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Vertical distribution of denitrification end-products in paddy soils.
Knowledge of denitrification process and its end-product at various depths of paddy soil is very important for our understanding of its role in mitigating reactive N and indirect nitrous oxide (N2O) emission. In this study, the end-products of denitrification were determined at four depths in a long-term field lysimeter experiment in southeast China over a rice season. Three treatments were included: (1) chemical fertilizer (NPK); (2) NPK plus pig manure (NPKM); and (3) NPK plus straw (NPKS). The concentration of dissolved N2O increased with soil depth across all treatments and the highest concentration of excess dinitrogen (N2) was observed at 0.2m depth, as was the highest dissolved organic carbon (DOC) content. Denitrification reduced the amount of nitrate by 48-54% in the paddy soil profile, especially at 0.2m depth (68-88%), whereas the lower reduction of NO3(-) (17-44%) in the subsoil (at 0.6 and 0.8m depth) was accompanied by a higher concentration of NO3(-). Our results demonstrated that DOC was the major limiting factor of denitrification in the subsoil. The application of pig manure markedly increased the amount of DOC in the surface soil, resulting in a high rate of denitrification, whereas the addition of straw had no effect on denitrification. The indirect emission factors for N2O (EF5-g, 0.001-0.006) were comparable with the default value (0.0025) reported by the Intergovernmental Panel on Climate Change. The low N2O production was probably caused by the complete reduction of N2O to N2, as reflected by the lower N2O/(N2O+N2) ratios in the paddy soil profile. Although the surface soil was identified as a hotspot for denitrification, a considerable amount of excess N2 was observed in the subsoil for all three treatments. We therefore conclude that the loss of N through denitrification may be significantly underestimated if only the surface soil is considered.
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