During nitrification, ammonia is changed back into nitrate.
Nitrification is a three-step process of oxidation of ammonia. It is an aerobic process performed by small groups of autotrophic bacteria (Nitrosomonas, Nitrobacter). First step- ammonia to hydroxylamine, Second step-hydroxylamine to nitrite, Third step-nitrite to nitrate. Nitrification together with ammonification is part of a nitrogen cycle.