Research Article
Miscanthus agronomy and bioenergy feedstock potential on minesoils Biofuels (2014)
David A.N. Ussiri* & Rattan Lal The US government has mandated production of 79 million liters of biofuel from lignocellulose biomass and advanced fuels by 2022. To meet this requirement, the Department of Energy is encouraging research to develop herbaceous lignocellulose-based bioethanol for use as transport fuel. Miscanthus × giganteus was introduced in the US during 1930s, and is being widely studied for its potential to produce large biomass yield with minimum input in different soils. Miscanthus × giganteus is highly productive, sterile rhizomatous C4 perennial grass adapted to a wide range of climatic and soil conditions. Since the early 1980s, this crop has been studied under various climate and soil conditions in Europe and used to produce heat and electricity by combustion. This paper summarizes its agronomy and the characteristics which make it a potential dedicated bioenergy crop suitable for the reclaimed minesoils of the Appalachian region: Ohio, West Virginia, Virginia, Pennsylvania, Maryland, Kentucky, and Tennessee. The area which has been mined in the Appalachian region is estimated at 1.1 million hectares and only about 5% has been fully reclaimed. Using minesoils for miscanthus bioenergy feedstock production minimizes competition for arable land and sequesters soil organic carbon in these degraded lands. Reclaimed minesoils in the Appalachian region has potential to produce 9.22 × 106 Mg yr−1 dry biomass of Miscanthus × giganteus feedstock.
Acronyms AWC SOC SOM RFS WUE NUE RMSs
available water capacity soil organic carbon soil organic matter renewable Fuel Standards water use efficiency nutrients use efficiency reclaimed minesoils
Energy security and climate change are the two major forces currently driving the search for non-fossil sources of energy which are economically viable and environmentally sustainable in the US. Biofuels are widely considered as the renewable energy sources which can fulfill the need of replacing fossil fuels for transportation and power generation without major economic investments in its production and distribution. For example, biomass can easily displace coal in power and heat generation
without major changes in the infrastructure. Also, bioethanol and biodiesel can be blended with existing fuels without the need for the supply infrastructure, and most of the newer car models are capable of using the reformulated fuels. Therefore, biofuels can provide energy independence and also mitigate global warming in short and medium term to allow transition to cleaner carbon (C), C-free, or C-neutral sources. Studies on future energy systems also expect bioenergy to play a significant role in decreasing fossil fuels dependence as well as mitigating carbon dioxide (CO2) emissions [1,2]. Biofuels can be “C-neutral” or even C-negative depending on the feedstock and technique for generating the energy. They produce energy while releasing C that was captured during the plant growth cycle to the atmosphere instead of non-cycling C that was stored in fossil reserves for millions of years. Therefore,
Carbon Management and Sequestration Center, School of Environment and Natural Resources, The Ohio State University, 2021 Coffey Road, Columbus, Ohio, USA *Author for correspondence: Email:
[email protected]
© 2015 Taylor & Francis
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Bioethanol produc on (Liters (× 109)
conversion of natural vegetation to agricultural land with the associated impacts on biodiversity and GHG emissions [4]. Furthermore, establishment of bioenergy plantations on arable land can exacerbate competition for land resources, water, capital, and labor with food production, resulting in food shortage and unstable prices. Biofuels have historically been more expensive to produce and use than fossil-fuel based energy. But since late 1970s, policy makers have attempted to overcome this economic impediment by creating various incentive packages and enacting policies to support US biofuel production and use [5]. One of such incentives is the Renewable Fuel Standard (RFS) of 2005 and 2007, which guarantees stable markets for biofuels in the US. Considerable research has been done in developing corn grains (Zea mays L.) as source of ethanol for fuel consumption in the US, which resulted in its boom from 2005 to the present (Figure 1) [6,7]. The majority of renewable liquid fuel consumption accounted for by biofuels mandated by RFS currently derives from corn grains produced from 210 production facilities which
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use of sustainably produced biomass recycles the atmospheric CO2 without adding new sources. It has significant greenhouse gas (GHG) mitigation potential provided that the biomass feedstock resources are developed sustainably, efficient production systems are used, and savings in CO2 emissions gained through biofuels use is not offset by an increase in emissions of other GHGs during the biomass production chain, processing, and transformation of biomass to usable energy. In addition to being environmentally cleaner energy, biofuels can contribute to stabilization of farmers' incomes, maintain ecological and social sustainability [3], while also sequestering soil organic carbon (SOC) if proper feedstock and land management systems are used for energy crop production. However, large-scale production of biofuels could also have disadvantages. It could lead to direct and indirect
Key terms Bioethanol: fuel mainly produced by sugar fermentation process and used as petrol substitute for road transportation Appalachian region: a 530,900 km2 geographical area surrounding Appalachian mountain in the US stretching from southern New York to Alabama. It includes 13 states whose economy is highly dependent on coal mining, forestry, and agriculture Marginal land: land with undesirable characteristics that may limit agricultural crop production Mined land reclamation: restoration of land disturbed by mining activities
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Figure 1. Relationship between corn price and bioethanol production in USA. Bioethanol production data adapted from Renewable Fuel Association [6]. Corn prices adapted from United States Department of Agriculture, National Agricultural Statistics Service (USDA-NASS) www.nass.usda.gov.
