2nd generation biofuels from short rotation plantations are less efficient in climate-change mitigation than reforestation within reasonable timeframes

Second generation biofuels (2G biofuels) produced from lignocellulosic biomass are often considered as integral part of a future sustainable transport system. Provided that substantial areas of agricultural land can be diverted from food and feed production without impairing food security, energy plantations managed in short rotation appear as a promising option for supplying large amounts of biomass feedstock. However, instead of using agricultural land for energy plantations, it could also be reforested, thereby acting as a long-term carbon (C) sink that also results in climate benefits. This paper provides a systematic comparison of the long-term C benefits from 2G biofuels produced from plantation biomass with the C sink strength of natural succession on arable land (i.e. renaturation).
C benefits of the two options are calculated on a per-km2 basis. The dynamics of C accumu-lation in natural vegetation as well as plantations (i.e. biomass yields) strongly depend on site-specific natural conditions. We apply a global perspective and assume that the considered area is distributed among ecological zones and climate zones exactly like actual global cropland areas. To this end, global raster data (5 arc minutes resolution) on cropland distribution, ecosystem and climate zones are merged, and representative C accumulation curves for renaturation in 11 world regions and on global scale are derived. Due to uncertainties with regard to biomass yields of energy plantations, harvest losses and future conversion efficiencies of 2G biofuel processes, a Monte Carlo simulation is carried out with these parameters being varied in ranges derived from literature data.
Results show that natural succession is highly likely to be superior (i.e. to result in higher C benefits) for timeframes up to 50 years. Hence, it takes more than 50 years of continuous land use as short rotation plantation and substitution of fossil fuels with 2G biofuels until this alternative results in higher cumulated C benefits than renaturation. This finding is robust to uncertainties related to yields and technological progress in 2G biofuel production. We conclude that allowing agricultural land to revert to its natural state must seriously be considered as low-cost climate mitigation strategy and alternative to biofuel production.