N2O Emissions

N2O emissions may originate on several occasions along the biofuel life cycle (Reijnders and Huijbregts 2005). The production of N fertilizers is often accom­panied by the emission of N2O. NOx emissions linked to burning fossil fuels may be deposited as N compounds in soils, and there, they may be partly converted by microorganisms into N2O. N inputs in cropping are also partly converted into N2O. It is usually assumed that the latter process is, directly and indirectly, responsible for most of the N2O emission linked to the transport biofuel life cycle. The actual quantity of the N2O emission, given a specified input of N and soil, is, however, the subject of a lively debate (Mosier et al. 1998; Crutzen et al. 2007). According to the estimates of Crutzen et al. (2007), 3-5% of the N input into growing biofuel crops will be converted into N2O. Mosier et al. (1998) have presented data suggesting that direct N2O emissions from agricultural fields associated with biofuel cropping may be about 1.25% of added fixed nitrogen. In addition, they argue that fixed nitrogen lost from agricultural fields may also be subject to microbial conversion to N2O (estimated at 2.5% of fixed N lost).

Local conditions may have a significant impact on actual N2O emissions. The presence of soil moisture matters (Rebelo de Mira and Kroeze 2006; Guo and Zhou 2007; Scheer et al. 2008). Higher temperatures tend to be conducive to higher emis­sions of N2O (Ding et al. 2007; Scheer et al. 2008). So are higher levels of soil organic carbon (Guo and Zhou 2007; Liu et al. 2007). In some soils, nitrate is rel­atively favourable to N2O production, but in other soils, it is rather ammonia (Guo and Zhou 2007; Liu et al. 2007; Scheer et al. 2008). Moreover, it may be noted that N2O emissions may change when the climate changes. Temperature and precipi­tation are significant determinants of N2O emissions, and as temperature and pre­cipitation are expected to change when the climate changes, N2O emissions from biofuel cropping may be different in the future from what they are now (Novoa and Tejeda 2006). In view of variability, there is a case for a rather wide range for the conversion of fixed N into N2O: 1.5-5%.

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