《森林生态学》课程教学资源（生态学热点）Issues in ecology - 17 生物燃料 Ecological Dimensions of Biofuels.pdf

© The Ecological Society of America • esahq@esa.org esa 1 ISSUES IN ECOLOGY NUMBER SEVENTEEN SPRING 2013 Ecological Dimensions of Biofuels Clifford S. Duke, Richard V. Pouyat, G. Philip Robertson, and William J. Parton SUMMARY 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. We conclude: • 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. Cover photos: Examples of biofuel feedstocks. Clockwise starting on the upper left: (a) Switchgrass (Panicum virgatum), a prairie grass native to North America (b) Two species of algae (Cyclotella and Oocystis) (c) Sunflowers (d) Hybrid poplars (crosses between two or more species of Populus), (e) Corn field. Photos credits: (a) Peggy Greb, USDA-ARS (b) Robert O. Megard, University of Minnesota (c) Edward McCain, USDA-ARS (d) Stephen Ausmus, USDA-ARS (e) Flickr user fishhawk

2 esa © The Ecological Society of America • esahq@esa.org Introduction Policy makers are increasingly looking to renewable energy sources as environmentally friendly and sustainable replacements for fossil fuels used for transportation. Biofuels, which are liquid fuels derived from a variety of sources—for example, row crops, trees, algae, and food waste— appear to hold promise to reduce our dependence on fossil fuels and to reduce net emissions of greenhouse gases (GHG) from mobile sources that contribute to global warming. Many countries, including the United States, have set targets for biofuels production (ethanol and biodiesel) and mandated the blending of ethanol with gasoline. These policies, combined with economic incentives and tariffs, have driven a worldwide expansion in the production of various crops for use as transport fuels. At present, biofuels are primarily derived from a small number of plant materials, or feedstocks, primarily corn and sugar cane, but the variety of materials is expanding. Worldwide biofuel use for transport is expected to nearly double by 2017 over 2005–2007 levels.1 A target adopted by the European Union (EU) in 2009 requires 10% of fuels for transport to be from renewable sources by 2020. In the U.S., ethanol, which accounts for more than 99% of biofuels produced, is made almost exclusively from corn grown on prime agricultural land. With rising demand and legislative targets for biofuels, increasing output will require boosting yields of existing crops, bringing more land into biofuel crop production, and/or developing new feedstocks (see Box 1). Provisions in U.S. legislation, including the U.S. Energy Independence and Security Act of 2007 and the Food, Conservation and Energy Act of 2008 (known as the Farm Bill), set new targets and incentives for biofuels production. In amending an earlier Renewable Fuel Standard (RFS), the Energy Independence and Security Act called for a nine-fold increase in renewable fuel production by 2022. Gasoline for road transportation would have to consist of 20% biofuels by this date. The U.S. Environmental Protection Agency (EPA) in 2010 issued new production targets for various biofuels and detailed schedules for meeting them (see Box 2.) The 2008 Farm Bill increased targets in the U.S. and established tax credits, grants, and other provisions to encourage the expansion and use of transport biofuels, with new emphasis on fuels derived from cellulosic sources. If produced in a sustainable fashion, biofuels could reduce demand for fossil fuels, in turn reducing needs for imported oil and mitigating climate change by limiting GHG emissions. As with any agricultural crop, however, biofuel crops can affect water supply and quality, soil biogeochemistry, land use, and biodiversity. Erosion, nutrient runoff, habitat loss, the loss of beneficial species, and the spread of invasive species are all potential risks of expanded production. Scientists and policy makers are examining how potential adverse effects on natural resources can be avoided or mitigated in order to meet biofuels production goals while enhancing environmental, social, and economic sustainability. Research has also focused on the impacts of various economic policies intended to stimulate biofuels production, the economics of growing crops for transportation fuel, and the potential impact on worldwide food production. As the need for food increases with a burgeoning global human population, biofuel crops compete for arable land and may lead to higher food prices. While acknowledging the potential economic and social implications, the objectives of this report are to summarize the environmental effects of biofuels, illustrate some of the uncertainties about those effects, and identify topics for an