In-season nitrogen (N) applications in corn ( Zea mays L.) can improve fertilizer N use efficiency. However late-season applications after canopy closure may increase ammonia (NH₃) volatilization, reducing soil N availability and contributing indirectly to greenhouse gas emissions. We quantified ammonia (NH 3 ) volatilization and crop response to fertilizer N source, placement, rate and timing treatments across 15 environments in Illinois, Indiana and Ontario (2021–2023). Treatments consisted of urea and urea-ammonium-nitrate (UAN) injected below the soil and surface-placed with or without a urease inhibitor (UI). Canadian and U.S. sites both applied 146 kg N ha −1 at V13 with U.S. sites also applying at V5. Additionally, U.S. applied 56 kg N ha −1 at both V5 and V13 growth stages. Injection of UAN produced the lowest losses (∼3 kg N ha −1 ) regardless of N source, while UIs reduced volatilization by 65% and 36% for urea and UAN, respectively. Without UIs, surface-applied UAN emitted 35 kg N ha⁻¹ less than urea. N timing only influenced volatilization from urea and was reduced at V5. Cation-exchange capacity (CEC) and post-application rainfall most strongly predicted volatilization. Volatilization declined as CEC and rainfall (events ≥ 30 mm) increased, although high-CEC soils needed more rainfall for equivalent suppression. Positive grain yield and N uptake response to UIs or injection only occurred when corn N demand exceeded the applied N rate, and volatilization loss potential was > 46 kg N ha⁻¹ . Otherwise, agronomic benefits were negligible. These findings can refine environmental models and guide late-season N management under diverse pedo-climatic conditions. • 15-site field study quantified NH 3 loss from late-season (V13) N in maize (2021–23) • Urease inhibitors lowered NH 3 loss by 65% (urea) and 36% (UAN) • NH 3 loss greater for urea at V5 than V13; UAN NH 3 loss unaffected by timing • Rainfall events ≥ 30 mm suppressed NH 3 loss but more rain needed when CEC > 23 • Time to first rain, number of rain events > 30 mm and CEC explained 40% of NH 3 variation
Kang et al. (Mon,) studied this question.