Position Statement on. Biofuel. crops.org USDA-ARS

Transcription

1 Position Statement on Biofuel crops.org USDA-ARS

2 Introduction Over the past decade, unstable petroleum prices and Federal policies aimed at reducing U.S. dependency on oil imports have led to rapid growth in biofuel demand. In turn, these policies have led to an increase in the production of agricultural commodities that serve as feedstock for biofuels. The production of bioenergy from biomass feedstocks is unique as it is the only renewable energy source that can be converted to a liquid transportation fuel. The Crop Science Society of America (CSSA) recognizes the significant role of biofuels as a renewable energy source and as a facilitator of rural economic growth. Our scientists and students play a vital role in this process by contributing new technologies and management strategies that assist energy producers in achieving or surpassing current renewable fuel standards. In addition, crop science and its component disciplines integrate basic and applied research in order to achieve a holistic, systems-based approach to producing sustainable biofuel feedstock. The integration of public and private sector contributions into the development and transfer of related technologies allows our crop scientists to continually optimize the production of feedstocks. In addition, the future sustainment of feedstock production for the enhancement of rural economies can only be accomplished by capitalizing on multi-sector relationships, training, and continued advances in crop science. Background The 2005 Energy Policy Act (EPAct) established The Renewable Fuel Standard (known as RFS1), which authorized the production of renewable biofuels in order to reduce U.S. reliance on foreign oil. RFS1 required 7.5 billion gallons of renewable fuel to be blended into gasoline by This goal was met with great success and in 2010 the U.S. produced more than 13 billion gallons of ethanol for blending. (see figure below) The 2007 Energy Independence and Security Act (EISA) took RFS1 a step further and increased the goal for blending renewable fuels into transportation fuel to 36 billion gallons by 2022 (known as RFS2). RFS2 also established new categories of renewable fuel (i.e. advanced biofuel, biomass-based diesel, and cellulosic biofuel) and required life-cycle greenhouse gas (GHG) performance threshold standards that demonstrate lower GHG emissions than petroleum. Because of these changes, conventional biofuels (specifically corn ethanol) and advanced biofuels are now categorized separately. By January 2012, the United States had 209 ethanol plants on line with a total operating capacity of 14.0 billion gallons per year. 1 However, reaching 36 billion gallons by 2022 will require the identification of more efficient technologies, the cultivation of new feedstocks, and the implementation of efficient and sustainable cropping systems. CSSA s objectives for this position statement include: identifying challenges and opportunities within the biofuels sector, providing practical knowledge from the field that can enhance and inform biofuel production, and inviting a discussion to develop solutions for meeting the challenges of increasing renewable fuel supplies. CSSA looks forward to feedback from government, industry, science, and concerned citizen stakeholders regarding these issues. istock

3 Challenges 1. Meeting the Goals of RFS2 For each compliance year under the EISA, the U.S. Environmental Protection Agency (EPA) is required to determine the applicable renewable fuel percentage standards. The final percentage standards for 2011 and 2012 require blending the following: Meeting these targets for bioenergy production will require major advancements in the feedstock supplies, conversion technologies, and a rapid evolution of the cellulosic biofuel feedstock supply chain. 4 Conventional bioenergy feedstocks contain starch and sugars that easily ferment to produce ethanol and are used in combustion. Corn grain starch, in particular, is converted into glucose via enzymatic hydrolysis and subsequent glucose fermentation to make ethanol. Currently, corn ethanol comprises about 10% of the volume and 7% of the energy content of domestic gasoline consumed. 5 Yet, like all other biofuels, corn ethanol is simply one component of a diverse energy portfolio. At current corn grain production levels, the United States cannot substitute more than 15% of the country s transportation fuels with ethanol. 