A series of experiments showed the quantity and composition of nitrogen lost in gaseous form from fertilizers in soil is largely determined by the conditions of denitrification. Loss of nitrogen from ammonium sulfate or calcium nitrate was mainly through the release of nitrous oxide and molecular nitrogen, while nitrogen was released from sodium nitrite in the form of nitric oxide. Under anaerobic conditions and at neutral soil pH in the presence of glucose, the more reduced gaseous forms of nitrogen were released. However, the oxides of nitrogen predominated under conditions unfavorable for denitrification. The nitrogen oxides were not the terminal products of nitrogen conversion (nitrates and nitrites). By a process of dissimilation, the nitrogen oxides acted as electron acceptors for microorganisms, being converted to N2O and N2. The reduction of NO generally led to the formation of N2O as an intermediate, and depended on pH, aeration, and the presence of an energy source for the denitrifying organisms.