Agricultural soils receive regular N applications of mineral N or organic fertilizers (farmyard manure, sewage sludge) and in some systems by intercropping, or crop rotations, with leguminous N fixing plants. The application of N fertilizers stimulates nitrification and denitrification activity. Under optimal soil conditions, when soil moisture and soluble carbon content are not limiting, the response to mineral N fertilizer application is very fast, resulting in peaks of NO, N2O, or N2 emissions within hours of application. These emission peaks can be 100 times larger than background emissions, but are short-lived and usually last for 1-3 weeks, before returning to near background emissions. The magnitude and shape of the peak and the dominant N gas emitted are controlled by the aeration of the soil and the availability of organic carbon. A mineral fertilizer provides a readily available N source for denitri-fiers to utilize immediately, resulting in a sharp response, as shown in Figure 2 in 2002. In 2003, there was very little rainfall during fertilization. The N2O emission peak was delayed until rainfall, but was smaller, because of uptake of fertilizer N by plants and microbial biomass during the period unsuitable for denitrification. Organic N fertilizers provide not only a source of N, but also a source of carbon; the bulk of it needs to be mineralized before it is available for denitrification. This is the reason for the delayed, but much broader N2O emission peak due to denitrification in Figure 2.
In other soils, nitrogen is supplied by mineralization of organic material or deposition of atmospheric nitrogen. Overall denitrification rates are much lower than that for agricultural soils.
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