6 Although the United States has developed ethanol production from corn, and in some cases energy cane, additional feedstock sources are needed from an increasingly varied set of crops and land in order to produce the volume of biomass necessary to meet the EISA s goals by Cellulosic Standards First generation renewable biofuel Advanced biofuels Biomass-based diesel Cellulosic ethanol Million Gallons While the U.S. is on track to meet the 2022 mandates for conventional biofuels and biodiesel, there is a great deal of uncertainty regarding the development of cellulosic biofuels. In 2010 and 2011, the total volume of cellulosic biofuels qualifying under the RFS2 standard for 2010 and 2011 was zero gallons. 7 In 2012, the EPA raised the mandate for cellulosic biofuels by 2.65 million gallons over the 2011 level to a standard of million gallons. 8 However, this target falls far short of the 500 million gallons originally planned for Further, where the projected volume is less than the mandate required by EISA, the EPA is required to make waiver credits available for sale to obligated parties (generally, an obligated party is any refiner or importer of gasoline or diesel fuel in the U.S.). In 2010, obligated parties purchased 12,186 RFS2 cellulosic biofuel waiver credits. 9 According to a 2011 report by The National Academies, [without] major technological innovation or policy changes, the RFS2-mandated consumption of 16 billion gallons of ethanolequivalent cellulosic biofuels is unlikely to be met in In addition, at the time of publication, no commercially viable biorefineries exist[ed] for converting lignocellulosic biomass to fuels. 11 The report goes on to conclude that the capacity for producing cellulosic biofuels to meet the RFS2 consumption mandate will not be available unless innovative technologies are developed that unexpectedly improve the cellulosic biofuel production process, and technologies are scaled up and undergo several commercial-scale demonstrations in the next few years to optimize capital and operating costs. 12 The CSSA agrees with The National Academies findings and further concludes that without increased funding and support for research in crop science, we will fail to stimulate the advancements in bioenergy production that are necessary to develop and optimize feedstocks and sustainable cropping systems in order to meet the 2022 mandate. 3. Sustainable Biofuel Production and Lifecycle GHG Analyses The EISA set the first U.S. mandatory lifecycle GHG reduction thresholds for each of the four types of renewable fuel thereby requiring a percentage improvement compared to a baseline of gasoline and diesel. 13 According to the EISA, renewable fuels produced at new facilities require a 20% reduction in lifecycle GHG emissions, biomass-based diesel or advanced biofuel require a 50% reduction, and cellulosic biofuels require a 60% reduction. 14 In addition, the EISA provided EPA some flexibility in adjusting these percentage thresholds under certain circumstances. 15 Lifecycle GHG Thresholds Specified in EISA* (percent reduction from 2005 baseline) Renewable fuel, 20% Advanced biofuel, 50% Biomass-based diesel, 50% Cellulosic biofuel, 60% * renewablefuels/420f10007.pdf

4 The EISA defines lifecycle GHG emissions as follows: The term lifecycle greenhouse gas emissions means the aggregate quantity of greenhouse gas emissions (including direct emissions and significant indirect emissions such as significant emissions from land use changes), as determined by the Administrator, related to the full fuel lifecycle, including all stages of fuel and feedstock production and distribution, from feedstock generation or extraction through the distribution and delivery and use of the finished fuel to the ultimate consumer, where the mass values for all greenhouse gases are adjusted to account for their relative global warming potential. 16 The National Risk Management Research Laboratory further describes life-cycle analysis as a technique used to assess the environmental aspects and potential impacts associated with a product, process, or service, by: compiling an inventory of relevant energy and material inputs and environmental releases; evaluating the potential environmental impacts associated with identified inputs and releases; and interpreting the results to help inform decisions. However, the models originally used were not designed for this purpose. 17 In July 2011, EPA attempted to address this issue and finalized a rule deferring application of the greenhouse gas regulations to biomass emissions for three years. 18 In its explanation for deference, the EPA cited the complexity and uncertainty of attempting to determine the net carbon cycle impact of particular facilities combusting particular types of biomass feedstocks. 