Both Arundo and Miscanthus energy crops have been studied and utilised for a while, but the latter has had the benefit of a higher popularity – until now. Over the last few years, scientists and bioengineers have assessed the two crops against each other. The purpose of these studies was to examine the input needs and the output potential of the plants; these figures have been especially useful when estimating the costs of biogas, biomass, bioethanol, pellets and briquettes, paper or furniture projects. Ultimately, it has been found that Arundo is the prevailing plant over Miscanthus on multiple fronts – these factors will be discussed below.
Potential candidates for clean and renewable energy sources: Arundo and Miscanthus
Figure 1: Second-year Miscanthus x giganteus planting in October near Lexington, KY, USA
Strategies for the expansion of clean and renewable energy sources are required for our better future. This growing demand for biomass and the use of renewable energy is being encouraged by EU directives and US-EPA acts. For that reason, bioenergy strategies are built that require tall, rapidly-growing grasses such as Miscanthus × giganteus (further on Miscanthus) and Arundo donax (further on Arundo, but also known as “giant reed, giant reed grass, giant cane, reed cane, giant tropical plant, Spanish reed”). These two species could fill this renewable energy niche.
Miscanthus (Figure 1) is a leading bioenergy crop that is well discussed in the literature and considered as one of the most promising energy crops.
Compared to other candidate energy crops, Arundo (Figure 2) is less studied. While the plant is adaptable to different kinds of environments, soils and growing conditions, in combination with the high biomass production and the low input required for its cultivation, give to Arundo many advantages when compared to other energy crops. The objective of the present paper is to examine the characteristics of Arundo and Miscanthus in terms of biomass production and biomass conversion to bio-energy and bio-products.
Figure 2: Arundo donax flowering around October
Biomass production of Arundo and Miscanthus
Plant propagation
Miscanthus as well as Arundo are sterile plants that normally do not produce seeds.
Figure 3: Micropropagation of Arundo donax
Alternatively, they can be propagated asexually from their vegetative parts, such as the rhizome and stem. There are several disadvantages of using rhizome for propagation i.e. higher weight, higher transportation cost. Furthermore, rhizomes are pulled out from soil, thus virus, bacteria fungi, nematode or any other pests may infect these parts of the plant. Rhizome applies a delayed germination survival strategy that has an impact on the homogeneity of the energy plantation.
Much more advanced propagation is from axillary buds used in vitro propagation technologies (Figure 3). Arundo BioEnergy Ltd utilises a worldwide patented micro propagation method that is a unique, peerless, state of the art technology based on sustainable embryogenic and somatogenic tissue culture. By this method, virus free, genetically stable, vigorous, homogenous bare root plantlets on an industrial scale can be developed.
Growth and planting requirements
Both Arundo and Miscanthus are adapted to a wide range of soils, even low quality soils, such as marginal land and post-mining land. However, well-drained soil is preferred for best growth. Arundo was reported to be more resilient to drought and moisture stresses than Miscanthus, especially in rhizome persistence. Besides, Arundo was found to tolerate wider ranges of pH and salinity than Miscanthus.
A plowing depth of 40-45 cm is suggested for rhizomes of Arundo, while a plowing depth of 20-30 cm is recommended for rhizomes of Miscanthus. The recommended planting density for Arundo is 10,000 plants/ha, while Miscanthus requires 30,000 plants/ha.
So compared to Miscanthus, Arundo can adapt to a broader range of environments, but may require a slightly higher energy input for planting and Arundo requires less propagules compared to Miscantus.
Irrigation, fertilisation and weed control
During establishment in the field, Arundo and Miscanthus are more susceptible to water shortage stress. Thus, irrigation has been shown to improve establishment rates. After developing strong root systems, they normally don’t need additional water, whilst fertilisation is also not necessary. With these circumstances, Arundo and Miscanthus are unlikely to be outcompeted by weed.
Harvesting
Figure 4: Harvesting Miscanthus
Generally, a late harvesting date when the water content is lower than 30% is recommended in order to decrease the cost for harvesting and drying. Two harvests per year are feasible for Arundo but not recommended for Miscanthus.
