Which process oxidizes ammonium compounds into nitrates and nitrites?

Nitrification is the key process that transforms ammonium compounds into nitrates and nitrites. This biological oxidation occurs primarily in soil and involves two main steps. First, certain bacteria, such as those in the genera Nitrosomonas, convert ammonium (NH4+) into nitrite (NO2-). In the second step, other bacteria, like Nitrobacter, further oxidize nitrite into nitrate (NO3-). This process is crucial in the nitrogen cycle, as it facilitates the availability of nitrogen in a form that plants can easily absorb for their growth and metabolism.

In contrast, nitrogen fixation refers to the conversion of atmospheric nitrogen (N2) into ammonium, while decomposition involves breaking down organic matter, which can lead to the release of ammonium but not the conversion into nitrates or nitrites. Fermentation is a metabolic process that occurs in anaerobic conditions, producing energy without the need for oxygen, and is unrelated to the oxidation of nitrogen compounds. Therefore, nitrification is specifically the process that accomplishes the oxidation of ammonium to nitrates and nitrites, making it the correct answer.