To determine N turnover and losses during aerobic composting of animal manure, a 41-d laboratory study was performed on pig manure composting with three additive treatments (Straw: pig manure + straw only; Lime: pig manure + straw + quick lime; and SSP: pig manure + straw + single superphosphate). The NH₄-N pool in the pig manure was initially labeled with ¹⁵N to determine the fate of manure NH₄-N during composting. The composts were subsequently applied to soil to investigate the effects on soil mineral N and to trace the ¹⁵N during 60 d of incubation at 25°C. Of the initial manure ¹⁵NH₄-N, approximately 30, 90, and 20% was lost by NH3 volatilization during composting in the Straw, Lime, and SSP treatments, respectively. Concurrently, 62, 16, and 41% of initial ¹⁵NH₄-N was immobilized in the respective treatments. When the composts were applied to soil, the mineral N in soil with SSP compost was higher throughout the incubation than in soil with Straw and Lime composts. This was because of higher mineral N content in the SSP compost on application and higher net N mineralization from that compost in the soil. In soil with Straw compost, N mineralization and immobilization were slow or effectively in balance. In soil with Lime compost, net N immobilization was strong in the fi rst 10 d, but then net N mineralization dominated the remaining period of soil incubation. Overall, adding lime before composting reduced the NH₄-N content in the compost and the amount available in soil, while adding superphosphate increased the NH₄-N content in both. Therefore, superphosphate addition increased the potential fertilizer value of composted pig manure.