Evaluation of intensive "4R" strategies for decreasing nitrous oxide emissions and nitrogen surplus in rainfed corn

The "4R" approach of using the right rate, right source, right timing, and right placement is an accepted framework for increasing crop N use efficiency. However, modifying only one 4R component does not consistently reduce nitrous oxide (N₂O) emissions. Our objective was to determine if N fertilizer applied in three split applications (Sp), by itself or combined with changes in N source and rate, could improve N recovery efficiency (NRE) and N surplus (NS) and decrease N₂O emissions. Over two corn (Zea mays L.) growing seasons in Minnesota, N₂O emissions ranged from 0.6 to 0.9 kg N ha-1. None of the treatment combinations affected grain yield. Compared with urea applied in a single application at the recommended N rate, Sp by itself did not improve NRE or NS and did not decrease N₂O. Combining Sp with urease and nitrification inhibitors and/or a 15% reduction in N rate increased NRE from 57 to > 73% and decreased NS by > 20 kg N ha^-1. The only treatment that decreased N₂O (by 20-53%) was Sp combined with inhibitors and reduced N rate. Emissions of N₂O were more strongly correlated with NS calculated from grain N uptake (R² = 0.61) compared with whole-plant N uptake (r2 = 0.39), possibly because most N losses occurred before grain filling. Optimizing both application timing and N source can allow for a moderate reduction in N rate that does not affect grain yield but decreases N₂O. Grain-based NS may be a more useful indicator of N₂O emissions than whole-plant-based NS.