The extent and rate of NO3-N production from organic fractions of different types of urban wastewater biosolids were studied in a soil incubation experiment. At least five N fractions with different NO3- accumulation properties were identified in digested biosolids. A proportion of the organic N, and the NH4-N pool, was rapidly converted to NO3-. Other organic fi actions were slowly mineralized, reflecting the degree of stabilization. Approximately 90% of the available N (60% of total N) was converted to NO3- after 4000 d degrees C; 30% of the total N resisted mineralization. Immobilization of soil mineral N was the dominant process for dewatered undigested biosolids. A net release of mineral N equivalent to 15% of the total N content occurred after 4000 d degrees C with this type of biosolids. Composted biosolids were barely mineralized and less than 10% of the applied N was converted to NO3- after 160 d of incubation in soil at 25 degrees C, implying a minimal risk of NO3- leaching. Nitrate accumulation from fawn wastes followed similar patterns to the wastewater biosolids, although NO3- production was potentially more variable compared with stabilized biosolids. The centralized collection and treatment of urban wastewater should contribute to a more consistent agricultural fertilizer A N management model based on thermal-time is proposed for recycling biosolids to improve N utilization and minimize NO3- leaching.