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2
Biomass Resources for Liquid Transportation Fuels
INTRODUCTION
America’s transportation systems will undergo major and multifaceted
transformations as the nation addresses human-driven climate change, the availability and
cost of liquid transportation fuels, and the need for energy security. Plant biomass has the
potential to play an important role in America’s energy future. Plants convert solar
energy to chemical energy naturally for their growth and development through the
process of photosynthesis. Plant biomass can be produced sustainably and converted into
liquid transportation fuels via biochemical conversion (Chapter 3) or thermochemical
conversion (Chapter 4). Liquid transportation fuels derived from biomass feedstock are
often referred to as biofuels. The amount of biomass that can be produced in an area
depends on the local availability of sunlight, water, and other resources. In principle,
biofuels are attractive alternatives to gasoline because they are made from renewable
feedstocks and can decrease the net release of greenhouse gases by the transportation
sector. Although those benefits are important, they must be viewed in the context of other
societal needs that are also met by the nation’s land base, especially needs for food, feed,
fiber, potable water, carbon storage in ecosystems, and preservation of native habitats and
biodiversity. Responsible development of feedstocks for biofuels and expansion of
biofuel use in the transportation sector would be economically, environmentally, and
socially sustainable. This chapter addresses the questions raised in the statement of task
regarding the following:
• The quantities of biomass that could potentially be produced and collected in a
sustainable manner for use as feedstocks for liquid transportation fuels.
• The input and costs involved in growing and harvesting the crops or in collecting
the feedstock and delivering it to a biorefinery for production of liquid transportation
fuels.
• The land-use, agricultural, price, greenhouse-gas, and other environmental
implications of biomass production for liquid fuels.
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• Research and development needed to advance production of biomass feedstock
for transportation fuels.
The chapter examines the quantities of different types of biomass that can be
harvested or produced while minimizing competition between food and fuel and
minimizing adverse environmental effects. It also assesses the total costs of various
feedstocks that will be delivered to a processing plant for conversion to biofuel. The
panel considered societal needs on the basis of recent analyses that have explored
tradeoffs between using land for biofuel production and using it for food, feed, fiber, and
other ecological services that land resources provide.
CURRENT BIOMASS PRODUCTION FOR BIOFUELS
Biofuel produced in the United States is overwhelmingly dominated by ethanol
made from corn grain; biodiesel derived from soybean oil makes up most of the
remainder. In the 2007 crop year (from September 2, 2007, to August 31, 2008), 3.0
billion bushels of corn, or 24 percent of the year’s harvest, was used to produce 8.2
billion gallons of ethanol (NCGA, 2008). Around 450 million gallons of biodiesel was
also produced, about 90 percent of which was derived from the oil extracted from 275
million bushels of soybean, 11 percent of the year’s harvest (USDA-NASSa, 2008; NBB,
2008). On an energy-equivalent basis (in British thermal units), corn grain ethanol and
soybean biodiesel together made up 2.1 percent of the liquid transportation fuel used in
the United States in 2007 (EIA, 2007).
The social, economic, and environmental effects of domestic biofuels have been
mixed. Diverting corn, soybean oil, or other food crops to biofuel production could
induce competition between food, feed, and fuel, but increases in crop price have helped
to revive rural economies. From farmers’ and small rural communities’ perspective,
development of ethanol plants has created greater local demand and higher prices for
corn grain (and for soybean through parallel efforts associated with production of
biodiesel). Local investment in and control of these plants have also provided well-
paying employment opportunities that reinvigorated many small midwestern
communities, but some argue that the number of jobs added to the local economy is
overestimated (Low and Isserman, 2009). For farmers, the increase in corn grain prices,
which averaged $2.36 ± 0.40 per bushel of grain (25 kg) in 1973–2005 but $3.04 and
$4.00 per bushel in 2006 and 2007 (USDA-NASSa, 2008), was of great importance. The
increased prices were results of an increased global demand for corn as animal feed and
for grain ethanol production. Higher commodity prices have also led to markedly higher
values of fertile farmland and adversely affected low-income consumers in the United
States and abroad and the drawing of land out of the U.S. Conservation Reserve Program
(CRP). On a global scale, high commodity prices are expected to accelerate clearing of
rain forest and savanna. There is growing concern about the use of grain for fuel instead
of food. Other environmental concerns, especially the loss of nitrogen by leaching
(Donner and Kucharik, 2008), have also been pointed out. Corn and soybean are
renewable biofuel feedstocks, but large amounts of fertilizer and pesticide are often
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needed to grow them (Hill et al., 2006). The resulting greenhouse-gas and other pollutant
effects of those practices can be harmful to human health and the environment.
