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DAYCENT National-Scale Simulations of Nitrous Oxide Emissions from Cropped Soils in the United States

^a USDA-ARS-NPA-SPNR, Natural Resources Research Center, 2150 Centre Avenue, Building D, Suite 10, Fort Collins, CO 80526-8119
^b Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80523
^c Agricultural and Biological Engineering, University of Florida, PO Box 110570, Gainesville, FL 32611-0570
^d ICF Consulting, 1725 I Street NW, Suite 1000, Washington, DC 20006
^e Terrestrial Sciences Section, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80305

^* Corresponding author (delgro{at}nrel.colostate.edu)

Received for publication April 29, 2005. Until recently, Intergovernmental Panel on Climate Change (IPCC) emission factor methodology, based on simple empirical relationships, has been used to estimate carbon (C) and nitrogen (N) fluxes for regional and national inventories. However, the 2005 USEPA greenhouse gas inventory includes estimates of N₂O emissions from cultivated soils derived from simulations using DAYCENT, a process-based biogeochemical model. DAYCENT simulated major U.S. crops at county-level resolution and IPCC emission factor methodology was used to estimate emissions for the approximately 14% of cropped land not simulated by DAYCENT. The methodology used to combine DAYCENT simulations and IPCC methodology to estimate direct and indirect N₂O emissions is described in detail. Nitrous oxide emissions from simulations of presettlement native vegetation were subtracted from cropped soil N₂O to isolate anthropogenic emissions. Meteorological data required to drive DAYCENT were acquired from DAYMET, an algorithm that uses weather station data and accounts for topography to predict daily temperature and precipitation at 1-km² resolution. Soils data were acquired from the State Soil Geographic Database (STATSGO). Weather data and dominant soil texture class that lie closest to the geographical center of the largest cluster of cropped land in each county were used to drive DAYCENT. Land management information was implemented at the agricultural–economic region level, as defined by the Agricultural Sector Model. Maps of model-simulated county-level crop yields were compared with yields estimated by the USDA for quality control. Combining results from DAYCENT simulations of major crops and IPCC methodology for remaining cropland yielded estimates of approximately 109 and approximately 70 Tg CO₂ equivalents for direct and indirect, respectively, mean annual anthropogenic N₂O emissions for 1990–2003.

Abbreviations: GHG, greenhouse gas • IPCC, Intergovernmental Panel on Climate Change • NASS, National Agricultural Statistics Service • NPP, net primary production • SOM, soil organic matter • STATSGO, State Soil Geographic Database