The seed-to-wheel emissions of the transport biofuels considered here are substan­tial. The most important current transport biofuels (bioethanol from starch and sugar crops and biodiesel from edible vegetable oil crops) are often not a substantial im­provement over current fossil transport fuels or do even worse. Thus, the question arises as to what the possibilities are for reducing this impact. Much of the impact tends to come from the production of feedstock for transport biofuel production. So the possibilities for the reduction of environmental impact associated with this stage of the transport biofuel life cycles will be considered here.

Increasing soil carbon stocks while growing feedstocks may reduce the emission of biogenic carbonaceous greenhouse gases. Reduced tillage, the use of cover crops and/or fallows, including nitrogen fixers, and the return of residues and application of other organic matter, such as manure and household composts, may contribute to such an increase in C stocks (Nandwa 2001; Bationo and Buerkert 2001; D^az — Zorita et al. 2002; Cowie et al. 2006; Reijnders and Huijbregts 2007,2008b). The net emissions of greenhouse gases due to changes in ecosystem carbon stocks linked to land use change following from expansion of feedstock cropping may be lowered or even reversed by growing feedstocks on soils with low aboveground carbon stocks (Germer and Sauerborn 2007). Also, increases in yield achieved at relatively low inputs of fossil fuels and improved efficiencies in converting feedstock to biofuel may reduce net greenhouse gas emissions (Gibbs et al. 2008).

In the case of CH4 emission due to anaerobic conversion linked to the process­ing of biomass, capture of CH4 and application thereof in energy generation will help (Reijnders and Huijbregts 2008a). When biofuels are burned in power plants, there is the option of CO2 sequestration in abandoned gas and oil fields or aquifers, which may lead to net biogenic C sequestration (Mathews 2008). The emission of N2O may be reduced by a better N efficiency of agriculture. Precision agriculture and subsoil irrigation techniques may be conducive to a better N efficiency (Reijn — ders and Huijbregts 2008b). Kim and Dale (2008b) have shown that in conventional corn cropping, there is an environmentally optimal N application rate which en­hances profitability to farmers. Reduction of greenhouse gas emissions linked to the production of synthetic N fertilizer may be possible by including N-fixing crops.

Thomsen and Haugaard-Nielsen (2008) have, for instance, proposed wheat under­sown with clover grass for the production of biomass to be subj ected to simultaneous saccharification and fermentation.

As to the emissions linked to fossil fuels, over time there have been significant re­ductions in the fossil fuel inputs into the production of the currently most important transport biofuel — ethanol due to efficiency gains (Hill et al. 2006; Macedo et al. 2008) — and a further significant reduction linked to efficiency gains is expected (Macedo et al. 2008).

Also, in feedstock processing, there is scope for the replacement of fossil fuels by agricultural residues, especially in the case of high-yielding crops (Reijnders and Huijbregts 2008a; Reijnders 2008). For instance, in Thailand, coal is an important fuel in the conversion of sugar cane molasses into ethanol (Nguyen et al. 2008), and coal can be replaced by residues of sugar cane or oil palm fruit processing. In the case of ethanol from sugar cane and within limitations linked to the need for maintenance of soil C stocks, the possibility exists to additionally produce electricity from bagasse (a sugar cane residue) for use elsewhere (Macedo et al. 2008).

In the case of sugar cane production, emissions of a variety of pollutants can be reduced when cane burning is replaced by mechanical harvesting (Macedo et al. 2008; Machado et al. 2008). Improving the nutrient efficiency of biofuel cropping (for instance by precision agriculture) may reduce nutrient emissions from arable land, and there is also scope for reduced pesticide emissions (e. g. Muilerman 2008).