Livestock agriculture accounts for 65 per cent of global N2O emissions (Steinfeld et al, 2006). This N2O is largely produced by the soil microbial processes of nitrification and denitrification, which convert ammonium and nitrate nitrogen cycling through the animal/plant/soil system into N2O (Figure 6.1). In livestock agriculture, N enters this system mainly through N fertilizer applications and biological N fixation by legumes. However, a relatively large proportion of these N inputs is subsequently recycled within the system via grazing ruminants that generally utilize very little of the N in feed and excrete between 75 to 90 per cent of ingested N (Whitehead, 1995). This N is excreted either directly on to pasture as urine and dung, or in animal housing systems and then applied to land as effluent or manure.