As discussed in Chapter 2, N2O emissions are highly variable in space and time, due to the numerous controlling factors of nitrification and denitri-fication. However, the two key variables that regulate N2O emissions from soils of livestock systems are generally soil N availability and soil aeration (de Klein and Eckard, 2008). The combined effect of these two factors is illustrated by measurements of N2O emissions from a sheep-grazed pastoral soil in New Zealand (Plate 6.1) and denitrification losses from a grazed ryegrass/clover pasture in Australia (Figure 6.4). These results clearly indicate that the coincidence of high soil nitrate N and high soil moisture content (low aeration) greatly enhanced N2O emissions. Most options for mitigating N2O emissions from livestock systems therefore focus on reducing the availability or input of soil N, particularly under wet conditions (Figure 6.5).
Temperature can also be important, with higher N2O emissions occurring at higher temperatures. However, its effect is generally more pronounced at a regional or national scale, i.e. temperature-induced differences in N2O emissions are generally observed between regions rather than at a farm scale, and temperature is therefore not often considered as a factor to target in relation to N2O mitigation at a farm level.
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