Co‐generated fast pyrolysis biochar mitigates green‐house gas emissions and increases carbon sequestration in temperate soils

Abstract Char is a product of thermochemical conversion of biomass via pyrolysis, together with gas (syngas), liquid (bio‐oil), and heat. Fast pyrolysis is a promising process for bio‐oil generation, which leaves 10–30% of the original biomass as char. Char produced for soil application, is defined biochar ( BC ), and it may increase soil C storage, and reduce soil emissions of greenhouse gases ( GHG ), such as N 2 O and CH 4 –potentially making fast pyrolysis bioenergy generation a C‐negative system. However, differences in production conditions (e.g., feedstock, pyrolysis temperature and speed, post handling, and storage conditions) influence the chemical properties of BC and its net effect when added to soils. Understanding if fast pyrolysis BC can increase C sequestration and reduce GHG emissions will enable full assessment of the economic value and environmental benefits of this form of bioenergy. We characterized a BC produced by fast pyrolysis for bio‐oil generation and examined GHG ( CO 2 , N 2 O and CH 4 ) efflux, C partitioning using δ 13 C, and soil C sequestration across four temperate soils and five BC rates; 0%, 1%, 5%, 10%, and 20% w/w. The fast pyrolysis process created a highly aromatic, low N, ash‐rich BC with a O : C ratio of 0.01, which we expected to be highly recalcitrant. Across soils, CO 2 emissions increased linearly and N 2 O emissions decreased exponentially with increasing BC addition rates. Despite still being actively respired after 2 years, total BC ‐derived C‐ CO 2 comprised less than the BC volatile C content (4%). Expressed as CO 2 equivalents, CO 2 was the primary GHG emitted (97.5%), followed by N 2 O . All GHG emissions were small compared to the total SOC sequestered in the BC . Fast pyrolysis produced a highly recalcitrant BC that sequestered C and reduced GHG emissions. The recovery and soil application of BC would contribute to a negative carbon balance for this form of bioenergy generation.