Methods to quantify the nitrogen derived from the fertilizer in maize applying blends of controlled-release and NBPT-treated urea

The use of N-15 tracers in blends of N-fertilizers contributes to determining the N derived from each N-fertilizer in the plant, improving the recommendation in the field. However, to evaluate the contribution of two N-15-labeled fertilizers to maize nutrition is an expensive process, and it is challenging to manufacture N-15-labeled controlled-release urea (CRU) considering a polymer-sulfur coated urea (PSCU). The objective of this study was to evaluate, in a field experiment, two methods to quantify the N derived from the PSCU (NdfPSCU) in maize plant using two blends (70% PSCU + 30% urea (U) and 30% PSCU + 70% U) applied in three ways (incorporated, broadcast and split application). The first method quantifies the NdfPSCU by subtracting the total N uptake in the N treatment by the sum of the N uptake in the control treatment (without N-fertilizer application) and the N derived from the U (NdfU) in the plant quantified with N-15. The second method quantifies the NdfPSCU using N-15 (precise and direct method). Despite the first method is less expensive than the second it underestimated the NdfPSCU in four maize growth stages in Rhodic Eutrustox soil, being necessary the use of N-15 in the PSCU to quantify the NdfPSCU with blends of PSCU + U.