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Miscanthus agronomy and bioenergy feedstock potential on minesoils Research Article
were in operation at the end of the 2013 [6,8], mostly in the Midwest “Corn Belt.” Corn grain production has high nutrients, water, and energy requirements, hence, the net GHG reduction due to corn grain bioethanol substitution for gasoline is 90% for herbaceous and agricultural residues
Hydrolysates are highly fermentable with lower inhibition Steam explosion
Cost-effective for hardwood biomass feedstock
Hemicellulose fraction is partly degraded
Inhibitory sugar degradation byproducts can be formed Biological
White rot and soft rot fungi
Organisms will degrade lignin Energy requirement is low Mild reaction conditions
groundwater and stream pollution due to excessive N and P leaching [64,66]. The increased bioenergy markets may become financial incentivized to either convert existing croplands to grow dedicated energy crops, which may lead to indirect land use change for the displaced crop and livestock production, or cause direct land use change by moving into forest and livestock production lands. Some of the pressing questions are what types of land can be used for sustainable biofuel production, where are the lands, how much is available, and what is the land currently used for? The answers to these questions will provide the basis for justification of the potential of biofuel and form the basis for evaluating the associated long-term environmental and economic impacts of biofuel production. The beneficial production of biofuel feedstock is that which do not compete with food crops for limited land resources, water scarcity, and nutrients, does not lead to land clearing, and offer a real GHG reduction. Fast growing perennial grasses have higher water use; therefore shifting from annual crops to perennial
Long reaction time of 2 to 4 weeks
grasses may lead to reduced water storage. However, the impact will vary depending on climatic zone [67]. In contrast, perennial grasses may be used for managing the water table in poorly drained soils [68]. But if poorly managed, fertilizer application, especially during establishment years, may lead to water pollution due to leaching [66]. Generally, proper land management under perennial grasses reduces movement of pollutants compared to annual crop systems due to growth in early spring [69,70] and their extensive root system which enables lower nutrients requirements than annual crops. A high yielding bioenergy crop such as miscanthus may be used to minimize the land demand for biofuel production. Table 3 shows the land requirement for selected bioenergy crops to produce 136 billion L required under RFS by year 2022. Miscanthus Miscanthus is a genus of C4 perennial grass which originates from East Asia. The most important species for bioenergy purposes are Miscanthus sinensis (2n = 2x = 38),
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dinucleotide phosphate-malic enzyme [NADP-ME]) type enables it to maintain high yielding potential in various climates and environments [75,76], and tolerate % of harvested cold climate better than almost all US cropland other C 4 plants [77]. It also has a 22.9 low nutrient requirement [21], high 34.8 water use efficiency (WUE) [78], and 13.8 the ability to sequester C and miti24.9 gate CO2 increase [79]. Other traits 67.9 that make miscanthus potentially 8.7 valuable are efficient photosynthetic mechanism, low fertilizer demand compared to annual crops, especially N, and rapid growth rate.
Table 3. Biomass yield of selected bioenergy crops, potential ethanol production land area needed for different energy crops to meet 136 billion L of bioethanol required under Renewable Fuel Standards.
Feedstock Corn grain Corn stover Total corn (grain + stover) Switchgrass Miscanthus
Harvestable biomass (Mg DM ha−1)
Ethanol production (L ha−1
Land area needed
10.2 7.4 17.6 10.4 3.8 29.6
4266 2805 7071 3936 1438 11205
31.0 47.2 18.7 33.7 92.1 11.8
Miscanthus sacchariflorus (2n = 2x = 76), and the sterile triploid between these, namely, Miscanthus × giganteus (2n = 2x = 57). Miscanthus sinensis has tuft forming rhizome with thinner stems, it relies primarily on seed for the reproduction, and is commonly found in montane environments that frequently get cold winters. Miscanthus sachariflorus has a broad creeping and thick stemmed rhizome. It is more adapted to warmer and wet climates where it is fast growing and has high productivity. The original hybrid M. × giganteus which arose from a rare natural hybridization of the two species has intermediate rhizomes and stems characteristics. It has more vigorous growth and higher yields than both M. sinensis and M. sacchariflorus [21] and, being a cross of a diploid and a tetraploid species, it produces a sterile triploid (i.e., having three sets of chromosomes which cause sterility due to uneven number of chromosomes) hybrid. This hybrid was first introduced to Europe by Aksel Olsen originally from Yokohama, Japan, to Denmark as a sterile hybrid ornamental garden grass in 1935. Karl Foster cultivated it and observed that it had vigorous growth [71]. It was originally named Miscanthus sinensis “Giganteus” hort [72]. It has been wrongly identified by various names such as M. sinensis Anderss. “Giganteus”, M. sacchariflorus var. brevibarbis (Honda) Adati, and M. ogiformis Honda [73], but it was confirmed to be an allotriploid hybrid with a chromosome number of 3n = 57, which combines genomes from diploid M. sinensis (2n = 38) and tetraploid M. sacchariflorus (4n = 76) by using DNA sequencing, amplified fragment polymorphism (AFLP), and florescent in situ hybridization [73] and subsequently classified as Miscanthus x giganteus Greef & Deuter ex Hodkson & Renvoize [74]. Miscanthus species, especially M. sinensis, have long been used as forages for horses and special breeds of cattle, and also for structural materials in Japan and China [22]. Only recently, M. × giganteus became the primary hybrid of choice as potential energy crop [22]. Its C4 photosynthesis pathway (nicotinamide adenine
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Plant genetics
Taxonomically, the genus Miscanthus is classified under Poaceae family, Panicoidae subfamily, and the Andropogoneae tribe, dominated by tropical grasses, including high economic value species such as corn sorghum and sugar cane. Within this tribe it is placed the subtribe Saccharinae [80]. The genus Miscanthus has been variably listed to comprise from 11 to 25 species [81, 82]. One type, M.× giganteus, has been identified for its potential biomass feedstock and placed in its own taxonomic classification based on its combination of propagation, growth, and developmental traits that other miscanthus species do not exhibit [83]. Miscanthus as an energy crop
The choice of biofuel crop is important for subsequent conversion processes and for energy yield. Different bioenergy crops have different environmental impacts due to variations in nutrient-use efficiency (NUE), WUE, pesticide demand, SOC sequestration, and biodiversity. Perennial grasses are environmentally beneficial compared to annual bioenergy crops due to elimination of the annual cultivation cycle which increases the potential for SOC sequestration. They also achieve rapid growth with the potential to produce high biomass yields with lower fertilizer and pesticide input, higher GHG mitigation, and biodiversity protection [28,52 ,84]. Lifecycle analysis (LCA) of heat, electricity, and liquid biofuel production indicate high energy savings and substantial reductions in GHG emissions for perennial crops [85]. For example, estimates of biofuel production from switchgrass indicate 94% GHG reduction compared with gasoline and 540% more renewable than nonrenewable energy consumed [86]. These advantages have been recognized, and large expansion of land for such crops is anticipated. Heaton et al. [87] outlined the characteristics of an ideal dedicated energy crop as a
Miscanthus agronomy and bioenergy feedstock potential on minesoils Research Article
