Crop Residues

Crop residues (e.g., corn stover and small grain straw) are sometimes excluded when discussing cellulosic energy crops per se, but because of the vast area upon which they are grown and their current role in the development of cellulosic energy systems, this chapter will review several important attributes of this “herbaceous” feedstock. Crop residues are potential feedstock sources for second-generation biofuel production. These materials, along with dedicated energy crops (e.g., switchgrass [Panicum virgatum L.], Miscanthus [Miscanthus × giganteus]), are considered to have greater potential for biofuel production than current first-generation feedstock (i.e., corn grain) [1–3]. Production of ethanol and other fuel sources from these lignocellulosic materials is receiving increased financial support for research and development [4–6]. Furthermore, biofuel production from crop residues provides a multipurpose land use opportunity where grain can be harvested to meet food and feed demands, while a sustainable portion of the residues provide a potentially available biofuel feedstock. Corn stover, the aboveground plant material left in fields after grain harvest,was identified as an important biomass source in the Billion-Ton Study (2005 BTS) [7]. The vast area from which this feedstock could potentially be harvested was confirmed by USDA National Agricultural Statistics Service (NASS) data showing that between 2005 and 2011, corn was harvested in the U.S.A. from an average of 32 460 000 ha each year [8]. Wheat straw was the other dominant residue identified in the 2005 BTS, and from 2005 through 2011, wheat was harvested in the U.S.A. from an average of 20 037 000 ha each year. Based on thesevast harvest areas, the 2005 BTS projected total annual corn and wheat residue production to be approximately 250 and 90 million Mg, respectively, with a sustainable removal of 82 and 12 million Mg after accounting for that needed to mitigate wind and water erosion. The 2005 BTS projections of available crop residue immediately raised concern among many soil scientists because harvesting residues as a biofuel feedstock or for any other purpose (e.g. animal feed) will decrease annual carbon input and may gradually diminish soil organic carbon (SOC) to a level that threatens the soil’s production capacity [9]. Concerns within the U.S. Corn/Soybean Belt were accentuated knowing that for many soils artificial drainage, intensive annual tillage, and less diverse plant communities have already reduced SOC by 30–50% when compared to pre-cultivation levels [10]. Returning a portion of crop residues to replenish SOC was deemed essential for sustainability [11–16] because crop residues influence many vital soil, water, and air functions. Many scientists stated that caution must be used to ensure that harvesting residue for any use does not compromise ecosystem services or decrease overall soil productivity. Furthermore, others argued that for several current cropping systems, soil erosion and organic matter depletion indicate that crop residue returns to the soil are already insufficient [17, 18]. As a result of soil resource sustainability concerns raised by the 2005 BTS, a follow-up report (2011 BT2) was developed by the U.S. Department of Energy (DOE) to include (1) a spatial, county-by-county inventory of potentially available primary feedstocks, (2) price and available quantities (i.e. supply curves) for individual feedstocks, and (3) a more rigorous treatment and modeling of resource sustainability [19]. The 2011 BT2 recognizes the importance of crop yield variation and the need to balance the economic drivers with ecologically limiting factors [20]. Table 8.1 presents some of the estimated feedstock supplies for various crop residues at selected price levels. These values are also consistent with several other estimates including those used for the U.S. National Academy of Science (NAS) study on Liquid Transportation Fuels from Coal and Biomass [21]. The 2011 BT2 also provides a more realistic overview of total crop residue availability and sets some achievable research and development goals for available feedstock supplies by creating various production scenarios that strive for higher crop yields and integrate multiple cellulosic energy crops into potential production systems. Several assessments examining the multiple roles that crop residues have for maintaining multiple ecological functions have been published since the 2005 BTS [22–30]. Therefore, this chapter focuses on current corn stover and wheat straw research designed to address concerns raised by those previous reviews and to help ensure that commercial bioenergy develops in an economically, environmentally, and socially acceptable manner