Characterize the following processes in the nitrogen cycle: fixation, ammonification, nitrification, and denitrification.

Answer: The four main processes in the nitrogen cycle are nitrogen fixation, ammonification, nitrification, and denitrification. Nitrogen fixation converts nitrogen gas (N2) from the atmosphere into ammonia (NH3) or ammonium (NH4+), which can be taken up by plants. Ammonification involves the breakdown of organic nitrogen-containing compounds into ammonium (NH4+) by decomposer microorganisms. Nitrification is a two-step process in which ammonia (NH3) or ammonium (NH4+) is converted into nitrite (NO2-) and then into nitrate (NO3-) by nitrifying bacteria. Finally, denitrification is the process in which nitrates (NO3-) are reduced to nitrogen gas (N2), nitrous oxide (N2O), and other nitrogenous gases by denitrifying bacteria, returning nitrogen gas to the atmosphere and completing the nitrogen cycle.