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Water Implications of Biofuels Production in the United States 4 Agricultural Practices and Technologies to Reduce Water Impacts The challenges of water use and water quality presented in the earlier chapters raise the question, “What are the promising new agricultural practices being developed that might help cut water use and minimize pollution associated with the production of biomass?” In fact, there are many such practices and technologies that, if used, can significantly reduce the impact of agricultural activities on water resources. It is important to acknowledge that many technologies have already been developed and applied to various crops, with corn being the best example. The challenge will be to modify them for cellulosic feedstocks. WHAT ARE THE AVAILABLE PRACTICES AND TECHNOLOGIES? Irrigation Techniques Efficient application of irrigation water is one the most important ways to mitigate any effects that increased biofuels production may have on water resources. There are several irrigation techniques that reduce the amount of water applied per unit of biomass produced, thus improving irrigation efficiency regardless of crop type. For example, subsurface drip irrigation systems minimize the amount of water lost due to evaporation and runoff by being buried directly beneath the crop and applying water directly to the root zone, thus keeping the soil surface dry (Payero et al., 2005). Real-time soil moisture and weather monitoring—the former through microwave remote sensing—are emerging technologies that can potentially help improve the scheduling of irrigation. Rainfall harvesting, efficient irrigation water transport, and use of reclaimed water can also lead to more efficient agricultural water use. These techniques would be effective for both corn and cellulosic ethanol crops. The overall effect of improved irrigation techniques on the regional
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Water Implications of Biofuels Production in the United States water budget will vary on a case-by-case basis. For example, if application efficiencies lead to less water being withdrawn from an aquifer, this would leave more water in long-term groundwater storage for future use. On the other hand, if lower water withdrawals from a stream only serve to make additional water available for junior water rights holders, the net effect on the regional water budget might be negligible. Soil Erosion Prevention As pointed out in the previous chapter, soil erosion can impair the water quality of streams and rivers and also contribute to nutrient pollution. Surface cover, especially in conjunction with conservation buffers, is crucial in reducing sediment in runoff and limiting soil erosion (Figure 4-1). Farmers can employ a number of conservation tillage techniques that leave some portion of crop residues on the soil surface. In “no-till” systems, as the name implies, crops are simply planted into the previous year’s crop residues. In “strip-till” systems, less than full-width tillage is conducted, leaving a relatively high amount of crop residue between rows. For corn, the stalks and cobs left in the field after the grain has been harvested—called the corn stover—can potentially be converted to cellulosic biofuel, but leaving them on the fields can greatly reduce soil erosion. The effects of crop residue management on soil erosion can be represented by the “cover-management factor” (C) in the U.S. Department of Agriculture’s Revised Universal Soil Loss Equation. Because soil loss varies directly with C, a lower value corresponds to lower erosion estimates. In Table 4-1, the C-factor is estimated to be 0.02 for perennial grass, 0.04 for continuous corn when 100 percent of the corn stover is left in the field, and 0.55 for continuous corn when 95 percent of the residue is removed. Thus, from the standpoint of water quality with regard to erosion, sediment, N loss, P loss, and pesticide loss, it is clear that perennial grasses or polyculture (a form of agriculture in which one raises multiple species of crops at the same time and place) would have a great advantage over continuous corn, especially if most of the stover is removed. Overall, conservation tillage appears to have had a positive effect on erosion. For example, in 1985, incentives were put in place to encourage adoption of conservation tillage practices. According to data from the National Resources Inventory (NRI), maintained by the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service, overall annual cropland erosion fell from 3.06 billion tons in 1982 to about 1.75
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Water Implications of Biofuels Production in the United States FIGURE 4-1 Terraces, conservation tillage, and conservation buffers, Woodbury County, northwest lowa. SOURCE: Photo by Lynn Betts, USDA Natural Resources Conservation Service. http://photogallery.nrcs.usda.gov/.
