Effect of nitrogen source, placement and timing on the environmental performance of economically optimum nitrogen rates in maize

The goal of most fertilizer decision support tools is to help maize farmers estimate the most profitable nitrogen (N) rate for a given fertilizer N/grain price ratio, known as the economically optimum N rate (EONR). While maximizing profitability, to our knowledge the environmental performance of the EONR has not been fully assessed using a process-based model that can jointly predict yield, N uptake, and multiple N loss pathways (leaching, nitrous oxide emissions, volatilization). The objective of this study was to construct a full N budget when the EONR is applied, and to measure how N management influences both grain yield and the environmental performance of the EONR. The DeNitrification and Decomposition model (DNDC) was calibrated and validated using measurements from a long-term N rate trial from Elora, ON, Canada (2009-2016). DNDC was then used to simulate N applications at the EONR when two different N sources (urea and urea ammonium-nitrate), two N placements (broadcast or incorporated) and four N timings (100 % at planting, 100 % at V6, 50 % at planting and 50 % at V13, and 50 % at V6 and 50 % at V13) were used. Depending on N management, mean EONR (2009-2016) was highly variable, ranging from 158 to 185 kg N ha(-1) while grain yield was stable across N management choices. The use of urea over urea ammonium-nitrate (UAN), and the decision to broadcast versus incorporate, increased the EONR, yield-scaled N/ losses at the EONR, and N surplus. When applied inseason, N applications modestly reduced leaching N losses at the EONR but did not significantly impact the amount of N applied at the EONR or yield-scaled N losses. In all 16 management combinations, simulated mean (2008-2016) N surplus never exceeded 50 kg N ha(-1), and yield-scaled N losses never exceeded 8 kg N Mg-1 grain. In conclusion, the EONR delivers strong environmental performance relative to established benchmarks for N surplus and yield-scaled N losses, while N management decisions such as N source and N placement will still affect in-season N losses and consequently the amount of N applied at the EONR.