Miscanthus giganteus


Miscanthus × giganteus, the giant miscanthus, is a sterile hybrid of Miscanthus sinensis and Miscanthus sacchariflorus. It is a perennial grass with bamboo-like stems that can grow to heights of more than in one season. Just like Pennisetum purpureum, Arundo donax and Saccharum ravennae, it is also called elephant grass.
Miscanthus × giganteus' perennial nature, its ability to grow on marginal land, its water efficiency, non-invasiveness, low fertilizer needs, significant carbon sequestration and high yield have sparked a lot of interest among researchers, with some arguing that it has "ideal" energy crop properties.
Some argue that it can provide negative emissions, while others highlight its water cleaning and soil enhancing qualities. There are practical and economic challenges related to its use in the existing, fossil based combustion infrastructure, however. Torrefaction and other fuel upgrading techniques are being explored as countermeasures to this problem.

Use areas

Miscanthus × giganteus is mainly used as raw material for solid biofuels. It can be burned directly, or processed further into pellets or briquettes. It can also be used as raw material for liquid biofuels or biogas.
Alternatively, it is possible to use miscanthus as a building material, and as insulation. Materials produced from miscanthus include fiberboards, composite miscanthus/wood particleboards, and blocks. It can be used as raw material for pulp and fibers as well as molded products such as eco-friendly disposable plates, cups, cartons, etc. Miscanthus has a pulp yield of 70–80% compared to dry weight, due to the high holocellulose content. The pulp can be processed further into methylcellulose and used as a food additive and in many industrial applications. Miscanthus fiber provides raw material for reinforcement of biocomposite or synthetic materials. In agriculture, miscanthus straw is used in soil mulching to retain soil moisture, inhibit weed growth, and prevent erosion. Further, miscanthus' high carbon to nitrogen ratio makes it inhospitable to many microbes, creating a clean bedding for poultry, cattle, pigs, horses, and companion animals. Miscanthus used as horse bedding can be combined with making organic fertilizer. Miscanthus can be used as a healthy fiber source in pet food.

Life cycle

Propagation

Miscanthus × giganteus is propagated by cutting the rhizomes into small pieces, and then re-planting those pieces below ground. of miscanthus rhizomes, cut into pieces, can be used to plant 10–30 hectares of new miscanthus fields. Rhizome propagation is a labor-intensive way of planting new crops, but only happens once during a crop's lifetime. New and cheaper propagation techniques is underway, which seem to increase the multiplication factor from 10–30 to 1000–2000.
A halving of the cost is predicted.

Management

A limited amount of herbicide should only be applied at the beginning of the first two seasons; after the second year the dense canopy and the mulch formed by dead leaves effectively reduces weed growth.
Other pesticides are not needed. Because of miscanthus' high nitrogen use efficiency, fertilizer is also usually not needed. Mulch film, on the other hand, helps both M. x giganteus and various seed based hybrids to grow faster and taller, with a larger number of stems per plant, effectively reducing the establishment phase from three years to two. The reason seems to be that this plastic film keeps the humidity in the topsoil and increases the temperature.

