Chapter 3 - Miscanthus: A Promising Biomass Crop

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Abstract

The C4 grass Miscanthus × giganteus is of increasing interest as a biomass feedstock for renewable fuel production. This review describes what is known to date on M. × giganteus from extensive research in Europe and more recently in the US. Research trials have shown that M. × giganteus productivity is among the highest recorded within temperate climates. The crop's high productivity results from greater levels of seasonal carbon fixation than other C4 crops during the growing season. Genetic sequencing of M. × giganteus has identified close homology with related crop species such as sorghum (Sorghum bicolor (L.) Moench) and sugarcane (Saccharum officinarum L.), and breeding of new varieties is underway. Miscanthus × giganteus has high water use efficiency; however, its exceptional productivity causes higher water use than other arable crops, potentially causing changes in hydrology in agricultural areas. Nitrogen use patterns are inconsistent and may indicate association with N fixing microorganisms. Miscanthus × giganteus has great promise as an economically and ecologically viable biomass crop; however, there are still challenges to widespread commercial development.

Introduction

Increasing the share of world energy that comes from renewable sources is critical to stabilizing the global climate (IPCC, 2007). Among renewable energy sources, only biomass can provide fuel and electricity in a form and scale that is compatible with existing transportation and power generation infrastructure (DOE, 2006). Unlike wind and solar energy, biomass can be converted directly into liquid fuel by a variety of conversion routes, as is current practice with petroleum, or it can be stored to generate electricity on-demand, as is the current practice with coal. It also provides raw material for renewable alternatives to fossil-based products. Biomass is also the only available source of renewable carbon for products currently made from fossil carbon sources.

How much biomass is needed? Of the 105 exajoules (EJ, 1018 J) of energy consumed in the US in 2008, only 4% or 4.1 EJ came from biomass sources, mainly from combustion of wood residues for heat and power by paper manufacturers (DOE, 2009). Energy consumption is expected to increase by 14% by 2034, to 120.8 EJ (DOE, 2010). Multiple acts of legislation currently under consideration in the US could further increase renewable energy demand 10–40%, leading it to comprise 14% of the total US energy demand, or 17 EJ y 1, by 2035 (DOE, 2010).

Over 900 million Mg of biomass per year is needed to produce 17 EJ y 1, assuming biomass to contain 18 MJ kg 1 (Jenkins et al., 1998) and energy conversion to be 100% efficient. Of course, conversion of biomass energy into useful forms like liquid fuels or electricity is not 100% efficient, and typical efficiencies range between 30% and 70%, depending on methods and accounting (Brown, 2003, Jenkins et al., 1998, Mohan et al., 2006). Assuming an average conversion efficiency of 50%, the US will require more than 1.8 billion Mg of biomass per year to meet renewable energy demands through bioenergy, or a little more than 50% of the entire US maize crop in 2009 (NASS, 2010).

Even if only a portion of US renewable energy comes from biomass, it will still have a major impact on cultivated and natural lands. The feasibility and impact of large-scale biomass production have been intensely debated and investigated in recent years (Dohleman et al., 2010, Dornburg et al., 2010, Fargione et al., 2008, Hertel et al., 2010, Hill et al., 2009, Kim et al., 2009, Levasseur et al., 2010, Reijnders, 2010, Smeets & Faaij, 2010, Solomon, 2010, Taheripour et al., 2010). Despite a wide range of conclusions, it is generally agreed that (1) resources are limited and (2) future agricultural systems must be sustainable.

It is reasonable to propose that crops that produce high biomass yields per unit land area be used to meet bioenergy demand, since they will require less land than low-yielding crops, and this is a key principle of biomass crop development (Heaton et al., 2008b). For example, the high-yielding perennial Miscanthus × giganteus could require 87% less land to produce the same amount of biomass as a low-input, high-diversity mixture of prairie species, because the yield of the M. × giganteus monoculture is nearly eightfold greater (Heaton et al., 2008a). However, while yield might be a driving selection criterion, it is not the only one, and future crop systems must be evaluated on their environmental and social functions, in addition to traditionally valued economic functions (Boody et al., 2005, Schulte et al., 2006).