19 Some studies have concluded that the total emissions from biomass-based energy generation depend primarily on the type of resource used, the production methods employed to grow and harvest the resource, and the time required to re-grow the biomass resource. 20 Because of this complexity, life-cycle analysis is a key factor of research design when developing and improving current feedstocks or designing cropping systems appropriate for biofuel feedstock production. As a result, although yield levels are critical, crop scientists also consider ways to improve input efficiency in feedstock production systems. Diverse teams of researchers have found that nutrient recovery and efficiency is possible in starch and cellulosicbased bioenergy production. A review of inputs over time in corn has documented modest to significant improvements in energy use and carbon emissions per bushel. 21 Additionally, switchgrass experiments show that approximately 78% of the nitrogen fertilizer input required can be recovered with the cellulosic ethanol cropping system. Crop development provides the resources and knowledge necessary for ensuring consistent and reliable advances in feedstock yield and quality. In the crop development process, specific cultivar lines are selected by examining traits for fertilizer and water use efficiency, yield, and biomass quality. An ideal feedstock will have a good ratio of chemical and biological components for the greatest yield, the finest fuel quality, the lowest transportation costs, and the lowest lifecycle GHG emissions. Crop breeders, physiologists, and biotechnology scientists balance traits valuable for feedstock quality such as disease and pest resistance in the field with logistical properties such as dry and wet weights. In addition, through the judicious use of both traditional crop breeding and biotechnological approaches, crop scientists can determine the most effective and proven means of increasing yields. These improvements are vital to the long-term sustainability and economic viability of bioenergy production systems. 4. Food and Fuel Security Increased demand for biofuels has raised concerns about rising food prices as well as food and fuel security. However, bioenergy crops can be integrated into food and feed cropping systems or established as designated systems that subsist exclusively as sustainable feedstock. Recent advances raise the possibility that biofuels can be made from non-edible plants engineered to grow on land that has been abandoned for agricultural use in order to avoid compromising food security. 22 Corn and sugarcane research have demonstrated that grain or cane can be harvested for food or feed while also producing residue that can be used as a digestible cellulosic feedstock. 23 In addition, germplasm for these crops has been assessed for key agronomic and biofuel feedstock traits across a large gene pool and improvements have been made. For example, genetic research in energy sorghum has elucidated a set of maturity genes that are easily manipulated and strongly influence biomass yield potential of the crop. 24,25 Research can enhance food and fuel security by optimizing crop production on arable land, using crop residues as feedstocks, or by developing designated feedstock production on marginally productive cropland. Agricultural researchers, crop science educators, and crop professionals play key roles in training farmers about crop rotations and nutrient management techniques that enable intense production while minimizing environmental consequences. 26 Herbaceous perennial energy crops generally demand less fossil fuels, herbicides, and fertilizer than traditional row crops because they do not need to be planted annually. 27 While more research is necessary, it is apparent that these perennial cropping systems can produce large quantities of biomass while also providing important water and wildlife related ecosystem services. Annual bioenergy crops are also important to the complete production system. These crops provide production in the establishment year, protect against crop failure, and avoid the stand establishment issues that can occur in perennial crops. Recent development and deployment of sweet sorghum and biomass sorghum hybrids have exhibited great potential as high yielding and complementary biomass feedstocks. 28 In general, annual bioenergy crops are used in crop rotation systems that allow for their integration into existing food/feed production systems.

