CHEMICAL INTERACTIONS BETWEEN SOIL-N AND ALKALINE-HYDROLYZING N-FERTILIZERS

Chemical interactions between soil N and alkaline-hydrolysing N fertil izers labelled with N-15 were studied in the laboratory using twelve g amma-irradiated soils. Fertilizer was recovered in the soil organic N fraction via the process of NH3 fixation. NH3 fixation at day 7 varied from 1.8 to 4.6% of the N added as aqua ammonia at 1000 mg kg-1 soil. The amount of NH3 fixed increased with increasing rates of applicatio n of NH3(aq) and urea. The rate of NH3 fixation decreased with time, w ith more than 55% of the total NH3 fixation in 28 days occurring in th e first week following application of 2000 mg urea-N kg-1 soil. Soil p H and NH3 fixation varied in response to N source, and increased in th e order of di-ammonium phosphate <urea < aqua ammonia at equivalent N concentrations. The alkaline hydrolysis of indigenous organic N occurr ed simultaneously with NH3 fixation, resulting in the release of unlab elled ammonium (deamination) and a real added nitrogen interaction in all but two of the soils studied. The release of NH4+ initially increa sed up to a pH of 7.5, was inhibited between pH 8.5 and 9.0, but incre ased thereafter. The balance (N(bal)) between NH3 fixation and deamina tion was either positive or negative, depending on the pH of the ferti lized soil, which was directly related to N source and concentration f or a given soil.