Duke, C. S., R. V. Pouyat, G. P. Robertson, and W. J. Parton. 2013. Ecological dimensions of biofuels. Issues in Ecology 17:1-17.
Biofuels, liquid fuels derived from biological materials such as crop plants, forest products, or waste materials, have been widely promoted as a means to reduce dependence of our transportation systems on fossil fuels and to reduce emissions of the greenhouse gases (GHG) that contribute to global warming. The primary forms of biofuels are ethanol, made from sugars, starches, cellulose, and other plant structural components, and biodiesel, made from oils produced by plants. Many countries, including the United States and members of the European Union, have adopted production and use targets for biofuels. The promise of biofuels as a renewable, environmentally friendly energy source, combined with these mandates, has driven a worldwide expansion in their production. However, many questions remain about how to produce biofuels without causing new and unanticipated environmental impacts. In this report we summarize the environmental effects of biofuels, illustrate some uncertainties about these effects, and identify topics for an integrated research program aimed at clarifying tradeoffs and reducing uncertainties in planning for sustainable biofuels production. Our considerations include effects on GHG emissions, soil carbon, water supply and quantity, land use, and biodiversity.
• Estimated net GHG emissions from biofuels production can be lower than those of fossil fuels. However, this is highly dependent on feedstock (raw material) choice, fuel and fertilizer inputs, whether biofuel crops replace native vegetation, and whether the soil is tilled. Further, emissions estimates for a given feedstock vary among studies, contributing to uncertainty about GHG effects.
• The effects of biofuels production on water supply and quality are a function of the feedstock choice and production method. High intensity agricultural crops such as fertilized and irrigated corn can contribute nitrogen and phosphorus pollution to adjacent waterways and downstream, and can place substantial demands on regional water supplies. Perennial cellulosic crops such as switchgrass and mixed prairie grasses can substantially reduce these impacts.
• Today’s grain-based biofuel crops compete with food crops for prime agricultural land. Pressure is growing to expand grain-based biofuels production onto marginal agricultural lands or land currently in the U.S. Department of Agriculture (USDA) Conservation Reserve Program. These lands support diverse wildlife communities and conversion is likely to affect some species of concern. Land conversion is also a major source of GHG production, especially when native habitats are destroyed. Land use impacts can be reduced by selecting feedstocks that do not displace food crops or require conversion of native habitats for production.
• Impacts on wildlife abundance and diversity depend on the feedstock choice and whether production takes place on existing agricultural lands or on newly cleared land. At a landscape scale, more diverse feedstock crops are associated with greater biological diversity, while monocultures decrease it. Some plants being considered as sources of biofuels are potentially invasive, requiring consideration of potential impacts on habitats adjacent to the biofuels crop.
Biofuels production presents a wide range of potential impacts and benefits, with substantial uncertainty associated
with different choices among sources and production methods. Society must carefully consider the environmental tradeoffs of different biofuels sources, and of biofuels compared with other energy sources, including fossil fuels. An integrated research program that explores optimal crop selection, agricultural landscape design, effects on GHG emissions, soils, and biodiversity, and economic and social factors, is necessary to fully inform decisions about these tradeoffs. Appropriately designed, a biofuels production system can be a sustainable and resilient source of energy for the long term.
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