perennial plant that: uses available resources efficiently, has high WUE, has low fertilizer requirement, sequesters SOC in the soil, and is non-invasive. Miscanthus × giganteus possesses almost all of these characteristics, as well as producing substantially higher biomass yield in marginal lands. As a result, it has attracted much interest as a biomass crop across Europe and US due to its high productivity even in cool temperate climates such as that of Northern Europe and North America [88]. Under optimum conditions, M. × giganteus can grow to heights of up to 7 m and produce predicted dry matter yields as high as 45 Mg ha−1 yr−1. In addition to being a perennial grass, two other features which make it attractive are: (1) translocation of minerals to the rhizomes at the end of the growing season, minimizing nutrients removal, and the amount of fertilizer needed to obtain high biomass yields, and (2) adaptation to a wide range of climates and soil conditions. Concerns over fossil dependence, beginning in the 1970s, increased interests in biomass energy as a substitute for imported crude oil in Europe and the US. In their search for an ideal bioenergy crop, the US started the Herbaceous Energy Crops Research Program (HECP) under the Department of Energy (DOE) in 1984, which screened several potential herbaceous plant and selected switchgrass as a model perennial grass to concentrate the research efforts on. Similarly, in Europe several perennial grasses were screened and miscanthus was one of the plants chosen for a more extensive research program [21]. The sterile clones from Denmark were spread across the European Union (EU) for trials [81,89] and it has been studied in Europe since 1983 [81]. It has been studied under different programs at national and EU level [21,90]. Two prominent EU projects – Miscanthus Productivity Network (MPN) and European Miscanthus Improvement (EMI) – are the influential programs on the availability of data and current understanding of Miscanthus spp. Miscanthus was not included in the initial screening of potential biomass crop in the US under the DOE in the early 1980s. The early focus was on switchgrass as a model herbaceous species beginning in 1983 [91]. However, Heaton et al. [92] modeled the productivity of M. x giganteus and predicted that it was likely to produce more biomass per unit area and inputs than switchgrass under similar conditions. Field trials were initiated at three sites in Illinois in 2001 by the University of Illinois at Urbana-Champaign [93]. Two- to four-fold superior yields for M. × giganteus variety than switchgrass were later confirmed [94]. Following these successes, research on miscanthus has expanded rapidly to other states and universities across the US. The EU programs focused on various aspects including biomass productivity potential, propagation and
establishment, management practices, harvesting and handling of biomass, breeding, and screening for different genotypes. These programs demonstrated that miscanthus has very vigorous growth and its biomass can be harvested in one cut in a delayed harvest, allowing it to dry out in the field. Three major limitations which were identified from these programs were: (1) narrow genetic base, (2) poor overwintering in colder climates of northern Europe, and (3) high costs of establishment, since M. × giganteus is a self-sterile triploid hybrid which does not form fertile seeds, and can only be propagated by rhizomes or micro propagation [21]. Propagation by using rhizomes increases the survival rate after the first winter, however. Planting methods have been developed that reduces the establishment costs by about 80% [89,95], and companies now offer specially developed machinery for planting rhizomes. Rhizomes harvested from mother fields are currently commercially available. Breeding and cloning for the desirable energy qualities and cost reduction still remain important development goals for achieving profitable biomass production from miscanthus, however. Although it conflicts with the intention to produce sterile genotypes to avoid the undesired spread into natural areas, the option of developing varieties of M.× giganteus with high yielding potential which can be established from viable seeds may be worth pursuing with the intent of further reducing establishment and production costs and increasing the profitability of biofuels production. One option which has been proposed in the US is hybridization of diploid M. sinensis and tetraploid M. sacchariflorus to produce viable F1 triploid seeds which may enable seed propagation without the risk of seed spread, but this still needs to be developed [96]. Suitable sites for production
Miscanthus × giganteus is adapted to a wide range of soils from sandy to heavy clay with high soil organic matter (SOM) content. The plant does well in sites that are well drained with medium to high fertility and pH ranging from 5.5 to 7.5. Although miscanthus grows best on some well drained soils, it also tolerates heavy soils and periodic flooding [97]. However, planting miscanthus in heavy clayey soils with flooding conditions may not suit winter harvest. When grown on marginal lands (e.g., mine lands) the yields are reduced. However, the yield is still better compared to other perennial grasses in the same location. Miscanthus can also grow on alkaline soils with pH > 8 but the crop establishment is poor [98]. Miscanthus has high WUE; however, substantial amounts of water are needed to sustain the maximum growth. In a water-stressed miscanthus, root growth constitutes the greatest proportion of the biomass gain [99].
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Planting and management
Due to its self-sterility, M. × giganteus must be planted by vegetative propagation by rhizome cuttings or micropropagation [21]. Most M. × giganteus are currently propagated by dividing the underground rhizomes, which form nodes, internodes, and buds similar to that of the aboveground stems. The underground rhizome system also serves as an underground overwintering storage organs reservoir of nutrients and for the initial aboveground growth. It produces new shoots from rhizome buds in spring, which use the reserve nutrients stored in the rhizome to initiate growth. The age of the M. × giganteus mother plant appears to be a factor in propagation and establishment success. The five-year-old plant can produce 88% successful clumps compared with 52% and 25% success for nine-year-old and one-year-old stumps, respectively [100,101]. Rhizome propagation can be performed in the late fall after plants have senesced and the biomass removed in early spring prior to emergence [102]. It is recommended for planting in spring on a clear field. Spring tilling and herbicide treatment prior to planting is necessary to control weeds and also to clear weed seed bank. Strip tillage is recommended in areas prone to soil erosion. Heavy layers of crop residues may impact the tiller emergence. The recommended tillage depth is at least 15 cm [97]. Recommended planting depth for rhizomes is 5–10 cm deep and of 75 × 75 cm inter-row and between rows spacing, respectively. Planting density of one to four rhizomes m−2 (i.e., 10,000 to 40,000 rhizomes ha−1) are reported for Europe [81], while in Illinois, US, 10,000– 12,000 rhizomes ha−1 are reported to be successful [100]. Additionally, Lewandowski et al. [81] noted that irrigation during the planting year may improve establishment rates. Weed control by herbicides or mechanical tilling for new planting is necessary for good establishment. Weed control by herbicides is only needed in first to second year stand, and only occasionally in case of development of certain persistent weeds during the lifecycle of the crop [21], otherwise there is no need for weed control after the third year after planting, since miscanthus is very competitive. It generally needs 3–5 years growth before it can yield maturely. Survival in the first year is highly dependent on the environment. The major risk to viability is the drop in soil temperatures to below -3 °C at 5 cm soil depth with mortality rates of up to 50% [95]. This appears to be a problem with the first year plants only, however [94,95], since survival for the established plants with winter air temperatures as low as -29 °C have been observed [100]. Miscanthus × giganteus begins growth from the dormant winter rhizomes when soil temperatures reach ∼9 °C, and leaf expansion occurs at temperatures between 5 and 10 °C [21], but 10 °C is considered as a standard
