N 2 O : Direct emissions from agricultural soils

  • Keith, Smith, Lex B
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Abstract

The OECD/IPCC/IEA phase II development of the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC Guidelines) methodology for agricultural sources of N2O (IPCC, 1997; Mosier et al., 1998) includes methodologies for calculating both direct and indirect emissions of N2O related to agricultural production. It takes into account anthropogenic N inputs including synthetic fertilizers, animal wastes and other organic fertilizers, biological nitrogen fixation by crops, cultivation of organic soils, and mineralization of crop residues returned to the field. Direct sources include those where N2O is emitted directly to the atmosphere from cultivated soils and fertilized and/or grazed grassland systems. Indirect emissions result from transport of N from agricultural systems into ground and surface waters through drainage and surface runoff, or emission as ammonia or nitrogen oxides and deposition elsewhere, causing N2O production. Worldwide consumption of synthetic N fertilizers has increased by about 150% since 1970 to about 82 Tg N y-1 in 1996. Animal wastes used as fertilizer supplied an estimated additional 65 Tg N y-1 in 1996, compared with 37 Tg N y-1 in 1950. This increase in N use is now widely recognised as a major factor in the increase in N2O emissions indicated by increases in atmospheric concentration. The evidence points to a further major increase from agricultural sources in the future, with increases in food production. Much research published since the adoption of the present default emission factors (1.25% of N applied as synthetic and organic fertilizers, crop residues etc, and 2% of the N deposited by grazing animals) strongly suggests that seasonal weather fluctuations and management variables (e.g. the timing of irrigation), and crop type in a given region, have a large impact on fluxes. This poses the question of whether an analysis of existing data may lead to modified emission factors. This will be an important theme of the workshop discussions. The alternative to experimental determination of these factors is modelling, and the potential for the use of models in this context is reviewed. Difficulties with complex models, including the issue of the scales on which they operate, may make simplified summary models a desirable option for developing updates of emission factors. This paper reviews the problems associated with the acquisition of activity data, quantification of uncertainties, and the achievement of completeness in source identification and calculation. It also identifies corrections that are needed to the present IPCC Guidelines, and examines the issue of accounting for mitigation measures. In relation to this last topic, it is concluded that careful attention needs to be given to the impact of measures taken to mitigate CO2 or CH4 emissions on those of N2O. With regard to inventory quality, it is concluded that a key concept is independent, objective review to assess the effectiveness of the internal QC programme, and to reduce or eliminate any inherent bias in the inventory processes. Several types of external reviews, or audits, may be appropriate for the inventory of direct N2O emissions from agricultural soils.

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Keith, Smith, Lex, B. & B. B. (1999). N 2 O : Direct emissions from agricultural soils. Retrieved from http://www.ipcc-nggip.iges.or.jp/public/gp/bgp/4_5_N2O_Agricultural_Soils.pdf

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