ISSUES IN ECOLOGY NUMBER SEVENTEEN SPRING 2013 Box 2.Types of Biofuels Defined by EPA Ethanol derived from comgrain cumently would fall in this category] Target:Total of 36 billion gallons by 2022(minimum of 15 Baal additional) rgetTotalof216ongalonsby2022mnmumcf4Bgaa0ional Cellulosic biofuel:Renewable fuel derived from any cellulose,hemicellulose,or lignin (components of stems.stalks,and woody parts enewable"fuel Target 16 billion gallons by 2022. mass-based diesel Any diesel fuel made from we bio gallons by 2012 and beyond. field to tailpin of the analysis,for the field will also conserve soil carbon.Com and operation of the biofuel refn A recent modeling study shows that when below are based primarily on LCAs. ,there are larg of cabon.The Direst Efferts reduce GHG ha mpened enthusiasm ends on com an CCne e of GHG emissions of both comn ethanoland carbon remains stored in th Soil organic ctices for GHG reductions relative to petroleum of up cultivation cellulosi coeaeiRmuihtedorelcasemoretoe he land dedi- plants,require no cated to comn is currently tilled.Growing com Perennial biofuel crops such as switchgrass or without tillage (no-till),alteratively,con- mixed prairie grasses actually can reduce The Ecological Society of Americaesahq@esa.org esa 5
© The Ecological Society of America • esahq@esa.org esa 5 ISSUES IN ECOLOGY NUMBER SEVENTEEN SPRING 2013 in the case of biofuels, from field to tailpipe. Results of LCAs depend partly on the spatial and temporal boundaries of the analysis, for example on the inclusion (or not) of factors such as the GHG emissions resulting from the construction and operation of the biofuel refinery. Conclusions of different studies therefore vary depending on the boundaries selected and the models used. The studies summarized below are based primarily on LCAs. Direct Effects A growing body of environmental evidence on GHG production has dampened enthusiasm for corn as a feedstock. This is due in part to the way corn is cultivated and in part to the biology of the plant itself. Corn, like many other annual crops, depends on fossil-fuel inputs for planting, harvesting, and ethanol production. Moreover, a major factor in the GHG balance of biofuel systems is how much carbon remains stored in the soil. Soil organic carbon is also necessary to maintain soil productivity. Conventional farming practices for annual crops involve tilling the soil, which substantially reduces soil organic carbon from pre-conversion levels. When the soil is tilled, microbes are stimulated to release more stored carbon into the atmosphere as carbon dioxide (CO2). According to the U.S. Economic Research Service, about 70% of the land dedicated to corn is currently tilled. Growing corn without tillage (no-till), alternatively, conserves soil carbon. Crop residue (corn cobs and corn stover - the leaves and stalks), left on the field will also conserve soil carbon. Corn production requires high inputs of nitrogen fertilizer. High nitrogen fertilization can cause corn stover to degrade more readily to CO2. 2 A recent modeling study shows that when conservation reserve or native grassland is converted to corn production using conventional tillage, there are large losses of soil carbon. The use of no-till practices greatly reduces soil carbon loss. Other agricultural practices that can reduce GHG emissions from grassland conversion include the use of slow-release fertilizer and nitrification inhibitors that have the potential to reduce soil nitrous oxide (N2O) fluxes by more than 50% (Box 3). There are roughly similar N2O emissions for corn and soybeans, and lower values for fertilized switchgrass, according to EPA. Estimates of GHG emissions of both corn ethanol and soybean biodiesel production are often only slightly lower, and sometimes higher1 , than petroleum, although some analyses suggest that best practices, if enacted, could provide GHG reductions relative to petroleum of up to 50%. In contrast to grain cultivation, cellulosic feedstocks require fewer fertilizer inputs and, because they are perennial rather than annual plants, require no tilling. Consequently they store or “sequester” carbon in the soil. Perennial biofuel crops such as switchgrass or mixed prairie grasses actually can reduce Box 2. Types of Biofuels Defined by EPA For regulatory purposes, the U.S. Environmental Protection Agency defines biofuels. Fuels developed to meet U.S. production targets must meet the requirements of those definitions. Renewable fuel: Fuel produced from renewable biomass and that is used to replace or reduce the quantity of fossil fuel present in a transportation fuel. It must also achieve a life cycle GHG emission reduction of at least 20%, compared to the gasoline or diesel fuel it displaces. [Ethanol derived from corn grain currently would fall in this category.] Target: Total of 36 billion gallons by 2022 (minimum of 15 Bgal additional). Advanced biofuel: Renewable fuel other than ethanol derived from corn starch, with life cycle GHG emissions at least 50% less than the gasoline or diesel fuel it displaces. Includes sugarcane ethanol, cellulosic ethanol, and algal biodiesel, for example. Target: Total of 21 billion gallons by 2022 (minimum of 4 Bgal additional). Cellulosic biofuel: Renewable fuel derived from any cellulose, hemicellulose, or lignin (components of stems, stalks, and woody parts of plants) each of which must originate from renewable biomass. It must also achieve a life cycle GHG emission reduction of at least 60%, compared to the gasoline or diesel fuel it displaces. Cellulosic biofuel generally also qualifies as both “advanced biofuel” and “renewable” fuel. Target: 16 billion gallons by 2022. Biomass-based diesel: Any diesel fuel made from renewable biomass feedstocks or from vegetable oils or animal fats. Its life cycle GHG emissions must be at least 50% less than the diesel fuel it displaces, and it cannot be co-processed with a petroleum feedstock. Target: 1 billion gallons by 2012 and beyond
