The change of NH⁺₄ to NO^-₃ is called as

Nitrification is the biological oxidation of ammonia (NH⁺₄) to nitrite (NO^-₃) followed by the oxidation of the nitrite to nitrate. Both the processes occur in the presence of different microorganisms. It is an important step in the nitrogen cycle in the soil. It is an aerobic process performed by small groups of autotrophic bacteria.