Agricultural soils treated with residual wastewater and lime-stabilized biosolids require the knowledge of N mineralization to improve biosolids management and to avoid contamination impacts. Wastewater treatment plants are generating 95,000 ton biosolids per year at Ciudad Juarez, Chih., Mexico which are sent to the municipality landfill. This situation is representing a serious risk to environmental health. The objective of this study was to determine the rates of N mineralization for three soil textures under field conditions. The experiments were conducted at the Juarez Valley using a randomized complete block design with four replications. Biosolids loading rates were 0, 200, 400 and 600 kg Total Kjeldahl N (TKN)/ ha. The Ion Exchange Resin (IER)-Soil Core Method was used to evaluate N mineralization under field conditions. The ion exchange resins consisted of a Na-saturated cation exchange resin (Ionac C-249), and a Cl-saturated anion exchange resin (Ionac ASB-1). Aluminum tubes 15 cm long by 4.8 cm in diameter were installed in the field. A tube was placed at each experimental plot, resin bags were taped to the inside bottom of the tube and it was filled with the soil plus biosolids rate. Resins bags were incubated at various time intervals: 26, 47, 81, 110, and 183 days. Regression models were fitted using the Marquardt iterative method of the SAS nonlinear (NLIN) curve fitting procedure. The total amount of N mineralized were significantly different among treatments for the clay soil (p<0.01) and loam soil (p<0.10), but there was not significant effect for the sandy soil. Rates of N mineralization varied from 0.145 to 0.47 mg N-NH4+NO3/kg soil/day. TKN was higher in biosolids amended soils than unamended soils from the three sites, also N mineralization was estimated by differences in soil TKN before and after biosolids application. A comparison of both methods (resins and TKN differences) seems to be a good strategy to generate rates of N mineralization when the soils are irrigated with wastewater.