Corn grain ethanol and soybean biodiesel are viewed by some as intermediate
fuels in the transition from oil to advanced biofuels made from cellulosic biomass. As a
biofuel feedstock, cellulosic biomass has numerous advantages over food and feed crops,
including its availability from sources that do not compete with food and feed production.
Biomass can be reclaimed from municipal solid-waste streams and from residual
products of some forestry and farming operations. It can also be grown on idle or
abandoned cropland, on which food or feed production is already minimal. Growing
cellulosic biomass can require less fossil fuel, fertilizer, and pesticide inputs than growing
corn and soybean (Tilman et al., 2006), especially if legumes (nitrogen-fixing plants) are
included in the mix (NRC, 1989). In addition, cellulosic biomass can serve not only as a
feedstock for biofuel production but also as a source of the heat and power required for
biorefineries and thus displace fossil fuels and fossil-fuel-derived electric power (Morey
et al., 2005). Therefore, this chapter focuses on the biomass resources available for
cellulosic biofuel production.
Sustainable Production of Biomass for Conversion to Biofuels
Globally, about 12 billion acres of land are used for agriculture, about 4 billion
cultivated and the remainder used for grazing. Any substantial expansion of agriculture to
accommodate dedicated biofuel crops via the direct conversion of natural ecosystems—
such as native rainforests, savannas, and grasslands—into cropland could threaten those
ecosystems and reduce their biodiversity. Biofuels can also indirectly cause land to be
cleared when fertile agricultural soils or food crops are used for biofuels. Such indirect
land clearing provides land used to grow “replacement” food crops. Moreover, intact
ecosystems are major storehouses of carbon: terrestrial vegetation stores as much carbon
as the atmosphere, and terrestrial soils store twice as much (Schlesinger, 1997). Dry
biomass—whether wood of trees, hay, or corn stover—contains about 45 percent carbon.
On combustion or decomposition, every ton of dry biomass contributes about 1.5 tons of
carbon dioxide (CO2) to the atmosphere. In many cases, conversion of intact ecosystems
to grain or fuel crop production could incur losses of biomass and soil carbon to the
atmosphere as CO2 that greatly exceeds the greenhouse-gas savings associated with
biofuel production on such lands for many years (Box 2-1) (Fargione et al., 2008;
Searchinger et al., 2008).
Biofuels offer opportunities for greenhouse-gas reductions, but large amounts of
cellulosic biomass will be needed. Sustainably produced biomass would be derived from
various agricultural or forestry residues, from current waste streams, or from dedicated
fuel crops grown on agricultural reserve land or on land so degraded that it is no longer
cost-effective for commodity production (Tilman et al., 2006). The United Nations
Environment Programme and other sources estimate that globally there are about 400-
500 million hectares of such land (Campbell et al., 2008; Field et al., 2008).
Collecting agricultural residues and producing biofuel crops both have
environmental benefits and costs. Removing biomass and crop residues, such as corn
stover, could increase soil erosion by wind and water and deplete soil carbon reserves,
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ultimately affecting water entry, retention, runoff, nutrient cycling, productivity, and
other critical functions. Depending on the crop, soil type, and terrain, various amounts of
biomass or crop residues need to be left on a field to mitigate soil erosion and sustain soil
carbon and nitrogen (Wilhelm et al., 2007). The proportion of biomass that has to be left
on the soil surface to prevent erosion is higher for annual crops than for perennial crops
because of the tillage generally used to establish a new crop each year. Perennial crops,
especially grasses, have dense long-lived root systems that can maintain soil resources.
When ecosystems are cleared of perennial vegetation and converted to annual row
crops, soil carbon stores tend to decline by 30-50 percent until a new equilibrium is
reached (Davidson and Ackerman, 1993). Removal of plant residues, such as corn stover
or wheat or rice straw, without offsetting practices, such as growing cover crops or
decreasing tillage intensity, could reduce soil carbon to a lower equilibrium. A portion of
the crop residue is needed for erosion and nutrient management and to sustain soil
organic carbon, which is the carbon fraction associated with all types of organic matter,
including plant and animal litter, microbial biomass, water-soluble organic compounds,
and stabilized or recalcitrant organic matter (Stevenson, 1994; Johnson et al., 2006a).
