Stacking nitrogen management practices: Combining double-slot fertilizer injection with urease and nitrification inhibitors improves yields and reduces ammonia and nitrous oxide emissions

Ammonia loss following nitrogen fertilization can degrade air quality and impact human health, whereas nitrous oxide (N₂O) can contribute to global warming and climate change. Mitigation practices that target only one N-loss pathway can lead to pollution swamping; hence, practices targeting both N-losses are required. A 3-year field study examined fertilizer N-placement (broadcast urea, single-slot injection of urea ammonium nitrate [UAN], double-slot UAN injection) and N-metabolization inhibitors (with/without urease and nitrification inhibitors) impacts on NH₃ and N₂O losses and corn yields. Ammonia volatilization was reduced (p < 0.05) by 26% with single-slot UAN injection (10.6 kg N ha⁻¹) and by 63% with double-slot UAN injection (5.32 kg N ha⁻¹) compared to broadcast urea (14.3 kg N ha⁻¹). Dual urease and nitrification inhibitors reduced NH₃ volatilization (0.84–3.86 kg N ha⁻¹) by 57%–92% compared to no inhibitors (5.32–14.3 kg N ha⁻¹). When no inhibitors were applied, N₂O emissions from slot injection (6.43–7.62 kg N ha⁻¹) were 2.6–3.1 times greater than from broadcast urea (2.43 kg N ha⁻¹). Dual inhibitors reduced N₂O emissions by 43% from 6.43 to 3.66 kg N ha⁻¹ with double-slot injection. Double-slot UAN injection increased corn grain yields (9.73 t ha⁻¹) by 12%–13% compared to single-slot UAN injection (8.71 t ha⁻¹) and broadcast urea (8.6 t ha⁻¹). Double-slot UAN injection effectively decreased NH₃ losses and increased corn grain yields, but dual N inhibitors were required to also reduce N₂O. Hence, combined productivity and environmental benefits were accrued only when fertilizer containing urease and nitrification inhibitors was combined with double-slot injection.