An important N source for rice (Oryza sativa L.) is mineralization of the organic forms of N present in the soil. The correct N determination will help to optimize the amount of fertilizer used. Our objective was to calculate an index that relates crop N uptake from previous N fertilization and N mineralized in rice paddy soils. To do so, we used two rice paddy soils, Alfisol and Vertisol, in Chile under different anaerobic incubation times and temperatures. We determined both soil mineralization and residual effect of fertilization in the previous season under field and laboratory conditions in an Alfisol and a Vertisol managed with a rice monocrop. Anaerobic incubations were carried out at 20 and 40 °C; and at five different times (0, 7, 14, 21, and 28 d). Our results indicate that residual fertilization affects N mineralization, which increases in both paddy soils when compared with the control without N. In the Alfisol it fluctuates between 6.1 and 13.3 mg N-NH4+ kg-1, whereas in the Vertisol ranged between 18.4 and 28.0 mg N-NH4+ kg-1. Crop N uptake as a residual effect of applied N ranged between 29.3 and 33.9 kg N ha-1 in the Alfisol and 28.3 and 45.8 kg N ha-1 in the Vertisol. Thus, N mineralization is mainly affected by incubation time, temperature, and N fertilization rate of the previous crop season; it can be represented by quadratic models with high determination values, R² = 0.73** and R² = 0.78**, for Alfisol and Vertisol, respectively. Moreover, we found that the best incubation time and temperature were 7 d at 20 °C and 21 d at 40 °C, for the Altisol and Vertisol, respectively. In addition, there are potential savings in the next rice crop season from 0.86 to 0.43 kg N applied for each 1 mg N kg-1 mineralized in the Alfisol and Vertisol, respectively. We conclude that crop N uptake was related to mineralized N and that the equation here developed that relates them is a good predictor to optimize fertilizer use in rice paddy fields.