Nonlinear nitrous oxide (N2O) response to nitrogen fertilizer in onfarm corn crops of the US Midwest

Row-crop agriculture is a major source of nitrous oxide (N2O) globally, and results from recent field experiments suggest that significant decreases in N2O emissions may be possible by decreasing nitrogen (N) fertilizer inputs without affecting economic return from grain yield. We tested this hypothesis on five commercially farmed fields in Michigan, USA planted with corn in 2007 and 2008. Six rates of N fertilizer (0–225 kg N ha ) were broadcast and incorporated before planting, as per local practice. Across all sites and years, increases in N2O flux were best described by a nonlinear, exponentially increasing response to increasing N rate. N2O emission factors per unit of N applied ranged from 0.6% to 1.5% and increased with increasing N application across all sites and years, especially at N rates above those required for maximum crop yield. At the two N fertilizer rates above those recommended for maximum economic return (135 kg N ha ), average N2O fluxes were 43% (18 g N2O–N ha 1 day ) and 115% (26 g N2O– N ha 1 day ) higher than were fluxes at the recommended rate, respectively. The maximum return to nitrogen rate of 154 kg N ha 1 yielded an average 8.3 Mg grain ha . Our study shows the potential to lower agricultural N2O fluxes within a range of N fertilization that does not affect economic return from grain yield.