Ammonia oxidizing bacteria utilize ammonia as an electron source and use oxygen as a terminal electron acceptor. In this form of aerobic respiration, the ammonia is getting .

oxidized

Explanation:

Oxidation is defined as a loss of electrons, increase in oxidation number, the addition of oxygen, or the removal of nitrogen. On the other hand, the reduction is defined as the gain of electrons, decrease in oxidation number, the removal of oxygen, or the addition of hydrogen.

Ammonia being utilized as an electron source means that the compound is an electron donor in the redox reaction while oxygen is acting as an electron acceptor. Hence, in this form of respiration, ammonia is getting oxidized while oxygen is getting reduced.

compost can be added to topsoil as a:

pesticide

substitution is the answer in this case