The high yields of Miscanthus × giganteus as a bioenergy crop has attracted much recent interest (U.S. Department of Energy, 2011). On about two-thirds of the 18 M ha potentially available for bioenergy crops in the US, M. × giganteus could potentially provide sufficient biomass to meet the goal of replacing 133 billion liters of petroleum of the Energy Independence and Security Act (Heaton et al., 2008; Somerville et al., 2010). In Europe, M. × giganteus is emerging as a major combustion feedstock as its largest single thermal electricity power generation station, Drax B, moves to 50% renewable. As increasing experience is gained with this new crop, will it deliver the yields, sustainability, and environmental benefits suggested by earlier studies? This Virtual Special Issue brings together recent articles addressing elements of this key question, the answer to which is critical to choice of crop and effective land use for expanding bioenergy production (Table 1).
|Study||Type of study||Environmental factors studied||Environmental effect|
|Drewer et al. (2012)||Field||GHGa||✚|
|Gauder et al. (2012)||Field||GHG||✚|
|Sanscartier et al. (2013)||Model||GHG||✚|
|Wang et al. (2012)||Model||GHG||✔|
|Gopalakrishnan et al. (2012)||Model||GHG, nitrate leaching||✚|
|Finnan et al. (2012)||Model||LCA||✚|
|Lesur et al. (2013)||Field||Nitrate leaching||✚|
|Mishra et al. (2013)||Model||Soil carbon||✚|
|Qin et al. (2012)||Model||Soil carbon||✚|
|Zatta et al. (2013)||Field||Soil carbon||✔|
|Zimmermann et al. (2012)||Field||Soil carbon||✔|
|Godard et al. (2013)||Model||Soil carbon, GHG||✚|
Based now on more than 25 years of European and just over 10 years North American biomass production experience with this crop, it can be concluded that M. × giganteusis emerging as a common choice for new commercial bioenergy projects both at pilot and commercial scale.
Recent publications in GCB Bioenergy help us to better understand the sustainability of M. × giganteus bioenergy production, in terms of greenhouse gas (GHG) emissions, soil carbon accumulation, and nitrate leaching in comparison with fossil fuels and bioenergy produced from other crops. These studies range from observational and experimental fieldwork to modeling and at scales from small plots to national.
Several articles deal with greenhouse gas emissions resulting from Miscanthus production. Gauder et al. (2012) compared CH4, CO2, and N2O emissions from N-fertilized and unfertilized M. × giganteus, coppiced willow, and energy maize in a one-year study. They found that CH4 contributed very little to GHG emissions; the soil in this study actually acted as weak CH4 sinks for all crops. Furthermore, CO2 emissions tended to be seasonal with higher emissions in warmer seasons than in cooler seasons, and, like CH4, were correlated with soil temperatures. Emissions of N2O were increased with N fertilization for the M. × giganteus and energy maize, but less so for the coppiced willow. Overall, because of the high yields of the unfertilized M. × giganteus (average annual productivity of 16.8 dry Mg ha−1 over 5 years) and minimal N2O emissions, the authors recommended the production of unfertilized or ‘moderately N-fertilized’ M. × giganteus at their experimental site to maximize yield and minimize N2O emissions. Similarly, Drewer et al. (2012) found that CH4 emissions were negligible and N2O emissions were related to N-fertilization. In this study, the N2O emissions from wheat production were 200% greater than the unfertilized M. × giganteus and 4000% greater than the unfertilized willow. Moreover, the N2O emissions from the oilseed rape were 800% greater than the unfertilized M. × giganteus and 16000% greater than the unfertilized willow. However, when the perennial M. × giganteus and willow systems were fertilized to rates equaling those of the annual crops, these advantages were greatly diminished.
In another study, Finnan et al. (2012) compared GHG emissions of replacing peat biomass burned in power plants with imported palm kernel shells, imported olive cakes, locally produced willow, and locally produced M. × giganteus. The authors used the European Union Strategic Environmental Assessment to compare the consequences of these different substitutions and reported fewer adverse environmental outcomes from substituting peat with locally produced willow and M. × giganteus than would occur with the imported biomass. In particular, modeled GHG emissions, as measured in tonnes of CO2 equivalents, from bioenergy produced from willow and M. × giganteus were 5.9% and 7.1% of the peat emissions. Emissions from both palm kernel shells and olive cakes were greater than those from peat.
