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a Dep. of Environmental Science and Technology, Univ. of Maryland, College Park, MD 20742
b Dep. of Crop, Soil and Environmental Sciences, Univ. of Arkansas, Fayetteville, AR 72701
c Dep. of Ecology and Evolutionary Biology, Cornell Univ., Ithaca, NY 14853
d Inst. of Marine Sciences, Univ. of North Carolina at Chapel Hill, Morehead City, NC 28557
e Dep. of Biological and Agricultural Engineering, Kansas State Univ., Manhattan, KS 66506
* Corresponding author (sharpley{at}uark.edu).
Received for publication November 10, 2007.
ABSTRACT
Renewable fuel production, particularly grain-based ethanol, is expanding rapidly in the USA. Although subsidized grain-based ethanol may provide a competitively priced transportation fuel, concerns exist about potential environmental impacts. This contribution focuses on potential water quality implications of expanded grain-based ethanol production and potential impacts of perennial-grass–based cellulosic ethanol. Expanded grain-based ethanol will increase and intensify corn production. Even with recommended fertilizer and land conservation measures, corn acreage can be a major source of N loss to water (20–40 kg ha–1 yr–1). A greater acreage of corn is estimated to increase N and P loss to water by 37% (117 million kg) and 25% (9 million kg), respectively, and measures to encourage adoption of conservation practices are essential to mitigate water quality impairments. Dried distiller's grains remaining after ethanol production from corn grain are used as animal feed and can increase manure P content and may increase N content. Cellulosic fuel-stocks from perennials such as switchgrass and woody materials have the potential to produce ethanol. Although production, storage, and handling of cellulosic materials and conversion technology are limitations, accelerating development of cellulosic ethanol has the potential to reduce dependence on grain fuel-stocks and provide water quality and other environmental benefits. All alternative fuel production technologies could have environmental impacts. There is a need to understand these impacts to help guide policy and help make programmatic and scientific decisions that avoid or mitigate unintended environmental consequences of biofuel production.
Abbreviations: CRP, Conservation Reserve Program • DDG, dried distillers grain • MRB, Mississippi River Basin
A rapidly accelerating emphasis is being placed on the conversion of grains into ethanol as costs and instability of petroleum supplies for oil production have increased. Ethanol and other liquid biofuels were used throughout the 20th Century and were critical in the development of early internal combustion engines and automobiles. The first internal combustion engine ran on ethanol, as did the first versions of the Model T Ford, and the diesel engine was originally conceived to run on peanut oil (http://en.wikipedia.org/wiki/Biofuel). Although the energy picture of the 20th Century was dominated by fossil fuels, worldwide use of ethanol has grown rapidly over the past two decades due to fossil-fuel supply instability and unreliability, price increases, and concerns about growing pollution from emissions (Fig. 1 ). Since the 1970s, Brazil has led the way in developing ethanol as a major fuel source. More recently, the USA has become a major producer of ethanol, with production doubling from 8 billion L yr–1 (B L yr–1) in 2002 to 15 B L yr–1 in 2005 and increasing further by 25% to 20 B L yr–1 in 2006 (Fig. 1) (Institute for Agriculture and Trade Policy, 2006).
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The recently passed federal energy bill provides additional incentives for ethanol and biodiesel production (Renewable Fuels Association; see http://www.ethanolrfa.org/). Most current ethanol production is concentrated in the US Midwestern states of Illinois, Nebraska, Iowa, Minnesota, South Dakota, Wisconsin, and Kansas (Renewable Fuels Association; see http://www.ethanolrfa.org/). Although much of the projected growth in ethanol production remains in the Midwest, numerous plants have been proposed in the East. For example, several plants have been proposed in Chesapeake Bay watershed states, and several plants have been approved and/or are under construction, such as the 850 M L yr–1 plant in Chesapeake, VA, the 750 M L yr–1 plant under construction in western Pennsylvania, two plants with more than 400 M L yr–1 capacity in Baltimore Harbor, and a 200 M L yr–1 plant near Ithaca, NY (Chesapeake Bay Commission, 2007; Fuoco, 2007). Because ethanol cannot be transported through current pipeline systems, construction of such facilities near major population centers is likely.
