Ammonium oxidation is a biological process where ammonium ions (NH4+) are converted into nitrite (NO2-) by specific bacteria, primarily from the group known as ammonia-oxidizing bacteria (AOB). This process is a crucial step in the nitrogen cycle, helping to transform nitrogen compounds in the environment. Following ammonium oxidation, the nitrite produced can be further oxidized to nitrate (NO3-) by another group of bacteria called nitrite-oxidizing bacteria (NOB). This sequential transformation is essential for maintaining soil fertility and supporting plant growth, as nitrate is a key nutrient for plants.