integrated research program that could reduce uncertainties in planning for sustainable biofuels production. We first summarize the potential direct and indirect effects of biofuels production on GHG emissions and soil carbon. Subsequent sections describe impacts on water use and quality, biodiversity, including the effects of land conversion, monoculture agriculture, invasive species, and the potential tradeoffs between biofuel energy yield and biodiversity. We propose elements of an integrated research program that could reduce the identified uncertainties. Finally, we link a set of princiEcological Dimensions of Biofuels Clifford S. Duke, Richard V. Pouyat, G. Philip Robertson, and William J. Parton ISSUES IN ECOLOGY NUMBER SEVENTEEN SPRING 2013

© The Ecological Society of America • esahq@esa.org esa 3 ISSUES IN ECOLOGY NUMBER SEVENTEEN SPRING 2013 Box 1. Types of Biofuels and the World’s Major Producers Biofuels are a renewable energy source derived from biological materials such as crops, wood, or algae grown specifically for fuel purposes or from wastes such as forest or agricultural residues and municipal waste. Biofuels come in two primary forms: ethanol and biodiesel. Ethanol, the most widely used renewable transportation fuel, is an alcohol fuel made from sugars found in grains or derived from starches, cellulose, and other plant components. Virtually all of the transportation biofuel produced in the U.S. today is ethanol derived from corn grain. The U.S. also imports ethanol, largely sugarcane-based ethanol from Brazil, which together with the U.S. supplies 90% of the world’s fuel ethanol.a Owing to increased domestic production and tariffs on imported ethanol, imports have declined in recent years.b Ethanol is generally blended with gasoline, although some cars can run on pure ethanol. In 2009, ethanol made up about eight percent of the U.S. motor vehicle gasoline market, which is nearly double the market in 2006. A wide variety of perennial grasses, including both native prairie species such as switchgrass and exotic species such as miscanthus, are being studied for their potential as biofuel crops. Because cellulosic ethanol can be made from any number of plant species, including trees, mixtures of species can potentially be grown to optimize environmental benefits in addition to fuel production. Such mixtures might comprise two to three species of native grasses and forbs grown together to enhance prairie restoration. Cellulosic biofuels constitute less than a half-percent of current biofuels production in the U.S., although this proportion is expected to grow rapidly, in part due to incentives for development that acknowledge their superior environmental benefits. Biodiesel, now made largely from palm and vegetable oils, fats, or greases, can be blended with diesel fuel or used directly in diesel engines. Biodiesel is currently produced largely in the EU and particularly in Germany, although biodiesel production is expanding worldwide, particularly in Southeast Asia, the U.S., and parts of South America and Europe. Biodiesel from the EU is made primarily from canola (also known as rapeseed), although the U.S. and Brazil, among others, have also used soybeans for biodiesel. Oil palm plantations have expanded in Southeast Asia for production for biodiesel (see Table 1). Algae constitute another potential biofuel feedstock. Algae grown in outdoor ponds or enclosed in containers (“photobioreactors”) are typically harvested, dewatered, and dried for their lipids and oils, which can be used to make biodiesel, ethanol, or other hydrocarbons. Some producers are developing systems in which the algae excrete the desired product, for example ethanol, into the culture medium and the product is then extracted from the medium without the need for harvesting of the algae. a World Bank. 2008. Biofuels: the promise and the risks, in World Development Report 2008. http://siteresources.worldbank.org/INTWDR2008/Resources/2795087-1192112387976/WDR08_05_Focus_B.pdf b Renewable Fuels Association. 