The diagram shows a simplified representation of the nitrogen cycle in a terrestrial ecosystem.
Which of the following correctly pairs a process with its description?
Process WWW is denitrification, which occurs under aerobic conditions.
Process XXX is nitrogen fixation, converting organic proteins to ammonium compounds.
Process YYY is nitrification, an oxidation reaction carried out by nitrifying bacteria in aerated soil.
Process ZZZ is ammonification, converting nitrate ions back into atmospheric nitrogen gas.