Bioenergy from plants, particularly from perennial grasses and trees, could make a substantial contribution to alleviation of global problems in climate change and energy security if high yields can be sustained. Here, yield traits in a range of key bioenergy crops are reviewed, from which several targets for future improvement can be identified. Some are already the focus of genetically modified (GM) and non-GM approaches. However, the efficient growth strategies of perennial bioenergy crops rely on newly assimilated and recycled carbon and remobilized nitrogen in a continually shifting balance between sources and sinks. This balance is affected by biotic (e.g. pest, disease) and abiotic (e.g. drought) stresses. Future research should focus on three main challenges: changing (photo)thermal time sensitivity to lengthen the growing season without risking frost damage or limiting remobilization of nutritional elements following senescence; increasing aboveground biomass without depleting belowground reserves required for next year's growth and thus without increasing the requirement for nutrient applications; and increasing aboveground biomass without increasing water use.

Bioenergy from plants and the sustainable yield challenge

Plants to power: bioenergy to fuel the future.

Miscanthus x giganteus (miscanthus) and Arundo donax L. (giant reed) are two perennial crops which have been received particular attention during the last decade as bioenergy crops. The main aim of the present study was to compare the above-ground biomass production and the energy balance of these perennial rhizomatous grasses in a long-term field experiment. The crops were cultivated from 1992 to 2003 in the temperate climate of Central Italy with 20,000 plants ha −1 , 100–100–100 kg N, P 2 O 5 , K 2 O per hectare, and without irrigation supply. For each year of trial, biomass was harvested in autumn to estimate biometric characteristics and productive parameters. Besides, energy analysis of biomass production was carried out determining energy output, energy input, energy efficiency (output/input) and net energy yield (output–input). Results showed high above-ground biomass yields over a period of 10 years for both species, with better productive performances in giant reed than in miscanthus (37.7 t DM ha −1  year −1 vs 28.7 t DM ha −1  year −1 averaged from 2 to 12 years of growth). Such high yields resulted positively correlated to number of stalks (miscanthus), plant height and stalk diameter (giant reed). Moreover, these perennial species are characterised by a favourable energy balance with a net energy yield of 467 and 637 GJ ha −1 (1–12 year mean) for miscanthus and giant reed respectively. With such characteristics, both grasses could be proposed as biomass energy crops in Southern Europe with a significant and environmentally compatible contribution to energy needs.

Comparison of Arundo donax L. and Miscanthus x giganteus in a long-term field experiment in Central Italy: Analysis of productive characteristics and energy balance

Negative emissions technologies (NETs) in general and bioenergy with CO2 capture and storage (BECCS) in particular are commonly regarded as vital yet controversial to meeting our climate goals. In this contribution we present a whole-systems analysis of the BECCS value chain associated with cultivation, harvesting, transport and conversion in dedicated biomass power stations in conjunction with CCS, of a range of biomass resources – both dedicated energy crops (miscanthus, switchgrass, short rotation coppice willow), and agricultural residues (wheat straw). We explicitly consider the implications of sourcing the biomass from different regions, climates and land types. The water, carbon and energy footprints of each value chain were calculated, and their impact on the overall system water, carbon and power efficiencies was evaluated. An extensive literature review was performed and a statistical analysis of the available data is presented. In order to describe the dynamic greenhouse gas balance of such a system, a yearly accounting of the emissions was performed over the lifetime of a BECCS facility, and the carbon “breakeven time” and lifetime net CO2 removal from the atmosphere were determined. The effects of direct and indirect land use change were included, and were found to be a key determinant of the viability of a BECCS project. Overall we conclude that, depending on the conditions of its deployment, BECCS could lead to both carbon positive and negative results. The total quantity of CO2 removed from the atmosphere over the project lifetime and the carbon breakeven time were observed to be highly case specific. This has profound implications for the policy frameworks required to incentivise and regulate the widespread deployment of BECCS technology. The results of a sensitivity analysis on the model combined with the investigation of alternate supply chain scenarios elucidated key levers to improve the sustainability of BECCS: (1) measuring and limiting the impacts of direct and indirect land use change, (2) using carbon neutral power and organic fertilizer, (3) minimising biomass transport, and prioritising sea over road transport, (4) maximising the use of carbon negative fuels, and (5) exploiting alternative biomass processing options, e.g., natural drying or torrefaction. A key conclusion is that, regardless of the biomass and region studied, the sustainability of BECCS relies heavily on intelligent management of the supply chain.