Yield

Miscanthus is close to the theoretical maximum efficiency at turning solar radiation into biomass,
and its water use efficiency is among the highest of any crop.
It has twice the water use efficiency of its fellow C4 plant maize, twice the efficiency as the C3 energy crop willow, and four times the efficiency as the C3 plant wheat.
This combined efficiency makes miscanthus fields energy dense. Since miscanthus has an energy content of 18 GJ per dry tonne, the typical UK dry yield of 11–14 tonnes per hectare produce 200–250 gigajoules of energy per hectare per year in that particular region. This compares favorably to maize, oil seed rape, and wheat/sugar beet,
underlining the differences between first and second generation bioenergy crops. In the USA, M. x giganteus has been shown to yield two times more than switchgrass.
Hastings et al. note that "ield trials have shown that for many locations in Europe M. x giganteus has the largest energy yield of all potential bioenergy crops in terms of net MJ ha −1 , and the highest energy‐use efficiency, in terms of the energy cost of production, due to its relatively high yields and low inputs ". The main competitors yield wise is willow and poplar, grown at short rotation coppice or short rotation forestry plantations. In the northern parts of Europe, willow and poplar approach and sometimes exceed miscanthus winter yields in the same location. FAO estimate that forest plantation yields range from 1 to 25 m3 "green" wood per hectare per year globally, equivalent to 0.4–12.2 dry tonnes per hectare per year. Russian pine have the lowest yield, while eucalyptus in Argentina, Brazil, Chile and Uruguay, and poplar in France/Italy, have the highest For natural temperate mixed forests, Vaclav Smil estimates somewhat lower average sustainable yields ; 1.5–2 dry tonnes per hectare.
The miscanthus peak yield is reached at the end of summer but harvest is typically delayed until winter or early spring. Yield is roughly 33% lower at this point because of leaves drop, but the combustion quality is higher. Delayed harvest also allows nitrogen to move back into the rhizome for use by the plant in the following growing season.
In Europe the peak dry mass yield has been measured to, depending on location, with a mean peak dry mass yield of 22 tonnes. Yields are highest in southern Europe; Roncucci et al. quote dry mass yields of 25–30 tonnes generally for that area under rainfed conditions. With irrigation, trials in Portugal yielded 36 tonnes, Italy 34–38 tonnes, and Greece 38–44 tonnes. Trials in Illinois, USA, yielded. Like in Europe, yields increase as you move south. In general, Vaclav Smil estimates roughly a doubling of net primary production of biomass in the tropics compared to the temperate regions of the world. For Micanthus x giganteus specifically, there are no scientific trials available yet regarding yields in the tropics, but other elephant grass types have been shown to yield up to 80 tonnes per hectare, and commercial napier grass developers advertise yields of around 100 dry tonnes per hectare per year, provided there is an adequate amount of rain or irrigation available.

Yield – arable land

Felten et al. found a mean winter/spring yield of during a 16-year trial on arable land in Germany. McCalmont et al. estimate a mean UK yield of 10–15 tonnes if harvested in the spring, while Hastings et al. estimate a "pessimistic" UK mean yield of 10.5 tonnes.
Nsanganwimana et al. summarize several trials, and give these numbers:
is land with issues that limits growth, for instance low water and nutrient storage capacity, high salinity, toxic elements, poor texture, shallow soil depth, poor drainage, low fertility, or steep terrain. Depending on how the term is defined, between 1.1 and 6.7 billion hectares of marginal land exists in the world. For comparison, Europe consists of roughly 1 billion hectares, and Asia 4.5 billion hectares.
Quinn et al. identified Miscanthus x giganteus as a crop that is moderately or highly tolerant of multiple environmental stressors, specifically, heat, drought, flooding, salinity, and cool soil temperatures. This robustness makes it possible to establish relatively high-yielding miscanthus fields on marginal land, Nsanganwimana et al. mention wastelands, coastal areas, damp habitats, grasslands, abandoned milling sites, forest edges, streamsides, foothills and mountain slopes as viable locations. Likewise, Stavridou et al. concluded that 99% of Europe's saline, marginal lands can be used for M. x giganteus plantations, with only an expected maximum yield loss of 11%. Since salinity up to 200 mM does not affect roots and rhizomes, carbon sequestration carry on unaffected. Lewandowski et al. found a yield loss of 36% on a marginal site limited by low temperatures, compared to maximum yield on arable land in central Europe.
The authors also found a yield loss of 21% on a marginal site limited by drought, compared to maximum yields on arable soil in central Europe. Using yield prediction software Miscanfor, 30 days of soil dryness is the mean maximum amount of time a miscanthus crop can endure before wilting, while 60 days is the maximum before its rhizomes are killed and the crop has to be replanted. In addition to adequate rainfall, soil water holding capacity is important for high yields, especially in dry periods—in fact Roncucci et al. reports approximately two times better yield for miscanthus planted in silty clay loam compared to sandy loam soil after a relatively normal growing season precipitation wise, and approximately six times better yield after a growing season containing severe drought. The authors note that in soils with poor water holding capacity, irrigation in the establishment season is important because it allows the roots to reach far deeper underground, thereby increasing the plants' ability to collect water. Irrigation can also increase yield if applied during dry growing seasons. The authors argue however that in soils with good water holding capacity, irrigation can potentially be avoided if rainfall exceeds 420 mm. Stričević et al. make a similar point for crops in Serbia. The soil in this area is generally well wetted at the start of the growing season because of snow melt. If the roots go deep and the soil has good water holding capacity, 300–400 mm rainfall during the season is enough for good yields. The authors note however that if there are no water constraints at all, that is, if the crops are irrigated, you can actually expect twice the yield.
Nsanganwimana et al. found that M. x giganteus grows well in soils contaminated by metals, or by industrial activities in general. For instance, in one trial, it was found that M. x giganteus absorbed 52% of the lead content and 19% of the arsenic content in the soil after three months.
The absorption stabilizes the pollutants so they don't travel into the air, into ground water, neighbouring surface waters, or neighbouring areas used for food production. If contaminated miscanthus is used as fuel, the combustion site need to install the appropriate equipment to handle this situation. On the whole though, " Miscanthus is suitable crop for combining biomass production and ecological restoration of contaminated and marginal land."
Because of miscanthus' ability to be " productive on lower grade agricultural land, including heavy metal contaminated and saline soils " Clifton-Brown et al. conclude that miscanthus can " contribute to the sustainable intensification of agriculture, allowing farmers to diversify and provide biomass for an expanding market without compromising food security."

