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1.2.3. Sources and Sinks of Nitrous Oxide
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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