SOLUBILIZATION OF SOIL ORGANIC-N BY ALKALINE-HYDROLYZING N-FERTILIZERS

Interactions between N-15-labelled fertilizers applied at concentratio ns representative of the fertilizer microsite and the solubility of th e nitrogenous component of soil organic matter were investigated in la boratory experiments. Soil organic N was solubilized in a gamma-irradi ated soil due to addition of NH3(aq), and the fertilizer-induced loss of unlabelled total N in the extracted soil (DELTATU(s)) increased wit h increasing N fertilizer concentration and soil pH. DELTATU(s) was li nearly correlated with ammoniacal-N concentration and the pH of the fe rtilized soil within the range of 7.5-10 (r = 0.94). Total organic N i n the soil extract (OT(e)) increased rapidly up to day 14 following ad dition of 2000 mg urea-N kg-1 soil, but was then stable up to day 28. OT(e) of a range of soils increased from between 5 and 148 to between 15 and 368 mg N kg-1 soil after application of 1045 mg NH3-N kg-1 soil . While up to 25% of the organic N was solubilized by the fertilizer i n nine soils, the change in total organic N in the extracts (DELTAOT(e )) of three soils was not significant. The highest DELTAOT(e) of 399 m g N kg-1 soil (35.4% of soil organic N) was measured after application of 2000 mg NH3-N kg-1 soil. pH and DELTAOT(e) decreased in the order of NH3(aq) > urea > di-ammonium phosphate > ammonium sulphate at equiv alent rates of N addition. A negative DELTAOT(e) was measured followin g application of ammonium sulphate. DELTAOT(e) was correlated with the pH of the fertilized soil but not ammoniacal-N concentration for diff erent N fertilizer sources.