The relationship, on a global basis, between the amount of N fixed bychemical, biological or atmospheric processes entering the terrestrialbiosphere, and the total emission of nitrous oxide (N 2 O), has beenre-examined, using known global atmospheric removal rates and concentrationgrowth of N 2 O as a proxy for overall emissions. For both thepre-industrial period and in recent times, after taking into account thelarge-scale changes in synthetic N fertiliser production, we find an overallconversion factor of 3ndash;5 from newly fixed N to N 2 O-N. We assume thesame factor to be valid for biofuel production systems. It is covered onlyin part by the default conversion factor for "direct" emissions fromagricultural crop lands (1) estimated by IPCC (2006), and the defaultfactors for the "indirect" emissions (following volatilization/depositionand leaching/runoff of N: 0.35ndash;0.45) cited therein. However, as weshow in the paper, when additional emissions included in the IPCCmethodology, e.g. those from livestock production, are included, the totalmay not be inconsistent with that given by our "top-down" method. When theextra N 2 O emission from biofuel production is calculated in"CO 2 -equivalent" global warming terms, and compared with thequasi-cooling effect of "saving" emissions of fossil fuel derivedCO 2 , the outcome is that the production of commonly used biofuels, suchas biodiesel from rapeseed and bioethanol from corn (maize), depending on Nfertilizer uptake efficiency by the plants, can contribute as much or moreto global warming by N 2 O emissions than cooling by fossil fuel savings.Crops with less N demand, such as grasses and woody coppice species, havemore favourable climate impacts. This analysis only considers the conversionof biomass to biofuel. It does not take into account the use of fossil fuelon the farms and for fertilizer and pesticide production, but it alsoneglects the production of useful co-products. Both factors partiallycompensate each other. This needs to be analyzed in a full life cycleassessment.
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