Removing plant residues for any purpose would decrease the annual carbon input,
gradually diminish soil organic carbon (Figure 2-1), and threaten the soil’s production
capacity (Johnson et al., 2006a). Therefore, a “systems” approach1 is required for
sustainable biomass production to ensure that its production has a low impact on global
food, feed, and fiber production and that addressing the biofuel problem does not
aggravate other critical challenges, including soil, water, and air quality; carbon
sequestration; greenhouse-gas emissions; rural development; and wildlife habitat.
1
A “systems approach” to agriculture is a holistic or integrated framework that recognizes the connectivity
of multiple processes that occur on the farm and in the ecosystem and that reach across spatial, temporal,
and trophic dimensions and scales. The systems approach examines the connections and interactions
between the different components that make up a system so that the relative effects of change on each
component can be understood.
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BOX 2-1
EFFECTS OF LAND-USE CHANGE ON GREENHOUSE-GAS EMISSIONS
Recent studies have focused on the life-cycle greenhouse-gas emissions from different
biofuels compared with gasoline or diesel. Despite some important disagreement, the prevailing
view is that corn grain ethanol emitted less greenhouse gas than gasoline, that biofuels from
sugarcane provided an even greater benefit, and that cellulosic ethanol, once commercialized,
would further increase the benefit (Farrell et al., 2006; Hill et al., 2006; Wang et al., 2007).
Some earlier studies recognized that carbon sequestration achieved by changing
practices to reduce carbon on a landscape could be offset by increased carbon releases on other
landscapes, which would result in a smaller net decrease or even a net increase in total carbon
emission (Murray et al., 2004; IPCC, 2006). However, emissions from change in land use were
not explicitly included in the comparative analyses of different biofuels in the life-cycle
assessment. If land is cleared and used to grow plants for biofuels, much of the carbon stored in
the biomass and some of that in the soil is released as CO2. A more complete life-cycle analysis
than the earlier biofuel analyses would deduct the carbon lost into the atmosphere from land-
clearing and no longer being stored in an ecosystem. That approach is being implemented by the
U.S. Environmental Protection Agency under the 2007 renewable fuels standard mandated in the
Energy Independence and Security Act (EISA).
Fargione et al. (Fargione et al., 2008) determined the greenhouse-gas carbon released in
converting forest or grassland to biofuel production, which they called the “carbon debt,” and the
years of biofuel production required to “pay back” the debt. They argued that the carbon debt
would arise from land where upland and lowland forest in Southeast Asia was converted to
produce palm oil, where various forms of cerrado forest in Brazil were converted for production of
biodiesel fuel, and where CRP grasslands were converted to corn for ethanol—scenarios with
payback periods of 48 years (the CRP) to over 400 years (lowland palm oil).
If land converted to biofuel production had been sequestering carbon, as would occur
with regrowing forests and conservation grasslands, it is also necessary to consider the
greenhouse-gas effects of forgoing the benefits that would have occurred on the same land if it
had not been used for biofuel production. Similarly, in a global agricultural system, if land used for
food production is converted to biofuel production, some portion of the decrease in food
production will be replaced by cultivation elsewhere and to a substantial extent through the
conversion of noncropland to cropland (Searchinger et al., 2008). This indirect cause of
converting forest and grassland to cropland also creates a carbon debt that needs to be
accounted for to evaluate effects on greenhouse-gases fully.
Searchinger et al. (2008) used the international model developed by the Center for
Agricultural and Rural Development at Iowa State University and the Food and Agricultural Policy
Research Institute at Iowa State University and the University of Missouri-Columbia to estimate
emissions from such indirect land-use changes. They found that each acre of corn diverted to
ethanol in the United States would result in roughly 0.8 acre of new cropland worldwide. They
concluded that U.S. corn grain ethanol increased greenhouse-gas emissions over 30 years by 93
percent relative to gasoline, and that it would take 167 years to pay back the carbon debt. Ethanol
made from switchgrass grown on former corn land increased emissions over 30 years by 50
percent relative to gasoline.
Estimating effects of land-use change on greenhouse-gas emissions requires a
worldwide agricultural land-use model, a basis of estimating which ecosystems will contribute to
new cropland, and a basis of estimating carbon release per hectare. Each step contains
uncertainties. The path through which land conversion takes place is complicated, requiring
consideration of animal-feed byproducts of biofuel production, crop-switching, reduction in food
demand as a result of higher prices, likely regions of expansion, different yields in different
countries, and alternative means of increasing production (for example, increased fertilizer use,
drainage, or irrigation). All those factors interact and require at least a partial-equilibrium model
for analysis. Because of the complexity, the exact magnitude of indirect carbon debt is difficult to
determine with great certainty.
