Estimation of Nitrous Oxide Emissions from US Grasslands

2 O) emissions from temperate grasslands are poorly quantified and may be an important part of the atmospheric N2O budget. In this study N2O emissions were simulated for 1052 grassland sites in the United States using the NGAS model of Parton and others (1996) coupled with an organic matter decomposition model. N2O flux was calculated for each site using soil and land use data obtained from the National Resource Inventory (NRI) database and weather data obtained from NASA. The estimates were regionalized based upon temperature and moisture isotherms. Annual N2O emissions for each region were based on the grassland area of each region and the mean estimated annual N2O flux from NRI grassland sites in the region. The regional fluxes ranged from 0.18 to 1.02 kg N2O N/ha/yr with the mean flux for all regions being 0.28 kg N2O N/ha/yr. Even though fluxes from the western regions were relatively low, these regions made the largest contribution to total emissions due to their large grassland area. Total US grassland N2O emissions were estimated to be about 67 Gg N2O N/yr. Emissions from the Great Plains states, which contain the largest expanse of natural grassland in the United States, were estimated to average 0.24 kg N2O N/ha/yr. Using the annual flux estimate for the temperate Great Plains, we estimate that temperate grasslands worldwide may potentially produce 0.27 Tg N2O N/yr. Even though our estimate for global temperate grassland N2O emissions is less than published estimates for other major temperate and tropical biomes, our results indicate that temperate grasslands are a significant part of both United States and global atmospheric N2O budgets. This study demonstrates the utility of models for regional N2O flux estimation although additional data from carefully designed field studies is needed to further validate model results.