Interchange between inorganic and organic nitrogen in soils

Immobilization of fertilizer nitrogen and mineralization of soil organic nitrogen were measured in three soils receiving ammonium sulfate or calcium nitrate labelled with the N15 tracer. In the presence of added straw deficient in nitrogen the rate of nitrogen immobilization was strongly temperature dependent, whereas the total quantity of nitrogen eventually incorporated in the organic fraction was little affected by temperature. Nitrifying bacteria were able to compete effectively for ammonium with the heterotrophic population in all three soils. Little remineralization of tagged nitrogen occurred, suggesting progressive stabilization of immobilized nitrogen. Ammonium-nitrogen was immobilized in greater quantity than nitrate-nitrogen in all soils. An appreciable fraction of the ammonium-nitrogen was fixed within the crystal lattice of clay minerals in two of the soils. The data indicate that this clay-fixed ammonium was completely unavailable to nitrifying bacteria or other microorganisms. Absolute mineralization and immobilization rates were calculated for periods of maximum activity at the beginning of the experiments. The highest mineralization rate observed was 12 ppm nitrogen per day, compared with a concurrent immobilization rate of 16 ppm per day. The data indicate that turnover rates of soil organic nitrogen are relatively low, and suggest that when nitrogen fertilizers are applied, part of the nitrogen remains unavailable to plants for many years.