5 Sustainable cropping systems are economically viable without compromising future agricultural land use. These systems maintain or enhance soil productivity, water quality, and agroecosystem functions. Research has shown that bioenergy cropping systems achieve sustainability by providing an additional source of income to farm operations while also mitigating GHG emissions, building soil health, and increasing biomass yields. 29 Today s systems are being redesigned to improve nutrient, water, and energy-use efficiencies. Because of the innovative work of crop scientists and their colleagues in other disciplines, bioconversion processes are being configured to recover key plant nutrients from biomass and soil that allow for increased nutrient recycling, closed nutrient cycles and reduced energy and economic costs of production Building Equity and Demand in the Marketplace with Economic and Social Sustainability Where there is a demand to fill, farmers will respond. There is currently a distinct need for a marketplace where consumer demand for biofuel feedstock is consistent and obvious to farmers. This means making the economic benefits of feedstock production clear and easy to understand. While the corn and sugar cane ethanol market is burgeoning, the cellulosic sector is significantly hampered by low product supply. Market barriers in this sector include cost and competitive uses, large capital investment requirements for conversion facilities, a lack of risk management tools for bioenergy crops (including emerging crops and uses), and an inadequate supply-chain infrastructure. These barriers could, if left unaddressed, limit the United States capability to meet the blending goals laid out in the EISA for Cropping system production efficiency is a key area of concern for the Crop Science Society of America. The CSSA Grand Challenges for research released in 2011 underscore the importance of increasing production efficiencies in bioenergy systems. The Grand Challenges state that biofuel feedstock production research should: develop sustainable biofuel feedstock cropping systems that require minimal land area, optimize production, and improve the environment. As a result, there is a need to: modify crop compositions according to processing requirements; increase yield in low-input production systems; understand plant response to changes in the environment, in tandem with changes to composition for accurate modification; understand the ecosystem services (carbon sequestration, water quality, wildlife habitat, etc.) from perennial bioenergy crop production on arable and marginal lands; and develop new production systems that thrive in lowinput situations. For more information, please view the grand challenge at: The costs of biomass residue production are not trivial and include the cost of harvest, labor, fertilizer, and pesticide inputs. In the case of dedicated feedstocks, the production costs also include site preparation, fertilization, and maintenance. These expenses are influenced by yield per acre, fuel price, fertilizer price, labor, equipment availability, and financing. Market conditions must offer a feedstock price that compensates for these costs and creates incentives for farmers to integrate bioenergy feedstocks into their production systems. To address this issue, the Department of Energy s Biomass Multi-Year Program Plan is targeting conversion processes that can produce additional biofuels cheap enough to compete with gasoline at the pump, while at the same time offering sufficient feedstock prices to support economically viable biomass production. 31 Michael Casler

6 Market level solutions to maximize the biofuel value chain at the local level while improving rural development will need to include: alignment of biomass production with the scale of cellulosic biorefineries; efficient use of existing locally-owned conventional equipment to store and handle densified biomass; reduced transportation costs; reduced biorefinery conversion costs; and development of biofuel production and marketing systems. Numerous opportunities for collaboration between public and private sectors exist that could accelerate the development of the bioeconomy without compromising the long-term sustainability of economic, environmental, or social factors of the sector. These challenges are surmountable at the local level, and the economic and environmental opportunities associated with bioenergy production, especially those in rural areas, appear to far outweigh the uncertainties. 6. A Trained Bioenergy Workforce New conversion technologies have been developed based on the assumption that the bioeconomy will create a domestic supply of renewable biofuel and increase rural economic development. 