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Water Implications of Biofuels Production in the United States TABLE 4-1 Effect of Cropping System on Cover-Management Factor C of the Revised Universal Soil Loss Equation Cropping System C Perennial grass or polyculture .02 Continuous Corn Grain only removed .04 Continuous Corn—75% residue removed .16 Continuous Corn—95% residue removed .55 NOTES: C-Factor is used to reflect the effect of cropping and management practices on erosion rates. SOURCE: R.Cruse, lowa State University, personal commun., July 12, 2007. Results calculated using the integrated crop and livestock production and biomass planning tool I-FARM, available online at http://i-farmtools.org. billion tons in 2003, a reduction of over 40 percent (http://www.nrcs.usda.gov/TECHNICAL/NRI/). The data presented in Table 4-1 suggest that stover removal might best be combined with some kind of soil conservation practice. Nutrient Pollution Reduction There are various nutrient management techniques that can reduce the amounts of N and P in stream runoff and groundwater. One technique is using enhanced efficiency fertilizers that match nitrogen fertilizer applications to the nitrogen uptake patterns of various crops. Another is injecting the fertilizer below the soil surface, which will result in reduced runoff and volatilization. Controlled release fertilizers have water-insoluble coatings that prevent water-soluble nitrogen from dissolving. These techniques increase the efficiency of the way nutrients are supplied to and are taken up by the plant, regardless of the corp. Precision Agriculture Tools Precision Agriculture (PA) can be defined as “an integrated information- and production-based farming system that is designed to increase long term, site-specific and whole farm production efficiency, productivity and profitability while minimizing unintended impacts on wildlife and the environment” (U.S. House of Representatives H.R.2534). The approach can be used to manage feedstock production inputs on a site-specific basis such as land preparation for planting, seed, fertilizers and nutrients, and pest
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Water Implications of Biofuels Production in the United States control. PA has the potential to reduce waste, increase profits, and maintain environmental quality. PA is actually a return toward pre-industrial revolution site-specific agriculture while retaining the economies of scale of large operations. As such, it accounts for spatial variability in a field on a micro scale. This variability can include soil pH, soil moisture, soil depth, soil type, soil texture, topography, pest populations, nutrient levels, organic matter content, expected yield, etc. Key tools that have catalyzed the development of PA include Global Positioning Systems (GPS), Geographic Information Systems (GIS), yield monitoring and mapping, real-time in-situ soil testing, crop scouting, remote sensing of crop and soil status, real-time weather information, map-based variable-rate technology (VRT), and sensor-based VRT. One PA technology is the Low-Energy Precision-Application Irrigation System (LEPA), an example of which can be seen below in Figure 4-2. The FIGURE 4-1 Low-Energy Precision-Application Irrigation System. SOURCE: USDA/ERS (2004).