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temperature for consistent leaf extension [103,104]. This temperature is considerably lower than that for corn, and therefore miscanthus has a longer growing season than corn. Temperature is the most important factor that regulates leaf expansion. Optimum photosynthetic capacity is achieved at temperatures >12°C. Photosynthesis in miscanthus is much less sensitive to cool climate than other C4 species such as corn. Tillering increases rapidly during May–July, reaching up to 40 stems per plant [105]. However, rhizome propagated plants tend to have fewer but stronger shoots than micro-propagated plants. Crop productivity is limited by late emergence and early spring radiation in cool temperate regions. For earlier emergence, shoots must be tolerant to frost. In general, M. sinensis clone (Sin-H9) has lower thermal requirements and higher frost tolerance [106], suggesting that yield gains in cool temperate zones could be achieved if these traits can be combined with high shoot density, radiation-use efficiency, and rapid leaf extension, the best traits of M. × giganteus. Miscanthus accumulates biomass rapidly in summer and peaks around September [94,107]. Biomass yields are often the highest in late summer, but the moisture content and nutrients removal rates are also the highest. Fertilizer need
The importance of fertilizer application is not clear in the literature. Even though the higher yields have been reported in fertile soils [75], higher yields are also realized on marginal soils if other environmental conditions (e.g., temperature and soil moisture) are favorable [94]. Generally, N input needs are minimal due to efficient nutrient recycling. Most of N is translocated into the roots, rhizomes, and litter by the winter [108]. Efficient translocation helps to ensure adequate nutrients supply for the following season and reduce the need for additional fertilizer [109]. It also minimizes the inorganic compounds in the harvested feedstock which would become fuel pollutants [110]. Stands grown at Rothamsted, UK, showed no response to added N over a 14-year period during which all biomass was removed each year [111,112]. However, in the warmer and drier climates of Italy, Portugal, and Turkey, M. × giganteus produced greater than 35 Mg ha−1 under irrigation and N fertilization, documenting yield responses with N fertilization [113,114]. In Turkey, the dry matter yield for the first year ranged from 1.5 to 1.6 Mg ha−1, and increased for two consecutive years, reaching 13.4 to 26.7 Mg ha−1 in the third year since the establishment. The highest yield was obtained with 100 kg N ha−1 fertilization [113]. In the semi-arid Mediterranean region, irrigation and N fertilization was applied at 100 and 50 kg ha−1, and in the fourth and fifth year no irrigation or fertilizer was applied. The dry matter yield increased from
Miscanthus agronomy and bioenergy feedstock potential on minesoils Research Article
2.5 Mg ha−1 in the first year to 26.9 Mg ha−1 in the third year, which was maintained in the fourth and fifth year without irrigation or fertilizer input. Rate of fertilizer input had no effects on yields, however [114]. Miguez et al. [115] conducted a meta-analysis of the effects of management factors on growth and biomass production based on reported biomass and management in Europe and observed that M. × giganteus responded to N fertilization only after the third growing season, while planting density had a significant effect on the second growing season. The analysis of thermal time against dry biomass showed that the dry biomass was stable among other management and environmental factors across many locations in Europe [115]. In Illinois, US, minimal yield gains enough to stop long-term yield decline observed for 13-year-old trials have been estimated [116]. Overall, these data on N response are variable and mostly related to the soil fertility of the site and genotypic variations [92,108,117]. High biomass yields mean that significant plant nutrients will be removed regardless, necessitating return of nutrients for long term sustainability. For example, autumn and spring harvest of 150–250 kg N ha−1 (0.3–0.8% N in dry matter) and 17–100 kg N ha−1 (0.1–0.8% N in dry matter), respectively, have been recorded [118,119]. Even though the potential for plant nutrients removal is high due to high yields, the N fertilizer response has been limited, and varies among sites [112 ,120]. To balance the nutrient removal, it has been suggested that M. giganteus most probably hosts N fixing organisms that could provide up to 250 kg N ha−1 yr−1 [112]. This hypothesis has been supported by the presence of nitrogenase enzyme activity from whole rhizomes and isolated bacteria [112]. Therefore, the nutrient requirements for miscanthus are low compared to other crops due to high nutrient absorption by the extensive rooting system, high NUE, significant nutrient recycling which minimize nutrient removal during biomass harvest, and possibly N fixation by bacteria. Harvesting
The crop is typically harvested after it senesces, remobilizing the nutrients to the rhizomes, and crop dying have occurred in the period after killing frost but prior to spring growth. It is harvested when the stems are fully dried out in the field. Retranslocation of nutrients into rhizomes makes nutrient efficiency of miscanthus higher compared to other agricultural crops [107]. Miscanthus × giganteus is not typically harvested in a year it is planted due to low yields and possible negative impacts on its survival during the crops' critical first winter. For Illinois, US, the harvesting has been estimated to be between November and March depending on the demand for feedstock and ability to access fields
under wintery snow conditions. Biomass yield drops by about 25% during the dying period and up to 25% additional harvestable biomass dry matter can be lost as leaves drop [79,94] and nutrient translocation to perennial rhizomes and roots [107,121]. In European trials, genotypic variation in plant height is associated with flowering time such that the tallest genotypes flowers later; late flowering is associated with late senescence and higher yields [75]. In more northerly regions, the frost kills the leaves and reduces translocation of nutrients to rhizome [75]. Diverse field trials have demonstrated the harvestable yields ranging from 5 to 55 Mg ha−1. The aboveground biomass of 30 Mg ha−1 have been obtained in mid-September in England and Ireland, which declines to about 20 Mg ha−1 by February due to leaf-fall for the 3–4-year-old stands [107]. In Illinois, US, an average yield range of 30–40 Mg ha yr−1 across three sites have been demonstrated [94]. Miscanthus × giganteus outperformed modern lines of corn grown side by side by 59% in Illinois, US, though corn is heavily fertilized and no fertilizer is added to M. × giganteus during the comparison season [122]. Illinois has recorded some of the highest yields of corn in the world. In Kansas, the yield M. × giganteus increased significantly from first year of establishment to second year, reaching 12.8 Mg ha−1 in the second year compared to corn yields at 10.1 Mg ha−1 [123]. However, the nutrients removal by miscanthus was not affected by the yield increase. Nutrients removal was the highest for corn grains [124]. The results indicated that higher annual grain yields will also result in higher nutrients (N, P, and K) removal, but miscanthus biomass yield increase will not result in increased nutrient removal. Higher biomass yield, low input demand, and low nutrients removal during biomass harvest are the main factors why recent analyses concluded that M. × giganteus in warmer temperate zone is the bioenergy crop that seems to deliver the highest GHG mitigation [84,112]. The root biomass – up to 20 cm depth – and rhizomes varies over the course of the year, showing a pattern of nutrients mobilization with depletion of biomass in spring and early summer followed by increase that peaked in early November in the UK [107], Germany [109], and the Midwestern region of US [125]. Translocation of nutrients at the senescence ensures low nutrients removal with biomass harvest. Harvesting technique is currently the area of active research. Typically, modern hay-harvesting equipment including cutters, conditioners, and balers are suitable for the harvesting of M. × giganteus, but the equipment needs to be operated at a slower speed than in hay due to the density and toughness of miscanthus stems. The lifetime of M. × giganteus stand can range from 15–30 years [92,103], and can be split into yield building
