Plants and Fuel

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Graduate Student Jacob Jungers looks to the future atop prairie biofuel bales.
David Tilman

Experimental studies by Cedar Creek scientists revealed that high-diversity mixtures of perennial prairie plants grown on nutrient-poor lands with no fertilization or irrigation have potential for use as a biofuel crop. Such a crop could offers many advantages over food-crop based biofuels, including net carbon storage, lower land use requirements, and reduced particulate emissions.

The search for alternate energy sources is growing in economic importance as fossil fuel supply rapidly depletes and concerns about its environmental effects grow. Various forms of biofuel have been incorporated into the United States industry, especially ethanol and biodiesel derived from monocultures of corn and soybean. Another promising source of biofuel is high-diversity prairie biomass. On the surface, prairie biomass solves some of the problems posed by using corn and ethanol biomass. Where corn and soybeans use fertile cropland, and may encourage the conversion of wooded ecosystems to agricultural land, prairies grasses can grow on abandoned degraded agricultural land. Where corn and soybeans require large amounts of pesticides and fertilizers, diverse prairie communities are more resistant to pest invasion and legumes provide nitrogen. Where corn and soybeans need to be replanted annually, prairie grasses regrow from below-ground root systems. (Tilman et al SCIENCE 2009) To further explore the viability of using prairie biomass as a biofuel source, and to compare its efficiency with corn and soybean biofuel, Cedar Creek scientists calculated the net energy gain from corn, ethanol, and prairie plots of varying biodiversity. They also calculated certain environmental effects, such as greenhouse gas reduction (Hill et al. PNAS 2006).

Prairie biofuel analysis: Randomly selected combinations of 1, 2, 4, 8, or 16 perennial herbaceous grassland species were planted in 152 prairie plots. Plots were grown with low inputs, unfertilized, and irrigated only during establishment; all plots were burned in early spring to remove aboveground biomass before growth began. Aboveground biomass was harvested and sampled annually. Net energy gain was calculated as estimated energy use subtracted from estimated energy yield. Energy use was estimated as the amount of energy used in prairie biomass production and in converting prairie biomass to biofuel. Energy used in prairie biomass production was estimated assuming standard agricultural practices for growing, harvesting, and transporting prairie biomass. Energy used in converting prairie biomass to biofuel was modeled in three scenarios: co-fired with coal, converted to ethanol, or gasified and converted to both synfuel and electricity. To estimate GHG reduction, GHG emissions were added to the GHG savings both from displacing fossil fuel and from the net GHG sink that occurs when prairie grasses store carbon underground in deep root systems. (Tilman et al. SCIENCE 2006)

Biofuels derived from low-input high-diversity (LIHD) mixtures of native grassland perennials can provide more usable energy, greater greenhouse gas reductions, and less agrichemical pollution per hectare than can corn grain ethanol or soybean biodiesel. Because LIHD biomass can be produced on abandoned agricultural lands, LIHD biofuels need neither compete for fertile soils with food production nor encourage ecosystem destruction. LIHD biomass can produce carbon-negative biofuels and can reduce agrichemical use compared with food-based biofuels. Moreover, LIHD ecosystem management may provide other ecosystem services, including stable production of energy, renewal of soil fertility, cleaner ground and surface waters, wildlife habitat, and recreation. (Tilman et al. SCIENCE 2006, 2009)

Effects of plant diversity on biomass energy yield and CO2 sequestration for low-input perennial grasslands. (A) Gross energy content of harvested aboveground biomass (2003–2005 plot averages) increases with plant species number. (B) Ratio of mean biomass energy production of 16-species (LIHD) treatment to means of each lower diversity treatment. Diverse plots became increasingly more productive over time. (C) Annual net increase in soil organic carbon (expressed as mass of CO2 sequestered in upper 60 cm of soil) increases with plant diversity as does (D) annual net sequestration of atmospheric carbon (as mass of CO2) in roots of perennial plant species. Solid curved lines are log fits; dashed curved lines give 95% confidence intervals for these fits.
Tilman- et al Science 2006
For further reading: 
Hill, J.; Nelson, E.; Tilman, D.; Polasky, S.; Tiffany, D. 2006. Environmental, economic, and energetic costs and benefits of biodiesel and ethanol biofuels. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 103:11206-11210.
Tilman, D.; Hill, J.; Lehman, C. 2006. Carbon-negative biofuels from low-input high-diversity grassland biomass. SCIENCE 314:1598-1600.
Tilman, D.; Hill, J.; Lehman, C.; 2007 Response to comment on "Carbon-negative biofuels from low-input high-diversity grassland biomass". SCIENCE 316:1598-1600.
Tilman, D; Socolow, R; Foley, J; Hill, J; Larson, E; Lynd, L; S, Pacala; Reilly, J; Searchinger, T; Somerville, C; Williams, R; 2009 Beneficial biofuels - the food, energy, and environment trilemma. Science 325:270
Tilman, D; Socolow, R; Foley, J; Hill, J; Larson, E; Lynd, L; S, Pacala; Reilly, J; Searchinger, T; Somerville, C; Williams, R; Beneficial biofuels - the food, energy, and environment trilemma. 2009. Science 325:270 2009
Fargione, J; Hill, J; Tilman, D; Polasky, S; Hawthorne, P, "Land clearing and the biofuel carbon debt", SCIENCE, p. 1235, vol. 319, 10.1126/science.115274 2008
Fargione, J; Hill, J; Tilman, D; Polasky, S; Hawthorne, P, "Biofuels: Putting current practices in perspective - Response", SCIENCE, p. 1420, vol. 320 2008
Fargione, J; Hill, J; Tilman, D; Polasky, S; Hawthorne, P, "Biofuels: Effects on land and fire - Response", SCIENCE, p. 199, vol. 321. Published 2008
Hill, J; Polasky, S; Nelson, E; Tilman, D; Huo, H; Ludwig, L; J, Neumann; Zheng, H; Bonta, D; Climate change and health costs of air emissions from biofuels and gasoline. 2009. Proceedings of the National Academy of Sciences 106:2077 2009
Tilman, D; Socolow, R; Foley, J; Hill, J; Larson, E; Lynd, L; S, Pacala; Reilly, J; Searchinger, T; Somerville, C; Williams, R; Biofuels: Steer clear of degraded lands-Response. 2009. Science 326:1346 2009
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