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Nevertheless, the analyses of Searchinger et al. (2008) show that conversion of fertile
farmlands to biofuel production is likely to have caused substantial indirect greenhouse-gas
release via land conversion to pasture or row crops and that indirect effects cannot be ignored in
determining the full life-cycle greenhouse-gas effects of biofuels. The best way to minimize such
indirect effects is to avoid using for biofuel production fertile lands that are well suited for food and
feed production.
One way to deal with indirect land-use conversion, followed by the EISA, is to require
calculation of indirect land-use change for each source or type of biofuel and to mandate only the
biofuels that achieve specified reductions in greenhouse-gas emissions relative to those from
gasoline. The requirement has the obvious benefit of encouraging only biofuels that, on balance,
reduce greenhouse-gas emissions. One limitation of the approach is the failure to consider
effects on food production and prices or other environmental effects of agricultural expansion,
including loss of biodiversity and other ecosystem services. An alternative approach that takes
into account both food and carbon limitations would mandate or provide incentives only for
biofuels that present little risk of substantial emissions from land use change. Such a policy would
emphasize biofuel production from waste products or from feedstocks grown on marginal land
(that is, areas that sequester little carbon or produce little food but can produce biomass for
biofuels). Such a policy would be designed to avoid greenhouse gases, biofuel-food competition,
and other potential environmental effects of agricultural expansion on water quality or quantity
and biodiversity. It would also avoid the difficulty of estimating greenhouse-gas emissions from
indirect land-use conversions accurately.
In summary, the greenhouse-gas benefit of biofuels compared with petroleum-based
fuels depends not only on direct greenhouse-gas emissions from biofuels during their life cycle
(that is, from the growth of biomass to the production and burning of the biofuels) but on any
indirect emissions that might be incurred by changes in land use. The appropriate quantification
of indirect greenhouse-gas emissions is being debated. Policies could play an important role in
ensuring that the biofuels produced provide environmental benefits.
A Landscape Vision of Feedstock Production
The rapidly emerging technologies to develop and use lignocellulosic materials
for production of bioenergy and bioproducts might offer an opportunity to reduce the
environmental footprint of the transportation sector and improve the environmental
sustainability of agriculture. For example, periodically mown perennial biomass crops
could be used to reduce some of the agricultural production “externalities” if they are
planted as buffer strips and in locations that would help to reduce soil erosion, improve
water quality, sequester carbon, and provide wildlife habitat (Tilman et al., 2006;
Doornbosch and Steenblik, 2007; Ernsting and Boswell, 2007; Fargione et al., 2008;
Searchinger et al., 2008).
Implementation of a landscape approach for producing biofuel feedstocks while
addressing some of the externalities associated with agriculture could be made more
feasible by precision agriculture (Giles and Slaughter, 1997; Tian et al., 1999; Ferguson
et al., 2002; Khosla et al., 2002; Robert, 2002) and other changes (Zhang et al., 2002;
Berry et al., 2003; Dinnes et al., 2004). Examples of how watershed-scale or landscape-
scale management could potentially address those multiple concerns while supplying the
necessary volume of biofuel feedstocks are presented in Appendix E.
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BIOMASS RESOURCES
The following is an assessment of biomass resources for liquid fuel production
using technologies available and management practices known in 2008 and projected to
be available in 2020. It is predicated on two fundamental principles: (1) that biomass
production for liquid fuels should not compete for land on which existing crop is
produced for food, feed, or fiber or compete for pasture land that will be needed to feed a
growing and increasingly affluent population, even with yield increases, and (2) that the
environmental impact on land used for biomass production should be no worse than that
of its previous use and provide greater benefits wherever possible (for example, reducing
fuel loads in fire-prone areas, managing volumes of urban waste, and increasing soil
carbon sequestration in restoration of native grassland ecosystems). Although many other
possible visions of biomass availability that do not hold as closely to those two principles
are possible, the panel chose to conduct its assessment with those two in mind.2 Among
the biomass sources considered are corn stover, straw from wheat and seed grasses (for
example, bluegrass and fescue), traditional hay crops (for example, alfalfa and clover),
normal and high-yielding fuel crops, woody residues, animal manure, and municipal solid
waste. Advantages of and concerns about each of the feedstocks are described below. The
resource amounts that could be made available by using technologies and management
practices of 2008 and the resource amounts projected to be available by 2020 are also
described.