Yield – comparison with other renewables

To calculate land use requirements for different kinds of energy production, it is essential to know the relevant area-specific power densities. Smil estimates that the average area-specific power densities for modern biofuels, wind, hydro and solar power production are 0.30 W/m2, 1 W/m2, 3 W/m2 and 5 W/m2, respectively. The average human power consumption on ice-free land is 0.125 W/m2, although rising to 20 W/m2 in urban and industrial areas.
The reason for the low area-specific power density for biofuels is a combination of low yields and only partial utilization of the plant.
Regarding ethanol production, Smil estimates that Miscanthus x giganteus fields generate 0.40 W/m2 when utilized for this purpose. Corn fields generates 0.26 W/m2. In Brazil sugarcane fields typically generate 0.41 W/m2. With the highest large-scale plantation yields in the industry, sugarcane fields can generate 0.50 W/m2. Winter wheat generates 0.08 W/m2 and German wheat generates 0.30 W/m2. When grown for jet fuel, soybean generates 0.06 W/m2, while palm oil generates a healthier 0.65 W/m2. Jathropa grown on marginal land generate 0.20 W/m2. When grown for biodiesel, rapeseed generate 0.12 W/m2. In contrast to miscanthus cultivation and solid fuel production, typical liquid biofuel feedstocks and fuel production require large energy inputs. When these inputs are compensated for, power density drops further down: Rapeseed based biodiesel production in the Netherlands have the highest energy efficiency in the EU with an adjusted power density of 0.08 W/m2, while sugar beets based bioethanol produced in Spain have the lowest, at only 0.02 W/m2.
Combusting solid biomass is more energy efficient than combusting liquids, as the whole plant is utilized. For instance, plantations producing solid biomass for combustion generate more than double the amount of power per square metre compared to corn fields producing for ethanol, when the yield is the same: 10 t/ha generates 0.60 W/m2 and 0.26 W/m2 respectively. For large-scale plantations with pines, acacias, poplars and willows in temperate regions, Smil estimates yields of 5–15 t/ha, equivalent to 0.30–0.90 W/m2. For similarly large plantations, with eucalyptus, acacia, leucaena, pinus and dalbergia in tropical and subtropical regions, his estimate is 20–25 t/ha, equivalent to 1.20–1.50 W/m2. In Brazil, the average yield for eucalyptus is 21 t/ha, but in Africa, India and Southeast Asia, typical eucalyptus yields are below 10 t/ha.
Oven dry biomass in general, including wood, miscanthus and napier grass, have a calorific content of roughly 18 GJ/t. When calculating power production per square metre, every t/ha of dry biomass yield increases a plantation's power production by 0.06 W/m2. As mentioned above, Smil estimates that the world average for wind, hydro and solar power production is 1 W/m2, 3 W/m2 and 5 W/m2 respectively. In order to match these power densities, plantation yields must reach 17 t/ha, 50 t/ha and 83 t/ha for wind, hydro and solar respectively. This seems achievable based on the yield data in the preceding sections. To match the world average for biofuels, plantations need only to produce 5 tonnes of dry mass per hectare per year.
Note however that yields need to be adjusted to compensate for the amount of moisture in the biomass. The moisture of biomass straw or bales varies with the surrounding air humidity and eventual pre-drying measures, while pellets have a standardized moisture content of below 10% and below 15%. Likewise, for wind, hydro and solar, power line transmission losses amounts to roughly 8% globally and should be accounted for. If biomass is to be utilized for electricity production rather than heat production, note that yields has to be roughly tripled in order to compete with wind, hydro and solar, as the current heat to electricity conversion efficiency is only 30-40%. When simply comparing area-specific power density without regard for cost, this low heat to electricity conversion efficiency effectively pushes at least solar parks out of reach of even the highest yielding biomass plantations, power density wise.

