Susan Asam, ICF International, 1725 Eye St. NW, Suite 1000, Washington, DC 20006, Stephen Del Grosso, USDA ARS NPA SPNR, 2150 Centre Ave, Building D, Suite 100, Fort Collins, CO 80526, and Tom Wirth, U.S. EPA, 1200 Pennsylvania Avenue, N. W, Mail Code: 6207J, Washington, DC 20460.
Agricultural soils were responsible for the majority of U.S. N2O emissions in 2004 (about 261.6 Tg CO2 Eq.). The Inventory of U.S. Greenhouse Gas Emissions and Sinks uses a process-based biogeochemical model (DAYCENT) to develop annual estimates of direct and indirect N emissions due to agricultural soil management as well as more simplistic Intergovernmental Panel on Climate Change (IPCC) Inventory methodologies to estimate components not simulated by DAYCENT. DAYCENT simulates crop growth, soil organic matter decomposition, greenhouse gas fluxes, and key biogeochemical processes affecting N2O emissions. The simulations are driven by model input data generated from daily weather records, land management surveys, and soil physical properties. This presentation will discuss how the process-based model improves on the methodologies used in previous Inventories. Though the modeling approach requires more refined activity data, it is expected to increase the accuracy of the estimates by accounting for land-use and management impacts and their interaction with environmental factors. The presentation will cover collection of activity data, development of modeling scenarios, and apportionment of N emissions to IPCC land-use categories (e.g., Croplands Remaining Croplands, Grasslands Remaining Grasslands). It will also discuss underlying assumptions, model limitations and gaps, and planned improvements.