Summary By using 15 N techniques, effects of urea and KNO 3 on processes and products of inorganic nitrogen transformation
in Hydragric paddy soil and Gleyic paddy soil, collected from Wuxi and Yingtan, respectively, were investigated under flooding conditions at 25℃. The results showed that coupled
nitrification and
denitrification of 15 NH + 4 occurred in both paddy soils. Oxidation of 15 NH + 4 in the Gleyic paddy soil amended with 15 N labeled urea alone was terminated at the product of 15 NO - 2, but denitrification product 15 N 2 was observed, implying a high possibility of aerobic denitrification. The decrease in 15 NO - 3 concentration in both soils, followed the first order reaction. The reaction constant of the Hydragric paddy soil was about 10 fold that of the Gleyic paddy soil. Addition of urea raised soil pH and stimulated DNRA process in the Hydragric paddy soil, and both denitrification and DNRA processes in the Gleyric paddy soils. However, denitrification was a dominant process in 15 NO - 3 transformation. The differences in the effects of urea and KNO 3 on the processes and products of inorganic nitrogen transformation in paddy soils would primarily result from their different effects on soil pH and secondarily from concentration effect.