BACKGROUND

Although the cumulative radiative forcing estimates for nitrous oxide (N₂O) are lower than either carbon dioxide or methane, N₂O contributes substantially to total radiative forcing by the Earth's atmosphere. Per unit mass, the radiative effectiveness of N₂O is 298 times more than carbon dioxide, making each kilogram of N₂O 298 times more relevant. The Intergovernmental Panel on Climate Change (IPCC) reported in 2007 that N₂O has increased by 10% since pre-industrial time periods, but lasts in the atmosphere for approximately 114 years. In soil, bacteria produce N₂O during the processes of nitrification and denitrification. During nitrification, ammonium is converted to nitrate, and N₂O is a byproduct. Denitrification, the reduction of nitrate to nitrogen gas (N₂), is an anaerobic process. Nitrous oxide is an intermediate product for many denitrifyers but can be the end product for some denitrifying bacteria (Robertson and Grace 2004). More information about nitrous oxide can be found on the U.S. EPA website: http://www.epa.gov/nitrousoxide/index.html.

Nitrous oxide is an important greenhouse gas because of its high relative radiative effectiveness. Two factors influence relative radiative effectiveness, which are physical chemistry (including radiation absorption properties) and lifetime of a molecule in the atmosphere. Physical chemistry of a molecule determines the infrared (IR) wavelength absorbed. Gases with absorption bands in the non-visible portion of the IR spectrum, particularly between 1,000-1,200 wavenumbers, have the highest radiative forcing effect. Carbon dioxide absorption peaks occur at 2350 and 650 wavenumbers while nitrous oxide absorption peaks occur at 2,200 and 1,250 wavenumbers.

Agricultural soils high in available nitrogen are a major contributor of N₂O to the atmosphere. Reactive (biologically available) nitrogen in the biosphere is twice as high as pre-industrial times, largely due to agricultural practices of fertilization and increased growth of nitrogen fixing crops (Vitousek et al. 1997). A good general source about human alteration of the global nitrogen cycle can be found on the Ecological Society of America website: http://www.esa.org/science_resources/issues.php.