
NO2−).
This step is essential because nitrites are then further converted to nitrates by Nitrobacter.
Nitrates (
NO3−) are the main form of nitrogen absorbed by plants.
Rhizobium is associated with root nodules of legumes and fixes atmospheric nitrogen.
Azotobacter is a free-living nitrogen-fixing bacterium in soil.
These organisms do not perform nitrite formation from ammonium compounds.
Therefore, the correct option is clearly Nitrosomonas.
Answer:C. Nitrosomonas