Figure 21.6 The nitrogen cycle. The nitrogen cycle begins with nitrogen gas in the atmosphere. The process of nitrogen fixation converts it into a form that producers can use. The fixed nitrogen can then be assimilated into producers and consumers; it ultimately decomposes into ammonium through the process of mineralization. The ammonium can be converted into nitrite and then nitrate through the process of nitrification. Under anaerobic conditions, the nitrate can be converted into nitrogen gas through the process of denitrification.