Corn Stover
In 2007, 13.1 billion bushels of corn grain was harvested in the United States
from 86.5 million acres of cropland. Assuming a 1:1 ratio of dry weight of corn grain to
stover (Johnson et al., 2006a), the amount of stover produced was estimated to be 370
million tons. Not all the corn stover can be used to produce biofuel, however, because
this crop residue is also a “resource” that farmers use to mitigate wind and water erosion
and maintain soil organic matter. The amount of stover that needs to be left on the land
for those purposes depends on the tillage practice being used as soil is being prepared for
planting by plowing, ripping, or turning (Johnson et al., 2006a; Wilhelm et al., 2007).
Perlack et al. (Perlack et al., 2005) estimated that no-tillage requires 0.35-0.5 ton of
stover per acre to protect against wind and water erosion, but that amount of crop residue
is not sufficient to control soil erosion if more aggressive tillage is used (Figure 2-2) and
is not sufficient to sustain soil organic matter (soil carbon). To maintain soil organic
matter, 2.3-5.6 tons/acre needs to be left in the field, depending on crop rotation and
tillage practice (Wilhelm et al., 2007). Maintaining soil organic matter is crucial for
2
These criteria are consistent with those of Johnson et al. (2007a,b), who concluded that (1) biomass
feedstocks should come first from wastes that would otherwise go to landfills, (2) agricultural residues
should be harvested only when the needs for protecting soil from wind and water erosion and loss of soil
organic carbon have been met, (3) dedicated fuel crops should be developed regionally to meet local needs,
and (4) management strategies must ensure that soils do not lost their ability to provide food, feed, fiber,
and fuel.
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sustaining soil structure, water entry and retention, nutrient cycling, biological activity,
and other critical soil processes.
Erosion control and maintenance of soil organic matter are critical factors to be
considered in the estimation of the sustainable amount of corn stover that could be
harvested to produce biofuel. The national average corn grain production in 2008 was
151 bushels/acre (USDA-NASS, 2008b). If erosion is to be controlled and soil organic
matter maintained, the potential harvestable corn stover even with no-tillage practices is
reduced from 3.58 tons/acre to 0.06-1.25 tons/acre depending on the crop rotation (Figure
2-2). If more intensive tillage equivalent to moldboard plowing is used, as is the case for
about 17 percent of U.S. corn cropland, all the corn stover in the corn-soybean rotation is
required for maintenance of soil organic matter, and only 0.2 ton/acre would be available
as a feedstock if corn is grown continuously. Using the higher harvestable value3 of 1.25
tons/acre and recognizing that only 70 percent of the available cropland area at most
would be planted continuously in corn because of disease, insect, and other factors, the
maximum sustainable amount of corn stover available as biofuel feedstock in 2007 would
have been 75.7 million tons. That value, rounded to the nearest million tons, was used for
the panel’s baseline estimate for the amount of corn stover that could be harvested
sustainably. The panel’s projection of 112 million tons available by 2020 was calculated
in a similar manner and allowed for increased yield as a result of genetic improvement
and improved management. (See Appendix F for details of those estimates.)
Wheat Straw and Seed-Grass Straw
Wheat straw and grass straw can be biofuel feedstocks as the technology to
convert them to liquid fuels develops. Banowetz et al. (2008) estimate that the Pacific
Northwest states of Idaho, Oregon, and Washington could provide at least 6.5 million
tons of crop residues with the wheat straw and grass straw yields in 2007 after the
appropriate amount of wheat straw and grass straw are left on the field to protect soil
resources. However, those straws are often distributed across the landscape at an average
available density of about 1 ton/acre, so transporting them to centralized biomass-
processing plants would probably increase transportation costs and transportation-
dependent greenhouse-gas releases. One approach for overcoming those limitations
would be to establish localized preprocessing and densification centers. Similar estimates
by Nelson (2002) for Kansas, Texas, Ohio, Illinois, and Missouri projected the
availability of another 8.8 million tons of wheat straw for harvest from the Great Plains
each year. The panel estimated that 15 million dry tons of wheat and grass straws per
year could be available for fuel production on the basis of earlier studies. It assumed a 20
percent increase in available wheat and grass straws by 2020.