Diverse cropping systems that fill all available environmental niches can provide more ecosystem services such as nutrient cycling, water retention and filtration and biodiversity than annual monocultures, but they are inherently more difficult to manage for biomass production because each species prefers different conditions in a given year (Russelle et al., 2007, Tilman et al., 2006). High-yielding perennials that are on the field for most of the year can offer a compromise by simplifying crop management over diverse mixtures while still providing ecosystem services (Heaton et al., 2004b, Schmer et al., 2008).

‘Sustainable’ has many definitions, most of them contentious with reference to agriculture. A useful metaphor to discuss sustainability is the ‘sustainability stool’. The legs of the stool are environmental, economic and social sustainability; if an agricultural system has inadequate performance in any of the three areas, the system will eventually collapse (Douglass, 1984). Perennial energy crops potentially can provide a solid foundation for sustainability with performance that is equal to or improved over that of annual arable crops.

Of the three legs of the sustainability stool, economic sustainability of agriculture receives the most attention. Globally, there has been a trend away from diverse crop rotation to simplified annual crop systems that has been accompanied by increases in yield and farm labour productivity, made possible through increased reliance on synthetic fertilizer, pesticides and subsidy payments for crops in surplus (Bullock, 1992, Malezieux et al., 2009, Schulte et al., 2006, Tegtmeier & Duffy, 2004). Beginning with the Soil Conservation Act of 1935, the US government has, like many developed countries, paid farmers to set aside land from arable cropping, and instead plant it to perennials as a soil conservation tool. As demand grows for highly productive land to produce food, feed, fibre and now fuel, however, the value of these government programmes fades in comparison to what a farmer can earn by producing a subsidy-protected grain crop.

Traditionally, it has been difficult and nebulous to value the ecosystem services provided by perennial agriculture (Farber et al., 2002, Liu et al., 2010, Porter et al., 2009), and without a harvested product for sale, perennials usually lose against annual crops in the marketplace. With the advent of a clear demand for energy from perennial biomass, farmers and conservationists may have their cake and eat it too, as the crops grown can be harvested and sold for a profit while still providing ecosystem services similar to those from set-aside land.

How does the economic return of biomass crops compare to that of traditional arable crops in the US? James et al. (2010) calculated the break-even price for a farmer in the Midwestern US to switch to a range of perennial energy crops and found that currently, none was economically viable against continuous maize production on highly fertile land. However, they evaluated M. × giganteus using current prices for rhizomes ($1.80 ea) and a future price anticipating improved production practices ($0.05 ea) and found that of all the crops evaluated, future M. × giganteus is more profitable than continuous maize, with a break-even price of only $45 Mg 1 (James et al., 2010). In on-farm trials with co-operators in Nebraska, South Dakota and North Dakota, Perrin et al. (2008) found that switchgrass could be grown at a commercial scale for about $50 Mg 1. By comparison, the costs for continuous maize production on prime farmland in Iowa are about $150 Mg 1 in 2010 (Duffy, 2010), suggesting that perennial crops are profitable and will be economically sustainable even on prime farmland in the US.

Perennial plants have long been associated with good environmental performance and improved ecosystem health. Without the disturbance of annual soil tillage above- and below-ground biomass accumulates, perennials protect and hold the soil against wind and water erosion while increasing soil quality and organic matter (Blanco-Canqui, 2010, Luo et al., 2010). An increased proportion of perennials in the landscape are also associated with an increase in biodiversity, as perennials provide habitat for animals and insects (Malezieux et al., 2009, Schulte et al., 2006). Additionally, perennial crops can increase the quantity and diversity of mineral nutrients available in the rhizosphere by establishing complex and often long-term relationships with the microbial community (Davis et al., 2010, Nehls et al., 2010).

