Bioenergy from Plants and the Sustainable Yield Challenge

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Blackwell Publishing Ltd

Tansley review
Bioenergy from plants and the sustainable yield challenge

Author for correspondence: Angela Karp Tel: +44 (0)1582 763133 Fax: +44 (0)1582 760981 Email: angela.karp@bbsrc.ac.uk Received: 21 November 2007 Accepted: 4 February 2008

Angela Karp and Ian Shield
Centre for Bioenergy and Climate Change, Plant and Invertebrate Ecology Department, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UK

Contents
Summary I. II. III. IV. Introduction Bioenergy, biomass and biofuel crops Bioenergy yield traits Bioenergy composition traits 15 15 16 16 22 V. VI. Sustainable bioenergy production from crops Increasing bioenergy yields in a sustainable way 23 24 26 27 27

VII. Conclusions and perspectives Acknowledgements References

Summary
Key words: bioenergy, biofuels, biomass, lignocellulosic, perennial grasses, poplar, willow. 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. New Phytologist (2008) 179: 15–32 © The Authors (2008). Journal compilation © New Phytologist (2008) doi: 10.1111/j.1469-8137.2008.02432.x

I. Introduction
Two main drivers have pushed renewable energy production to the top of global agendas: climate change and energy

security. Energy consumption worldwide increased 13-fold in the 20th century, tripling since 1960, which is faster than the increase in population size (Hein, 2005). Concerns heighten about how such rising demands can continue to be met by finite

www.newphytologist.org

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Tansley review

Table 1 Definitions of bioenergy terms as used in this review Term Bioenergy Biomass Lignocellulose Bioenergy crops Biomass crops Biofuel crops Biopower crops First generation Second generation Third generation Definition Production of any form of renewable energy from biological sources Biological mass from which energy can be produced Subset of biomass that comprises the structural components (e.g. cell walls) A generic term embracing crops grown for both power and transport markets Crops grown for biomass production for either market Crops grown for transport fuels Crops grown for heat or power Crop/fuel chains based on existing conversion technologies Crop/fuel chains based on developing conversion technologies Crop/fuel chains based on emerging/future technologies Comment Focus on plants, but algal, animal and microbial sources are also important Includes not only harvestable mass but also residues

The terms ‘bioenergy’ and ‘biofuels’ are often confused, but a distinction is required (see below)

Often also referred to as first or second generation (see below) No term is in common use for crops grown for power generation, but this can be adopted Currently exclusively from sugar, starch or oil crops, also grown for food The aim is to switch to lignocellulosic feedstocks Includes hydrogen...
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