Cole crops can pose a high risk of N losses after harvest because 202-247 kg N ha^-1 remains in crop residues which rapidly mineralize. Previous field and laboratory studies showed lower soil mineral N with organic carbon amendment of used cooking oil (OCA-oil) after broccoli harvest. However, the quantity of N-derived from broccoli crop residue must be separated from indigenous soil N or N fertilizer to accurately assess N dynamics. Thus, a ¹⁵N tracer study was conducted to assess the fate of broccoli crop residue-N (CR-¹⁵N) after harvest and into the subsequent year, with and without OCA-oil. In spring 2011, 342 kg N ha^-1 of 5% atom excess enriched ¹⁵N urea was incorporated in 1.35m² micro-plots prior to broccoli transplanting in a randomized complete block with four replications in two systems. At broccoli harvest, treatments of ¹⁵N crop residue incorporated with or without OCA-oil at 5 Mg ha^-1 were established. Spring wheat was grown in 2012. Plant (broccoli, spring wheat) and soil (0-30, 30-60 cm) samples were collected for N and ¹⁵N analysis. Two weeks after broccoli harvest in autumn, OCA-oil reduced soil mineral CR-¹⁵N by 19 kg ha^-1 and increased microbial biomass CR-¹⁵N by 21 kg ha^-1 compared to the control (P<0.05), indicating immobilization of CR-¹⁵N and a reduced potential for N losses. Although, no differences were observed between the control and OCA-oil for total soil CR-¹⁵N in autumn or spring after broccoli harvest, by spring wheat harvest OCA-oil had greater total and mineral soil CR-¹⁵N compared to the control by 43.5 (P<0.1) and 0.75 (P<0.05) kg ha^-1, respectively. Thus, OCA-oil appeared to reduce soil CR-¹⁵N losses over a year, without impacting spring wheat yield or plant N content. Therefore, growers should apply OCA-oil after cole crop harvest to minimize the risk of N losses.