Opportunities and Challenges of Maximizing Nitrogen Use Efficiency.

World-wide use of synthetic fertilizer-N has increased to 105 million tons annually.  Because of environmental and health concerns caused by excess available N in the soil and water plus over fertilization leads to a lower economic return of the N applied,  numerous strategies have been used to increase N use efficiencies. Strategies to improve fertilizer-N use efficiency include site specific N management, the determination of the crop N status during the growing season and apply N fertilizer synchronized with crop N demand, use of nitrification inhibitors, controlled release fertilizer, placement of N fertilizers, and plant breeding. Although all these strategies have the potential to increase synthetic fertilizer-N use efficiencies, on a  world-wide basis the average recovery of synthetic fertilizer-N by crops remains between 50 and 60 %, independent similar whether based on the N balance method or on long-term 15N fertilizer recoveries studies. There are agro-ecosystems which have reported high N fertilizer use efficiencies. Analogous on yield gap, here we would like to propose the concept of the NUE gap: the maximum NUE that can be obtained under field conditions; the highest NUE reported; and the actual or average NUE reported.