Biochar reduces total greenhouse gas emissions from oilseed rape cultivation, mainly due to C sequestration quantified by the concept of avoided atmospheric CO2 load – a theoretical life cycle assessment study.

Biochar is related to multiple greenhouse gas (GHG) mitigation potentials, namely nitrous oxide (N₂O) mitigation, carbon (C) sequestration and the possible green electricity produced in the pyrolysis process. Whereas the mechanisms behind potential N₂O mitigation effects of biochar are still unclear, the mechanisms behind C sequestration is agreed to be caused by C compound conversion towards aromatic structures in the biochar C as a consequence of the heating treatment in the pyrolysis process. The specific recalcitrance of the biochar is, however, difficult to estimate. The possible electricity production from the syn-gas and bio-oil pyrolysis products depends on the feedstock and process temperature, and can contribute considerably to total system GHG mitigation. However, the multiple effects on GHG balances of biochar and pyrolysis represent a complexity, which may best be analysed by a life cycle assessment (LCA) approach. In this study, the average Danish oilseed rape cultivation was set as the reference scenario in an LCA of cultivation related GHG emissions. The reference was compared with two scenarios with theoretical inclusion of pyrolysis and biochar, meaning that the oilseed straw residue was transported to a pyrolysis plant and the biochar was returned to the field in a corresponding amount (ca. 1 Mg ha^-1). Transportation, additional field operations, N₂O mitigation, electricity production and C sequestration was included, and the latter was calculated by using the concept of avoided atmospheric CO₂ load. The latter approach 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 with the reference scenario, mainly due to increased C sequestration. The study suggests that even low application rates and rather conservative biochar C recalcitrance estimates lead to considerably reductions in GHG emissions from oilseed rape and expectedly other crops.