Carbon sequestration

Soil carbon input/output

Plants sequester carbon through photosynthesis, a sunlight-driven process where CO2 and water are absorbed and then combined to form carbohydrates. The absorbed carbon is released back to the atmosphere as CO2 when the harvested biomass is combusted, but the belowground parts of the plant remain in the soil and can potentially add substantial amounts of carbon to the soil over the years. Belowground carbon does not stay below ground forever however; " soil carbon is a balance between the decay of the initial soil carbon and the rate of input ."
Plant derived soil carbon is a continuum, ranging from living biomass to humus, and it decays in different stages, ranging from months to hundreds of years. The rate of decay depends on many factors, for instance plant species, soil, temperature and humidity, but as long as fresh new carbon is input, a certain amount of carbon stays in the ground—in fact Poeplau et al. did not find any " indication of decreasing SOC accumulation with age of the plantation indicating no SOC saturation within 15–20 years." Harris et al. estimate 30–50 years of SOC change following a land use change between annual and perennial crops before a new SOC equilibrium is reached. The amount of carbon in the ground under miscanthus fields is thus seen to increase during the entire life of the crop, albeit with a slow start because of the initial tilling and the relatively low amounts of carbon input in the establishment phase. Felten et al. argue that high proportions of pre- and direct-harvest residues, direct humus accumulation, the well-developed and deep-reaching root system, the low decomposition rates of plant residues due to a high C:N ratio, and the absence of tillage and subsequently less soil aeration are the reasons for the high carbon sequestration rates.

Net annual carbon accumulation

A number of studies try to quantify the net amount of miscanthus-induced below-ground carbon accumulation each year, after decay is accounted for, in various locations and under various circumstances.
Dondini et al. found 32 tonnes more carbon per hectare under a 14 year old miscanthus field than in the control site, suggesting a combined mean carbon accumulation rate of, or 38% of total harvested carbon per year. Likewise, Milner et al. suggest a mean carbon accumulation rate for the whole of the UK of 2.28 tonnes per hectare per year, given that some unprofitable land is excluded. Nakajima et al. found an accumulation rate of 1.96  tonnes per hectare per year below a university test site in Sapporo, Japan, equivalent to 16% of total harvested carbon per year. The test was shorter though, only 6 years. Hansen et al. found an accumulation rate of 0.97 tonne per hectare per year over 16 years under a test site in Hornum, Denmark, equivalent to 28% of total harvested carbon per year. McCalmont et al. compared a number of individual European reports, and found accumulation rates ranging from 0.42 to 3.8 tonnes per hectare per year, with a mean accumulation rate of 1.84 tonne, or 25% of total harvested carbon per year.

Transport and combustion challenges

Overview

Biomass in general, including miscanthus, have different properties compared to coal, for instance when it comes to handling and transport, grinding, and combustion. This makes sharing the same logistics, grinding and combustion infrastructure difficult. Often new biomass handling facilities have to be built instead, which increases cost. Together with the relatively high cost of feedstock, this often leads to the well-known situation where biomass projects have to receive subsidies to be economically viable.
A number of fuel upgrading technologies are currently being explored, however, that make biomass more compatible with the existing infrastructure. The most mature of these is torrefaction, basically an advanced roasting technique which—when combined with pelleting or briquetting—significantly influences handling and transport properties, grindability and combustion efficiency.

Energy density and transport costs

Miscanthus chips have a bulk density of only 50–130 kg/m3, bales 120–160 kg/m3, while pellets and briquettes have a bulk density of 500 and 600 kg/m3 respectively. Torrefaction works hand in hand with this trend towards a denser and therefore cheaper to transport product, specifically by increasing the product’s energy density. Torrefaction removes the parts of the biomass that has the lowest energy content, while the parts with the highest energy content remain. That is, approximately 30% of the biomass is converted to gas during the torrefaction process, while 70% remains, usually in the form of compacted pellets or briquettes. This solid product contains approximately 85% of the original biomass energy however. Basically the mass part has shrunk more than the energy part, and the consequence is that the calorific value of torrefied biomass increases significantly, to the extent that it can compete with energy dense coals used for electricity generation. Vaclav Smil states that the energy density of the most common steam coals today is 22–26 GJ/t.
The higher energy density means lower transport costs, and a decrease in transport-related GHG emittance. The IEA has calculated energy and GHG costs for regular and torrefied pellets/briquettes. When making pellets and shipping them from Indonesia to Japan, a minimum of 6.7% energy savings or 14% GHG savings is expected when switching from regular to torrefied. This number increases to 10.3% energy savings and 33% GHG savings when making and shipping minimum 50mm briquettes instead of pellets.
The longer the route, the bigger the savings. The relatively short supply route from Russia to the UK equals energy savings of 1.8%, while the longer supply route from southeast USA to the Amsterdam-Rotterdam-Antwerp area is 7.1%. From southwest Canada to ARA 10.6%, southwest USA to Japan 11%, and Brazil to Japan 11.7%

