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Large-Scale Use of Harvest Residues for Biofuel Production: a Bad Idea
20 Dec, 2007 01:31 pm
Plans for large scale use of harvest residues to produce biofuels such as ethanol and biodiesel will be detrimental to soil quality. They will negatively affect the future productivity of soils and thereby the future supply of harvest residues and food.
Cellulose and hemicellulose can be enzymatically converted into ethanol, which can be included in petrol. Lignocellulose can also be gasified and then converted into biodiesel.
Harvest residues, such as straw and cornstalks, are a convenient source of lignocellulose for biofuel production. They can be easily gathered during harvest operations, and there are large amounts thereof: worldwide about 4 billion tons each year.1 Indeed, many plans for second generation transport biofuels assume that in the future harvest residues will be a major of the main feedstock.
However there are now two papers in press1,2 that warn against this development, both pointing out the importance of harvest residues in maintaining soil quality. Adding harvest residues to soils adds to the stock of soil organic carbon. The latter is important for soil structure. It limits erosion, is important in the provision of plant nutrients and for the water binding capacity of soils. Thus, soil organic matter is a major determinant of crop productivity. Indeed, low levels of soil organic carbon contribute much to poor agricultural yields in large parts of sub-Saharan Africa and the tropical and subtropical areas of Asia. Even limited withdrawal of crop residues may lead to a reduction in soil organic carbon. And with climate getting warmer, more harvest residue should be added to maintain present carbon levels in soils, because respiration of organic matter tends to increase when temperature increases.
R. Lal (Ohio State University), one of the authors1 warning against the large scale use of harvest residues for biofuel production, states that removal of crop residues exacerbates soil degradation and aggravates food insecurity. He favours adding more crop residues to soils because this can increase food production. As lignocellulosic feedstock for biofuels he rather favours grasses, such as switchgrass, short rotation woody perennials such as poplar and willows, and herbaceous species such as Miscanthus. Plantations for such species can in his view be established on degraded soils, agricultural marginal lands, and disturbed lands such as mined soils.
My paper points out that, within limits, the scope for residue removal may be increased when annual crops generate relatively large amounts of biomass (and residues).2 When harvest residues are used for ethanol production, preferentially materials that contain relatively high levels of cellulose and hemicellulose should be selected for removal. Also, one may consider returning to the field suitable ‘waste’ from processing crop residues that is rich in refractory organic compounds such as lignin.
Though advice on the way forward diverges, both critical papers agree that large scale use of harvest residues for biofuel production will be detrimental to soil quality. Such large scale use will negatively affect future crop productivity, and thereby also the future supply of harvest residues and food.
1R. Lal, Crop residues as soil amendments and feedstock for bioethanol production. Waste Management
2L. Reijnders, Ethanol production from crop residues and soil organic carbon. Resources, Conservation and Recycling