Agricultural and forestry residues and organic portions of municipal solid waste can have a negative impact on the environment as they decay. On the other hand, converting these residues to ethanol can offer immediate and sustained GHG advantages and simultaneously enhance domestic fuel production (Paustian et al. 1998; Tilman et al. 2001; DiPardo 2002; Wyman 2003b; Zhang et al. 2007; BR&Di 2008; Fargione et al. 2008; Smith et al. 2008). In general, cellulosic ethanol can be a low carbon fuel and provide a valuable replacement for gasoline from petroleum. Although production and combustion of ethanol adds CO2 to the atmosphere, an equivalent amount of CO2 can be taken up as the next rotation of agriculture feedstocks is grown to replace that used to produce ethanol (Wyman 2003a2 . Thus, cellulosic ethanol provides an opportunity to recycle carbon instead of continually building up carbon in the atmosphere as fossil fuels do, and it has been estimated that ethanol from corn stover could reduce GHG emissions by over 80% compared to petroleum — derived fuels (Wang et al. 1999). Exporting the excess power produced by burning lignin and other portions of cellulosic biomass not utilized for making ethanol can reduce the amount of coal used to produce electricity in the grid, potentially resulting in negative emissions of CO2 compared to the status quo (Wyman 1994a). Moreover, because the large amount of virtually pure CO2 (around 300 kg CO2 per dry ton of corn stover) pro­duced during fermentation could be sequestered more easily than being considered for capturing CO2 from burning coal, ethanol could actually become a negative GHG emission fuel. Because these agricultural residues are associated with food production, they will be grown whether we use them for making fuel or not, largely avoiding many of the concerns about indirect land use now being hotly debated (EPA 2005; BR&Di 2008; Fargione et al. 20082 .

Even if GHG benefits are demonstrable, other sustainability and environmental consider­ations must be addressed for use of agricultural residues. In particular, removing agricultural residues from the land can impact soil cultivation and the needs for fertilizer, pesticides, and other chemicals, all of which can impact soil, water quality, air quality, site productivity, and GHG emissions (Kim and Dale 2004; EPA 2005; Smith et al. 2008). Some residues such as sugarcane bagasse, rice hulls, rice straw, and corn fiber that are typically removed from the field anyway can be employed without additional negative consequences when properly managed, and their use results in little, if any, net additional demands for cropland, fertilizer, pesticides, or water. On the other hand, leaving corn stover, wheat straw, and other plant matter remaining after food is harvested on the field helps maintain soil organic and inorganic matter and protect against erosion (Lal 2006). The amount of sustainably harvestable residues varies with location and depends upon climate, soil texture, rain fall, and the production practice used (Tilman et al. 2002; BR&Di 2008). For example, conventional till production of corn leaves more residues in the field than no-till systems (Kim and Dale 2005; BR&Di

2008) . However, estimates of the amounts of residues that can be removed sustainably are still being refined.