32,33 However, to launch many of these technologies into commercialized production, there must be an available agricultural labor force. The contribution of biofuels production to increasing jobs and income will be substantial according to several economic reports {Swenson, 2007, Estimating the future economic impact of corn ethanol production in the U.S.}. 34 An estimate of the effects of increasing corn ethanol production to 14.5 billion gallons per year includes: 9000 direct jobs with payroll of $0.5 billion; payments for inputs of $17 billion requiring 11,600 jobs and payroll of $0.63 billion; and household purchases by workers of $3.5 billion sustaining 26,700 jobs and payroll of $1 billion. Total economic contributions include 47,200 jobs with $2.2 billion in labor income. If RFS2 biofuel targets are met, production would more than double, and job and income increases would be on the same order of magnitude. Training and deploying a qualified workforce for the bioenergy industries is no small task. These industries require personnel with a diverse and transformative set of skills ranging from crop physiology and plant breeding to economics. Certified professionals and scientists that can offer consulting services, develop new feedstocks appropriate to conversion technologies, and continually refine the approaches for producing bioenergy crops and cropping systems are increasingly in demand. CSSA has identified two main areas that require ongoing support from public and private institutions in order to ensure the success and growth of this sector: 1) continued support for basic and applied research and extension programs, and 2) support for STEM education programs and workforce training programs that will support the development of a qualified workforce for the bioenergy industry. Research, education, and extension programs within the United States Department of Agriculture s Research, Education, and Economics Mission Area are currently in place that develops educational and training materials appropriate to the needs of the bioenergy sector. CSSA urges the federal government to acknowledge and build upon the essential research conducted in such programs to build better science, technology, engineering, and mathematics education approaches that support the training of the next generation of experts who will be essential for developing and shaping the bioenergy sector. In order to succeed, more university-industry partnerships at the bachelor, master, and PhD levels are needed. Scholarships, fellowships, or internships could be funded by public or joint public-private foundations. Increased emphasis on recruitment needs to occur by interacting with guidance counselors and high-school level educators so that students enter the pipeline early. Community colleges also play a significant role in training students for the industry as not all jobs require technical training at or above the bachelor level. Further development of the science-business interface is needed to educate faculty, students, and post-doctorates about commercialization strategies, including the marketing of innovative ideas for the purpose of acquiring capital, venture or otherwise. Federal support for better use of university-based, industry-supported innovation incubators would help to advance the development of the sciencebusiness interface. Conclusion Today s advances in crop science provide farmers, agricultural consultants, and feedstock processers with robust options for sustainable bioenergy feedstock systems. To continue to meet future demands, collaborative and innovative transdisciplinary teams of researchers and private sector experts need to be supported to improve current feedstocks, identify new feedstocks, and design approaches to maximize the efficiency of bioenergy cropping systems. Additionally, accurate scientific information on productivity and life-cycle analysis models is needed to guide policy and regulations governing bioenergy use. These efforts will enhance the use-efficiency of agronomic inputs and limit greenhouse emissions from these systems.

