Forest residues are waste biomass, generated from activities including logging, thinning and wood product manufacturing. Using them as biofuels is attractive as it can enhance resource utilization, reduce wildfire risks and generate revenue for landowners. Moreover, in contrast to using dedicated biofuel feedstocks, it avoids land-use change and the greenhouse gas (GHG) emissions that can come with it. In light of this lack of land-use change, shifts in soil organic carbon (SOC) stock are often not considered by regulatory agencies in the calculation of the GHG footprint of biofuels made from forest residues. Now, using life-cycle assessment, Yuan Yao and colleagues at Yale University report that SOC changes caused by removal of forest residues contribute substantially to the GHG emissions associated with forest-residue-derived biofuels.

The researchers focus on the production of diesel and gasoline fuels via pyrolysis of pine residues in the southern United States. Their life-cycle analysis integrates models of forest growth and biorefinery processes with a model that estimates changes in SOC. The team find that changes in SOC over 100 years due to removal of forest residue account for 20–66% of the life-cycle GHG emissions of the biofuels. The results underline the importance of accounting for variations in SOC when assessing the environmental impact of biofuels, even when they are made from waste and in the absence of land-use changes.

森林残留物是由伐木、间伐和木制品制造等活动产生的废弃生物质。将它们用作生物燃料很有吸引力，因为它可以提高资源利用率，减少野火风险并为土地所有者创造收入。此外，与使用专用生物燃料原料相比，它避免了土地利用变化以及随之而来的温室气体（GHG）排放。鉴于缺乏土地利用变化，监管机构在计算由森林残留物制成的生物燃料的温室气体足迹时通常不考虑土壤有机碳（SOC）储量的变化。现在，通过生命周期评估，Yuan Yao 和耶鲁大学的同事报告说，清除森林残留物引起的 SOC 变化极大地增加了与森林残留物衍生的生物燃料相关的温室气体排放。

研究人员专注于通过美国南部松树残渣的热解生产柴油和汽油燃料。他们的生命周期分析将森林生长和生物精炼过程的模型与估计 SOC 变化的模型相结合。研究小组发现，100 年来，由于清除森林残留物而导致的 SOC 变化占生物燃料生命周期温室气体排放量的 20-66%。结果强调了在评估生物燃料对环境影响时考虑SOC变化的重要性，即使它们是由废物制成并且没有土地利用变化。