The dramatic increase in ethanol production and proposals for further increases in production have fueled a rise in corn prices. In February 2007, as farmers finalized planting decisions, corn futures prices were above $4.00 a bushel for fall 2007 delivery. By October 2007, prices for near-term delivery of corn had dropped to $3.60 to $3.80 per bushel, but futures prices for delivery in the fall of 2008 and 2009 were $4.10 to $4.20 per bushel, creating a strong likelihood of continued expansion and intensification of corn production in those years (Chicago Board of Trade, 2007). In comparison, 2005 and 2006 corn futures prices averaged $2 to 2.5 per bushel (Chicago Board of Trade, 2007). In fact, the Center for Agriculture and Rural Development projected large increases in corn prices and acreage and proportionally greater use of corn for ethanol production (Table 1 ) (Elobeid et al., 2006). Such projected growth is unprecedented and raises many questions regarding feed availability and price for US animal production and the food system, including processors, retailers, and consumers. The rapid growth of grain-based ethanol production has major water quality implications for lakes, rivers, and coastal marine ecosystems in much of the USA, particularly along the Northern Gulf of Mexico (Mississippi-Atchafalaya discharge region) and Atlantic Seaboard, including the two largest estuaries, the Chesapeake Bay and the Albemarle-Pamlico Sound.
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The Environmental Consequences
There are serious concerns about large increases in surface and ground water discharge of N and P (Howarth et al., 2002) because primary production of downstream waters is known to be stimulated by the addition of these nutrients (Howarth and Marino, 2006; Mankin et al., 2003; Nixon, 1995; Ryther and Dunstan, 1971). Nutrient-enhanced primary production, or eutrophication, of estuarine and coastal waters is a key cause in the incidence of harmful algal blooms (Paerl, 1988, 1997; Richardson, 1997), oxygen depletion (hypoxia), and overall fisheries habitat decline (National Research Council, 2000; Rabalais and Turner, 2001). These impairments will increase in frequency and severity in receiving Atlantic and Gulf of Mexico coastal waters that are already stressed by the unwanted symptoms of nutrient over-enrichment (Boesch et al., 2001). Over 60% of the coastal rivers and bays in the USA are moderately to severely degraded due to nutrient enrichment (National Research Council, 2000). Although excessive N loading is the main culprit in estuarine and coastal eutrophication (Boesch et al., 2001; Nixon 1995), P loading leads to severe degradation and impairment of freshwater lakes, rivers, and some estuarine and coastal waters, especially those also receiving high N loads (Fisher et al., 1999; Howarth and Marino, 2006; Sylvan et al., 2006).
Increased Corn Acreage
Increased demand for grain for ethanol production led to nearly 7 million ha or a 15% increase in corn acreage in the USA from 2006 to 2007 (USDA-NRCS, 2007). Corn planting projections for 2007 indicate much of this increase comes from continuous corn replacing soybeans [Glycine max (L.) Merr.], with additional acreage coming from land currently in the Conservation Reserve Program (CRP), hay, and pasture. Wisner (2007) projected that up to 2.9 million ha of CRP land may be converted to corn production. Elobeid et al. (2006) estimated an ethanol-related long-term increase of 7.3 million ha in corn acreage. In this paper we use this acreage to estimate water quality impacts, although more recent projections suggest this may more closely represent the 2007 acreage rather than a longer-term increase.
Corn is an inherently inefficient N user in that 40 to 60% is generally not taken up by the crop, and N loads to downstream aquatic ecosystems from corn-dominated landscapes are typically 20 to 40 kg N ha–1 yr–1 (Balkcom et al., 2003; Randall et al., 2003). Soybeans average 15 to 30 kg P ha–1 yr–1 (Chesapeake Bay Program, 2006). For P, average losses in runoff from corn (2–15 kg P ha–1 yr–1) tend to be greater than from soybean (1–8 kg P ha–1 yr–1) (Carpenter et al., 1998; Kimmell et al., 2001; Sharpley and Rekolainen, 1997). The loss of P from perennials and hay crops (0.2–2 kg P ha–1 yr–1) is generally less than annuals due to decreased runoff volumes and lower crop P requirements contributing to smaller amounts of P (fertilizer or manure) being added (Sharpley et al., 2001; Smith et al., 1992). Water-quality model simulations of converting CRP or perennial grasses to cropland confirm that delivered N and P loads increase by more than double the percentage land area converted (Mankin et al., 1999, 2003). Assuming fertilizer application rates remain the same, annual nutrient loads are estimated to increase by 117 million kg N (37% increase) and 9 million kg P (25% increase) (Table 2 ). Most of the change will occur in the Mississippi River Basin (MRB), and once fertilizer leaves fields in this basin, most of the N and P are delivered downstream to the Gulf of Mexico (Alexander et al., 2000; David et al., 2006).