Water absorption and transport costs

Torrefaction also converts the biomass from a hydrophilic to a hydrophobic state. Water repelling briquettes can be transported and stored outside, which simplifies the logistics operation and decreases cost.
All biological activity is stopped, reducing the risk of fire and stopping biological decomposition like rotting.

Uniformity and customization

Generally, torrefaction is seen as a gateway for converting a range of very diverse feedstocks into a uniform and therefore easier to deal with fuel.
The fuel's parameters can be changed to meet customers demands, for instance type of feedstock, torrefaction degree, geometrical form, durability, water resistance, and ash composition.
The possibility to use different types of feedstock improves the fuel's availability and supply reliability.

Grindability

Unprocessed M. x giganteus has strong fibers, making grinding into equally sized, very small particles difficult to achieve. Coal chunks are typically ground to that size because such small, even particles combust stabler and more efficient. While coal has a score on the Hardgrove Grindability Index of 30–100, unprocessed miscanthus has a score of 0. During torrefaction however, " the hemi-cellulose fraction which is responsible for the fibrous nature of biomass is degraded, thereby improving its grindability." Bridgeman et al. measured a HGI of 79 for torrefied miscanthus, while the IEA estimates a HGI of 23–53 for torrefied biomass in general. UK coal scores between 40 and 60 on the HGI scale.
The IEA estimates an 80–90% drop in energy use required to grind biomass that has been torrefied.
The relatively easy grinding of torrefied miscanthus makes a cost-effective conversion to fine particles possible, which subsequently makes efficient combustion with a stable flame possible. Ndibe et al. found that the level of unburnt carbon " decreased with the introduction of torrefied biomass", and that the torrefied biomass flames " were stable during 50% cofiring and for the 100% case as a result of sufficient fuel particle fineness."

Chlorine and corrosion

Raw miscanthus biomass has a relatively high chlorine amount, which is problematic in a combustion scenario because, as Ren et al. explains, the " likelihood of corrosion depends significantly on the content of chlorine in the fuel ." Likewise, Johansen et al. state that " the release of Cl-associated species during combustion is the main cause of the induced active corrosion in the grate combustion of biomass." Chlorine in different forms, in particular combined with potassium as potassium chloride, condensates on relatively cooler surfaces inside the boiler and creates a corrosive deposit layer. The corrosion damages the boiler, and in addition the physical deposit layer itself reduce heat transfer efficiency, most critically inside the heat exchange mechanism. Chlorine and potassium also lowers the ash melting point considerably compared to coal. Melted ash, known as slag or clinker, sticks to the bottom of the boiler, and increase maintenance costs.
In order to reduce chlorine content, M. x giganteus is usually harvested dry, in early spring, but this late harvest practice is still not enough of a countermeasure to achieve corrosion-free combustion.
However, the chlorine amount in miscanthus reduces by approximately 95% when it is torrefied at 350 degrees Celsius.
Chlorine release during the torrefaction process itself is more manageable than chlorine release during combustion, because " the prevailing temperatures during the former process are below the melting and vaporization temperatures of the alkali salts of chlorine, thus minimizing their risks of slagging, fouling and corrosion in furnaces."
For potassium, Kambo et al. found a 30% reduction for torrefied miscanthus. However, potassium is dependent on chlorine to form potassium chloride; with a low level of chlorine, the potassium chloride deposits reduce proportionally.

