Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories N2O: Direct Emissions from Agricultural Soils 361 N 2O: DIRECT EMISSIONS FROM AGRICULTURAL SOILS ACKNOWLEDGEMENTS

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 cro