NITRIFICATION IN FOREST SOIL AFTER REFERTILIZATION WITH UREA OR UREA AND DICYANDIAMIDE

The effects of urea refertilization on nitrification activity and the influence of a nitrification inhibitor, dicyandiamide, added together with urea were studied in an acid forest soil. Net nitrification and mineralization of N were determined by aerobic incubation of soil samples and numbers of autotrophic nitrifers by the most probable number (MPN) method. Refertilization with urea enhanced nitrification activity more than single urea application. The higher number of autotrophic nitrifiers in the previously fertilized soil compared to the soil receiving urea for the first time indicated that a nitrifier population was built up after the first urea addition. Dicyandiamide application seemed to help sustain raised soil nitrogen concentrations after urea fertilization since in the previously fertilized soils the ammonium concentrations were higher after urea and dicyandiamide treatment than after refertilization with urea alone.