High Crop Nitrogen-Use Efficiency Does Not Translate into Low Environmental Losses in the Midwestern US.

Increasing nitrogen (N)-use efficiency (NUE) is necessary for improving crop production and environmental quality. Cropping system NUE is often evaluated based on the long-term balance between the inputs of N into soil and its removal in harvested yield. A long-term N surplus (i.e. N inputs > N yield) is widely accepted as indicative of environmental N losses. Here, we evaluate this assumption in the context of the predominant rainfed maize and soybean cropping systems of the Midwestern US. We used a process-based model (APSIM), trained and tested with experimental datasets from Iowa, US, to calculate N balances across cropping systems that differed in fundamental biophysical controls on N dynamics. Modeling results indicated that despite the high NUE (87%) and a low N surplus (21 kg N ha^-1 yr^-1) in these systems, the actual environmental N losses (47 kg ha^-1 yr^-1) were substantially greater than what would be predicted by these metrics. Because the soil N pool in these systems is not in long-term equilibrium (-26 kg N ha^-1 yr^-1), the N surplus did not adequately characterize environmental N losses. Additionally, efficiency improvements resulting from the inclusion of a rye cover crop, which mitigated environmental N losses by 22%, were not captured by the N surplus method. This suggest that mitigating the high environmental losses in these systems requires more than reducing the N surplus alone, but also enhancing the N recycling with practices such as cover crops. Results also highlight the need for developing NUE metrics that can concurrently evaluate the efficiency attributable to improvements in both fertilizer management and N cycling. This would aid in the identification of win-win management scenarios that increase production while mitigating environmental N losses.