The larger and active root system of perennial grasses is particularly effective at scavenging available nutrients and preventing them from leaching with draining water where they may act as pollutants (Allan, 2004, Randall et al., 1997). In the US Environmental Protection Agency's recent Science Advisory Board report on hypoxia in the Gulf of Mexico, the high losses of nitrate from current corn–soybean production systems on tile-drained landscapes in the Mississippi River Basin were clearly identified as a major source of the nutrients causing hypoxia (EPA, 2008). These losses occurred even when best management practices were applied. In that report, it was suggested that perennials were the best option to substantially reduce nitrate losses, but such a shift was unlikely, given current agricultural policies.

In a more specific example of how nitrate losses from current production systems could be reduced using perennials, Hatfield et al. (2009) evaluated a watershed in central Iowa. They observed that mean annual NO3–N concentrations in water have been increasing since 1970 in spite of no significant change in N fertilizer use for the past 15 years, and a decrease in cattle and hog production in the watershed. Upon evaluation of regional crop yields, land-use change and precipitation, they found that an increase in land planted to maize and soybean, at the expense of perennial pasture, were highly correlated with the increase in NO3–N concentrations. The authors concluded that the narrow window of nutrient uptake in maize–soy systems allowed more nutrients to leave the system, even though the amount of fertilizer applied was steady and crop yields were increasing. One suggested solution to reduce nutrient loading in the watershed was to plant more perennials with water use patterns that complement those of maize–soy (Hatfield et al., 2009).

Biomass energy may help revitalize languishing rural economies (Solomon, 2010). Even as industrial agriculture has delivered record crop yields and gross revenue in the past 50 years, farmer employment and profit have deteriorated (Fig. 1). The US Department of Agriculture (USDA) reports that a rural society that used to be characterized by small farms supported by farm sales has changed to large, concentrated farms, and over 40% of documented farms are in the ‘residential/lifestyle’ category. While the majority of US farms are still small farms, over 50% of their operators are retired or rely on another job as their principal occupation (NASS, 2007). Conversely, large farms, that is, those with revenue over $100,000 per year, comprise only 15% of all US farms, yet account for 88% of sales. In short, only a fraction of farmers can still make a living from farming (Duffy, 2008), and this is reflected in the steady decline of rural populations (US Census Bureau, 1990).

Job creation in the renewable energy economy supports the social sustainability of biomass cropping systems. A review of clean energy finance by the Pew Charitable Trust found global investment up by 230% since 2005, despite the largest economic downturn in at least 50 years, and clean energy investments are expected to grow to $200 billion by 2010 (The Pew Charitable Trusts, 2010). ‘Green jobs’ have been touted as the solution to the economic and environmental woes of many countries, and have received priority in economic recovery spending. Despite inconsistent government support, there are already more green jobs than biotechnology-related jobs, though biotech has seen steady government support (Fig. 2) (The Pew Charitable Trusts, 2009).

The low bulk density of biomass makes it inherently inefficient to transport (Fales et al., 2007, Rentizelas et al., 2009, Shinners & Binversie, 2007), necessitating local processing and handling, thus ensuring distributed jobs within regions irrespective of the fuel produced. In an analysis of case studies in Brazil and the Ukraine, Smeets and Faaij (2010) found that instilling a ‘strict’ set of sustainability criteria, for example, restriction of child labour, education of the workforce and mandatory healthcare, had positive community impacts with only a limited effect on the cost of bioenergy production from perennials. This was largely attributed to the reduced costs of perennial agriculture compared to annual row cropping systems.

Miscanthus is a genus comprising 14–20 species of perennial, C4 grasses native to eastern Asia, N. India and Africa (Clayton et al., 2008, Hodkinson et al., 2002a, Scally et al., 2001). As described in a review by Stewart et al. (2009), Miscanthus species have long been used for grazing and structural materials in China and Japan and have only recently become of interest for energy. Long recognized for their ornamental value, and as a germplasm source of stress tolerance in sugarcane breeding, Miscanthus species are now found and commonly naturalized in North and South America as well as in Europe, Africa, Asia and Europe (Clayton et al., 2008, Scally et al., 2001).

