• Open Access

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

Authors

  • Catherine E. Stewart,

    Corresponding author
    1. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, USA
    • USDA/ARS, Soil-Plant-Nutrient Research Unit, Fort Collins, CO, USA
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  • Jiyong Zheng,

    1. State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A & F University, Yangling, Shaanxi, China
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  • Jorin Botte,

    1. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, USA
    2. Department of Soil and Crop Sciences, Colorado State University, Fort Collins, CO, USA
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  • M. Francesca Cotrufo

    1. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, USA
    2. Department of Soil and Crop Sciences, Colorado State University, Fort Collins, CO, USA
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Correspondence: Catherine E. Stewart, tel. + 970 492 7270, fax + 970 492 7213, e-mail: catherine.stewart@colostate.edu

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 N2O and CH4 –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 (CO2, N2O and CH4) efflux, C partitioning using δ13C, 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, 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.

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