Reducing and managing uncertainties. The uncertainties implicit in GHG accounting can be divided into those that predominantly stem from the approach taken (what actually happens), and those that are more technical in nature (e.g. scientific uncertainty in key emission factors and in indirect impacts). Much of the uncertainty lies in attempting to understand what level of detail is required in the monitoring and accounting procedures to provide a valid average for a field or farm level operation. There are also issues of fundamental scientific uncertainty where insufficient knowledge is available to provide an adequate level of precision. Despite these uncertainties sometimes being possibly large enough to change the outcome of the GHG balance, considerable knowledge will be gained through learning-by-doing. Indeed, it may not be possible to gain sufficiently broad data sets through any other means. The coupling of the GHG Calculator’s development to the development of the farm audits has already helped to identify the nature and scope of the uncertainties and practical methods to account for and ameliorate a wide range of these factors as detailed in Kindred et al, 2008. The biggest uncertainty surrounding GHG intensity concerns N2O emissions. The IPCC approach advocated in the proposed RTFO Carbon reporting methodology (DfT 2007) provides the simplest, most transparent and defensible basis for quantifying N2O emissions and may be suitable in the first instance. It is appropriate that emissions are driven by N fertiliser application. However, emissions from organic N sources (manures/compost/sludge), organic soils and crop residues are currently ignored. Accounting for N2O emissions from these sources using an adapted IPCC approach seems likely to allow the fastest progress. Regard will have to be given to potential consequences, intended or not, of on-farm practices that could result from untried accounting procedures. These issues will need to be reviewed before economic incentives are derived from low carbon intensities, or perverse practices could be encouraged. In terms of producing a conservative methodology for dealing with N2O emissions, it is recommended here that: • Organic additions are accounted for using the IPCC approach on the basis of available N content rather than total N content. • Crop residues are accounted for using the IPCC approach assuming a modest N addition that is included irrespective of yield, N fertiliser and whether or not straw is removed. • That appropriatel...
