Denitrification and leaching losses of nitrogen (N) in maize production result from dynamic and complex interactions among weather, soil hydrology, crop water and N uptake, and management practices. Current tools for N management do not directly account for the dynamic behavior of soil N, limiting our ability to more efficiently manage N. Using dynamic simulation models as nutrient management tools represents a major step forward in the management of agricultural nutrient flows.  We have developed the Precision Nitrogen Management or PNM model, composed of a dynamic simulation model of soil N transformations and soil N/water transport (LEACHN; Hutson, 2003) linked to a maize N uptake/growth model (Sinclair and Muchow, 1995). Our goal is to apply the PNM model to improve N use efficiency and reduce N losses in maize production. To achieve this goal, we are developing and testing two N management tools with the PNM model. First, a new Nitrate Leaching Index (NLI) for maize production is being constructed for the Northeast US. The new NLI will be based on multi-year PNM model simulations using archived weather data and input files representing a range of soil types and management practices. Second, we are developing an N management tool for in-season N application guidelines. Current guidelines for in-season N applications don’t account for the well-documented variability in economic optimum maize sidedress N rates. This variability can be quantified with a well-calibrated dynamic simulation model of the soil-maize system. Using the PNM model, we generated adjustments to the recommended in-season N rates for maize in the 2004 and 2005 growing seasons for different climate regions in New York State. We are developing a Web-based version of this tool that will automatically access high resolution climate data (Northeast Regional Climate Center at Cornell University) and allow farm- or field-specific in-season N recommendations