Life Cycle Assessment (LCA), which considers all processes along the supply chain, has been applied to evaluate the net balance of different ecosystem services in several studies. In 2009, in the UK, the cost of delivering M. × giganteus to the energy station was 3.76£ GJ−1. This was less than fuel oil (6.14£ GJ−1) and was more expensive than gas (2.81£ GJ−1) and coal (2.05£ GJ−1) (Wang et al., 2012). The cost of M. × giganteus could become more competitive with increased yields(Wang et al., 2012) and shorter transport distances (Wang et al., 2012; Godard et al., 2013). Miscanthus also produced fewer GHG emissions than oil (Wang et al., 2012). A study in Canada reported that in each of five electricity generation scenarios, M. × giganteus produced fewer life-cycle emissions (g CO2 eq k Wh−1) than coal (Sanscartier et al., 2013). In addition, in scenarios of high productivity or/and soils currently poor in carbon, M. × giganteus would further reduce GHG emissions by soil C accumulation. Godard et al. (2013), wrote, ‘Miscanthus was a net GHG sink due to high soil-C sequestration rates …’ providing a ‘…negative global-warming impact…’ in a study that compared the LCA of flax shives, M. × giganteus, cereal straw, linseed straw, and triticale used for boiler combustion. Similar to Wang et al. (2012), Godard et al. (2013) found transportation to be the top contributor to carbon emissions.
Nitrate leaching to watercourses is a major environmental impact for many modern crop systems. Lesur et al. (2013) evaluated leaching over the first 3 years of M. × giganteus production on 38 fields covering a wide range of soils. Compared to other crops, they found no or little leaching across all sites. The greatest nitrate leaching occurred during the first winter after planting, on sites having sandy, shallow soils and on sites where establishment failed, suggesting that improved management and site selection could diminish these losses even further.
Using the biogeochemical Terrestrial Ecosystem Model, Qin et al. (2012) examined six land-use change scenarios of conversion from annual food crops to energy crops, to determine change in carbon balance, bioenergy production, and agricultural yield. When comparing the total of the combination of corn grain and stover with M. × giganteus, the authors reported net primary production of 713 g C m−2 for the corn and 1489 g C m−2 for the M. × giganteus and harvestable biomass of 17.2 Mg ha−1 for the corn and 24.5 Mg ha−1for the M. × giganteus. Moreover, switching from wheat to M. × giganteus was found to increase biomass production and reduce CO2 emissions (Qin et al., 2012). In the context of environmental and economic benefits, M. × giganteus was found the best option where a feedstock was needed for energy in comparison to both annual crops and switchgrass.
Another biogeochemical model, Denitrification-Decomposition, was used to simulate crop yield, nitrous oxide emissions, and nitrate leaching when growing switchgrass, M. × giganteus, and native prairie grasses as buffer strips on land adjacent to conventional row-crop sin the US (Gopalakrishnan et al., 2012). Based on their simulations, the authors reported that all of the energy-crop buffers had significant environmental benefits due to reduced nutrient runoff, nitrate leaching and nitrous oxide emissions compared to traditional cropping systems, but M. × giganteus gave the highest biomass and energy yield.
Several researchers have examined the effects on soil carbon of growing M. × giganteus. The proportion of C4 Miscanthus-derived carbon in soil organic carbon was considerable after 6 years of producing Miscanthus hybrids and M. × giganteus on a site that had previously been a C3 grassland in the UK (Zatta et al., 2013). However, the amount of soil organic carbon stock derived from Miscanthus grasses did not increase from the initial C3 grassland stock over 20 years. Yet, substituting the aboveground portions of Miscanthus spp. for fossil fuels could produce an overall C mitigation benefit of 73–108 Mg C ha−1 (depending on Miscanthus hybrid) over 6 years (Zatta et al., 2013). In contrast, Zimmermann et al. (2012) found considerable carbon sequestration under 2-year-old M. × giganteus in Irish farm plantings; and found that the carbon lost during initial land conversion to M. × giganteus could potentially be replaced within 4–5 years. Similar to Zimmermann et al. (2012), Mishra et al. (2013) found that the adoption of Miscanthus in the US on land traditionally used to produce annual crops, would result in positive soil organic carbon sequestration due to cessation of tillage and increased biomass carbon input into the soil system from this more productive plant.
Whether evaluating greenhouse gas emissions, nitrate leaching, or soil carbon sequestration, these recent studies re-inforce and greatly expand the early observations that M. × giganteus would have very positive effects on the environment, or at minimum, have effects not worse than those from the change in land use. In addition, several studies have reported that due to the high productivity of M. × giganteus, less land is required to reach US bioenergy goals than for switch grass or maize (Heaton et al., 2008; Miguez et al., 2012; Arundale et al., 2014). Based on its productivity, as well as the impact it has on the environment in areas where it can be successfully produced, M. × giganteus should be regarded as the energy crop of choice. To date much focus has been given to replacing annual crops and managed grassland with M. × giganteus where major direct environmental benefits are apparent. Attention now must turn increasingly to marginal, abandoned and noncrop land, where models and limited trials suggest that considerable yields would be obtained.