The production and supply of nitrogen (N) fertilizer is the largest single energy input to Ontario corn production systems and results in substantial environmental losses. The 4R Nutrient Stewardship program has potential for reduction of N losses to water and air, but this impact needs to be quantified under field conditions. We are conducting a 3-year experiment using micrometeorological methods applied to farm-scale experimental plots (4 x 4 ha) and year-round measurements, including the assessment of non-growing season N losses to address this research gap. Funding is provided by the Canadian Fertilizer Institute and Agriculture and Agri-Food Canada AgriInnovations Program (AAFC/AIP Program) as part of the project ‘A Canadian Research Network to Improve 4R Nutrient Stewardship for Environmental Health and Crop Production Profitability’. In a complementary project funded by OMAFRA/Univ. of Guelph Partnership we are using a modelling approach to evaluate the effectiveness of 4R mitigation practices to reduce N2O emissions and the environmental footprint of corn production systems province-wide.