CO₂ emissions from crop residue-derived biofuels

To the Editor —

In the May issue of Nature Climate Change, Liska et al.¹ presented a comprehensive analysis of the soil organic carbon (SOC) loss due to harvest of corn residues for bioethanol production. We do not dispute the main findings that harvest of residues has a negative impact on SOC levels and that this impact should be addressed when evaluating the potential benefits of cellulosic biofuels. We do, however, find that the conclusion, that cellulosic biofuels increase CO₂ emissions, builds on an incomplete analysis and that the analysis could have reached the opposite conclusion had it been more complete.

Liska et al. refer to consequential life cycle assessment (LCA) as the reason why SOC loss must be incorporated in greenhouse-gas analyses of biofuels. Consequential LCA requires that mass balances are closed and if not, some impact allocation must take place. In cellulosic ethanol production based on agricultural residues 20–25% of the carbon in the biomass ends up in ethanol and half of that amount in CO₂. Approximately 40% is retained in the lignin residue and the rest (∼20–30%) in molasses/vinasse^2,3. Liska et al. surprisingly disregard a considerable part of that carbon mass and attribute all CO₂ interactions between the product system and the atmosphere to ethanol. The lignin fraction is not accounted for in the main comparison between cellulosic and fossil fuels (Fig. 3 in ref. 1). If the lignin fraction was used for electricity generation, the authors report a potential to save greenhouse-gas emissions worth 55 g CO₂ equivalent MJ⁻¹. Otherwise it can, due to its recalcitrance, constitute a valuable contribution to SOC if returned to the soil, as also noted, but not accounted for, by the authors. C5-molasses may be used to feed livestock or generate energy through anaerobic digestion, displacing in either case other production or energy use. This fraction is not accounted for at all. The feed fraction from corn residue ethanol is reported to make up ∼17% of the total greenhouse-gas displacement potential from cellulosic ethanol production⁴.

The analysis by Liska et al. shows that growing corn after corn, in itself, reduces SOC, and that the harvest of residues accelerates SOC loss. Owing to the exponential decay of carbon in soil (Supplementary Fig. 1 in ref. 1), time, so to speak, dilutes the average annual carbon emissions⁵. While Liska et al. chose a 5–10-year time perspective in their analysis, the IPCC recommends a 20-year perspective⁶, the same as the European Union Renewable Energy Directive⁷. And much LCA work applies a 100-year time perspective⁵. Applying any of these time perspectives to the analysis of Liska et al. would reduce the greenhouse-gas impact of cellulosic biofuel and render cellulosic biofuels capable of reducing CO₂ emissions and perhaps even meeting the Renewable Fuel Standard reduction target.

Loss of SOC from biofuel production is a critical issue for greenhouse-gas emissions and soil quality, and it should be addressed in both science and management. But it is highly important that all biogenic carbon is included in greenhouse-gas analyses and that relevant time frames are applied, which is not the case for the analysis by Liska and co-authors.