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phase – lasting from 2–5 years [81,126] depending on climate and planting density [104] – and plateau yield phase, where yield is maintained [21,81,111]. The plateau yields are related to length of the growing season, with higher yields associated with longer growing season [126]. Plateau yield is attained more rapidly in warmer than cooler climates when water supply is not limiting [104]. Variable harvestable biomass after achieving plateau yields are attributed to annual weather conditions including moisture availability and possibly K deficiencies, since K uptake is often high for both fertilized and unfertilized treatments. In Ireland, soil and tissue analysis indicated that K contributed more to the observed yield increases than did N and P fertilizers [79], which did not show any significant influence on yields. However, most studies reported followed M. × giganteus for only a short period ranging from three to five years. Only a few studies have reported yield of M. × giganteus for a longer term of 10 or more years [79,111]. The longer term trials at Rothamsted, UK (∼51 °N), and Cashel county, Tipperary, Ireland (52°N) in cool climates noted an increase in the harvestable biomass during the first five years of growth [79,111]. In warmer climates the plateau yield is often achieved in two to three years [75,94]. Effects of temperature on photosynthesis
The mechanism used for fixing CO2 during photosynthesis in C4 plants enables these species to increase CO2 concentration relative to O2, and in turn eliminate photorespiration [127]. The unique photosynthetic reactions in the C4 pathway are believed to concentrate CO2 in bundle sheath cells which are one of the critical factors accounting for various physiological features typical of C4 plants such as high light saturated photosynthesis rates, high quantum yield, absence of effects of O2 on photosynthesis, and negligible photorespiration [128,129]. Although more energy is required for each CO2 assimilated, this energy is less than that lost in photorespiration when temperature exceeds 25 °C [130]. As a result, C4 photosynthesis has higher efficiency of light, water, and N compared to C3 photosynthesis [131]. However, this advantage is rarely realized in cool climates, because many C 4 plants cannot maintain photosynthesis-functional leaves at cold temperatures below 15°C. The majorities of C4 species are of tropical and subtropical origin and, therefore, unsuited to cool temperate climates. However, there is a small number of C4 species native to cool climates which shows increased tolerance to low temperatures. Corn is a C 4 crop cultivated in cool temperate climates. Although corn is the most tolerant of major C4 crops in cooler temperate climates, the initial establishment of crop and subsequent canopy development is limited by temperature [131]. If leaves are developed at suboptimal
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temperatures, the photosynthetic apparatus of the leaves is impaired due to reduced levels of thylakoid proteins, and reduced activity of stoma enzymes, which results in light saturated CO2 uptake. Variation in photosynthesis capacity of leaves will influence canopy development and dry matter production [132]. Loss of photosynthetic capacity in temperatures below 15 °C is the key limitation for extending the range of corn in cool climates and to extend its growing season in the temperate zone [131, 133]. Miscanthus × giganteus is an exception in that it can maintain high photosynthesis rates when transferred to temperate zones of temperatures below 15 °C [132]. Unlike other C4 species, it is able to realize the photosynthetic potential of the C4 process without suffering any temperature impairment, except at the very end of the growing season [132]. It is very productive even at 52°N latitude in UK, where summer is insufficient for corn to grow through to grain yield, producing a peak biomass of 30 Mg ha−1 yr−1 and harvestable biomass of 20 Mg ha−1 yr−1, the highest yield recorded for cool temperate climates [88]. Miscanthus can develop leaves at lower temperatures than corn and is less prone to photo inhibition at chilling temperatures. Tests in both Europe and the US have shown that the species from Genus Miscanthus can maintain high CO2 assimilation at temperatures below 15 °C [134,135]. It can maintain high rates of photosynthetic C assimilation metabolism at 10°C which allows it to utilize more of the absorbed light energy [134]. In trials in the Corn Belt in the US, M. × giganteus has the ability to produce photosynthesis viable leaves during the cooler period at the end of the growing season within the temperate environment [121]. When transferred to lower temperatures, it can acclimatize at a molecular level that allows photosynthesis to continue, while corn cannot [134,135]. The biogeochemical mechanisms behind this tolerance are not fully understood. The enzyme pyruvate phosphate dikinase (PPDK) involved in CO2 fixation in the mesophyll cells of C4 plants initially falls when M. × giganteus plants are transferred from 25 to 14 °C environments as well as photosynthesis. But then this enzyme PPDK rises to the highest levels than that at 25°C with the stabilization of yields. In contrast, both yields and PPDK continue to decrease in corn [135]. Light interception and photosynthesis
Crop yields are closely linked to canopy photosynthesis during the growing season. Photosynthesis depends on the amount of radiation intercepted by leaves and the photosynthetic activity of the individual leaves. A mature stand of M. × giganteus is able to intercept 90% of useful photosynthetic radiation when leaf area index (LAI) reaches 3.2 [99]. On average, plants convert only
Miscanthus agronomy and bioenergy feedstock potential on minesoils Research Article
0.1% of the solar energy into biomass and, therefore, are less efficient and require a large land area for production of fuel feedstock [136,137]. However, in Illinois US, solar energy conversion efficiency of 1–2% without irrigation and only 25 kg N ha−1 applied in one season has been measured in M. × giganteus [94]. This is ∼60% more productive than corn in the Corn Belt zone [122]. Although photosynthesis is 60% higher in corn in midsummer, miscanthus develops a leaf canopy earlier than corn and maintains it later, and canopy photosynthesis is 44% higher in miscanthus, which corresponds with the observed biomass yield difference [121]. Pretreatment of Miscanthus for ethanol production