3
The panel uses the high harvestable value because it took a conservative approach to estimating the
amount of stover that has to be left in the field to maintain soil.
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Hay
U.S. hay production ranged from about 50 million tons in 1999 to about 142
million tons in 2006 (USDA-NASS, 2008b). The average yield in 2007 was 2.4 tons/acre.
Most hay is consumed as animal feed, but, as with corn grain, average yields are often
lower than many good producers achieve. On the basis of the 30-year record of increases
in hay yields, the panel estimated that 10 percent of the average production for 2003-
2007 (15 million tons) could be available for biofuel production without substantially
affecting the hay price and supply. The portion of the hay crop used as biofuel feedstock
was assumed to have very low nutritional quality for animal production because of
excessive weathering. The low-quality hay would be marketed only in areas where
biofuel plants provided an alternative marketing option to local farmers. The assumed
supply of hay for use as a biofuel feedstock is small because hay production is dispersed,
bulky, and expensive to transport. As with wheat and grass straws, the panel’s projection
of available hay for biofuels in 2020 was based on a 20 percent yield increase as a result
of better genetics and management practices.
Dedicated Fuel Crops
When this report was being written, most agricultural land in the United States
was being used for food, feed, hay, livestock, and forestry production or enrolled in the
CRP. This section considers the potential for producing biofuels on CRP lands to avoid
potential conflicts over land requirements for existing or future food, feed, and fiber
needs. Other lands—such as power-line rights of way, road rights of way, land classified
by the U.S. Department of Agriculture (USDA) as “idle” land, or lands abandoned by
agriculture sufficiently long ago as not to be classified—merit further study for their
potential to produce biomass crops. Although they are not formally considered in this
report, such lands might be used for production of dedicated fuel crops in lieu of or in
addition to the CRP lands discussed below. The potential yields from those lands,
however, have not been assessed, because few side-by-side studies of dedicated fuel
crops grown on lands of different fertility and climate have been performed.
The CRP compensates farmers for removing land from crop production for
environmental reasons (such as erosion control, water-quality improvement, and
provision of wildlife habitat by planting appropriate perennials) and economic reasons
(such as curbing production of surplus commodities and providing income support for
land owners) (USDA-FSA, 2008a). If the land has not been severely eroded or depleted
of essential nutrients and if expected rainfall patterns are not disrupted by increasing
climate variability, a portion of it could be used for dedicated perennial fuel-crop
production with appropriate site-specific agricultural practices. Planting an appropriate
species or mixture of perennials and harvesting them late in the growing season could
produce biofuel feedstock while potentially providing many of the same environmental
benefits envisioned for CRP land. Because some land was enrolled in the CRP because of
low yields of annual crops, the panel focuses on using such lands for perennials, which
generally are more efficient in using nutrients in resource-poor soil than are annuals.
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As of early 2008, about 35 million acres was enrolled in the CRP (USDA-FSA,
2008b). However, not all types of CRP land can be used for dedicated fuel-crop
production without losing their current environmental benefits. The different types of
conservation practices used on CRP land and those considered potentially compatible
with biofuel-crop production by this panel are listed in Table 2-1. The categories of
practices considered by the panel to be unavailable for biofuel-crop production included
those already in wooded areas, in wetland restorations, or containing particular wildlife
habitat. Using that classification, about 24 million acres of CRP land could potentially be
converted to appropriate dedicated fuel-crop production. Landowners, however, might
choose to leave land in the CRP for various reasons or to return it to food, feed, and fiber
production, an option that becomes more profitable as crop prices rise (Secchi and
Babcock, 2007).
Biomass yields depend on a host of factors, including location, choice of crop,
cultivation practices, fertility status, and seasonal weather patterns. Switchgrass (Panicum
virgatum) is the most immediately implementable and has been the focus of the
Department of Energy’s Bioenergy Feedstock Development Program for more than a
decade. Although more is known about switchgrass yields than about the yields of any
other proposed biofuel crop, the available data cannot yet adequately address the yields
likely to be achieved on potentially usable, typical CRP lands. A recent review of
published switchgrass yield trials across the United States showed an average annual
yield of 4.6 tons/acre (Heaton et al., 2004a). Farmers are more likely to plant the
cultivated varieties (cultivars) that had the highest yields in those trials, and a separate
tally of the two highest-yielding switchgrass cultivars in independent trials across the
United States showed an average of 6.1 tons/acre (McLaughlin and Kszos, 2005). Such
trials are generally used as the basis of models for predicting yields. Two studies
predicted an average yield of 5.4 tons/acre on existing cropland across the United States
with the use of best management practices (Graham and Walsh, 1999; McLaughlin et al.,
2002). Their predicted yield might not be achievable on CRP land if, as might often be
the case, its soil has degraded physical, chemical, and biological conditions or if it is
isolated in small fields or the terrain is not suitable for efficient mechanical harvesting. In
general, the land most likely to be put into switchgrass production (for example, CRP
acreage) tends to be of lower quality than test plots that are typically situated on fertile
ground. For example, McLaughlin et al. (McLaughlin et al., 2002) estimated switchgrass
yields on previously idled land to be 85 percent of those on land most recently in food
production.
