**1. Introduction**

With ever increasing global populations there is a rising demand for energy to support even modest changes in lifestyle. It has been recognized for some time now that with decreasing oil reserves on a global scale there is a need for alternative energy sources. Many of our needs for energy utilizing electricity can be met by alternatives to petroleum and coal-based power generation. Of particularly high potential is the efficient utilization of solar energy. According to Lewis and Nocera [1], the earth receives approximately 7000 times more energy from the sun than is utilized by all of mankind. There are several technologies that are being utilized, ranging from photovoltaic to focusing mirrors to super heat fluids for steam generation in the production of electricity. The continued development of these technologies, along with other types such as wind-driven turbines, geothermal, hydroelectric, and ocean wave motion for electricity production, will greatly lessen the demand on petroleum-based energy. However, a critical need is liquid fuels for transportation. The movement of people and goods over great distances is a vital part of the world economy.

Part of the answer may still lie in the utilization of solar energy; not in a direct manner to power vehicles (cars, trucks, trains, and airplanes), but what it has been doing for billions of years in providing energy to growing plants. Conversion of plant biomass to energy or the production of bio-based liquid fuels (biofuels) has received greater attention in the last couple of decades. Although there is a tremendous amount of potential energy stored in the total plant biomass as it goes through its normal life cycle, much of the current technology has focused on the utilization of grains (corn, cereals, and soybeans) or sugars from storage organs of specialty plants (sugar cane, sugar beets). This has allowed a rapid ramping up of liquid fuel production in the form of ethanol. The technology needed for this production was not something that required a lot of development, but was basically a matter of scale. After all the brewing industry

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has been utilizing this process for centuries. For corn grain and cereals, it is a matter of converting starch to glucose, a simple enzymatic process followed by the fermentation of glucose by yeast to ethanol. In the case of sugar cane or sugar beets, the same technology was already being utilized to efficiently remove the sugar (sucrose) from plant biomass and easily convert to sugars fermentable with yeast [2]. Even for the production of plant-derived biodiesel, the grains from oil-producing crops are pressed to release oils in which the fatty acids can be methyl- or ethyl–esterified, producing a suitable diesel alternative. Biodiesel lags well behind other types of biofuel production systems and seems to be focused primarily on the utilization of waste products from the food industry[3].

With current scenarios, the ethanol industry will have to compete with increasing demands on grains for feed and food [2]. A concern has been the diversion of land from food production to energy production and rightly so with increasing world populations. With this in mind, much attention has been directed to the conversion of cellulosic biomass to liquid fuels. This subject has been highly reviewed in the past few years, addressing a wide range of concerns and potential advantages. It is clear that crop residues will play a key role in meeting the projected total biomass needed to provide the amount of liquid fuel to meet the goal of replacing 30% of U.S petroleum consumption by 2030 [2]. Dedicated biofuel crops such as switchgrass and fast-growing poplar also figure prominently into meeting this goal. It is envisioned that the dedicated energy crops could be grown on marginal lands poorly suited for the high capacity needs of feed and food [4]. Recently Schmer et.al.,2008 [5]demonstrated that switchgrass grown in areas considered to be margin cropland could be an effective source of biomass for biofuels. It has been proposed that establishment of low input man made prairies could be an economical way of producing biomass for biofuels [6]. Although this could be a way to supply some of the required biomass it may fall well short of the amount needed per acre to make it a practical enterprise for harvest and transportation. Well-managed switchgrass plots on marginal croplands supplied higher estimated ethanol yields per acre (93% greater than poor management) [5]. Genetic improvement is a critical component to establish switch‐ grass as a major biomass source that can meet the demands for more biofuels [7]. It should be kept in mind that biofuel programs must fit into an agricultural system that maximizes the production potential of each acre of farmland while protecting the environment. In this respect switchgrass on marginal croplands could also provide a nutrient sink for nitrogen waste from animal production. Switchgrass needs little nitrogen input but as with any crop production increases with the application of nitrogen [5]. Well-managed switchgrass plots could extend the useful life of croplands no longer fit for typical row crop production. Perennial grasses such as switchgrass can provide runoff protection as buffer strips along streams and rivers to keep nutrients out of waterways and lakes, thus providing dual benefits.

Although there have been a wide range of crop residues proposed to contribute to the total biomass needed for biofuel production, corn stover would be the largest contributor. It has been estimated that corn stover would contribute as much as 20% of the total biomass requirement [2]. One of the concerns of removing crop residues is the long-term impact upon soils. Removing large portions of the residues leaves the soil surface vulnerable to wind and water erosion. Guidelines have been proposed for leaving sufficient biomass on the fields to keep this from becoming too much of a problem [8]. In addition, removing large portions of the biomass leads to a depletion of the soil organic carbon levels [9]. If sufficient amounts were left in place to meet these demands, this in turn would limit the amount of biomass for biofuel production [10]. With anticipated small profit margins, especially in the early going, there will be a temptation to remove more of the biomass, leaving the soils vulnerable to erosion and risking soil organic carbon depletion. Once these soils have reached high depletion levels, productivity will be severely restricted and returning them to better productivity will be a monumental task. Switching these lands to crops such as switchgrass that can do well in marginal soils would help the biofuels industry, but some of the most productive farmland for food and feed would be lost. This would most certainly sharpen the debate over land use for biofuels vs. food. No matter the approach it is clear multiple scenarios will need to be investigated to meet biomass for biofuel needs in a sustainable manner. The driving force behind future directions should be one of maintaining our existing high production lands while capturing increased value from lands that are should not be in continuous crop pro‐ duction. The challenge moving forward is to develop farming systems that are both economic and environmentally sustainable while meeting the increasing demands of food, feed, fiber, and now bioenergy. There is no doubt that crop residues, especially corn stocks, play a major part in making this vision a reality but as already pointed out it is walking a fine line between productivity and maintaining soil health.