Miscanthus is a lignocellulosic feedstock and is naturally recalcitrant to chemical and enzymatic hydrolysis. Therefore, its effective utilization in biofuels production requires the development of pre-treatment technologies which are necessary to separate the main constituents: cellulose, hemicellulose, and lignin. The pre-treatment is a key step for subsequent enzymatic hydrolysis and fermentation steps in order to maximize the production of bioethanol. Just as in other lignocellulosic biomass, the goals of pretreatment are: (1) to produce highly digestible solids with enhanced sugar yields during enzyme hydrolysis, (2) to avoid degradation of sugars to furans derivatives and carboxylic acids which are fermentation inhibitors, (3) to be cost-effective at both demonstration and commercial scales, and (4) to recover lignin and other pretreatment byproducts which can be converted into valuable co-products. The composition of cellulose, hemicellulose, and lignin in Miscanthus plays an important role in optimizing strategies for biofuels, chemicals, and bio-power production. Miscanthus contains ∼40%, ∼25%, and ∼26% by weight cellulose, hemicellulose, and lignin, respectively [138,139]. Harvesting miscanthus when stems are fully dried out – usually in February – generally leads to higher cellulose. The degree of polymerization (i.e., the number of glucose units that makes up one polymer) and crystallinity of the cellulose are the two limiting factors of the conversion of cellulose to bioethanol. It is generally accepted that crystalline regions of cellulose are more difficult to degrade than the amorphous domains due to strong intermolecular hydrogen bonding. Lignin, a cross-linked hydrophobic polymer, forms a protective sheath around carbohydrates: it protects against enzymatic hydrolysis to simple sugars [140], and its effective utilization for ethanol production necessitates the deployment of cost effective pretreatment technologies that are necessary to separate the polymers and enhance the enzyme digestibility of cellulose. Therefore, the purpose of pretreatment is to remove
as much lignin as possible. Several types of chemical pretreatment that have been tested for the miscanthus pretreatment, including: (1) alkali pretreatment, (2) dilute acid auto hydrolysis, (3) explosive pretreatments, (4) organic solvents (organosolv), and (5) ionic liquids. Reaction conditions, advantages, and disadvantages of different pretreatment techniques are summarized in Table 4. Alkali pretreatment is effective in removal of lignin from the biomass and, therefore, improves the reactivity of the remaining polysaccharides. It utilizes lower temperatures and pressures compared to other pretreatment technologies. However, the reaction durations are usually longer. Dilute acid hydrolysis, especially H2SO4, depolymerizes hemicellulose sugars with only a slight effect on cellulose and lignin. Pretreatment with dilute H2SO4 results in an increased surface area and hydrophilic nature which may enhance enzymatic action in miscanthus, but it may also increase the inhibitive effect of lignin [141]. Wet explosion combines steam explosion and wet oxidation in one treatment process which results in physical rupture of the biomass and chemical degradation. Ammonia (NH3) fiber expansion (AFEX) is a type of explosive pretreatment which utilizes concentrated NH3 at 70–180°C and pressures ranging from 15 to 70 bar. When pressure is explosively released, it effectively disrupts the structure of the biomass. Organic solvents (organosolv) are pulping processes which extract lignin from lignocellulosic feedstocks either with organic solvents or their aqueous solutions. It allows fractionation of lignocellulosic feedstocks into the three major components of cellulose rich pulp, organosolv lignin, and oligosaccharides. The production of high quality lignin is one of the advantages of using organic solvents pretreatment. Some organic solvents which have been tested for Miscanthus include acetone and formic acid (Milox) [142]. One of the disadvantages in most of the pretreatment techniques discussed above that promote depolymerization of lignin is the formation of degradation products which inhibit downstream processing of biofuels. To avoid this drawback, two stage pretreatments are considered the best option. The first step involves hydrolyzing hemicellulose, while the second step, which can involve much harsher pretreatment, can be used for separation and recovery of lignin. Potential of Miscanthus × giganteus as bioenergy in minesoils Miscanthus requires relatively low levels of nutrients compared to the annual crop, and have high yielding potentials on the marginal lands unsuitable for seasonal crops. The perennial lifecycle and deep rooting system provides valuable land cover to protect the
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Table 4. Summarized different pretreatment methods, their advantages and disadvantages. Pretreatment method
Reaction conditions for common pretreatment
Advantages
Disadvantages
Dilute acid
1–4% H2SO4, 130 °C, 15 minutes
High glucose yield Hydrolysis and recovery of
High cost of acids and need for acid
hemicellulose
recovery
High cost of corrosive resistant equipment
Alkali
1.5 M NaOH, 145 °C, 30 minutes
Wet explosion
O2, H2O2, 170 °C, 18 bars, 5 minutes
Efficient removal of lignin Low inhibitor formation Cost effective Lignin transformation and hemicellulose solubilization
Formation of inhibitors High cost of alkaline catalyst Alteration of lignin structure Sugars degradation High cost of corrosive resistant equipment
High yield of glucose and hemicellulose Organic solvents (Organisolv)
Ionic liquids
Ammonia fiber expansion (AFEX)
Autohydrolysis
Two-step pretreatments
Biological treatment
a) Formic acid/Acetic acid, H2O2 for 3hours at 107 °C. b) Milox: formic acid-H2O2-water
Efficient removal of lignin High glucose yield Low inhibitor formation Lignin recovery Ionic liquids with acetate, chloride, and Mild processing conditions phosphate ions are good solvents for Lignin and hemicellulose hydrolysis Miscanthus Ability to dissolve high loading of Ammonia at 2:1 (w/w) ammonia to biomass ratio, 160°C for 5 minutes
Oligo xylanes
Dilute acid + wet explosionDilute acid + organosolvEnzyme + organosolvAutohydrolysis + organosolv Soft rot fungi to remove lignin, brown rot to deconstruct hemicellulose
different biomass types Cellulose becomes more accessible Low formation of inhibitors Renders lignin inactive to enzymes High effectiveness for herbaceous material Hydrolysis and recovery of hemicellulose No need for catalysts Higher sugar yield Enhanced lignin dissolution
Environmentally friendly process Low energy requirement No chemical required
land from soil erosion, and can also sequester SOC in the sub-soil where it remains stored for longer periods. Its extensive rooting system, nutrient recycling between the rhizome and aboveground biomass, and nutrient recycling through leaf litter fall at senescence are valuable attributes that makes it attractive for marginal lands such as minesoils with depleted SOC and low soil fertility. It can also provide habitat and feed for wildlife [118]. It has the potential to provide significant fossil substitution [94], and GHG mitigation potential [79, 84]. Miscanthus also has a deep rooting system of more than 1 m deep, which can be a valuable attribute for SOC sequestration. However, production longevity in minesoils in the Appalachian region is uncertain. Its
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Recycling of solvent and/or catalysts High cost of corrosive resistant equipment
High cost of reagents High solvent costs Need for solvent recovery and recycling
High cost of ammonia High costs of corrosive resistant equipment
Alteration of lignin structure Solid mass left over ( consisting cellulose/lignin) will require disposal
High cost of acid and need for acid recovery
High cost of equipment Need for solvent recovery Slow process
ability to grow in marginal lands with low nutrients can minimize the competition with food crops for arable lands and avoids the potential for direct and indirect land clearing associated with biofuel expansion [143]. However, it is better suited to areas which receive at least 700 mm of rainfall per year, and biomass yield increases with increase in rainfall [74]. Application of N, P, and K could also improve the yield potential in minesoils depleted in plant nutrients. Nutrient translocation from the aboveground to the underground rhizomes in autumn at the crop's senescence preserves plant nutrients for next year's growth and minimizes the need for fertilizer application in minesoils. Leaf litter fall prior to harvesting also increases SOC input to minesoils.