Existing equipment, such as silage harvesters, can be used with slight modifications for harvesting Arundo and Miscanthus (Figure 4-5). However, specific stock shredders are recommended for harvesting Arundo with a diameter of 2-3 cm and moisture content higher than 40-50%.
Figure 5: Harvesting Arundo donax
Biomass, yield and energy balance
It is generally believed that Arundo has a higher biomass yield than Miscanthus (Angelini, 2009) (Di Nasso, 2011) (Di Tomaso, 2013). In case of fall harvest 15-41 dry tonne per ha, while in case of winter harvest 21-49 dry tonne per ha was reported (Di Nasso, 2011) (Ping, 2013) (Di Girolamo, 2013).
For Arundo, the yields generally increase from the first to the third year, reach a steady phase from the fourth to eighth year, and then start to decrease from the 9th year onward.
The biomass yield of Miscanthus was reported to be only 2.4 dry tonne per ha during the first year, while it increased to 14.28, 19.77, and 29.43 dry tonne per ha, respectively, from the second to the fourth year.
Furthermore, different management practices, soil types, and other environmental conditions also contribute to the variation of biomass yields reported in the literature. However, flooding was shown to have the weakest effect on Arundo and Miscanthus because both plants were reported to have a high tolerance to flooding. Comparing Arundo and Miscanthus, Arundo was found to be more tolerant to drought than Miscanthus, while Miscanthus had a slightly higher tolerance to flooding compared to Arundo.
The changes of annual energy input during a long-term cultivation of Miscanthus and Arundo was studied in a field experiment. More than half of the total energy input during the cultivation was used for the crop establishment during the 1st year. After planting operations were no longer needed, the annual energy input decreased for the following years. Due to the higher biomass yield that was obtained from Arundo, the total energy output and net energy output of Arundo were higher than those of Miscanthus.
Environmental impact of Arundo and Miscanthus
Invasive potential and controlling methods
Figure 6: Arundo donax on the Rio Grande River near Eagle Pass prior to the release of biological control agents. Image Credit: John Goolsby USDA-ARS
Both Arundo and Miscanthus have invasive properties (Figure 6). However, Miscanthus is believed to be less invasive. This assumption comes from adverse circumstances that Arundo was managed during its history. To find out more, read our article on ‘Is Arundo Invasive?’ On the other hand, there are no documented historical invasion events for Miscanthus due to its relatively new genetic modification and shorter cultivation history in new habitats.
Several ecological control methods have been shown to mitigate the invasive potential of both Arundo and Miscanthus.
Ecological control of Arundo invasion should include preventing flooding of its cultivation site, strict nutrient management in its surrounding areas, and removal of Arundo from riparian ecosystems adjacent to fire prone shrub lands (Ceotto, 2010) (Holt, 2005) (Coffman, 2007).
For Miscanthus, precautions should be taken if living (green) culms or rhizome fragments are removed from fields and transported through habitats containing either open water or abundant soil moisture in spring and summer. However, the whole culms and fragments of Miscanthus cannot produce shoots and roots after fall and winter cutting.
Effects on soil quality
Both Arundo and Miscanthus have the ability to grow on marginal land, thus prevent soil erosion and nutrient run off, and to improve soil quality. Arundo was also reported to enhance the bacterial and fungal growth in soil. In one study, Arundo was found to have a higher ability than Miscanthus in increasing soil organic carbon accumulation.
Phytoremediation
The use of energy crops for cost-effective remediation of contaminated soil, water, and sediments is a rapidly developing field. Features of Arundo and Miscanthus such as extensive root systems, high biomass production, and tolerance to high trace element concentrations, make them highly suitable for phytoremediation (Bonanno, 2012) (Iqbal, 2013).
Figure 7: Phytoremediation quality of Miscanthus
Both Arundo and Miscanthus (Figure 7) accumulated trace elements more in the below-ground organs (roots, rhizomes, etc.) than in aboveground parts (stems and leafs). This phenomenon is advantageous in the point of further utilisation, since aboveground biomass of Arundo and Miscanthus is used for biological conversion.