7 Examples of bioenergy crops and uses: Switchgrass is a perennial warm-season grass native to North America well adapted to marginally-productive cropland similar to land enrolled in the Conservation Reserve Program (CRP). In 1984, the U.S. DOE funded a multi-year screening of herbaceous species at 31 sites in the primary agro-ecoregions of the U.S. to determine their suitability for biomass energy production. Switchgrass, second generation feedstock, does require management to optimize productivity and maintain quality stands. Best management practices for switchgrass have been developed for most agro-ecoregions of the U.S. east of the 100th Meridian. Switchgrass genetics have been enhanced through population improvement. Energy cane is developed from early generations of the sugar cane hybrid breeding program which have lower sugar yields, but higher biomass yields, making it an ideal cellulosic feedstock. These energy cane hybrids produce 10 to 20 tons of biomass per acre in the Gulf Coast region and have persisted in most of the southeast as well. 35 Production practices for sugar cane can be easily adapted for energy cane. Miscanthus species are being evaluated for biofuel and electricity cogeneration with coal. Although the cost to plant Miscanthus giganteus is nearly $1,200 per acre and initial yields are low, it has a high return on investment because it requires fewer agronomic inputs than other annual row crops commonly grown in the Midwest. Additionally, in the second year of production, Miscanthus giganteus can produce six to eight tons of biomass per acre, 36 and 10 tons of biomass per acre by year three. 37 Right now, Miscanthus strains are being developed that are available for planting via seed. 38 While Miscanthus produces more biomass per unit than other herbaceous perennials, the cost of establishment and maintenance requirements may limit producer acceptance on a large scale until non-invasive, seeded varieties can be developed. Limitations also exist for this otherwise promising species because of the lack of field data available to develop coherent business plans or and life-cycle analysis. feedstocks, the biomass sorghum types accumulate significant quantities of structural carbohydrates and are intended to be used to complement those perennial feedstock production systems. All of these sorghums are annuals, meaning that they are established each year for production in that year. Short-rotation woody crops are also an attractive bioenergy feedstock. With the potential for production on marginal lands, woody crops are characterized by rotations of 3 10 years and offer logistical advantages for year-round delivery of biomass. Willow (in the Northeast) and hybrid poplar (in the North-Central and Pacific Northwest) are currently the more developed species. While biomass yields of 4 7 tons per acre are typically reported, elite varieties are currently being evaluated through a partnership with DOE. Preliminary results suggest that elite varieties may have significant yield improvements in certain regions of the country. Rich genetic diversity in both species promise continued gains in growth rates, and the potential to tailor properties for optimal conversion performance in moving forward. Woody crops are projected to play a major role in the Southeast. Exciting progress has been made with eucalyptus. This highly productive species yields up to twelve tons of biomass per acre but is sensitive to cold weather. However, cold-tolerance is encouraging and could expand the range of eucalyptus to a significant area across the region. Southern pine occupies more than 30 million acres of plantation land across the south and holds considerable promise as a biofuel feedstock. Production technologies specific to bioenergy feedstock have received only limited attention, although integrated systems which produce herbaceous energy crops between forested pine are of great interest. Also, recent advances in control of cell wall chemistry to reduce recalcitrance and research toward increased production of isoprenoid compounds for fuel suggest that southern pine will make important contributions to the region s biofuels industry. Sorghum as a bioenergy crop has been developed for three distinct conversion processes. Approximately 30% of U.S. grain sorghum is used for ethanol production. Sweet sorghum accumulates significant quantities of soluble, fermentable sugar in the stalk of the plant. This crop can be processed in a similar manner to sugarcane. It is currently being evaluated to extend the mill season in the Brazilian sugarcane industry and in targeted regions of the U.S., the production of both sweet sorghum and sugarcane (energycane) is essential to have a harvest season long enough to justify capital investment in infrastructure and conversion facilities. Like most of the perennial biomass istock/jan-otto

