The nitrogen cycle illustrates the mechanism for nitrous oxide emissions (U.S. Office of Technology Assessment 1991). Nitrous oxide (N2O) absorbs infrared radiation in the atmosphere, causing atmospheric warming, and can also combine with oxygen in the stratosphere to form NO, which reacts with ozone and decreases the ozone layer. Bouwman (1990) estimates the contribution of N2O to global warming at approximately 5 percent of the total radiative force.
In Climate Change: The IPCC Response Strategies, the Intergovernmental Panel on Climate Change (1991) estimates total global nitrous oxide emissions at 10 to 17.5 million metric tons per year. Reliable estimates of the amount due to fertilizer application are not yet available. Anthropogenic factors that affect emissions from fertilizer use include the type and amount of fertilizer applied, application technique and timing, tillage practices, chemical use, irrigation practices, vegetation type, and soil residual nitrogen. Chapter 4 of the U.S. Environmental Protection Agency report Policy Options for Stabilizing Global Climate provides data on nitrous oxide emissions by fertilizer type in table 4-11 (Lashof and Tirpak 1990). Figure 4-17 presents data on nitrogen fertilizer consumption.
Another contributing factor to the level of nitrous oxide emissions is leguminous crops that add nitrogen to the soil (Eichner 1991). In "Nitrous Oxide Emissions from Fertilizer Soils," Eichner estimates emissions from legumes at 23 to 315 Tg (1 Tg=1 million tons) of N2O-N in 1986. Table 1 illustrates key factors affecting nitrous oxide emissions, and table 2 and table 3 present data on fertilizer-derived nitrous oxide emissions from agricultural soils. These data were obtained from 104 field experiments conducted from 1979 to 1987 and from soil science literature.
Byrnes, Holt, and Austin (1993) report on a greenhouse experiment to measure nitrous oxide emissions in "The Emission of Nitrous Oxide upon Wetting a Rice Soil Following a Dry Season Fallow." The results indicate that fertilizer nitrogen additions to the rice crop had little effect on emissions, because nitrate developed from mineralization of organic nitrogen. Figure 3 presents data on nitrous oxide emissions with various fertilizer treatments to the rice crop.