Nitrate-N production in soil treated with different types of urban waste water biosolids was modelled with simple exponential relationships and accumulated thermal-time units (d degrees C) in a laboratory incubation study. The rate of NO3-accumulation was also governed by the amount of NH4-N and the stability of the organic N contained in the biosolids, both of which were influenced by the type of biosolids treatment. A smaller proportion of the organic N contained in anaerobically digested biosolids was mineralized compared with untreated material, although the organic N in stabilized biosolids was converted to NO3- more rapidly. Dewatering significantly reduced the extent of NO3- formation in biosolids-treated soil due to the removal of soluble NH4-N ions. Dewatered undigested biosolids initially immobilized native soil mineral N before a net release of NO3-N occurred Nitrate-N was produced from several pools of organic N of varying stability in biosolids-amended soil and models of these pools are compared. More information is required on the behaviour of the different forms of organic N in urban wastewater biosolids to assess the potential long-term production of NO3- in biosolids-treated agricultural land.