8 Endnotes 1 Renewable Fuel Association, 2 Under the Clean Air Act Section 211(o), as amended by the EISA of 2007, the Environmental Protection Agency (EPA) was required to set the annual standards under the Renewable Fuel Standard program (RFS) each November for the following year based on gasoline and diesel projections from the Energy Information Administration (EIA). renewablefuels/420f10056.htm 3 pdf CFR, Vol. 77, No. 5, Monday, January 9, web.pdf 5 United States Department of Energy. Report on the First Quadrennial Technology Review, P. 58 (Sept., 2011). ( gov/downloads/report-first-quadrennial-technology-review or energy.gov/sites/prod/files/qtr_report.pdf) 6 (Houghton et al., 2006) pdf CFR, Vol. 77, No. 5, Monday, January 9, htm 10 Renewable Fuel Standard: Potential Economic and Environmental Effects of U.S. Biofuel Policy, Committee on Economic and Environmental Impacts of Increasing Biofuels Production; National Research Council (2011). Page Renewable Fuel Standard: Potential Economic and Environmental Effects of U.S. Biofuel Policy, Committee on Economic and Environmental Impacts of Increasing Biofuels Production; National Research Council (2011). Page Renewable Fuel Standard: Potential Economic and Environmental Effects of U.S. Biofuel Policy, Committee on Economic and Environmental Impacts of Increasing Biofuels Production; National Research Council (2011). Page Clean Air Act Section 211(o)(1) 17 For example, GTAP was built to analyze trade policy; FAPRI was used to model agriculture policy and farm bill implications; and, FASOM was built by the EPA to analyze the forestry sector. 18 Deferral for CO2 Emissions from Bioenergy and Other Biogenic Sources Under the Prevention of Significant Deterioration (PSD) and Title V Programs, 76 Fed. Reg. 43,490 (July 20, 2011) (to be codified at 40 C.F.R. pts. 51, 52, 70, and 71). 19 Deferral for CO2 Emissions from Bioenergy and Other Biogenic Sources Under the Prevention of Significant Deterioration (PSD) and Title V Programs, 76 Fed. Reg. 43,490 (July 20, 2011) (to be codified at 40 C.F.R. pts. 51, 52, 70, and 71) at 43, MANOMET CTR. FOR CONSERVATION SCIS., BIOMASS SUSTAINABILITY AND CARBON POL Y STUDY (2010), Environmental Resource Indicators for Measuring Outcomes of On-Farm Agricultural Production in the United States, First Report, January Keystone Alliance for Sustainable Agriculture Field-to-Market_Environmental-Indicator_First_Report_With_ Appendices_ pdf 22 post/2012/05/plant-based-biofuels-could-power-nearly-a-third-ofvehicles/1#.t6e_ysxllgm 23 Lorenz, A.J., J.G. Coors, C.N. Hansey, S.M. Kaeppler, and N. de Leon.* Genetic Analysis of Cell Wall Traits Relevant to Cellulosic Ethanol Production in Maize (Zea mays L.) 24 Rooney, W.L., J. Blumenthal, B. Bean, and J.E. Mullet Designing sorghum as a dedicated bioenergy feedstock. Biofuels Bioprod. Bioref. 1: Murphy, R.L., R.R. Klein, D.T. Morishige, J.A. Brady, W.L. Rooney, F.R. Miller, D.T. Dugas, P.E. Klein, and J.E. Mullet Sorghum PRR37 (Ma1) Controls Flowering in Response to Photoperiod through Coincident Light/Clock Regulation. Proc. Natl. Acad. Sci. 108: Burney, J.A., S.J. Davis, and D.B. Lobell Greenhouse gas mitigation by agricultural intensification. Proc. Natl. Acad. Sci. USA 107: Schmer, M.R., K.P. Vogel, R.B. Mitchell, and R.K. Perrin Net energy of cellulosic ethanol from switchgrass. Proc. Natl. Acad. Sci. 105: Rooney, W.L., J. Blumenthal, B. Bean, and J.E. Mullet Designing sorghum as a dedicated bioenergy feedstock. Biofuels Bioprod. Bioref. 1: Somerville, C., H. Youngs, C. Taylor, S.C. Davis, and S.P. Long. (2010. Feedstocks for Lignocellulosic Biofuels. Science 329(5993): Anex, R.P., L.R. Lynd, M.S. Laser, A.H. Heggenstaller, and M. Liebman Potential for Enhanced Nutrient Cycling through Coupling of Agricultural and Bioenergy Systems. 31 DOE. april_2011.pdf 32 Berndes, G., M. Hoogwijk, and R. van den Broek The contribution of biomass in the future global energy supply: a review of 17 studies. Biomass Bioenerg. 25: Hoogwijk, M., et al Exploration of the ranges of the global potential of biomass for energy. Biomass Bioenerg. 25: Swenson, D Estimating the future economic impact of corn ethanol production in the U.S. In: Web papers. Iowa State University. p Perlack, R.D., and B.J. Stokes U.S. Billion-Ton Update: Biomass Supply for a Bioenergy and Bioproducts Industry. Oak Ridge National Laboratory, Oak Ridge, TN. p Perlack, R.D., and B.J. Stokes U.S. Billion-Ton Update: Biomass Supply for a Bioenergy and Bioproducts Industry. Oak Ridge National Laboratory, Oak Ridge, TN. p Heaton, E., T. Voight, and S.P. Long A quantitative review comparing the yields of two candidate C4 perrennial biomass crops in relation to nitrogen, temperature, and water. Biomass Bioenerg. 27: Mitchell, R.B., V. Owens, N. Gutterson, E.P. Richard, and J.N. Barney Herbaceous perennials: placement, benefits and incorporation challenges in diversified landscapes. In: R. Braun, D. Karlen, and D. Johnson, editors, Sustainable Alternative Fuel Feedstock Opportunities, Challenges and Roadmaps for Six U.S. Regions. Soil & Water Cons. Soc., Ankeny, IA. p Schmer, M.R., K.P. Vogel, R.B. Mitchell, and R.K. Perrin Net energy of cellulosic ethanol from switchgrass. Proc. Natl. Acad. Sci. 105: Mitchell, R., K.P. Vogel, and D. Uden The feasibility of switchgrass for biofuel production. Biofuels 3: Guilford Road, Madison, WI