Trials like those described above are typically conducted on small plots, and
although they are useful for evaluating ranking of cultivars best adapted to local
environmental conditions, the results are not necessarily indicative of what can be
expected of farm-scale production (Monti et al., 2009). Schmer et al. (2008) noted that
most biofuel-crop data are derived from small plots of less than 6 yd2 each, and they
assisted farmers in establishing farm-scale switchgrass trials in Nebraska, South Dakota,
and North Dakota. Average postestablishment yields in 2003-2005 were 2.7, 3.6, and 3.2
tons/acre in Nebraska, South Dakota, and North Dakota, respectively. In contrast, values
predicted on the basis of small-plot trials were 5.4, 5.1, and 4.4 tons/acre (Graham and
Walsh, 1999). Moreover, small-plot trials conducted concurrently in Nebraska with the
cultivars represented in the farm-scale trials yielded an average of 6.4 tons/acre, or over
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twice the average yield of the larger plots (Schmer et al., 2006; Vogel, 2007). Thus,
actual farm-scale fuel-crop production results in harvested yields about 35-50 percent
lower than those of small-scale plots. Lower yields in large-scale production might be a
result of farmers’ inexperience with the cropping system or differences in cropland
quality. But in the experiments of Schmer et al. (2008), farmers worked closely with the
researchers, and the land that was used had been in active annual crop production until it
was converted to switchgrass production.
An alternative biomass source is diverse mixtures of native prairie species—about
equal initial densities of legume species and warm-season grass species—and seems
likely to fare better in drier areas and on soils that are nitrogen-limited. In the only side-
by-side comparison done to date, a high-diversity mixture of perennial grasses, legumes,
and forbs had biomass yields about 200 percent greater than those of switchgrass
monocultures (Tilman et al., 2006). That one study, however, was done without
fertilization, with unimproved cultivars, and on a highly degraded soil of much lower
fertility than the land used in the studies of switchgrass and Miscanthus mentioned
earlier. Further field trials are necessary to assess the yield of switchgrass and mixtures of
perennial grasses. To provide a preliminary estimate of potential yield of perennial
grasses, the panel assumed that their yield is about 4 tons/acre, for two reasons: many
studies report yields of 2-6 tons/acre (Heaton et al., 2004a; Fike et al., 2006; Perrin et al.,
2008; Vadas et al., 2008), and producers are likely to use species or cultivars that have
high yields, and 4 tons/acre is about 60 percent of the high yield reported.
Another dedicated perennial fuel crop being evaluated and developed is
Miscanthus. Miscanthus is an exotic and potentially invasive grass species (unless sterile
hybrids that reproduce only vegetatively are used) from Asia that has high yield potential.
Recent European trials have resulted in average biomass yields of 10 tons/acre (Heaton et
al., 2004b). Yield trials in the United States have been limited to Illinois, where the
average yield was 13.2 tons/acre (Heaton et al., 2008), close to the 14.7 tons/acre
predicted for that state on the basis of European data (Heaton et al., 2004b). Miscanthus
has higher water requirements than switchgrass and therefore would have a more
restricted production range.
Although the initial result suggests that high-diversity mixtures rich in warm-
season grasses and cool-season legumes have the potential to be a viable source of
biomass on highly degraded land, further field trials are needed to test that possibility; to
determine the regions, soil types, and other conditions for which such mixtures,
switchgrass, Miscanthus, or other feedstock species would be superior biomass sources;
and to assess the effects of dedicated fuel crops on other ecosystem services. The panel
emphasizes that much work is needed to achieve greater confidence in any projections of
perennial grassland biomass production for biofuels.
Short-rotation woody crops, such as hybrid poplar and willow, could also provide
biomass while maintaining environmental benefits of the CRP (Johnson et al., 2007).