Conclusion

Li et al. conclude that the " process of torrefaction transforms the chemical and physical properties of raw biomass into those similar to coal, which enables utilization with high substitution ratios of biomass in existing coal-fired boilers without any major modifications."
Likewise, Bridgeman et al. state that since torrefaction removes moisture, creates a grindable, hydrophobic and solid product with an increased energy density, torrefied fuel no longer requires " separate handling facilities when co-fired with coal in existing power stations."
Smith et al. makes a similar point in regard to hydrothermal carbonization, sometimes called "wet" torrefaction.
Ribeiro et al. note that " torrefaction is a more complex process than initially anticipated" and state that " torrefaction of biomass is still an experimental technology ." Michael Wild, president of the International Biomass Torrefaction Council, stated in 2015 that the torrefaction sector is " in its optimisation phase ", i.e. it is maturing. He mentions process integration, energy and mass efficiency, mechanical compression and product quality as the variables most important to master at this point in the sector's development.

Environmental impacts

GHG savings

Yield and soil carbon content
The amount of carbon sequestrated and the amount of GHG emitted determine if the total GHG life cycle cost of a bio-energy project is positive, neutral or negative. Specifically, a GHG/carbon negative life cycle is possible if the total below-ground carbon accumulation more than compensates for the above-ground total life-cycle GHG emissions. Whitaker et al. estimate that for Miscanthus x giganteus carbon neutrality and even negativity is within reach. The authors argue that a miscanthus crop with a yield of 10 tonnes per hectare per year sequesters so much carbon that the crop more than compensates for both farm operations emissions and transport emissions. The chart on the right displays two CO2 negative miscanthus production pathways, represented in gram CO2-equivalents per megajoule. The green bars represents soil carbon change, the yellow diamonds represent mean values.
Emmerling et al. make the same point for miscanthus in Germany : "Miscanthus is one of the very few crops worldwide that reaches true CO2 neutrality and may function as a CO2 sink. Related to the combustion of fuel oil, the direct and indirect greenhouse gas emissions can be reduced by a minimum of 96% through the combustion of Miscanthus straw vs. 0.0032 kg CO2‐eq MJ−1 ). Due to the C‐sequestration during Miscanthus growth, this results in a CO2‐eq mitigation potential of 117%".
One should note that successful sequestration is dependent on planting sites, as the best soils for sequestration are those that are currently low in carbon. The varied results displayed in the chart highlights this fact.
Milner et al. argue that for the UK, successful sequestration is expected for arable land over most of England and Wales, with unsuccessful sequestration expected in parts of Scotland, due to already carbon rich soils. Also, for Scotland, the relatively lower yields in this colder climate makes CO2 negativity harder to achieve. Soils already rich in carbon include peatland and mature forest. Milner et al. further argue that the most successful carbon sequestration in the UK takes place below improved grassland. However, Harris et al. notes that since the carbon content of grasslands vary considerably, so does the success rate of land use changes from grasslands to perennial. The bottom graphic displays the estimated yield necessary to achieve CO2 negativity for different levels of existing soil carbon saturation.
The perennial rather than annual nature of miscanthus crops implies that the significant below-ground carbon accumulation each year is allowed to continue undisturbed. No annual plowing or digging means no increased carbon oxidation and no stimulation of the microbe populations in the soil, and therefore no accelerated carbon-to-CO2 conversion happening in the soil every spring.
Savings comparison
Fundamentally, the below-ground carbon accumulation works as a GHG mitigation tool because it removes carbon from the above-ground carbon circulation The above-ground circulation is driven by photosynthesis and combustion—first, the miscanthus fields absorb CO2 and assimilates it as carbon in its tissue both above and below ground. When the above-ground carbon is harvested and then burned, the CO2 molecule is formed yet again and released back into the atmosphere. However, an equivalent amount of CO2 is absorbed back by next season's growth, and the cycle repeats.
This above-ground cycle has the potential to be carbon neutral, but of course the human involvement in operating and guiding the above-ground CO2 circulation means additional energy input, often coming from fossil sources. If the fossil energy spent on the operation is high compared to the amount of produced energy, the total CO2 footprint can approach, match or even exceed the CO2 footprint originating from burning fossil fuels exclusively, as has been shown to be the case for several first-generation biofuel projects.
Transport fuels might be worse than solid fuels in this regard.
The problem can be dealt with both from the perspective of increasing the amount of carbon that is moved below ground, and from the perspective of decreasing fossil fuel input to the above-ground operation. If enough carbon is moved below ground, it can compensate for the total lifecycle emissions of a particular biofuel. Further, if the above-ground emissions decreases, less below-ground carbon storage is needed for the biofuel to become CO2 neutral or negative. To sum up, a GHG negative life cycle is possible if the below-ground carbon accumulation more than compensates for the above-ground lifecycle GHG emissions.
For first generation bio-energy crops, the greenhouse gas footprints were often large, but second generation bio-energy crops like miscanthus reduces its CO2 footprint drastically. Hastings et al. found that miscanthus crops " almost always has a smaller environmental footprint than first generation annual bioenergy ones ."
A large meta-study of 138 individual studies, done by Harris et al., revealed that second generation perennial grasses planted on arable land on average store five times more carbon in the ground than short rotation coppice or short rotation forestry plantations. Compared to fossil fuels, the GHG savings are large—even without considering carbon sequestration, miscanthus fuel has a GHG cost of 0.4–1.6 grams CO2-equivalents per megajoule, compared to 33 grams for coal, 22 for liquefied natural gas, 16 for North Sea gas, and 4 for wood chips imported to Britain from the USA.
Confirming the above numbers, McCalmont et al. found that the mean energy input/output ratios for miscanthus is 10 times better than for annual crops, while GHG costs are 20-30 times better than for fossil fuels.
For instance, miscanthus chips for heating saved 22.3 tonnes of CO2 emissions per hectare per year in the UK, while maize for heating and power saved 6.3. Rapeseed for biodiesel saved only 3.2.
Lewandowski et al. found that each hectare of Central European arable land planted with miscanthus can reduce the atmospheric CO2 level with up to 30.6 tonnes per year, save 429 GJ of fossil energy used each year, with 78 euros earned per tonne reduced CO2 —given that the biomass is produced and used locally.
For miscanthus planted on marginal land limited by cold temperatures, the reduction in atmospheric CO2 is estimated to be 19.2 tonnes per hectare per year, with fossil energy savings of 273 GJ per hectare per year. For marginal land limited by drought, the atmospheric CO2 level can potentially be reduced with 24 tonnes per hectare per year, with fossil energy savings of 338 GJ per hectare per year.
Based on similar numbers, Poeplau and Don expect miscanthus plantations to grow large in Europe in the coming decades.
Whitaker et al. state that after some discussion, there is now consensus in the scientific community that " the GHG balance of perennial bioenergy crop cultivation will often be favourable ", also when considering the implicit direct and indirect land use changes.

