Efficiency of fall versus spring applied urea‐based fertilizers treated with urease and nitrification inhibitors II. Crop yield and nitrogen use efficiency

Abstract

AbstractThe urease inhibitor [N‐(n‐butyl) thiophosphoric triamide (NBPT)] and the nitrification inhibitor (NI) 3,4‐dimethyl pyrazole phosphate have been reported to conserve urea‐based N fertilizers by reducing N losses. However, their effects on crop yield and N uptake are inconsistent and fall‐applied N fertilizers are usually less efficient than spring applications. We conducted a 2‐yr field study on contrasting soils [Carman sandy loam (CSL) and Portage clay loam (PCL)] on the effects of NBPT with and without NI on grain yield, grain N removal, and crop N uptake from fall and spring surface‐applied urea‐based fertilizers. Fertilizer treatments (75 or 100 kg N ha−1) were urea and urea ammonium nitrate (UAN) with and without NBPT or NBPT + NI. Canola (Brassica napus L.) and wheat (Triticum aestivum L.) yield, N removal, and N uptake were not consistently greater for urea and UAN treated with inhibitors than for untreated urea and UAN. The inhibitors’ effect on yield and N uptake was observed in urea treated with NBPT in CSL but not PCL. Although agronomic efficiency was significantly greater for spring‐applied untreated urea or UAN than fall‐applied urea or UAN with and without inhibitors in PCL, no significant difference appeared between fall‐applied urea or UAN treated with inhibitors and spring‐applied untreated urea or UAN in CSL. The N conserved by the inhibitors did not appear in the soil as nitrate‐N. Although inhibitors may reduce N losses, their use to increase yield and bridge the efficiency gap between fall‐ and spring‐applied urea‐based fertilizers may be site‐specific.