Woody-crop yield might be greater than average in New England and the northern
regions of the Great Lakes states (Graham and Walsh, 1999). (See also figures in
Milbrandt, 2005). Additional research to identify appropriate woody species on various
land types is needed.
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Finding 2-2
Improvements in agricultural practices and in plant species and cultivars will be
required to increase the sustainable production of cellulosic biomass and to achieve
the full potential of biomass-based fuels. A sustained research and development (R&D)
effort in increasing productivity, improving stress tolerance, managing diseases and
weeds, and improving the efficiency of nutrient use will help to improve biomass yields.
Recommendation 2-1
The federal government should support focused research and development
programs to provide the technical bases for improving agricultural practices and
biomass growth to achieve the desired increase in sustainable production of
cellulosic biomass. Focused attention should be directed toward plant breeding,
agronomy, ecology, weed and pest science, disease management, hydrology, soil physics,
agricultural engineering, economics, regional planning, field-to-wheel biofuel systems
analysis, and related public policy.
Crop residues; residues from pulp, timber, and other forestry operations; forest
thinnings; and some cover crops can be used to produce fuels that have much lower CO2
emission than fossil fuels if the biomass sources are harvested so as to preserve soil
carbon and nutrients and to minimize erosion. Some components of municipal solid
waste can also be used as cellulosic feedstock to reduce and reuse waste. Using biomass
as a resource for energy in a sustainable manner require holistic assessment of the effects
of biomass production or harvesting on soil, water, and air quality; food, feed, and fiber
production; carbon sequestration; wildlife habitat and biodiversity; rural development and
related issues; and the resulting supply of energy so that multiple concerns are addressed
simultaneously. If food crops or lands used for food production are diverted to produce
biofuel rather than food, additional land will probably be cleared elsewhere in the world
and drawn into food production. The greenhouse-gas emissions caused by such clearing
of land, especially forests, will decrease or even negate the greenhouse-gas benefits of the
resulting biofuels.
Finding 2-3
Incentives and best agricultural practices will probably be needed to encourage
sustainable production of biomass for production of biofuels. Producers need to grow
biofuel feedstocks on degraded agricultural land to avoid direct and indirect competition
with the food supply and also need to minimize land-use practices that result in
substantial net greenhouse-gas emissions. For example, continuation of CRP payments
for CRP lands when they are used to produce perennial grass and wood crops for biomass
feedstock in an environmentally sustainable manner might be an incentive.
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Finding 2-4
Depending on the locations in which it is grown and the management practices used
to produce it, the production of cellulosic biomass for fuels has the potential to
improve agricultural sustainability. Research that emphasizes the relationship between
cellulosic-biomass production and its surrounding landscape as a system is needed to
improve knowledge and understanding of the environmental effects of harvesting crop or
woody residues or growing the fuel crops and the potential ecosystem services that they
provide. Such research would require expertise in a wide array of topics.
Recommendation 2-2
A framework should be developed to assess the effects of cellulosic-feedstock
production on various environmental characteristics and natural resources. Such an
assessment framework should be developed with input from agronomists, ecologists, soil
scientists, environmental scientists, and producers and should include, at a minimum,
effects on greenhouse-gas emissions and on water and soil resources. The framework
would provide guidance to farmers on sustainable production of cellulosic feedstock and
contribute to improvements in energy security and in the environmental sustainability of
agriculture.
Large regions of the United States could produce sufficient biomass to provide
about 300,000 tons of biomass per year within a 40-mile radius of strategically located
biomass-conversion facilities. Biomass is also available in other regions but at lower
densities. The major U.S. regions that can deliver large quantities of biomass include
portions of the Northwest, the upper Midwest, and the East.
Finding 2-5
Biomass availability could limit the size of a conversion facility and thereby
influence the cost of fuel products from any facility that uses biomass irrespective of
the conversion approach. Biomass is bulky and difficult to transport. The density of
biomass growth will vary considerably from region to region in the United States, and the
biomass supply available within 40 miles of a conversion plant will vary from less than
1,000 tons/day to 10,000 tons/day. Longer transportation distances could increase supply
but would increase transportation costs and could magnify other logistical issues.
Recommendation 2-3
Technologies that increase the density of biomass in the field to decrease
transportation cost and logistical issues should be developed. The densification of
available biomass enabled by a technology such as field-scale pyrolysis could facilitate
transportation of biomass to larger-scale regional conversion facilities.
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