Table 2.

Potential environmental impacts of a globally distributed lignocellulosic biofuel production system.

Environmental outcome	Perennial crops (inc processing & conversion)	Crop residues (inc processing & conversion)	Forestry residues (inc processing & conversion)	Distribution of impacts
Greenhouse gas emissions	Most studies report (+) impact, i.e., GHG savings compared to fossil fuels [26] but actual impact on the ground depends on choices about land-use	Most studies report (+) impact, i.e., GHG savings compared to fossil fuels [26]; But including (−) impact of residue removal on soil carbon would change the assessment [27]	Studies reporting (+) impact, i.e., GHG savings challenged by others reporting high potential for (−) impact, i.e., increase in GHG emissions from iLUC (due to diversion of forestry residues from current uses in furniture, paper/pulp industries) or from total increase in use of forestry resources [27], [46]. Impact of residue removal on soil carbon also cited as (−) [27]	Impact on national carbon savings targets. Physical impacts of GHG changes will be felt on a global level.
Biodiversity	Some evidence of (+) impact on biodiversity [24]. Depends on indicators used for biodiversity, and what perennial crops are compared to: arable crops, grassland or leaving the land fallow. Or if compared with fossil fuel extraction, high (+) impact [26]	IEA (201) suggests it depends on whether these are ‘primary’ or ‘secondary’ residues. (−) impact for primary residues which are likely to be left as cover or ploughed back into soil with benefits for microflora. (+) impact for secondary residues which would be treated as waste. (−) impact may be ameliorated depending on how much residue is removed vs left behind	Similar concern about (−) impact due to role of forest residue cover in enhancing soil biodiversity. Here Schulze et al [46] suggest even thinning of forest cover (as opposed to large-scale deforestation) can be harmful	Locally specific impacts experienced in sites of biomass sourcing
Water, Soil & Air quality	Contrary results of + and – reported across studies. Review suggests more research is needed especially on under-researched aspects (e.g., impact of enzymes, catalysts used in conversion).	Contrary results of (+) and (−) across LCAs. Depends on allocation of impacts between crop and co-product. Frequently cited concern of (−) impact on soil structure and nutrient balance from removing residues (vs ploughing back to soil). Difficulties with scientific measurement of impacts on soil [26]	(−) impact on soil quality and structure from removing residues cited as concern Difficulties with scientific measurement of impact on soil quality [26]	Local impacts experienced in and around sites of biomass sourcing and conversion