Biodiversity

Below ground, Felten and Emmerling found that the number of earthworm species per square meter was 5.1 for miscanthus, 3 for maize, and 6.4 for fallow, and state that " it was clearly found that land-use intensity was the dominant regressor for earthworm abundance and total number of species." Because the extensive leaf litter on the ground helps the soil to stay moist, and also protect from predators, they conclude that " Miscanthus had quite positive effects on earthworm communities " and recommend that " Miscanthus may facilitate a diverse earthworm community even in intensive agricultural landscapes."
Nsanganwimana et al. found that the bacterial activity of certain bacteria belonging to the proteobacteria group almost doubles in the presence of M. x giganteus root exudates.
Above ground, Lewandowski et al. found that young miscanthus stands sustain high plant species diversity, but as the miscanthus stands mature, the canopy closes, and less sunlight reach the competing weeds. In this situation it gets harder for the weeds to survive. After canopy closure, Lewandowski et al. found 16 different weed species per 25 m2 plot. The dense canopy works as protection for other life-forms though; Lewandowski et al. notes that " Miscanthus stands are usually reported to support farm biodiversity, providing habitat for birds, insects, and small mammals ." Supporting this view, Caslin et al. argue that the flora below the canopy provides food for butterflies, other insects and their predators, and 40 species of birds.
Both Haughton et al. and Bellamy et al. found that the miscanthus overwinter vegetative structure provided an important cover and habitat resource, with high levels of diversity in comparison with annual crops. This effect was particularly evident for beetles, flies, and birds, with breeding skylarks and lapwings being recorded in the crop itself. The miscanthus crop offers a different ecological niche for each season—the authors attribute this to the continually evolving structural heterogeneity of a miscanthus crop, with different species finding shelter at different times during its development—woodland birds found shelter in the winter and farmland birds in the summer. For birds, 0.92 breeding pairs species per hectare was found in the miscanthus field, compared to 0.28 in the wheat field. The authors note that due to the high carbon to nitrogen ratio, it is in the field's margins and interspersed woodlands that the majority of the food resources are to be found. Miscanthus fields work as barriers against chemical leaching into these key habitats however.
Caslin et al. further argue that miscanthus crops provides better biodiversity than cereal crops, with three times as many spiders and earthworms as cereal. Brown hare, stoat, mice, vole, shrew, fox and rabbit are some of the species that are observed in miscanthus crops. The crop act as both a nesting habitat and a wildlife corridor connecting different habitats.