2012. www.ethanolrfa.org/pages/statistics viewed 7 December 2012. Figure 1. Examples of different biofuel types. a) field corn (conventional bioethanol); b) switchgrass (advanced bioethanol); c) sunflower (conventional biodiesel); d) green alga Botryococcus braunii (advanced biodiesel, bioethanol, biobutanol, aviation fuels). Photo credits: a) Warren Gretz / NREL b) Bob Nichols / USDA c) Peggy Greb / USDA. d) Tim Devarenne / Texas AgriLife Research. Table 1. Major biofuels and their sources. “Conventional” biofuels are those that dominate today’s marketplace. “Advanced” biofuels are new-generation fuels under experimental production. Type of Biofuel Feedstock Where produced Conventional bioethanol Corn United States, Canada Sugarcane South America (primarily Brazil), Central America, Asia, Africa Sugar beets Europe Cereals (e.g. milo, wheat, barley) Europe, Canada Cassava Asia, South and Central America Advanced bioethanol Cellulosic biomass • Grass (e.g. switchgrass, miscanthus, mixed species) In development • Short-rotation woody crops (e.g. poplar) • Plant waste (e.g. corn stover, wood waste) Conventional biodiesel Rapeseed (canola) Europe, Canada, Asia Soybean Europe, Canada, South and Central America, Africa, Asia, United States Sunflower Europe, Canada, Africa, Asia Palm South and Central America, Africa, Asia Jatropha South and Central America, Africa, Asia Castor South and Central America Advanced biodiesel, bioethanol, Algae In development biobutanol, aviation fuels Source: United Nations Environment Programme. 2009. Towards sustainable production and use of resources: assessing biofuels. http://www.unep.org/PDF/Assessing_Biofuels.pdf (a) (b) (c) (d)

ISSUES IN ECOLOGY NUMBER SEVENTEEN SPRING 2013 ples for biofuels and sustainability to a landscape approach designed to meet social, economic, and energy needs. Potential Environmental Effects The potential environmental effects of biofuels production have been examined using field measurements, laboratory experiments, computer models, and combinations of two or more of these methods. Not all studies agree with each other, and we discuss some of the reasons for differing conclusions. The effects of biofuels on the environment are many and complex; they vary greatly depending on initial conditions and assumptions. Greenhouse Gas Emissions and Soil Carbon GHG emissions resulting from biofuel production depend on 1) land clearing, if necessary, and whether the feedstock crop replaces native vegetation or another existing crop; 2) feedstock choice; 3) fuel and energy use for crop growth, harvest, and biofuels production; 4) water use and source; 5) the use of nitrogen fertilizers; and 6) soil turnover effects on carbon and nitrogen emissions (Box 3). The use of fossil fuels and nitrogen fertilizer has direct effects on emissions. Indirect effects can occur when biofuels production displaces another agricultural activity (e.g., cattle grazing in tropical regions which then expands into native rainforest). Whether biofuels production causes net GHG emissions, has no net GHG emissions, or takes up GHGs from the atmosphere (e.g., by storing carbon in plant roots and soil) depends on the entire life cycle of production and use. For this reason, researchers studying the effects of biofuels production on GHG emissions generally conduct life cycle analyses (or assessments), known as LCAs (Box 4). An LCA describes the impacts of a product at every step from start to finish— 4 esa © The Ecological Society of America • esahq@esa.org Terms and Definitions biofuels Liquid fuels derived from biological materials such as crop plants, forest products, or waste materials carbon debt The amount of carbon released as a result of land use conversion, for example from grassland or forest to crops for biofuels production. The carbon debt can be repaid over time if the biofuel produced has lower greenhouse gas emissions than the fossil fuel that it replaces (see reference 8). cellulosic Refers to fuel derived from vegetative plant tissue, composed primarily of cellulose, hemicellulose, and lignin (for example, crop residues, wood, and grass not harvested for grain); contrast to grain-based or algae-based biofuels. CO2 Carbon dioxide CRP Conservation Reserve Program of the U.S. Department of Agriculture EPA United States Environmental Protection Agency EU European Union feedstock The source material for biofuel, for example, algae, crop plants, waste materials, or wood foregone sequestration The carbon that would otherwise have been stored by an ecosystem in the absence of its conversion to biofuel cropping GHG Greenhouse gas hypoxia Oxygen deficiency indirect land use change Refers to the carbon cost of converting grassland or forest to food crops in order to replace the food production lost when cropland elsewhere is diverted to biofuels production no-till Farming without plowing (tillage); the prior crop’s residue is left on the soil surface to decompose. N2O Nitrous oxide RFS Renewable fuel standard. The first RFS was established by the U.S. Environmental Protection Agency under the Energy Policy Act of 2005. RFS2, an expanded version of the standard, was developed in response to the Energy Independence and Security Act of 2007. stover Corn leaves and stalks, a potential cellulosic feedstock USDA United States Department of Agriculture