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Use of By-Product Distillers Grain
The concentration of grain-based ethanol production in the Corn Belt has the potential to create areas of nutrient imbalances around ethanol production facilities and may create state-wide or regional imbalances (Perlack and Turhollow, 2003). Dry distiller's grain (DDG) (0.8– 0.9% P), a by-product of ethanol production, can be used in animal feed. Even with <20% DDG supplementation of dairy cow diets, this elevates ration P to 0.5% P (0.33–0.36% P is recommended), offsetting reductions over the past decade gained through feed management (National Research Council, 2001). Lawrence (2006) estimated a 60 to 90% adoption of the use of DDGs in animal rations in Iowa. The highest DDG percent of total ration was for beef cattle, with an estimated average of 33%. Dairy averaged 20%, and poultry and swine were 10%. The inclusion of DDGs in rations at rates such as these increases the P content of manure (Baxter et al., 2003; Maguire et al., 2004; Wu et al., 2001) and, if land applied, will increase the potential for P loss in runoff (Ebeling et al., 2002; Maguire et al., 2007; Sharpley et al, 2005).
Research indicates that recommended P requirements for beef and dairy cattle should be decreased (Erickson et al., 2002). This research shows that reducing dietary P from conventional levels (0.35% or more) to diets with no supplemental P (0.25%) improved animal P use efficiency, decreased the P excreted in manure, and did not adversely affect animal performance. The US Department of Agriculture–Natural Resources Conservation Service has established a national practice standard for feed management for dairy, and many state Natural Resources Conservation Service offices are implementing dairy feed management aimed at reducing P to, or below, an animal's recommended nutritional requirement (National Research Council, 2001). However, the rapidly expanding supply of DDGs (each 1 B L yr–1 of grain ethanol creates about 650,000 Mg of DDGs) has lowered the price of the material, and it is being widely incorporated into rations at substantial rates for beef and dairy cattle in the Midwest and East (Lawrence, 2006). This seems very likely to erode progress in managing P in dairy feed and will make government incentives and cost-share payments to improve feed management more expensive and less attractive to farmers (Simpson et al., 2004).
Drying distiller's grains is estimated to use 60% of the natural gas required at the ethanol facility (Elobeid et al., 2006), so feeding wet distiller's grain would result in major savings in natural gas use. Feeding wet distiller's grain requires co-location of animal production facilities near the ethanol plant to minimize transport cost. It is also possible that anaerobic digestion of the manure could produce enough methane to meet 25% of the ethanol plant's natural gas needs. If the distiller's grains are fed wet, then the natural gas demand would be cut by about 60%, so the manure-generated methane could produce 62% of the methane needed for uses other than to dry distiller's grains (Elobeid et al., 2006). Research indicates that wet corn gluten feed is higher than corn in net energy for weight gain in cattle (Ham et al., 1994). Wet and dried corn distiller's grains are excellent sources of protein for ruminants (Berg, 2006). The proposed uses for wet co-products and the potential to generate methane are creating interest in opening or expanding dairies or feedlots near ethanol facilities (Berg, 2006). These advantages of co-locating confined animal and ethanol facilities are offset by the environmental consequences of increased concentration of animal manures. Inclusion of the high-P manure that will come from animals fed with distiller's grains will increase the area of land required to apply these manures, which will exacerbate nutrient management planning in counties already short of an adequate land base to fully utilize available manure N and P (Kellogg et al., 2000; USDA-NRCS, 2003a).
