Oxygen is the primary controller of denitrification. Denitrification only operates when the soil atmosphere O2 concentration ranges from 4% to 17%. The main variables that control the soil O2 concentration are rainfall, drainage, soil compaction, soil texture, and soil respiration. Rainfall, lack of drainage, and compaction all reduce the O2 concentration in the soil and stimulate denitrification. Fine-textured soils, with high clay content, are more readily depleted of oxygen than a coarse-textured soil with high sand content. The lack of O2 of a soil is often expressed in terms of its water-filled pore space (WFPS). This variable is calculated from the bulk density and particle density of the soil and the water content. Denitrification operates when the WFPS is greater than 55%. The ratio of the products of denitrification N2O/N2 decreases as the WFPS increases. At the lower WFPS rates (<70%) most of the denitrification

Figure 2 The influence of mineral N fertilizer, manure, and rainfall on the emissions of N2O from intensively managed grassland. Measurements were made using static chambers (Figure 1) before and at intervals after fertilizer application. The differences in the shape and magnitude of the N2O peaks can be explained by differences in rainfall pattern. (a) In 2002, rainfall distribution before, during, and after fertilizer application ensured optimal conditions for N2O emission. (b) In 2003, fertilizer was applied during a 3 week dry period, and rainfall only started 10 days after fertilizer application. The much broader peaks associated with the manure treatment is a result of the organic carbon source supplied in manure, stimulating denitrification. Modified from Jones SK, Rees RM, Skiba UM, and Ball BC (2007) Influence of organic and mineral N fertilizer on N2O fluxes from a temperate grassland Agriculture, Ecosystems and Environment 121(1-2): 74-83.

reactions stop at N2O whereas at high WFPS (>70%) denitrification to N2 may dominate, provided the C and N supplies are not limiting.

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