In 1935, Aksel Olsen brought a sterile Miscanthus hybrid that was of horticultural interest back from Yokohama, Japan to Denmark, where it was cultivated by Karl Foerster and observed to have vigorous growth (Lewandowski et al., 2000, Linde-Laursen, 1993, Scally et al., 2001). Originally named Miscanthus sinensis ‘Giganteus’ hort. (Greef and Deuter, 1993), it has gone by many names, including M. giganteus, M. sinensis Anderss. ‘Giganteus’ and M. ogiformis Honda (Hodkinson et al., 2002c). By using DNA sequencing, AFLP and fluorescent in situ DNA hybridization, Hodkinson et al. (2002c) confirmed suspicions that it was an allotriploid (2n = 3x = 57) hybrid of M. sinensis and Miscanthus sacchariflorus and subsequently formally classified it with the Royal Botanic Gardens, Kew in the UK as M. × giganteus (Greef & Deuter ex Hodkinson & Renvoize) (Hodkinson et al., 2002b).

Following concern over fossil fuel dependence beginning in the 1970s, M. × giganteus was evaluated along with several other species for potential as a bioenergy crop. The sterile clone from trials in Hornum, Denmark was spread across Europe, and included in both public and private trials (Jorgensen & Schwarz, 2000, Lewandowski et al., 2000).

Miscanthus × giganteus has been studied across Europe since 1983 under a multitude of national and EU programmes (Jones & Walsh, 2001a, Lewandowski et al., 2000). Two EU-wide projects, the Miscanthus Productivity Network (MPN) and the European Miscanthus Improvement (EMI), have been particularly influential on the availability of Miscanthus data today (Fig. 3).

In 1992, the 3-year MPN began as part of the European Agro-Industry Research programme (contract no. AIR1-CT92-0294). With 17 partners in 10 countries, the MPN aimed to ‘…generate information on the potential of Miscanthus as a non-food crop in Europe’, (Jones and Walsh, 2001b). Most trials used similar methods to assess potential productivity associated with water, nitrogen and low temperature limitation across different environments. Harvest, storage and utilization of biomass were also studied, along with genotype screening of other Miscanthus species. Generally, the MPN found M. × giganteus to be broadly adapted to a wide range of growing conditions, but was not the optimal choice in all locations tested (McCarthy, 1992). For a complete description of MPN results, see Jones and Walsh (2001a).

Following on from the MPN, the EMI project began in 1997 to address the limitations imposed by a narrow genetic base within M. × giganteus clones and better match genotypes with environments (Lewandowski and Clifton-Brown, 1997). Similar in structure to the MPN, EMI focused on crop improvement by developing breeding methods and assessing the genotype × environment interaction of 15 selected Miscanthus genotypes in five countries (Clifton-Brown et al., 2001a). The EMI project successfully identified genotypic variation in environmental performance among Miscanthus genotypes and has paved the way for current private and public breeding programmes in the US and Europe (Clifton-Brown et al., 2008).

In contrast to Europe, Miscanthus species were not included in initial screening of potential biomass crops in the US. There, research, supported primarily by the US Department of Energy (DOE), focused on switchgrass (Panicum virgatum L.) as a model herbaceous species beginning in the 1980s (McLaughlin, 1992, Parrish & Fike, 2005, Sanderson et al., 1996). In fact, it was not until 2004 that Heaton et al. (2004b) used the model MISCANMOD, developed by Clifton-Brown et al. (2000) in Ireland, to project potential M. × giganteus productivity in the US. Following promising modelled productivity, Heaton et al. (2004a)Heaton et al. went on to show that M. × giganteus was likely to produce more biomass per unit input of water, nitrogen or heat, than would switchgrass under the same conditions, and thus field research in the US was warranted. Superior yield of M. × giganteus over switchgrass was later confirmed in the first replicated trials of M. × giganteus in the US, at three sites in Illinois where measured yields of M. × giganteus were two- to fourfold higher than those of switchgrass, var. Cave-In-Rock (Heaton et al., 2008a).

