The Effect of Partial Pressure of Oxygen on Some Soil Nitrogen Transformations1

Two New York soils were incubated under conditions permitting control of the partial pressure of oxygen in a stream of gas passing continuously through the soil sample. Ammonium and nitrate salts and organic materials were added in various amounts and combinations and distribution of inorganic nitrogen determined at the end of each experiment. In two experiments the N¹⁵ tracer was used to determine accurately the fate of added nitrate. Immobilization of added inorganic nitrogen occurred whether or not ample nitrogen was available to the soil population, probably due to preferential assimilation. Reduction of nitrate to ammonia was almost negligible at all oxygen concentrations, though slightly higher under anaerobic conditions than otherwise. Denitrification of added nitrate was inversely related to partial pressure of oxygen, but was of appreciable magnitude even under fully aerobic conditions. No evidence of nitrogen loss from the soil was obtained when ammonium salts were added. The experimental findings are compatible with the theory that nitrous oxide in the earth's atmosphere arises from the soil, and that denitrification is of major importance in the nitrogen cycle.