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.
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