(3) Conversion of ammonia to nitrites Explanation: Nitrification is the biological oxidation of ammonia or ammonium to nitrite followed by the oxidation of the nitrite to nitrate. The oxidation of ammonia into nitrite is performed by two groups of organisms, ammonia-oxidizing bacteria AOB) and ammonia-oxidizing archaea (AOA). Nitrification is an important step in the nitrogen cycle in soil.