Ecological dimensions of biofuels: state of the science
Are biofuels a renewable, environmentally friendly energy source? The Ecological Society of America reviews bioethanol and biodiesel in conventional production as well as feedstocks still in development. Biofuels in commercial scale production are made from the sugars and oils of food crops, and share the ecological impacts of high intensity agriculture. Corn, the primary biofuel source in the United States, demands a lot of fuel to produce fuel. It needs nitrogen fertilizer, fixed using energy-intensive industrial processes. Much of that nitrogen ends up in waterways, where it causes problems for fish and fisheries. In some of the drier western states, farmers are drawing down groundwater resources to irrigate corn for biofuel. Cornfields are usually tilled, which releases greenhouse gasses stored in soil, loses topsoil to erosion, and loses water to evaporation. Much hope has been placed in the "cellulosic" biofuels for their superior environmental benefits. Made from grasses, woody crops like poplar, crop silage and other plant wastes, cellulosic ethanol does not compete with the food supply for feedstocks, which consume fewer resources, and are potentially more compatible with wildlife. Mixes of perennial native grasses, for example, offer better habitat than monocultures and don't need intensive fertilizer, pesticide, and water inputs. But cellulosic ethanol contributes only 0.5% of current biofuel production, and still faces major implementation challenges to become commercially viable. Algal biofuels remain in development. The authors conclude the report with recommendations to get the most out of biofuels going forward, improving ecosystem services, reducing greenhouse gases, and providing new income for rural communities.
•Net greenhouse gas emissions: vary greatly by feedstock. Conversion of fallow, range, or wild lands to biofuel production releases greenhouse gases stored in soil. Tilling existing croplands also contributes greenhouse gasses. High intensity agriculture uses fuel for irrigating, fertilizing, sowing, harvesting, and transporting biofuel crops.
•Water: biofuel processing plants do not use much water, but some of the biofuel crops do. Perennial crops such as switchgrass and mixed prairie grasses do not demand the irrigation, nitrogen fertilizer and yearly soil tilling typical of high intensity corn production.
•Land use and wildlife diversity: biofuel crops compete with food crops. Demand for biofuels drives conversion of prime agricultural land, expansion into marginal agricultural lands and reopening of reserves. To meet current US energy demand through biofuels alone would require conversion of 41% of US land to corn, 56% to switchgrass, or 66% to rapeseed, but potentially only 3-13% to algae. Drought tolerance, fast growth and pest resistance, traits that make plants fine candidates for biofuel feedstocks, also make them fine candidates for becoming invasive. Some, such as the old world grass miscanthus, are already invasive in North America.