Human activities are probably the major cause of the increase in the atmospheric N2O concentration of 0.7ppb per year and of the increased release of NO into the atmosphere. Nitrous oxide is a greenhouse gas, which constitutes 6 per cent of the anthropogenic greenhouse effect (Forster et al, 2007), and also contributes to the depletion of stratospheric ozone (IPCC, 2001). Although the contributions of N2O from the different sources remain less well known than for all other greenhouse gases, it is generally accepted that the use of mineral N fertilizers and animal manure management are the major anthropogenic sources of N2O (Mosier et al, 1998; IPCC, 2001).

Many factors associated with crop, soil, water and N fertilizer management influence soil conditions and processes, and thus N2O emissions. The bacterial processes of denitrification and nitrification are the dominant sources of N2O and NO in most soil systems, while denitrification is also a sink for N2O. Nitrification is an aerobic process that occurs across all ecosystems. The availability of ammonium (NH4+) and O2 is the most important factor controlling soil nitrification (Firestone and Davidson, 1989). Denitrification is an anaerobic process, and rates can be highly variable across time and space. The major controls of biological denitrification include the availability of C and NO3~ and other N oxides, and the O2 supply (Tiedje, 1988). N2O can also be produced during chemical decomposition of HNO2 under limited O2 conditions and at low soil pH (Neff et al, 1995; Bremner, 1997; McKenney and Drury, 1997; Veldkamp and Keller, 1997).

Direct anthropogenic emissions of N2O occur in N-fertilized cropland and grassland, from grazed pastures, and from animal barns and manure storage systems. Indirect anthropogenic emissions occur through degassing of N2O from aquifers and surface waters, stemming from N2O dissolved in water draining through soils, or from denitrification in groundwater of nitrate leached from fertilized soils. Furthermore, NH3 volatilization and NO emissions from agricultural systems may lead to N2O emission, following redeposition on land or water. These nitrogen transformation processes, known as the 'nitrogen cascade' (Galloway et al, 2003), are illustrated in Figure 4.3.

In this chapter we first discuss different methods for estimating direct N2O emissions, including the EF approach, and empirical and process-based models. For indirect emissions we discuss different approaches to determine EFs. We then present a global inventory of fertilizer-N use and animal manure management systems and associated N2O emissions. This inventory is used in subsequent sections to discuss the magnitude of the various sources of N2O within the agricultural system, the various mitigation strategies for reducing N2O emissions, and their emission reduction potential. Finally, we present a summary and conclusions.

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