Figure 2. A schematic representation of the nitrogen cycle. Ammoniacal nitrogen enters the system through the fixation of atmospheric nitrogen by bacteria, the decomposition of organic matter by bacteria and fungi, or through fertilizer application. Ammonium is oxidized to nitrite and then nitrate by bacteria through a process known as nitrification. Plants take up (assimilate) nitrogen as either ammonium or nitrate. (Illustration from Wikimedia Commons, http://commons.wikimedia.org/).