Introduction: Impact of land use change for bioenergy production on greenhouse gas emissions

Abstract

The European Union has committed to increase the proportion of renewable energy from 9% in 2010 to 20% of total energy consumption by 2020. Bioenergy currently accounts for almost two-thirds of the total renewable energy in Europe and much of this comes from energy crops. One element of the rationale for growing energy crops is based on the understanding that these can be used as part of the portfolio of measures for mitigating greenhouse gas (GHG) emissions. This is based on the principle that carbon emitted during combustion is balanced by carbon fixed in photosynthesis. However, this is a gross oversimplification and bioenergy production is unlikely to be carbon neutral because of GHG emissions released during crop growth, field management, feedstock processing and transport. Additional to carbon emitted as CO2 to the atmosphere, emissions of other GHGs, particularly methane and nitrous oxide, have also to be taken into account. Clearly, a better understanding of the impact of the conversion of land from its more typical use for food production to bioenergy production is required before we can argue for the widespread adoption of crops for both energy production and GHG mitigation and this was the topic of a workshop sponsored by GHG-Europe (an EU FP7 project) in Dublin in October 2010. The papers that follow all arose from presentations at the meeting. The review of Don et al. (2012) is a compilation of existing knowledge of the GHG balances of major European bioenergy crops with a particular focus on dedicated perennial crops such as Miscanthus and short rotation coppice species. Although such second-generation crops currently account for only 3% of the current European bioenergy production, their wider use in future will have a major impact on N2O emissions as they emit 40% to >99% less than conventional annual crops due to both lower fertilizer requirements and a higher N-use efficiency. These perennial energy crops also have the potential to sequester additional carbon in the soil, particularly if established on former cropland. This is largely confirmed by the study of Zimmermann et al. (2012) who have undertaken a study of Miscanthus grown on commercial farms, although they also show that the amount of carbon stored varies considerably with soil type, crop management and previous land use. In another review, Njakou Djomo and Ceulemans (2012) make a comparative analysis of 15 studies of the carbon intensity (the amount of CO2 emitted per unit of biofuel produced) of biofuels caused by direct and indirect land use change. They find that the total land use change carbon intensity of bioethanol production from biofuels ranged from −29% to 384% of that of gasoline fossil fuel. In an experimental study in NE England, Drewer et al. (2012) show that N2O emissions from Miscanthus and willow were lower than for wheat and oilseed rape on the same land, but that perennial bioenergy crops only emit less GHGs than annual crops when they receive zero or very low rates of N fertilizer. Furthermore, the review by Monti et al. (2012), which focused on switchgrass (Panicum virgatum), another C4 perennial grass, indicates that in most life cycle analysis studies there are significant reductions in CO2 emissions associated with cultivation and processing, compared with conventional crops. Interestingly, this species also performed better than most other biomass crops in terms of N2O emissions. Further studies of the impact of first-generation biofuel production on N2O emissions are reported by Carter et al. (2012) for organic farming systems in Denmark where the greatest GHG reduction was obtained from biogas production or co-production of bioethanol and biogas on either fresh grass-clover or whole crops of maize. In contrast, biofuel production based on lignocellulosic crop residues of rye and vetch produced considerably smaller net GHG reductions. Finally, Mander et al. (2012) studied the special case of GHG emissions from abandoned peat extraction areas in Estonia cultivated with reed canary grass (Phalaris arundinacea). The associated decreases in GHG fluxes turned these areas from net sources to net sinks of carbon and the almost zero CH4 emissions from Phalaris plots was attributed to the high sulphur concentration in peat, which probably inhibits methanogenesis. Clearly further studies on other peat substrates with varying sulphur contents are required to assess the generality of these findings. Whilst the papers in this special issue largely support a dual role for bioenergy crops in GHG mitigation and energy production there are still a number of unanswered questions, including their long-term impacts on the sustainability of agricultural ecosystems, their effects on dissolved N and C losses, and the extent to which their dual functions may be constrained by climatic and soil factors. All of these factors could have important consequences for the widespread introduction of bioenergy crops.