Following promising initial results, a Strategic Research Initiative (SRI) was initiated at the University of Illinois at Urbana-Champaign to further investigate M. × giganteus in Illinois. Initial work by 14 investigators focused on a clone of M. × giganteus collected by the Chicago Botanic Garden and brought to the Urbana, Illinois campus in 1988 where it had thrived in a demonstration planting (Heaton et al., 2008a). This review will highlight research areas addressed by the SRI through support from the Illinois Council on Food and Agriculture Research from 2004 to 2009 (award 04-SRI-036) (Long, 2005). Research in Illinois has expanded exponentially in recent years, and has been the provenance of work on Miscanthus in the US, which has grown from non-existence 10 years ago to being underway in nearly every state today.

Focusing on M. × giganteus, this review will address modelled and observed productivity (Section II), the physiological basis for that productivity (Section III), breeding and genetic engineering efforts (Section IV), the environmental impacts of production (Section V) and the technical challenges to commercial production (Section VI).

Section snippets

European and US Trials

Here, we review the biomass production of M. × giganteus reported from trials over a wide geographic range, with emphasis on how yield varies with precipitation, temperature and soil conditions. While other reviews of Miscanthus productivity and suitability can be a good source of data that might be otherwise difficult to find (Jones & Walsh, 2001a, Lewandowski et al., 2000, Miguez et al., 2008, Smeets et al., 2009, Zub & Brancourt-Hulmel, 2010), our goal here is to provide an overview of M. ×

Physiology

As reviewed in the previous section, M. × giganteus has proved to be one of the most, if not the most, productive terrestrial plants in mid-latitude northern climates (35–60° N). The first replicated trials of this crop in the US showed yields of 30–40 Mg ha 1 y 1 across three sites in Illinois (Heaton et al., 2008a). In central Illinois, where some of the highest yields of maize in the world are recorded, M. × giganteus yielded 60% more shoot biomass, even though the maize crop was heavily

Taxonomy and Origins

The tribe Andropogoneae within the family Poaceae includes several species of natural and agricultural value, including the C4 grasses sorghum (S. bicolor L. Moench), maize (Zea mays L.) and sugarcane (S. officinarum L.). The subtribe Saccharinae includes the genera Saccharum L. and Miscanthus Anderss., species of which are currently under consideration as potential biomass crops for renewable energy production (Hodkinson et al., 2002a). These two genera are closely related with evidence

Environmental Impacts

Compared to annually cultivated crops, perennial grasses are often considered environmentally favourable because the more dense and continuous vegetative cover provides protection to the soil against erosion, may reduce runoff and nutrient loss and sequester carbon in the soil (Blanco-Canqui, 2010). Because perennials begin growth earlier in the year than annuals, perennial grasses are thought to be more synchronous with soil nutrient availability (mineralization) and plant uptake throughout

Technical Challenges to Commercial Production

Even though European researchers have studied M. × giganteus as a biomass feedstock since the early 1980s, and Illinois researchers have studied its use since the early 2000s, barriers remain to the commercial production of the grass. Given that the biomass potential of M. × giganteus is great for some temperate areas in North America, it is important that these hurdles be overcome in a timely fashion in order to avoid being unprepared should an energy crisis occur. These challenges occur in

Acknowledgements

This work was primarily supported by the Illinois Council on Food and Agriculture Research (Award 04-SRI-036). Additional support was provided by the Energy Biosciences Institute at the University of Illinois and the Iowa State University Department of Agronomy. The authors thank Dustin Schau for reviewing this manuscript.

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