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

Author
item Stewart, Catherine
item ZHENG, JIYONG - Northwest Agriculture And Forestry University
item BOTTE, JORIN - Colorado State University
item COTRUFO, M FRANCESCA - Colorado State University

Submitted to: Global Change Biology Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2012
Publication Date: 2/28/2013
Citation: Stewart, C.E., Zheng, J., Botte, J., Cotrufo, M. 2013. Co-generated fast pyrolysis biochar mitigates green-house gas emissions and increases carbon sequestration in temperate soils. Global Change Biology Bioenergy. 5:153-164.

Interpretive Summary: Biochar (BC) 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. When applied to soils, BC may increase soil C storage, and reduce soil emissions of greenhouse gases (GHG), such as N2O and CH4 --potentially making fast pyrolysis bioenergy generation a C-negative system. However, differences in production conditions (feedstock, pyrolysis temperature and speed, post-handling and storage conditions etc.) influence the chemical properties of BC and its net effect when added to soils. Understanding if the char produced from fast pyrolysis 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 (CO2, N2O and CH4) efflux, C partitioning using '13C, and soil C sequestration across four temperate soils and five BC rates; 0, 1, 5, 10 and 20% w/w. Expressed as CO2 equivalents, CO2 was the primary GHG emitted (97.5%), followed by N2O. 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.

Technical Abstract: Biochar (BC) 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. When applied to soils, BC may increase soil C storage, and reduce soil emissions of greenhouse gases (GHG), such as N2O and CH4 --potentially making fast pyrolysis bioenergy generation a C-negative system. However, differences in production conditions (feedstock, pyrolysis temperature and speed, post-handling and storage conditions etc.) influence the chemical properties of BC and its net effect when added to soils. Understanding if the char produced from fast pyrolysis 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 (CO2, N2O and CH4) efflux, C partitioning using '13C, 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, that we expected to be highly recalcitrant. Across soils, CO2 emissions increased linearly and N2O emissions decreased exponentially with increasing BC addition rates. Despite still being actively respired after 2 years, total BC-derived C-CO2 comprised less than the BC volatile C content (4%). Expressed as CO2 equivalents, CO2 was the primary GHG emitted (97.5%), followed by N2O. 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.