A better understanding of N availability in co-composted drilling wastes is required to evaluate the potential use of the composts as growth media. We investigated N dynamics in co-composted drilling wastes by examining the changes in the concentrations and partition of applied 15N in various soil N pools (, , dissolved organic N, microbial biomass N, and non-extractable N) in a 4-month greenhouse incubation experiment using 1-, 2-, 3-, and 4-year-old (referred to below as 1Y, 2Y, 3Y, and 4Y, respectively) composts, representing substrates with different quality. Regardless of compost age, after 4 months of incubation extractable N concentrations decreased (P<0.05), in contrast with the increasing pattern of the non-extractable N, indicating stabilization of the extractable N into the recalcitrant soil organic fraction. Fertilizer N application increased (P<0.05) extractable N concentrations. In the younger composts, a major part of the applied 15N was recovered in the non-extractable N fraction (44.0% for 1Y and 38.5% for 2Y) with little recovered as mineral N. On the other hand, a considerable percentage of the applied 15N (21.8% for 3Y and 18.8% for 4Y) was found in the pool in the older composts with relatively high mineral N but low organic C contents. This study shows that the dynamics of biologically available N and fate of applied N in the composts depend on compost quality such as mineral N and organic C contents, and compost C:N ratio. To use the co-composted drilling waste as growth media, different N management strategies need to be established for those composts with differed substrate quality.