Nitrogen Surplus and Nitrogen Losses in US Corn Production: Model Simulations and Management Implications.

There is a pressing need to find ways to increase the sustainability of corn production environments in the US. N Surplus is an index reflecting agricultural N inputs and the N removed by the crop at harvest. Previous field studies have shown that environmental N losses increase rapidly once a threshold of N surplus was reached. However, the factors that control N surplus are still inadequately defined. We address this research gap using multiple model simulations performed by the Adapt-N tool, accounting for N fertilizer inputs, crop N uptake and environmental N losses at 19 different locations in the US Midwest. The simulations spanned six years of climatic data (2010-2015) and compared three N management approaches: large preplant applications applied either at the fall or the spring, and a split application approach, where the majority of N is applied in-season. In all simulations N deficiency was minimized by an in-season addition of N, if recommended by the Adapt-N tool. Our results show that N surplus varies considerably both in space and in time, and is significantly affected by the timing of N application. In general, fall applications were found to generate the highest N surplus and simulated N losses. Shifting the timing of N application from fall to spring or to in-season application substantially reduced the simulated N surplus and the associated environmental N losses, a trend accentuated in locations with wetter climate. These results could be used to inform environmental policies and business models to reduce environmental costs associated with corn production in the U.S.