Biodiesel made from vegetable oil which may also serve as food or feed is usu­ally inferior to conventional diesel, when the arable soil is subject to tillage without addition of large amount of fresh C (e. g. residues, manure) and land use changes are factored in. Ethanol from starch and sugar crops is usually inferior to conven­tional gasoline when arable land is subject to tillage, ethanol production is powered by fossil fuels, land use change is factored in and Intergovernmental Panel on Cli­mate Change (IPCC) guidelines are applied. It has been argued that no foreseeable changes in agricultural or energy technology will be able to achieve meaningful benefits as to the emission of greenhouse gases if annual crop-based biofuels are produced at the expense of tropical forests (Gibbs et al. 2008).

Among the transport biofuels from sugar and starch crops, ethanol from sugar cane does relatively well. For Brazilian sugar-cane-derived ethanol produced in 2005/2006, a greenhouse gas emission has been published of somewhat more than 0.4 kg CO equivalent per litre ethanol (Macedo et al. 2008). The latter energetically equals about 0.751 gasoline, which has a life cycle CO2 emission of about 2.5 kg CO2 equivalent. The estimate of Macedo et al. (2008) includes an N2O emission from soils, though linked to a lower input of N fertilizer than used by Machado et al. (2008), and a greenhouse gas emission linked to burning ‘trash’. Changes in aboveground and belowground C were not accounted for by Macedo et al. (2008), and the value for the emission of N2O used by them was well below the 3-5% of N input suggested by Crutzen et al. (2007).

There is no clarity regarding the impact of sugar cane cultivation on soil C stocks. It has been suggested that a conversion of rainforest to pasture and then to sugar cane plantation reduced the soil C stock by about 40% (Groom et al. 2008). But there are also reports that current practices are not associated with reductions in soil C of current arable soils with limited tillage (La Scala et al. 2006; de Resende et al.

2006) . However, a major loss of carbon is associated with converting the wooded Cerrado to arable land for growing sugar cane. Fargione et al. (2008) estimate that it will take 17 years to pay back the carbon debt for this by producing sugar cane ethanol. If the IPCC guidelines are followed, ethanol from sugar cane for which the Cerrado savannah has been cleared would probably do somewhat better than fossil gasoline. If arable land remains in use for many decades, sugar cane ethanol will do substantially better than conventional gasoline.