Miscanthus agronomy and bioenergy feedstock potential on minesoils Research Article
Marginal lands
Concerns of large scale production of bioenergy crops competing for limited arable land for food, feed, and water resources have caused the stakeholders to examine many bioenergy feedstock production options. As of 2007, the US had about 166 million hectares (Mha) of cropland, of which 9% is classified as idle cropland or abandoned [144]. Approximately 4 Mha are devoted to corn production for bioenergy. In 2013, 42% (40.1 Tg) of the harvested corn grains was used for ethanol production. To meet the increasing demand for biofuel, use of the arable lands for bioenergy feedstock production is becoming a controversial issue and has created a food versus energy dilemma [143]. It raises major nutritional and ethical concerns, since growing crops for fuel utilizes arable land, water, and energy resources that could otherwise be used for food production for humans. Good public policy can ensure maintenance of food security by limiting use of food and feed for bioenergy. Competition of finite land and water resources between bioenergy crops and food and feed production can be alleviated by using arable lands with no restriction for food crops and growing dedicated bioenergy crops in the marginal lands. Use of such land eliminates competition with food crops, while also minimizing potential for direct and indirect land clearing associated with expansion of biofuel production. If properly managed, use of marginal land could increase wildlife habitat, improve water quality, and increase SOC sequestration in these soils [41,145]. Marginal lands have low inherent productivity for agriculture, are susceptible to severe degradation, and are of high risk for agricultural production, but they can benefit from perennial grasses which do not require annual tillage. Perennial grasses such as miscanthus have higher net primary production (NPP), require low inputs, and have high drought tolerance [146 ,147]. They can also sequester SOC and increase the rate of restoration of these lands. Marginal lands are generally enrolled in the Conservation Reserve Program (CRP) or under permanent grass cover as soil erosion control measures, and also serve as wildlife habitat, or rangeland [144]. The primary purpose of CRP is to remove highly erodible and environmentally sensitive croplands and pasturelands from production and restore them to grassland, trees, and other vegetative covers to reduce erosion and water pollution. An estimated 86 Mha or 23% of US cropland area is eligible to participate in CRP, and 12 Mha is already enrolled in CRP [148]. Those lands which are not under sustainable economic use (i.e., livestock production or to provide essential habitat for wildlife) may be underutilized compared to the environmental and economic potential of bioenergy production [50,
149].
Marginal lands in the US can be grouped under three categories: (1) agriculturally marginal lands (which includes CRP lands) – these are lands with low inherent productivity for agriculture and are prone to degradation; (2) abandoned farmlands – consisting of lands that were once used for agriculture or pasture but abandoned without being converted to forest or urban residential areas [150]; and (3) degraded lands that have been drastically disturbed such as by mining activities [151], some of these have been reclaimed and others have not been fully reclaimed. They are unable to completely recover naturally and, therefore, they are of low value and underutilized. Minesoils and bioenergy crop production potential
Minesoils are formed on landscapes altered by human activities of mining. Sometimes referred as spoils, anthropogenic soils, or Anthrosols [152], these soils exhibit profile characteristics, physical, chemical, and biological conditions that reflect anthropogenic perturbations of mining rather than natural soil forming processes [151,153]. Minesoils are generally characterized by heterogeneous mixture of rock fragments and sediment material. Surface mining (either strip mining, open pit, or mountain top removal, mainly for coal, sand, gravel, stone, gold, phosphate, and iron) results into drastic land disturbance and land degradation. In Ohio and most of the Appalachian region, surface mining for coal is the dominant anthropogenic activity which creates most of the existing minesoils. Coal mining in the US has disturbed an estimated 2.4 Mha since 1930 [154]. More than 1.0 Mha of the affected land is in the Appalachian region [155]. Surface mining, also known as opencast mining or strip mining, is the method of choice used for extracting coal when the coal seam is relatively shallow (40 to 50 m) and the geomorphology allows easy removal of overburden. However, once disturbed, complete restoration through natural recolonization and secondary succession cannot be achieved for most surface mined lands, thereby necessitating managerial reclamation. In recognition of this necessity, the US Federal Surface Mining Control and Reclamation Act (SMCRA) of 1977 (Public Law 95-87) was enacted to mandate reclamation of lands disturbed by coal mining [156]. Although SMCRA requires restoration of land to nearly original contour, there are flexibilities that allow land forms decision to be made on site-specific basis, which takes into account the post-mining land use. In the Appalachian region where flat lands are scarce, the creation of flat lands through mining is valuable commercially, especially for pasture and future agricultural
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land use. Most of minesoils which have been reclaimed after SMCRA in Ohio and in the Appalachian region are restored to grassland. The process involves application of topsoil to reconstruct suitable rooting media for vegetation establishment and soil amendments, commonly lime, if needed, fertilizers such as N, P, and K, and mulch, and establishment of quick and aggressive growing cool season mixture of grasses and legumes to provide ground cover and minimize soil erosion [157]. As a result of this reclamation technique, there are vast lands in the Appalachian region, and south eastern Ohio in particular, which have been converted to grasslands. It is estimated that about 1.1 Mha have been disturbed in the Appalachian region, and only 68,300 ha (6%) are either partially or completely reclaimed and bonds released [158]. Properties of minesoils
Surface mining causes drastic perturbations of the original soil profile, and the level of disturbance generally exceeds the natural resilience of soils, leading to severe landscape degradation. The heavy equipment used during the grading causes high compaction. In addition, severe loss of SOM surface mining and reclamation leads to decline in soil quality and functions. The SOM losses can be attributed to lack of inputs from plant litter, mechanical mixing of A, B, and C horizons during removal and handling of overburden material, soil erosion, leaching, and accelerated decomposition of SOM in stockpiled topsoil [151]. Minesoils are generally characterized by low levels of some key nutrients (N, P, and K), high bulk density, high rock fragments, low water holding capacity, and generally low biomass productivity [151,159,160]. Minesoils also have altered soil pH, concentration of soluble salts, and sometimes metal toxicity. Mine soils are highly variable in terms of physical and chemical properties. Some may have unfavorably high or low pH, low cation exchange capacity (CEC), and low fertility requiring soil amendments and intensive fertilization. These soils are often characterized by poor soil structure, high bulk density (1.55–1.86 Mg m−3), high rock fragments (33–45%), low water holding capacity, and low productivity [160,161]. Results of several studies [162] indicate that the physical condition of the reconstructed land is the most limiting factor in achieving successful revegetation and productivity in minesoils. However, available data suggest that fully restored minesoils have high potential for C sequestration compared to other terrestrial soils, especially during the first two decades following restoration [151,163, 164]. High potential for C sequestration makes them the potential target for perennial bioenergy crop production without competition for scarce arable croplands. Perennial bioenergy crops such as miscanthus can