Arundo is likely to have a better ability for trace element removal due to its higher biomass yield. On wetlands, Arundo achieved more than twice the amount of average dry biomass production over Miscanthus. Moreover, the N and P phyto-uptake of Arundo were also significantly higher than other plants.
Bioenergy and bioproducts made from Arundo and Miscanthus
For bioenergy production, the preferred features of a biomass feedstock depend on the end-use. Each conversion technology, e.g., combustion, gasification, pyrolysis, fermentation, and anaerobic digestion (AD), has different requirements for the heating value, ash content, moisture content, sugar yield, digestibility, and other characteristics of the feedstock.
Biological conversion
Pre-treatment
Both Arundo and Miscanthus are highly recalcitrant to enzymatic digestion, due to their considerable lignin content and the complex structure formed by the cell wall components. Thus, pre-treatment is a required step prior to enzymatic hydrolysis to enhance the availability of polysaccharides to the hydrolytic enzymes.
In general, better results have been found for Miscanthus than for Arundo. With some pre-treatment processes Miscanthus achieved sugar yields greater than 90% (Murnen, 2007) (Brosse, 2009) (Khullar, 2013) (Scordia, 2013).
Liquid fuels
For bioethanol production – after pre-treatment – baker yeast Saccharomyces cerevisiae is used for fermentation. Currently, there is only one published report that compares ethanol production between Arundo and Miscanthus using the same pre-treatment and hydrolysis procedure. Higher ethanol yield was obtained from Arundo than from Miscanthus mainly due to the higher sugar yield from Arundo than that from Miscanthus.
Gaseous fuels
Figure 8: The University of Guelph Ridgetown campus in Ontario has a 250-kW digester housed at the CARES, or Center for Agricultural Renewable Energy and Sustainability, research facility. PHOTO: KIM VANOVERLOOP
Biogas is made by anaerobe microorganisms through the metabolism of organic matter (Figure 8). Biomass from Arundo and Miscanthus can be converted into bio-methane by anaerobic digestion, and similar yields are obtained. From raw biomass 151-391 L per kg VS (volatile solid) for Arundo (Ragaglini, 2014) (Yang, 2014) (Barbanti, 2014) (Di Girolamo, 2014) and 175-333 L per kg VS for Miscanthus (Vasco-Correa, 2015) (Jurado, 2013) (Wahid, 2015). However, harvest time may affect the total energy potential per hectare by altering both the methane yield per unit biomass and the total biomass yield per hectare. For Arundo, a double harvest in early summer and late autumn was reported to increase the methane yield per hectare by 20–35% compared to the most productive single harvest. In contrast, recent work in Miscanthus has shown that the harvest date had little effect on the methane yield (Vasco-Correa, 2015) (Wahid, 2015). This implies that Miscanthus can be harvested at any time during the harvest window if it is to be used for the production of biogas.
Arundo and Miscanthus can also be converted into bio-hydrogen via dark fermentation. But limited data are available for comparing them.
Thermochemical conversion
Figure 10: Fast pyrolysis bio-oil
Figure 9: lab produced bio-char
With high temperature and controlled oxygen exposure, Arundo and Miscanthus can also go through thermochemical conversion processes, such as gasification, pyrolysis, and liquefaction to produce syngas, bio-oil, and bio-char (Figure 9-10), and solid residue. Compared with Miscanthus, studies about thermochemical conversion of Arundo are relatively limited. Among research, no significant difference was found between the two crops. Miscanthus appears to have higher bio-oil yield and lower gas yield, but no conclusion can be made with this limited data.
Bio-products made from Arundo and Miscanthus
Arundo and Miscanthus are promising as sources for bio-products as well. Arundo showed better performance than Miscanthus in production of particle boards, paper and XOS, but its lignin had a lower effectiveness than that of Miscanthus as a humic substrate.
Arundo has great potential in producing these bio-products compared with Miscanthus, and future researches are needed for more side-by-side comparisons. For further detail on other bio-products that could be produced from Arundo and Miscanthus, read our article ‘Arundo Donax Uses: 8 Ways to Utilise Giant Reed’.