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Can BECCS Deliver Sustainable and Resource-efficient Negative Emissions?

Biomass yield and energy balance of three perennial crops for energy use in the semi-arid Mediterranean environment

The potential growth and biomass productivity of Miscanthus×giganteus was investigated under constraint-free conditions during two years with appreciable different climatic conditions in central Greece, and under three different plant densities (0.66, 1, 2 pl m−2) and two different nitrogen dressings ( N 0 = 50 , N1=100 kg N ha−1). The field experimental design was a 3×2 factorial split plot in four blocks. Plant height, tiller number, leaf area index and dry weight per plant component were measured in subsequent harvests throughout the growing periods of 2001 and 2002. It was found that fertilisation within the studied rates did not affect growth and biomass productivity of the crop, and no interaction between fertilization and plant density was observed in any of the samplings. Contrary to fertilization, a significant effect of plant density was found, with the denser populated plants (10,000–20,000 pl ha−1) performing growth rates of 250–350 kg ha−1 d−1 for large parts of the growing period, reaching maximum dry biomass yields in excess of 38 t ha−1 in the more favorable year, 2002, and 28 t ha−1 in the warmer and drier year 2001. Such high yield potentials were explained by the particularly great assimilation rates of this crop that were also measured under light saturation and optimum temperature and water regimes. With such biomass yields (11.2–15.2 t ha−1 oil equivalent), obtainable particularly under modest fertilisation needs, Miscanthus sinensis should be considered as a very promising crop for biomass production in Greece in the near future.

Potential growth and biomass productivity of Miscanthus×giganteus as affected by plant density and N-fertilization in central Greece

Switchgrass produces high amounts of biomass that can be used for solid biofuel production. In this study, the dry biomass yield vs. N–P–K nutrient uptake relations as well as the N-mineralization and the N-fertilization recovery fraction for switchgrass (cv. Alamo) were determined under field conditions for three N-fertilization (0, 80 and 160 kg ha−1) and for two irrigation (0 and 250 mm) levels, in two soils in central Greece with rather different moisture status over the period 2009–2012. It was found that dry biomass yield on the aquic soil may reach 27–30 t ha−1 using supplemental irrigation, and remain at high levels (19–24 t ha−1) without irrigation. In the xeric soil, however, lower biomass yields of 14–15 t ha−1 may be produced with supplemental irrigation. The average N-concentration varies between 0.23% in stems and 1.10% in leaves, showing the very low needs in N. P-content varies between 0.16% in leaves and 0.03% in stems, whereas K-content fluctuates between 0.67% and 0.78%. Linear biomass yield-nutrient uptake relationships were found with high R2, pointing to nutrient use efficiencies of 240 and 160 kg kg−1, for N and K respectively. The base N-uptake ranged 70–84 kg ha−1 in the aquic to 60 kg ha−1 or less in the xeric soil. N-recovery fraction was about 30% in the aquic soil and lower in the xeric. Therefore, switchgrass is very promising for biomass production and its introduction in land use systems (especially in aquic soils of similar environments) should be seriously taken into consideration.

Switchgrass (Panicum virgatum L.) nutrients use efficiency and uptake characteristics, and biomass yield for solid biofuel production under Mediterranean conditions

Nicholas G. Danalatos

Papers

Potential growth and biomass productivity of Miscanthus×giganteus as affected by plant density and N-fertilization in central Greece

Switchgrass (Panicum virgatum L.) nutrients use efficiency and uptake characteristics, and biomass yield for solid biofuel production under Mediterranean conditions

Bioavailability of Trace Elements in Relation to Root Modification in the Rhizosphere

Maximizing oilseed rape's yield by glyphosate under Mediterranean conditions