Options and Sustainability Recommendations
The transition from row crops to perennial grasses using tools such as the CRP has the greatest potential to achieve major reductions in N and P loss (Baker et al., 2005). The high price of corn due to ethanol production provides a disincentive for cropland retirement or conversion to perennials. The costs of land retirement programs to the public will have to increase if they are to remain effective, and the demand for expanded corn production will likely lessen interest in enrollment regardless of the incentive. The pressure to expand corn acreage will reduce the acreage in perennial crops and some pastures, and idle lands will be brought into row crop production. This will clearly discourage establishment of perennial crops, including those to be grown for cellulosic ethanol, unless their value can be increased through multiple revenue streams associated with their production. In summary, the demand for corn to support grain-based ethanol production will reduce acreage in CRP, perennials, or idle land and could make future conversions to these low-impact uses more expensive and less likely (Potter et al., 2006). This is opposite to what is recommended as one of the most effective ways to reduce N and P loss to surface and ground waters (Committee on Environment and Natural Resources, 2000).
Cellulosic Feedstock Production
Although there is potential for cellulosic-based ethanol using perennial grasses, fast growing woody species, and manures and other wastes, the infrastructure to produce cellulosic ethanol is not as advanced or operational as for grain sources (Datar et al., 2004). Although millions of acres of corn exist to supply grain-based facilities, the 23,000 to 44,000 ha needed in a concentrated area to grow enough perennial grass or woody material to support a 200 M L yr–1 facility does not exist (Datar et al., 2004). The fermentation technology for cellulosic material is not perfected, and the fermentation process is different than for grain, so grain ethanol facilities cannot be easily converted to cellulosic ethanol production. Additional issues to be overcome are a disparity between the cost of biomass production and energy conversion (Epplin, 1996), scale or acreage of production for it to become economically competitive with other feedstocks (Walsh et al., 2003), and traditional nonrenewable sources (Cundiff and Shapouri, 1997). Although these factors combine to favor grain rather than cellulosic ethanol production, cellulosic-based production provides a long-term option that can generate multiple revenue streams for the farmer while providing a wide range of ecosystem services, which has the added benefit of improved surface and ground water quality (Jordan et al., 2007).
The use of corn stover and other crop residues in cellulosic ethanol production is also being promoted (Doering, 2007). However, removal of crop residues should only be done when soil erosion and associated N and P loss would not be exacerbated (USDA-NRCS, 2003b). Additionally, residue harvesting will accelerate reductions in soil organic matter content, which has the potential to reduce long-term productivity and increase runoff and N and P losses (Sims and Kleinman, 2005).
Switchgrass Production
Switchgrass (Panicum virgatum) is a warm-season perennial native prairie grass that produces large amounts of biomass in its top growth and in an extensive, deep root system. The use of switchgrass as a cellulosic fuel-stock for ethanol production offers some water quality benefits over a corn grain–stover energy strategy (Parrish and Fike, 2005). Although perennial grasses require some N and P fertilization, amounts needed are lower, and their extensive root systems make them more efficient nutrient users than annuals, particularly corn (McLaughlin and Kszos, 2005). With recommended fertilizer applications, N and P loss in surface runoff and tile drainage can be much lower (about 50–90%) than from corn–soybean rotations (Randall et al., 1997; Smith et al., 1992).
It typically takes two to three growing seasons without harvesting to establish switchgrass. Once established, switchgrass can grow for 20 yr or more without replanting if properly managed and fertilized (McLaughlin and Kszos, 2005). In fact, little or no response of switchgrass to added P has been observed in Iowa (Hall et al., 1982) and Texas (Muir et al., 2001) when harvested once a year. However, N management of switchgrass is complex, with optimal N rates ranging from 0 to >400 kg N ha–1 yr–1 as a function of uncertainties over the use of switchgrass as a cash crop and harvest frequency and timing, which affects N offtake and thereby residual soil N (Miller et al., 2007; Parrish and Fike, 2005). For instance, Thomason et al. (2004) observed maximum switchgrass yields in Oklahoma with 448 kg N ha–1, although more N (1100 kg ha–1) was removed in biomass with no N than with 896 kg N ha–1 yr–1 (996 kg N removed ha–1). Although late harvesting of switchgrass (i.e., November–December) can decrease the potential for N and P loss in runoff and leaching, there is some reduction in biomass harvested (Madakadze et al., 1999a; Sanderson et al., 1999). For example, during senescence, N, Si, and K are translocated from shoot to roots (Hargrave and Seastedt, 1994; Madakadze et al., 1999b), decreasing N removal (i.e., improving fertilizer-use efficiency) and mineral constituents that reduce the efficiency of thermo-chemical conversion and increase NOx and SOx emissions (McLaughlin et al., 1996).
