Nitrous oxide (N2O), another major greenhouse gas, has no significant sinks on land and is destroyed by chemical reactions in the upper atmosphere. Land surfaces are the main source of atmospheric N2O; thus, changes in land-use practices modify soil emissions and influence N2O concentration in the atmosphere (Kroeze et al., 1999). Uncertainty with respect to current magnitudes of sources and sinks of N2O (Prasad, 1997) and its atmospheric lifetime limit an accurate budget: Existing data on fluxes of N2O from soils and oceans are insufficient to quantify them in detail. Nevertheless, present-day global N2O emissions have been estimated to be about 14 Mt N yr-1 (Prasad, 1997). Roughly half of the global N2O emissions are anthropogenic (Davidson, 1991; Khalil and Rasmussen, 1992; Hutchinson, 1995; Prather et al., 1995; Prasad, 1997).
Microbiological processes in soils are the primary sources of N2O (Davidson, 1991, 1992; Shiller and Hastie, 1996). Table 1-3 lists estimated land use-related emissions of N2O on both a nitrogen content and a CO2-equivalent basis.2 Emissions from soils are enhanced under warm and wet conditions (e.g., those present in the soils of moist tropical forests) and when nitrogen fertilizers are applied in agriculture (Conrad et al., 1983; Winchester et al., 1988; Khalil and Rasmussen, 1992). Thus, changes in agricultural soil management and tropical forestry may alter N2O emissions from soils and influence its concentration in the atmosphere.
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