Solubilization of soil organic N by alkaline-hydrolysing N fertilizers

Interactions between¹⁵N-labelled fertilizers applied at concentrations representative of the fertilizer microsite and the solubility of the nitrogenous component of soil organic matter were investigated in laboratory experiments. Soil organic N was solubilized in a-irradiated soil due to addition of NH_3(aq), and the fertilizer-induced loss of unlabelled total N in the extracted soil (TU_s) increased with increasing N fertilizer concentration and soil pH. TU_s was linearly correlated with ammoniacal-N concentration and the pH of the fertilized soil within the range of 7.5-10 (r = 0.94).Total organic N in the soil extract (OT_e) increased rapidly up to day 14 following addition 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 in nine soils, the change in total organic N in the extracts (OT_e) of three soils was not significant. The highest OT_e of 399 mg N kg^–1 soil (35.4% of soil organic N) was measured after application of 2000 mg NH₃-N kg^–1 soil.pH and OT_e decreased in the order of NH_3(aq) > urea > di-ammonium phosphate > ammonium sulphate at equivalent rates of N addition. A negative OT_e was measured following application of ammonium sulphate. OT_e was correlated with the pH of the fertilized soil but not ammoniacal-N concentration for different N fertilizer sources.