Nitrification is an essential step in the nitrogen cycle of natural systems because it links organic matter degradation to fixed nitrogen loss. Ammonium released by ammonification is oxidized to nitrate by nitrification, and can then be reduced to dinitrogen gas by denitrification, resulting in net loss of fixed nitrogen from the system. Whether organic matter degradation results in net ammonium release depends largely on the quality of the organic substrate and interactions among members of the microbial community involved in nitrogen and organic matter cycling. In sediments, nitrogen cycle processes depend on the supply of organic matter and oxygen from overlying water. The nature of the net flux (which direction and which form of nitrogen) is a function of closely coupled reactions (ammonification-nitrification-denitrification) in the nitrogen cycle.