Biochar potentially mitigates greenhouse gas emissions from cultivation of oilseed rape for biodiesel

Winter oilseed rape (WOSR) is the main crop for biodiesel in the EU, where legislation demands at least 50% savings in greenhouse gas (GHG) emissions as compared to fossil diesel. Thus industrial sectors search for optimized management systems to lower GHG emissions from oilseed rape cultivation. Recently, pyrolysis of biomass with subsequent soil amendment of biochar has shown potentials for GHG mitigation in terms of carbon (C) sequestration, avoidance of fossil based electricity, and mitigation of soil nitrous oxide (N₂O) emissions. Here we analyzed three WOSR scenarios in terms of their global warming impact using a life cycle assessment approach. The first was a reference scenario with average Danish WOSR cultivation where straw residues were incorporated to the soil. The others were biochar scenarios in which the oilseed rape straw was pyrolysed to biochar at two process temperatures (400 and 800 °C) and returned to the field. The concept of avoided atmospheric CO₂ load was applied for calculation of C sequestration factors for biochar, which resulted in larger mitigation effects than derived from calculations of just the remaining C in soil. In total, GHG emissions were reduced by 73 to 83% in the two biochar scenarios as compared to the reference scenario, mainly due to increased C sequestration. The climate benefits were higher for pyrolysis of oilseed rape straw at 800 than at 400 °C. The results demonstrated that biochar has a potential to improve the life cycle GHG emissions of oilseed rape biodiesel, and highlighted the importance of consolidated key assumptions, such as biochar stability in soil and the CO₂ load of marginal grid electricity.