Bioenergy from crops is expected to make a considerable contribution to climate change mitigation. However, bioenergy is not necessarily carbon neutral because emissions of CO 2 , N 2 O and CH 4 during crop production may reduce or completely counterbalance CO 2 savings of the substituted fossil fuels. These greenhouse gases (GHGs) need to be included into the carbon footprint calculation of different bioenergy crops under a range of soil conditions and management practices. This review compiles existing knowledge on agronomic and environmental constraints and GHG balances of the major European bioenergy crops, although it focuses on dedicated perennial crops such as Miscanthus and short rotation coppice species. Such second-generation crops account for only 3% of the current European bioenergy production, but field data suggest they emit 40% to >99% less N 2 O than conventional annual crops. This is a result of lower fertilizer requirements as well as a higher N-use efficiency, due to effective N-recycling. Perennial energy crops have the potential to sequester additional carbon in soil biomass if established on former cropland (0.44 Mg soil C ha À1 yr À1 for poplar and willow and 0.66 Mg soil C ha À1 yr À1 for Miscanthus). However, there was no positive or even negative effects on the C balance if energy crops are established on former grassland. Increased bioenergy production may also result in direct and indirect land-use changes with potential high C losses when native vegetation is converted to annual crops. Although dedicated perennial energy crops have a high potential to improve the GHG balance of bioenergy production, several agronomic and economic constraints still have to be overcome.Keywords: biofuel, carbon debt, carbon footprint, land management, methane, Miscanthus, nitrous oxide, short rotation coppice, soil organic carbon Greenhouse gas saving with bioenergy -a European perspectiveThe European Union has committed to increase the proportion of renewable energy from 9% in 2010 to 20% of Correspondence: Axel Don,

The recent policies enacted by the EU foresee an increased interest in the cultivation of energy crops.Hence systematized information on new energy crops and cropping strategies is necessary to optimize their production quantitatively and qualitatively and to integrate them into traditional production systems. This kind of information will offer farmers new perspectives and options to diversify their farming activities. Some of these crops, however, may compete for land and resources with existing food crops, while others could be grown in marginal/degraded lands with consequent benefi cial effects on the environment. Therefore choosing the appropriate management components and species should be site specifi c and oriented to minimize inputs and maximize yields. In some cases, traditional food crops are used as dedicated energy crops with the advantage that their management practices are well known. On the other hand, the management of new dedicated energy crops, such as perennial herbaceous crops, often demands a range of structural features and tactical management approaches that are different to those commonly used for traditional food crops. Most of these crops are largely undomesticated and are at their early stages of development and improvement. In this work, state-of-the-art research and development of agronomic management and the production of a wide range of multipurpose future energy crop species are reviewed and where possible examples of appropriate crop management practices that would enhance energy yields are provided. Interesting lines of investigation are also suggested. W Zegada-Lizarazu et al. Review: Agronomy of energy crops W Zegada-Lizarazu et al. Review: Agronomy of energy crops 14:549-563. (2005). 63. Mücher CA, Bunce RGH, Jongman RHG, Klijn JA, Koomen A, Metzger MJ et al., Identifi cation and characterisation of environments and landscapes in Europe. Alterra Rapport 832, Alterra, Wageningen (2003).

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Uncertainty in predictions of long-term yields of perennial grasses makes business plans untenable 21 in the short run. Long-term data across varied environments, including marginal lands, will help in 22 preventing uncertainty while providing farmers and entrepreneurs with sound information to 23 estimate reliable and affordable strategies on what, where and how long to grow perennial grasses. 24 In the present study, the long-term yields (11 to 22 years) of switchgrass (Panicum virgatum L.), 25 miscanthus (Miscanthus × giganteus Greef et Deuter) and giant reed (Arundo donax L.) grown in 26 northern and southern Mediterranean environments are reported. Switchgrass was grown in Greece 27 and northern Italy, giant reed in southern and northern Italy, and miscanthus in southern Italy. 28 Furthermore, lowland and upland switchgrass ecotypes were compared in Greece. Despite similar 29 biomass productions (9.8 and 10.0 Mg DM ha -1 for uplands and lowlands, respectively), the upland 30 ecotypes showed a significantly higher yield stability (CV of 24% and 32% for uplands and 31 lowlands, respectively) over a 17-year period. Biomass yield varied considerably across years and 32 locations; giant reed outperformed switchgrass under northern Italy environment (21.2 and 13.6 Mg 33 DM ha -1 for giant reed and switchgrass respectively). Annual yield of switchgrass was 30% higher 34 in the north than south Mediterranean; miscanthus showed intermediate production compared to 35 giant reed and switchgrass (average of 22 years) and a CV similar to switchgrass. In summary these 36 results evidence that multi-location, long-term trials are strongly needed to reduce uncertainties on 37 crop yield variability and provide more accurate data from which optimized socio-economic and 38 environmental predictions can be achieved.

Sustainable intensification of agricultural systems has been suggested – in addition to reducing waste and changing consumption habits – as a way to increase food, feed, fuel, and fiber security in the twenty‐first century. Here we describe three primary strategies of agricultural intensification – conventional intensification, temporal intensification, and spatial intensification – and how they can be used to manage and integrate food and second‐generation crop portfolios. While each strategy has individual merits, combining them to meet case‐specific targets may achieve optimum results. Multiple experiments and examples from the USA and the EU illustrate the potential of combining these approaches for agroecological intensification that can provide ecosystem services while maintaining or increasing economic output, thus striking a balance between ‘land sparing’ and ‘land sharing’. Management strategies will vary by the types of markets available, e.g., food, fuel and/or ecosystem services, and the scale of markets supplied, e.g., small heat and power vs. large cellulosic ethanol. Future research should holistically and methodologically evaluate the trade‐offs between different management strategies. © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd