AUTOTROPHIC AND HETEROTROPHIC NITRIFICATION IN ACIDIC FOREST AND NATIVE GRASSLAND SOILS

The extent to which either autotrophic or heterotrophic nitrifiers were active in fresh and incubated acidic (pH 3.9-5.5) forested and grassland soils and in neutral cultivated soils, was investigated by estimates of most probable numbers (MPN) of autotrophic nitrifiers: by direct isolation of potential heterotrophic nitrifiers; and by using autotroph-specific inhibitors. Estimates of the MPN of autotrophic nitrifiers present in the cultivated soils were adequate to account for their rates of nitrification, but were much too low to account for the rates of nitrification found in the vegetated soils. Of 171 fungal and 258 bacterial cultures that were isolated, no fungi showed nitrifying activity; five bacteria produced traces of nitrite, of which one produced a trace of nitrate when grown on an ammonium-glucose medium. Nitrification was not detected when a peptone-glucose medium was used. Acetylene proved to be a more effective inhibitor of autotrophic nitrifiers than was sodium-chlorate. Acetylene inhibited all nitrification in a cultivated soil, 96% in a grassland soil and 93% in a forested soil. Although nitrification occurred readily at pH as low as 3.4 it was concluded that acidophilic autotrophs rather than heterotrophs were responsible for nearly all nitrifying activity in these soils.