Although it is necessary to locate ethanol production facilities near areas of switchgrass production to minimize transportation and storage costs, switchgrass has the potential to be grown on marginal lands or to be used as an edge-of-field buffer (Kort et al., 1998). The dense canopy and extensive root network of switchgrass can also reduce the propensity for runoff, erosion, and associated N and P loss (Lee et al., 2003; Sanderson et al., 2001; Self-Davis et al., 2003). Blanco-Canqui et al. (2004) observed a respective five- and four-fold decrease in N and P runoff from a bare, tilled Mexico silt loam in Missouri with a switchgrass compared with tall fescue buffers.
Switchgrass sequesters C, increases soil organic matter, and improves soil quality through its extensive, deep root system. Tufekcioglu et al. (2003) found that switchgrass in a riparian buffer had eight times the belowground biomass and up to 55% more total soil organic C than adjacent cropped fields. This sequestered organic C in a continuous perennial crop could be sold as C credits, while the aboveground biomass growth would be sold for ethanol production.
Finally, switchgrass can improve soil quality compared with row crops (McLaughlin and Kszos, 2005; Parrish and Fike, 2005), which can make a farmer eligible for payments under the US Department of Agriculture's Conservation Security Program. Even if payments are not received for soil quality improvement, long-term production of switchgrass will improve soil productivity and should increase future yield potential should the land be returned to row crop production. It seems that if fermentation and infrastructure constraints are overcome, the opportunity for income from multiple revenue sources generated through the provision of a range of ecosystem services could make switchgrass and other perennial grasses economically competitive with commodity row crops.
Conclusions
Grain-based ethanol production is expanding rapidly and is the primary factor leading to major increases in the amount of corn grown in the USA and elsewhere. Increased corn acreage and fertilizer application rates due to corn prices will increase N and P losses to streams, rivers, lakes, and coastal waters, particularly the Northern Gulf of Mexico and Atlantic coastal waters downstream of expanding production areas. Harvest of corn stover for cellulosic ethanol production would likely increase erosion (sedimentation) and nutrient loads, which will adversely affect these already nutrient-stressed waters. It is critical that a broad suite of conservation measures, particularly nutrient management, are rigorously implemented on new or more intensively managed corn lands, particularly under continuous corn production to partially offset an increased potential for nutrient loss (Simpson and Weammert, 2007); for example, the use of precision and variable-rate applications of fertilizers (Osborne et al., 2002; Scharf et al., 2006a) and the Late Spring Nitrate Test (Jaynes et al., 2004; Karlen et al., 2005). Additional measures include interseeding corn with cover crops (Kleinman et al., 2005) and the inclusion of buffers or riparian filter strips to minimize edge-of-field runoff of N and P (Lowrance et al., 1994).
Dried distiller's grain from fermentation is rapidly becoming available for use in animal rations, particularly for beef and dairy cattle. Due to the high P content of DDGs, the P content of manures will increase, which will further enhance the mobility in runoff of P applied in these manures (Kleinman et al., 2002, Sharpley et al., 2005). The use of DDGs in animal rations should not be done at such a level as to increase N and P contents of feeds above nutritional requirements. Otherwise, N and P contents of manure will be greater than using current feed management practices, enhancing the potential for nutrient enrichment of runoff. The likely concentration of animal facilities near ethanol production plants must be accompanied by sound nutrient management planning and other conservation measures to avoid the soil and water-quality consequences of N and P accumulation in areas used to land apply manures.
The development of efficient and competitive fermentation technologies and supporting infrastructure to allow development of a perennial grass or waste-based cellulosic ethanol industry could provide a long-term sustainable approach to ethanol production. A cellulosic renewable energy approach could provide multiple ecosystem services including energy, C sequestration, improved water quality and fisheries habitat, and improved soil quality and productivity. As markets develop for each of these services/products, perennial grass production for ethanol generation may become economically competitive with, or even superior to, grain or crop residue fuel-stocks for ethanol production. Any alternative fuel production technology may have impacts on water quality. As a result, it is important to make policy and programmatic and scientific decisions that avoid or mitigate the unintended environmental consequences of biofuel production during development of the industry to avoid the much higher costs of remediation and ecosystem restoration at a later date.
NOTES
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REFERENCES
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