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assimilate atmospheric CO2 into aboveground biomass and transfer some of it into the deeper soil horizons via their deep and extensive root system. Crop productivity constraints of minesoils
Minesoils physical properties are generally most important in determining the productivity. Although chemical properties of newly RMSs exert the stronger initial influence on the productivity, this influence lessens as the rapid weathering of recently exposed materials and pedogenic processes take effect. During weathering, both toxic and non-toxic elements are released to the soil solution. Vegetation establishment enables accumulation of SOC which re-establishes biological activity in the minesoils. However, physical properties dictate the overall productivity potential of the minesoils. Soil compaction resulting from the applied pressure by heavy equipment stabilizing the land during reconstruction of the seedbed is the most common problem for minesoils in the Appalachian region. Compaction is also done to minimize differential settlement forms that may result in depressions with the potential to remain saturated for extended periods of time. The locations within RMSs where the compaction zone develops has a critical bearing to the extent in which it may limit root growth and also the possibility of amelioration. In many sites in the Appalachian reclaimed grasslands, compaction occurs at the interface between graded spoils and redistributed topsoil. Compacted minesoils lack the continuous macropore network to facilitate water movement, aeration, and root extension. Compacted minesoils generally lack notable structure and exhibit high bulk density and cone penetrometer index. As a result of the lack of structure and macropore network, hydraulic conductivity is also low. The relationship between soil properties and plant growth is shown in Figure 5. Natural soil processes that improve soil physical conditions are slow, especially if the compaction is in the deeper depths [157]. Other constraints include inherently high coarse texture, low aggregate stability, and greater rock fragments [165]. Rock fragments are not desirable in minesoils top layers, since they reduce CEC, water holding capacity, inhibit root penetration, and may damage farm equipment. Minesoils in the Appalachian region typically have high coarse fragments – ranging from 35 to 70% rock fragments [166], which also results into low clay content – and often highly variable chemical properties [166,167]. Due to high coarse fragments and low clay contents, available water capacity minesoils in the Appalachian are generally lower and highly variable – ranging from 3 to 33%, with potential for adverse plant moisture stress especially during drought periods.
Miscanthus agronomy and bioenergy feedstock potential on minesoils Research Article
Figure 5. Relationship between minesoils reclamation, soil compaction, minesoils properties, and plant growth characteristics.
Potential productivity constraints for miscanthus in minesoils Available water capacity
Decreased yields due to moisture stress will likely be observed in years with lower than normal rainfall. Field studies have shown significant yield reduction in corn and forage grown on previously reclaimed mined sites during drier years [168]. These trends are also expected for miscanthus biomass yields. With normal to above normal rainfall, the biomass yields will generally be similar to the unmined lands with decreases in yields expected during drier years mostly due to water stress. Minesoils are prone to moisture stress in drier than normal years than undisturbed lands. Similar yield decrease has also been observed for miscanthus in West Virginia
in 2012 compared to previous year due to soil moisture stress [168]. Minesoils with greater than 40% coarse fragments, generally those with very shallow or no topsoil, have low silt and clay fractions and have low water holding capacity which may also limit plant growth. Application of soil conditioners such as biosolids or sludge can increase water retention in minesoils with low clay contents and SOM [159]. Sewage sludge and related biosolids are the end products of municipal wastewater treatment. These can be produced in various formulations including liquid effluent, sludge cake, air dried, or heat-dried. Biosolids are normally stabilized to reduce pathogen levels by aerobic or anaerobic digestion, addition of lime or alkali, and composting. Use of such materials can be highly beneficial for minesoils where topsoil has low SOM content. Organic
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amendments affect soil properties through direct effects of added organic matter (OM) and indirect by modifying soil physical, biological, and chemical properties. For example, N release is slow in organic form compared to readily available fertilizer inorganic N which may be prone to leaching and runoff. In addition, organic carbon (OC) in biosolids provides instant energy source which supports biological properties of soil, increases microbial activity, and an increase in SOM improves poor soil physical and hydrological conditions resulting from mining and reclamation operation. Biosolids also increase the nutrient availability in minesoils [169]. Compaction
Compaction restricts root growth and may also limit the available water for the plants during drier periods. Soils with bulk density > 1.7 Mg m−3 are common in minesoils. Soil compaction can cause perched water table which creates anaerobic conditions and limits root respiration. Limiting bulk density for selected crops in minesoils is presented in Table 5. Miscanthus roots can grow as deep as 250 cm below the soil surface [170,171]. Compaction in minesoils potentially can inhibit root development and possibly reduce the soil depth available for plant nutrients and water. One time deep tillage or chiseling below 45 cm is generally recommended for correcting soil compaction and improving soil physical properties to increase root penetration [157,162]. Deep tillage can be undertaken prior to rhizomes planting. Chiseling of the minesoils can reduce bulk density and increase root and shoot biomass for minesoils under pasture [162]. Chemical and biological properties
The important chemical properties for miscanthus and other plant growth include pH, nutrient availability, SOM content, soluble salts concentration, and metal toxicity. The spoil which is backfilled during reclamation is generally less weathered and the topsoil which is applied to establish rooting media has low SOC content as a result of stockpiling and storage during Table 5. Effect of bulk density on root penetration of various plants in minesoils. Plant species
Limiting bulk density
Soil texture
Corn
1.69–1.80 1.67–1.80 1.60 1.60 1.75 1.60 1.46–1.63 1.75
Silty clay Silty clay loam Silty clay loam Silty loam Sandy loam Silt loam Clay Sandy soil
Soybean Wheat Alfalfa Grain sorghum Sunflower
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mining operations. Therefore, minesoils have inherently lower plant nutrients and SOC content [151,160]. Low nutrients and higher acidity can be corrected by the application of fertilizers, biosolids, and liming. Organic fertilizers have the advantage of increasing SOM, which can also correct physical and biological limitations. Most RMSs have inherently lower microbial biomass, compared to undisturbed soils [164]. Microbial properties are important in decomposing the added OM and recycling of nutrients for plant growth. Addition of biosolids can improve the biological properties of the minesoils. Prior to establishment of miscanthus in RMSs, one time deep tillage is recommended to reduce the compaction and improve soil physical conditions. Application of soil conditioners such as biosolids, sewage sludge, animal manure, compost, and/or mulching is also recommended to increase water holding capacity, microbial activity necessary for nutrients recycling, and increase aeration. Liming is also recommended for sites with soil pH