Water quality

McCalmont et al. claim that miscanthus fields leads to significantly improved water quality because of significantly less nitrate leaching.
Likewise, Whitaker et al. claim that there is drastically reduced nitrate leaching from miscanthus fields compared to the typical maize/soy rotation because of low or zero fertilizer requirements, the continuous presence of a plant root sink for nitrogen, and the efficient internal recycling of nutrients by perennial grass species. For instance, a recent meta-study concluded that miscanthus had nine times less subsurface loss of nitrate compared to maize or maize grown in rotation with soya bean.

Soil quality

The fibrous, extensive miscanthus rooting system and the lack of tillage disturbance improves infiltration, hydraulic conductivity and water storage compared to annual row crops, and results in the porous and low bulk density soil typical under perennial grasses, with water holding capabilities expected to increase by 100–150 mm.
Nsanganwimana et al. argue that miscanthus improves carbon input to the soil, and promote microorganism activity and diversity, which are important for soil particle aggregation and rehabilitation processes. On a former fly ash deposit site, with alkaline pH, nutrient deficiency, and little water-holding capacity, a miscanthus crop was successfully established—in the sense that the roots and rhizomes grew quite well, supporting and enhancing nitrification processes, although the above-ground dry weight yield was low because of the conditions. The authors argue that M. x giganteus' ability to improve soil quality even on contaminated land is a useful feature especially in a situation where organic amendments can be added. For instance, there is a great potential to increase yield on contaminated marginal land low in nutrients by fertilizing it with nutrient-rich sewage sludge or wastewater. The authors claim that this practice offer the three-fold advantage of improving soil productivity, increasing biomass yields, and reducing costs for treatment and disposal of sewage sludge in line with the specific legislation in each country.

Invasiveness

Miscanthus × giganteus parents on both sides, M. sinensis and M. sacchariflorus, are both potentially invasive species, because they both produce viable seeds. M. x giganteus does not produce viable seeds however, and Nsanganwimana et al. claim that " there has been no report on the threat of invasion due to rhizome growth extension from long-term commercial plantations to neighbouring arable land."

Summary

There seem to be agreement in the scientific community that a shift from annual to perennial crops have environmental benefits. For instance, Lewandowski et al. conclude that analyses " of the environmental impacts of miscanthus cultivation on a range of factors, including greenhouse gas mitigation, show that the benefits outweigh the costs in most cases."
McCalmont et al. argue that although there is room for more research, " clear indications of environmental sustainability do emerge."
In addition to the GHG mitigation potential, miscanthus' " perennial nature and belowground biomass improves soil structure, increases water-holding capacity, and reduces run-off and erosion. Overwinter ripening increases landscape structural resources for wildlife. Reduced management intensity promotes earthworm diversity and abundance although poor litter palatability may reduce individual biomass. Chemical leaching into field boundaries is lower than comparable agriculture, improving soil and water habitat quality."
Milner et al. argue that a change from first generation to second generation energy crops like miscanthus is environmentally beneficial because of improved farm-scale biodiversity, predation and a net positive GHG mitigation effect. The benefits are primarily a consequence of low inputs and the longer management cycles associated with second generation crops.
The authors identifies 293247 hectares of arable land and grassland in the UK where both the economical and environmental consequences of planting miscanthus is seen as positive.
Whitaker et al. argue that if land use tensions are mitigated, reasonable yields obtained, and low carbon soils targeted, there are many cases where low-input perennial crops like miscanthus " can provide significant GHG savings compared to fossil fuel alternatives ."
In contrast to annual crops, miscanthus have low nitrogen input requirements, low GHG emissions, sequesters soil carbon due to reduced tillage, and can be economically viable on marginal land.
The authors agree that in recent years, " a more nuanced understanding of the environmental benefits and risks of bioenergy has emerged, and it has become clear that perennial bioenergy crops have far greater potential to deliver significant GHG savings than the conventional crops currently being grown for biofuel production around the world."
The authors conclude that " the direct impacts of dedicated perennial bioenergy crops on soil carbon and N2O are increasingly well understood, and are often consistent with significant lifecycle GHG mitigation from bioenergy relative to conventional energy sources."

Practical farming considerations

For practical farming advice, see Iowa State University's "Giant Miscanthus Establishment" PDF. See also the best practice manual jointly developed by Teagasc and AFBI.