Agroecosystems, Nitrogen-use Efficiency, and Nitrogen Management Kenneth G. Cassman, Achim R. Dobermann, and Daniel T. Walters examine nitrogen-use efficiency (NUE) in cereal production systems, focusing on maize, rice, and wheat, which account for over 60% of human dietary calories. These crops are crucial for future food supply due to their high yield, ease of cultivation, and low resource requirements. NUE is defined as the proportion of nitrogen inputs that are retained in harvested biomass, crop residues, or soil organic matter. Nitrogen not retained contributes to reactive nitrogen (Nr) pollution. On-farm NUE is often lower than in controlled experiments due to differences in scale and management practices. The authors emphasize the need to improve N fertilizer recovery efficiency (RE_N) to reduce Nr emissions and enhance profitability. Current RE_N values are low, with rice at 31% and maize at 37%, while wheat varies widely. The N-difference method is preferred for measuring RE_N due to fewer confounding factors. Aggregate data suggest improvements in N-use efficiency in some regions, but not in rice. Factors affecting N supply and demand include climate, crop physiology, and management practices. C4 plants like maize have higher N-use efficiency than C3 plants like rice and wheat. Indigenous N supply is a key factor, and its variability makes predicting N fertilizer requirements challenging. Improving NUE requires better synchronization between N supply and crop demand, which can be achieved through precision management, soil testing, and tailored fertilizer applications. In the U.S., improvements in maize NUE since 1980 are attributed to better management practices, while in Asia, rice NUE has not improved significantly. Field-specific management in rice systems has shown potential to increase yields and reduce fertilizer use. Research priorities include understanding NUE in major cereal systems, improving management practices, and addressing environmental impacts of nitrogen use. The authors stress the need for long-term research and policy efforts to enhance NUE, reduce environmental damage, and ensure food security.Agroecosystems, Nitrogen-use Efficiency, and Nitrogen Management Kenneth G. Cassman, Achim R. Dobermann, and Daniel T. Walters examine nitrogen-use efficiency (NUE) in cereal production systems, focusing on maize, rice, and wheat, which account for over 60% of human dietary calories. These crops are crucial for future food supply due to their high yield, ease of cultivation, and low resource requirements. NUE is defined as the proportion of nitrogen inputs that are retained in harvested biomass, crop residues, or soil organic matter. Nitrogen not retained contributes to reactive nitrogen (Nr) pollution. On-farm NUE is often lower than in controlled experiments due to differences in scale and management practices. The authors emphasize the need to improve N fertilizer recovery efficiency (RE_N) to reduce Nr emissions and enhance profitability. Current RE_N values are low, with rice at 31% and maize at 37%, while wheat varies widely. The N-difference method is preferred for measuring RE_N due to fewer confounding factors. Aggregate data suggest improvements in N-use efficiency in some regions, but not in rice. Factors affecting N supply and demand include climate, crop physiology, and management practices. C4 plants like maize have higher N-use efficiency than C3 plants like rice and wheat. Indigenous N supply is a key factor, and its variability makes predicting N fertilizer requirements challenging. Improving NUE requires better synchronization between N supply and crop demand, which can be achieved through precision management, soil testing, and tailored fertilizer applications. In the U.S., improvements in maize NUE since 1980 are attributed to better management practices, while in Asia, rice NUE has not improved significantly. Field-specific management in rice systems has shown potential to increase yields and reduce fertilizer use. Research priorities include understanding NUE in major cereal systems, improving management practices, and addressing environmental impacts of nitrogen use. The authors stress the need for long-term research and policy efforts to enhance NUE, reduce environmental damage, and ensure food security.