Rice is an important food in the world population's diet. As nitrogen (N) is the principal nutrient associated with the yield and its mineralization from the soil contribute to plant-available N, the use of biological or chemical methods are necessary for its estimation. Two paddy rice soils types of Chile were used for laboratory (anaerobic incubation) and field experiments: Quella (Vertisol) and Quilmen (Inceptisol), fertilized with three N rates - ureas of 0, 80 and 160 kg ha(-1). The mineralized inorganic N was measured and potentially mineralizable N (N-0) was calculated based on the first-order kinetic model. N mineralization was positively affected by N fertilization, but only affected grain yield in the Quilmen soil with a negative effect on the crop N recovery. Furthermore, crop N uptake was related with the incubation time at 14 and 21 days for the Quilmen and Quella soils respectively. Finally an N optimization model for those soils was proposed.