ISSUES IN ECOLOGY NUMBER SEVENTEEN SPRING 2013 Box 3.Biofuels and Nitrous Oxide Emissions biofuels should not c tribute to GHG the Nitrogen Sources ca ed to th most of e the ad t osttomhefeldtset,butsomeislost NO in down rays and GHG source in annual cropping systems. Figure 2.Nitrous oxide (N,O)emissions from agriculture. atmospheric GHG concentrations by trans. mercial-scale production systems.A compari- forming CO:in t the armosphere to stored so in switchgrass fields harvested annually over a algae had much higher GHG e ions than This is een reporte ne use the CO.that is bubbled into the water as a car hen The CHC of nding ways to to provide CO,and wastewater from wate reatment plants as a source of nitrogen and put high iv话 on at on of ith greater than for com or ns.An addi- the overall energy produced per dpower and fuel pro I. lancckchaif c crops can ction. s in soil carbon seques are lower t e of other biodiesel feed. cd cur if he 30-40 tock according to modeling Miscanthus,a used as cattle or fish fo or fur a an high rate of root hio oduction and doe need much fertili ns for native the sustainability of algal rod sources reductions reported for ethanol to make definitive 6 esa The Ecological Society of Americaesahq@esa.org
ISSUES IN ECOLOGY NUMBER SEVENTEEN SPRING 2013 6 esa © The Ecological Society of America • esahq@esa.org atmospheric GHG concentrations by transforming CO2 in the atmosphere to stored soil organic carbon. Soil organic carbon increased in switchgrass fields harvested annually over a five-year period.3 Similar carbon storage findings have also been reported for “low input, high diversity” vegetation.4 These crops, which require little or no fertilizer input, stored more than 30 times as much carbon in soil and roots as monoculture soybean and corn crops. After accounting for the release of CO2 from fossil fuel combustion during all phases of production and processing, the GHG emissions reduction attributable to low input high diversity biomass was 6 to 16 times greater than for corn or soybeans. An additional benefit to the GHG balance is that N2O output by cellulosic crops can be half that for grain feedstocks. Substantial increases in soil carbon sequestration could occur if the 30-40% of current U.S. lands used for corn were replaced with cellulosic perennial grasses, according to modeling studies. Miscanthus, a perennial grass native to Africa and Asia, for example, has high soil carbon storage, because it has a high rate of root biomass production and does not need much fertilizer. Projections for native North American switchgrass, on the other hand, are less optimistic. Growing switchgrass without fertilization significantly decreases soil carbon and nitrogen. Algal biofuels production systems are in the early stages of development, and while a number of pilot scale production facilities are being constructed, it is difficult to make definitive statements about the GHG emissions of commercial-scale production systems. A comparison of the potential environmental impacts of algae with other biofuel feedstocks found that algae had much higher GHG emissions than corn, switchgrass, or rapeseed.5 This is attributable to the use of petroleum-based fertilizers for algal culture and to energy required to produce the CO2 that is bubbled into the water as a carbon source for the growing algae. The GHG balance might be improved by finding ways to recycle flue gas from fossil fueled power plants to provide CO2 and wastewater from water treatment plants as a source of nitrogen and phosphorus for the algae. Co-location of algae production systems with power plants could also increase the overall energy produced per CO2 from the combined power and fuel production. According to EPA, air pollutant emissions associated with algal biodiesel production are lower than those of other biodiesel feedstocks and much lower than emissions from corn ethanol production. Waste material from algal biofuels production could potentially be used as cattle or fish food, or further digested to make syngas or other biofuels. A National Research Council report, Sustainable Development of Algal Biofuels in the United States, addresses the sustainability of algal biofuels in detail. Liquid fuels produced from forest products such as wood or other biomass residues may reduce net GHG emissions more than fuels produced from agricultural sources like fastgrowing poplars or other short-rotation woody plants. GHG reductions reported for ethanol produced from woody biomass, compared with gasoline, range from 51% to 107%.6 However, Box 3. Biofuels and Nitrous Oxide Emissions In theory, biofuels should not contribute to GHG buildup in the atmosphere because the plants grown for fuel take up carbon dioxide (CO2) as they grow, offsetting carbon released to the atmosphere when the fuels are burned. However, although CO2 gets most of the attention, there is another GHG of serious concern (see Figure 2). Only about half of the nitrogen applied to a grain crop is taken up by plants while the remainder is lost to the environment. Some of the nitrogen ends up in the atmosphere as nitrous oxide (N2O), a greenhouse gas 300 times more potent than CO2. Most of the N2O is lost from the field itself, but some is lost indirectly after nitrate leached from the fields is transformed to N2O in downstream waterways and wetlands. Row crop agriculture is the largest human source of N2O globally, and N2O loss is often the biggest GHG source in annual cropping systems. Figure 2. Nitrous oxide (N2O) emissions from agriculture. Nitrogen Sources Direct N2O Emissions Indirect N2O Emissions Synthetic N Fertilizer N Fixing Crops Other Sites Urine and dung Crop Residues Residue Burning N Volatilization and Re-deposition, and N Leaching and Runoff