Nitrous oxide (N2O) flux responds exponentially to nitrogen fertilizer in irrigated wheat in the Yaqui Valley, Mexico

The Yaqui Valley, one of Mexico's major breadbaskets, includes 230,000 ha of cultivated, irrigated cropland, with two thirds of the area planted annually to spring wheat (Tritictmt turgidum). Nitrogen (N) fertilizer applications to wheat have doubled since the 1980s, and currently average around 300 kg N ha(-1). Emissions of nitrous oxide (N2O), a potent greenhouse gas, increase following soil management activities, especially irrigation when N fertilizer is applied, and particularly when N fertilizer inputs exceed crop N requirements. Here we investigate trade-offs among N fertilizer inputs, spring wheat yields, and N2O emissions to inform management strategies that can mitigate N2O emissions without compromising yields, and link this to how farmers can generate carbon credits from N management to receive payment for more precise N use. We used static chambers to measure N2O fluxes from spring wheat at five N fertilizer rates (0, 80, 160, 240, and 280 kg N ha(-1)) during two growing seasons at CIMMYT in Ciudad Obregon, Sonora, Mexico. Average daily fluxes were between 1.9 +/- 0.5 and 13.4 +/- 2.8 g N2O-N ha(-1), with lower emissions at N rates below those that maximized yield, and substantially higher emissions at N rates beyond maximum yield; this exponential response is consistent with crops in temperate regions. Results suggest that current average N fertilizer rates (300 kg N ha(-1)) are at least double economically optimum rates, resulting in low crop N use efficiency: 36-39% at higher N rates as compared to 50-57% for economically optimum rates. N fertilizer rate reductions to the economic optimum rates here (123 and 145 kg N ha(-1) in 2013 and 2014, respectively) could have avoided N2O emissions equivalent to 0.5 to 0.8 Mg CO(2)e ha(-1) yr(-1) or, regionally, 84-138 Gg CO(2)e yr(-1) without harming yields. Insofar as fertilizer use in Yaqui Valley is likely similar to high-productivity irrigated cereal systems elsewhere, our results provide evidence for a global triple-win scenario: large reductions in agricultural GHG emissions, increased farmer income, and continued high productivity.