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

Chemical interactions between soil N and alkaline-hydrolysing N fertilizers labelled with N-15 were studied in the laboratory using twelve gamma-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 application of NH3(aq) and urea. The rate of NH3 fixation decreased with time, with more than 55% of the total NH3 fixation in 28 days occurring in the first week following application of 2000 mg urea-N kg-1 soil. Soil pH and NH3 fixation varied in response to N source, and increased in the order of di-ammonium phosphate <urea < aqua ammonia at equivalent N concentrations. The alkaline hydrolysis of indigenous organic N occurred simultaneously with NH3 fixation, resulting in the release of unlabelled ammonium (deamination) and a real added nitrogen interaction in all but two of the soils studied. The release of NH4+ initially increased up to a pH of 7.5, was inhibited between pH 8.5 and 9.0, but increased thereafter. The balance (N(bal)) between NH3 fixation and deamination was either positive or negative, depending on the pH of the fertilized soil, which was directly related to N source and concentration for a given soil.