After cole crop harvest, over 400 kg N ha(-1) may remain in the field as crop residues and soil mineral N. Thus, methods to reduce potential post-harvest N losses are needed. Urea with 5 % N-15 excess was incorporated in mini-plots to produce N-15 enriched broccoli (Brassica olecerea var italica L.). The fate of above-ground crop residue-derived N (N-15(residue-above)) and below-ground residual fertilizer or root biomass N (N-15(residue-below)) were studied from broccoli harvest (Aug and Sept 2011) to spring wheat (Triticum durum L.) harvest (July 2012), with and without an amendment of used cooking oil. The N-15(residue-below) remained mostly as organic N, was not influenced by the amendment, and was resistant to post-harvest losses. With the oil amendment, soil mineral N-15(residue-above) was reduced by 19 kg ha(-1) and microbial biomass N-15(residue-above) was increased by 21 kg ha(-1) 2 weeks after broccoli harvest, indicating immobilization of N-15(residue-above) and reduced potential N losses. At spring wheat harvest, amended soil had greater total, organic, and mineral N-15(residue-above) compared to the unamended control, by 44, 43, and 0.75 kg ha(-1), respectively. The amendment increased the recovery of N-15(residue-above) in the soil total N pool by 209 % at spring wheat harvest, and it did not affect spring wheat yields or plant N-15(residue-above) content. It is possible that the amendment facilitated the incorporation of N-15(residue-above) into organic compounds, which were less susceptible to losses. Growers should consider applying used cooking oil at harvest to minimize potential N losses and to increase the soil organic N fraction.
