Robertson, G. P., S. K. Hamilton, B. L. Barham, B. E. Dale, R. C. Izaurralde, R. D. Jackson, D. A. Landis, S. M. Swinton, K. D. Thelen, and J. M. Tiedje. 2017. Cellulosic biofuel contributions to a sustainable energy future: Choices and outcomes. Science 356:eaal2324.
Cellulosic crops are projected to provide a large fraction of transportation energy needs by mid-century. However, the anticipated land requirements are substantial, which creates a potential for environmental harm if trade-offs are not sufficiently well understood to create appropriately prescriptive policy. Recent empirical findings show that cellulosic bioenergy concerns related to climate mitigation, biodiversity, reactive nitrogen loss, and crop water use can be addressed with appropriate crop, placement, and management choices. In particular, growing native perennial species on marginal lands not currently farmed provides substantial potential for climate mitigation and other benefits.
BACKGROUND Cellulosic biofuels offer environmental benefits not available from grain-based biofuels and are a cornerstone of efforts to meet transportation fuel needs in a future low-carbon economy, even with electrified vehicles and other advances. Bioenergy with carbon capture and storage (BECCS) is also key to almost all Intergovernmental Panel on Climate Change mitigation scenarios that constrain end-of-century atmospheric CO2 to 450 parts per million. Some cellulosic feedstocks can come from industrial and agricultural by-products or from winter cover crops, but a substantial fraction must come from cellulosic biomass crops—perennial grasses and short-rotation trees planted for this purpose. Land requirements, however, are substantial and raise crucial questions about the environmental sustainability of a future bioenergy economy. First, if planted on existing croplands, will biofuel crops increase food prices or lead to the establishment of new cropland elsewhere, with concomitant climate harm? Second, will planting biofuel crops diminish biodiversity, especially if non-native or invasive species are cultivated on land with existing conservation value? Third, might perennial biofuel crops use more water than the vegetation they replace, leading to lower water tables and reduced surface water flows? And finally, if crops are fertilized, how much additional reactive nitrogen might be added to a biosphere already overburdened?
ADVANCES Recent empirical findings have shed considerable light on these questions. Broad generalizations are difficult, but we know now, for example, that planting perennial cellulosic biofuel crops on marginal lands—that is, land not currently used for food production because of low fertility, environmental sensitivity, or other reasons—can potentially avoid food-fuel conflict and indirect land-use change effects while providing substantial climate benefits. The direct carbon costs of establishing crops on such lands can be minimized by avoiding tillage and by avoiding land with large existing carbon stocks, such as forests and wetlands. Diverse plantings provide multiple ecosystem services including wildlife conservation, pollination, and pest protection that can benefit neighboring crops; relatively little plant diversity can provide disproportionately large benefits. Biofuel crops can be planted that require little if any nitrogen fertilizer, thus avoiding its environmental impact. And although different crops have different water-use efficiencies, most crops examined appear to evapotranspire about the same proportion of growing season rainfall, suggesting little impact on landscape water balances in humid temperate regions.It is also clear that there is no best crop for all locations even within a single region, and that all choices involve trade-offs. For example, highly productive non-native species can maximize climate benefits but harm biodiversity. Balancing trade-offs entails societal choices.
OUTLOOK Many questions about cellulosic biofuel sustainability remain. Still needed is an integrated understanding of the entire field-to-product enterprise sufficient to leverage synergies and to avoid trade-offs that can diminish environmental benefits. More specifically, and of particular importance, is the need for knowledge to facilitate the successful cultivation of highly productive native spec es on marginal lands, where plant growth is often limited by abiotic stressors. Harnessing the plant microbiome to help ameliorate environmental stress is a major untapped frontier, as is the potential for microbiome-assisted soil carbon gain. The promise of cellulosic biofuels for helping to create a more sustainable energy future is bright, but additional effort is required, including policies and incentives to motivate farmers to grow appropriate crops in appropriate places in sustainable ways. We must be careful to facilitate genuine climate mitigation that enhances rather than diminishes other ecosystem services. The planet deserves no less.
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