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.