An analysis shows that fuel made from wild, herbaceous vegetation grown on land currently unsuitable for cultivating field crops could contribute substantially to the United States' targets for biofuel production. See Letter p.514
The governments of more than 35 countries, including the United States, Brazil and members of the European Union, have established policies promoting the production and use of biofuels1. This is driven by an interest in becoming more independent of fossil-fuel imports and reducing the climate footprint of their economies. However, the climate benefit of replacing fossil fuels with biofuels is strongly disputed, because of the lack of compelling evidence to show that biofuels are indeed associated with much lower greenhouse-gas emissions than fossil fuels when the full life cycle of their production and use is taken into account. On page 514 of this issue, Gelfand et al.2 report that certain wild, herbaceous vegetation, growing on 'marginal' lands currently unsuitable for arable farming, can be used as a biofuel crop, and does substantially mitigate greenhouse-gas emissions compared with fossil-fuel use — even rivalling the benefits associated with growing traditional biofuel crops, such as maize (corn)*.
When assessing the potential climate benefits of biofuels, it is essential to consider the consequences of land-use change and of fertilization associated with growing biofuel crops3,4 — particularly any changes in the carbon stocks of affected ecosystems, and in the emissions of nitrous oxide, a potent greenhouse gas produced by soil bacteria. It is also crucial to determine whether the growing of biofuel crops poses local threats to biodiversity, or to water and nutrient cycling5.
Moreover, because biofuel feedstocks are currently produced mostly on fertile agricultural land, it has been questioned whether useful amounts of biofuels can be produced without threatening food production. The ensuing conflict of interest has been called the “food, energy and environment trilemma”6. To be acceptable to society, therefore, biofuel-production strategies must be shown to greatly mitigate greenhouse-gas emissions without jeopardizing food and animal-feed production through competition for land use, and to have a minimal effect on the environment.
Gelfand et al. compared the biofuel yields, greenhouse-gas emissions, changes in soil-carbon stocks, and energy consumption associated with field operations for six biofuel-cropping systems in the midwestern United States over a 20-year period. They then used these data in a rigorous life-cycle assessment of the climate benefits of the different systems. Because it is based on long-term data, this is the first convincing analysis of the impact of biofuel-production systems on global warming. By contrast, previous studies relied either on modelling or on short-term studies of a smaller number of systems.
The authors show that all the biofuel-cropping systems investigated are net sinks of atmospheric carbon dioxide if fossil-fuel offset credits are included in the analysis. These credits are the sum of all the CO2 emissions potentially avoided when fossil fuels are replaced with biofuels, taking into account both the production and the combustion of the fossil fuels7. Surprisingly, the researchers found that the greenhouse-gas mitigation of wild, perennial, herbaceous vegetation (Fig. 1) — specifically, successional vegetation, which naturally regrows in marginal areas such as abandoned, low-productivity arable land — was markedly higher than that of intentionally grown crops, including maize, alfalfa, poplar and a maize–soya bean–wheat crop rotation, and that energy production was comparable. Moreover, Gelfand et al. show that moderate levels of nitrogen fertilization could further boost biofuel yields of the wild vegetation system by about 50%, with only a marginal increase in nitrous oxide emissions.
A big advantage of such native successional systems over other biofuel crops is that they can be productive despite the soil and climate restrictions typically found in marginal lands. This suggests that marginal lands could be a viable alternative to fertile cropland for biofuel production — which would be extremely useful, given the limited land resources8,9.
To explore the regional implications of their study, Gelfand et al. used a computational approach to identify suitable marginal lands for biofuel production across ten states of the US Midwest. More specifically, they used information from a geographical database in a biogeochemical model to estimate the effects of soil and climate on biofuel yields.
One constraint on the production of biofuels is the need to minimize the energy consumed by the collection and transport of the crop. Gelfand and colleagues show that, given the distribution of marginal lands in the US Midwest, optimal biofuel production would be achieved if biomass is collected from within a region of 80-kilometre radius around refineries. Such a production strategy could yield approximately 21 billion litres of ethanol per year from 11 million hectares of marginal land. This is about 25% of the target mandated by the US Department of Energy's Biomass Program for cellulosic biofuel production in 2022 (cellulosic biofuel is that produced from lignocellulose, a major constituent of wood and grasses). It equates to an expected fossil-fuel offset of roughly 40 teragrams of CO2 equivalents each year (1 teragram is 1012 grams) — the same as the CO2 emissions from 10 million medium-sized cars, each with an annual run of 20,000 km.
So would a native successional biofuel crop be all good? Perhaps not. Gelfand and co-workers' study does not explicitly answer the question of whether all the marginal lands identified as suitable for biofuel production could be used without harming biodiversity and the environment. Moreover, land that is fallow today might be needed in the future for agricultural production, to offset the demands of the world's growing population.
Another question raised by the study concerns greenhouse-gas mitigation: for the biofuel-cropping systems under consideration, the authors found that, apart from fossil-fuel offset credits, increases in soil-carbon stocks are the major driver of climate benefits. But the rate of increase of soil-carbon stocks will slow down with time, so that the stocks reach an equilibrium level within a few decades10. It therefore seems that comprehensive assessments of the long-term climate impacts of biofuels will require the quantification of spatially and temporally explicit soil-carbon sequestration potentials.
*This article and the paper under discussion2 were published online on 16 January 2013.