Developing soybeans as a source of sustainable food and energy

Background/Question/Methods . In an effort to address the need for efficient, sustainable and environmentally responsible energy sources, we are developing soybeans as a source of both biofuel and food products. Soybeans are currently the second most cultivated crop in the US, behind corn. Soybean seeds are also currently cultivated for ethanol production, but growing seeds for ethanol requires converting land from food to energy production. To promote sustainable land use and a dual-use for soybeans as both a food and energy crop, we are investigating soybean crop residues (stems, enriched in cell wall polymers) as a source of biomass to produce biofuel through pyrolysis. Five cultivars of soybeans were grown in summer 2011 in a field plot in Rockbridge County, Virginia. Samples were removed at full height and again at reproductive maturity to measure growth traits of shoot components (main stem, branches, and fruits). Stem samples were also collected to measure cell volumes of thick- and thin-walled cells and to relate cell wall traits to biofuel yields. The goal for the first part of the project was to select a cultivar with cell wall traits known to promote high biofuel yield (high cellulose:lignin ratio) while maintaining seed yield.

Results/Conclusions . Maximizing biofuel yield from soybean stem residues requires identifying a cultivar that produces high yields of both stems and seeds. The five cultivars we planted showed significant variations in these traits. Cultivar Teejay produced the smallest (thinnest diameter, lowest dry mass) main stems while cultivar Hutcheson had the largest (widest diameter, lowest dry mass) main stems at the time of final harvest. While large stem diameters are an ideal trait when considering economics of fuel production, ease of harvest must also be considered as soybean stems with larger, tougher diameters are known to jam harvesting columbine machines. Hence, we are considering both size of stems and also yield of biofuel-producing cells in the stems. Cells that produce the most favorable ratio of high cellulose:lignin (parenchyma cells) for biofuel production through pyrolysis occur in the middle of the stem, surrounding the lower quality (lignin-enriched) vascular bundle cells. Our analysis of these structures revealed significant variation in amounts of parenchyma and vascular bundle cells, along with reproductive yield, among the cultivars. Our results indicate that cultivar-specific differences in stem biomass and stem cell wall traits should be considered when identifying crop plants such as soybeans for dual-purpose energy and food agriculture.