Influence of Used Cooking Oil On the Fate of Broccoli Crop Residue-Derived 15N in the Autumn.

Cole crops can pose a high risk of N losses after harvest due to the substantial quantities of N in crop residue (191-304 kg N ha^-1) which rapidly mineralize. Previous field and laboratory studies have found lower soil mineral N with the application of 5 Mg ha^-1 used cooking oil after broccoli harvest. However, the quantity of residue-derived N must be separated from indigenous soil N or N fertilizer to accurately assess reduction of crop residue-derived N losses. Thus, a ¹⁵N tracer study was conducted to label broccoli plants from early and late crop production to assess the fate of broccoli crop residue-derived N after harvest, with and without the application of used cooking oil. In spring 2011, 5% atom excess enriched ¹⁵N urea fertilizer (at 342 kg N ha^-1) was incorporated in the top 15 cm soil in 1.35m² micro-plots prior to broccoli transplanting. Seasonal management followed typical grower practices. At harvest, plant parts (head, leaves, stem, roots), rhizosphere soil, and soil samples were collected. After harvest, two treatments were established: (1) a crop residue incorporated control and (2) crop residue incorporated with used cooking oil. Soil samples were collected twice throughout autumn.  Minimal fertilizer  N losses occurred during the broccoli growing season but a significant amount of fertilizer-derived N (54%) remained in the field as above-ground crop residue (leaves and stem).   In early harvested broccoli only, used cooking oil resulted in significantly lower (by 45 to 68 %) mineral ¹⁵N- and quantifiably higher (by 80%) microbial biomass ¹⁵N-derived from crop residue, suggesting immobilization during autumn after early- but not late-broccoli harvest.  Environmental conditions later in autumn may have limited immobilization of crop residue-derived N. Thus, the application of used cooking oil may be a beneficial N management practice for cole crop producers to minimize N losses, particularly after early-harvested broccoli.