Explanation

Some ammonia ($NH_{3}$) is oxidized to form nitrites ($NO_{2}$) and nitrates ($NO_{3}$) through the action of autotrophic bacteria, e.g., Nitrosomonas and Nitrobacter. This process is called nitrification and the energy released supports the existence of nitrifying bacteria. The organic nitrogen of plants and animals is ultimately broken by the action of saprophytic fungi and bacteria to ammonia ($NH_{3}$) in a process called ammonification. Other types of anaerobic soil bacteria (e.g., Pseudomonas) act on nitrate in the process of denitrification by which nitrogen is liberated into the atmosphere. It involves the reduction of nitrate ions ($NO_{3}$) to nitrogen dioxide ($N_{2}O$), nitrogen monoxide (NO) or nitrogen ($N_{2}$).