Human activities such as agricultural fertilization and fossil fuel combustion have introduced a massive amount of anthropogenic nitrogen (N) in reactive forms to the environment.  As agricultural fertilization is the single largest anthropogenic N source, an integrated approach to understand the interactions among agriculture, atmosphere, and hydrology is essential in examining human-altered N cycling.  Here we present an integrated modeling system (IMS) with agriculture (EPIC - Environmental Policy Integrated Climate), atmosphere (WRF/CMAQ - Weather Research and Forecast model and Community Multiscale Air Quality), and hydrology (SWAT - Soil and Water Assessment Tool) models to assess the interactions among land-air-water processes.  The centerpiece of the IMS is the Fertilizer Emission Scenario Tool for CMAQ (FEST-C) which includes a Java-based interface and EPIC adapted to regional applications along with built-in database and tools.  The Linux-based interface guides users through EPIC simulations for any CMAQ grid domain over the conterminous United States (CONUS) and integration among the multimedia models.  

This presentation focuses on the description of the currently released FEST-C and the impact assessment of agricultural fertilization on air quality through an improved CMAQ bi-directional ammonia approach. As N deposition is also an important source altering N cycling, the influence of nitrogen deposition along with weather variability on cultivated soil N budget will also be examined for CONUS.  The system is applied over CONUS with a 12km resolution for 2010, 2011, and 2012.  EPIC simulations are conducted using WRF/CMAQ weather and N deposition for these years and cases adjusted to represent conditions in the early 1990s for assessing the impacts of N deposition reduction since 1990 due to tightened NO_X emission standards under the Clean Air Act (CAA).  SWAT integrated with EPIC and WRF/CMAQ are then applied to the Mississippi River Basin (MRB) to simulate watershed hydrology and water quality for these years under different N deposition conditions.  Preliminary results demonstrate that air quality linked with simulated agriculture improves NH₃ flux estimation and results in better performance for N cycling in the atmosphere.  The N budget in agricultural production is sensitive to weather variability and atmospheric N deposition with increased N fertilization and decreased N loss in areas with N deposition reduction.  In addition, the impact of N deposition reduction on N losses from MRB to the Gulf of Mexico will be analyzed and presented to show the effect of CAA policies on water quality for the biggest drainage basin in North America.