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Water Implications of Biofuels Production in the United States LEPA differs from other types of low-pressure nozzles and heads in several ways. Generally, it operates at lower pressures and has higher irrigation-water application and distribution efficiencies, which result in lower net water loss and energy use (Fipps and New, 1991). This system can also be set up to apply fertilizers and pesticides. Another promising technology involves using spectral radiometers that analyze crop color (Scharf et al., 2001). These can be mounted directly on fertilizer applicators and used to control variable-rate nitrogen applications. Systems that utilize sensors to assess color and health of crop plants, as well as variable-rate nutrient applications based on soil management zones and aerial photography, should find success with multiple types of feedstocks. Perennial feedstocks such as switchgrass or other native grasses would be in the field longer and thus should provide a greater opportunity to apply PA technologies. However, the application of technologies for efficient production of cellulosic biofuel will be determined by the economics of the specific production system. HOW CAN BIOTECHNOLOGY CONTRIBUTE? Biotechnology innovations can be important in at least three areas. The first and obvious one is in improving biomass feedstock development through molecular biology/genetic engineering as well as traditional crossing and selection of plants. This has long been done with a focus on optimization for food, but now there is a need to broaden the focus to optimizing for biofuels production. Currently, major companies are screening their corn germplasm for ethanol production efficiency (i.e., gallons of ethanol per bushel of corn), and work is progressing to transfer identified traits into commercial varieties (Mark Alley, Virginia Tech, personal commun., July 20, 2007). By optimizing for fuel as opposed to food, researchers can create biomass feedstocks that have a higher nitrogen-use efficiency, increased drought and water-logging tolerance, and improved root distribution characteristics—technologies that can be applied to both corn and cellulosic feedstocks. It should be noted that corn has a head start in this area in the form of a 15-year history of biotechnological development while even the basic tools of biotechnology for cellulosic crops remain in their infancy. An example of this can be seen in the rate of yield gain, in which corn yields increased at a rate of about 2.5 bushels per acre per year during this 15-year period (Troyer, 2006). Second, this new molecular genetics knowledge can be incorporated
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Water Implications of Biofuels Production in the United States into weather-sensitive crop models that can help design crop varieties to match climate conditions, as well as determine optimal management of crops in specific climate conditions, either in the present or in the future. Biotechnology innovations that will increase the water-use efficiency of both food and biofuel crops will be of great value, as the introduction of biofuels will in some regions lead to an increased demand for water that will also increase the value of drought-tolerant varieties of crops. Finally, biotechnology research and development can be important in improving lignocellulosic, microbial, and bioconversion as well as thermochemical conversion technologies. Although the cost of cellulolytic enzymes, which are used to break down these forms of biomass into biofuels, has decreased in recent years, sugar release from biomass still remains an expensive and slow step, perhaps the most critical in the overall process. Intensive research and development has produced a reduction in the cost of such enzymes by a factor of 10 to 30, down to 20 to 30 cents per gallon of ethanol produced. Although this decrease in price is an important advance, it is estimated that the enzyme cost will have to be further reduced to a level comparable to that of current approaches that produce ethanol from the starch in corn kernels at a cost of 3 to 4 cents per gallon of ethanol (Stephanopoulos, 2007). REFERENCES Fipps, G., and L.L.New. 1991. Water for Agriculture: Improving the Efficiency of Center Pivot Irrigation with LEPA. Report prepared for UNESCO Inaugural International Seminar on Efficient Water Use, Mexico, October 21, 1991. Available online at (http://www.unesco.org.uy/phi/libros/efficient_water/wfipps.html). Accessed on July 13, 2007. Payero, J.O., C.D.Yonts, S.Irmak, and D.Tarkalson. 2005. Advantages and Disadvantages of Subsurface Drip Irrigation. University of Nebraska-Lincoln Extension Publication EC776. Lincoln, NE: University of Nebraska-Lincoln. Available online at (http://www.ianrpubs.unl.edu/epublic/live/ec776/build/ec776.pdf). Accessed on July 13, 2007 Scharf, P., E.Souza, and K.Sudduth. 2001. Spectral Radiometer to Control Variable-rate N Applications for Corn. Proposal prepared for University of Missouri and U.S. Department of Agriculture (USDA) Agricultural Research Service. Columbia, MO: University of Missouri, Columbia. Available online at (http://aes.missouri.edu/pfcs/research/prop501.pdf). Accessed on July 13, 2007. Stephanopoulos, G. 2007. Challenges in engineering microbes for biofuels production. Science 315:801–804. Available online at (http://www.sciencemag.org/cgi/content/full/315/5813/801). Accessed on July 13, 2007. Troyer, F.A. 2006. Adaptedness and heterosis in corn and mule hybrids. Crop Science 39:528– 543. U.S. Department of Agriculture, Economic Research Service (USDA/ERS). 2004. Irrigation and Water Use: Glossary for Irrigation Systems and Land Treatment Practices. Available online at (http://www.ers.usda.gov/Briefing/WaterUse/glossary.htm). Accessed on July 13, 2007.
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