Controls and dynamics of biochar decay and soil microbial abundance, carbon use efficiency during long-term biochar-amended soil incubations

Authors: JIANG, XINYU - Colorado State University; DENEF, KAROLIEN - Colorado State University; Stewart, Catherine; COTRUFO, M. FRANCESCA - Colorado State University

Submitted to: Biology and Fertility of Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/2015
Publication Date: 8/19/2015
Publication URL: http://DOI:10.1007/s00374-014-1047-7
Citation: Jiang, X., Denef, K., Stewart, C.E., Cotrufo, M. 2015. Controls and dynamics of biochar decay and soil microbial abundance, carbon use efficiency during long-term biochar-amended soil incubations. Biology and Fertility of Soils. 52:1–14. doi:10.1007/s00374-015-1047-7.

Interpretive Summary: Biochar addition to soil has been proposed as a management strategy to sequester a recalcitrant form of carbon (C). However, there is growing evidence that biochar can be degraded by soil microbes and modify their abundance, community composition and activity. Yet we lack an understanding of how microbial use of biochar affects its persistence in soils, how biochar additions alter microbial soil C utilization, and how these dynamics differ across soils with differing physicochemical properties and biochar addition rates. We present results from a 30-month laboratory incubation, where oak-derived biochar was added at 0, 1, 5, 10, and 20% rate by weight to four soils varying in texture and soil organic carbon (SOC) content. We determined biochar C loss using natural abundance 13C isotope mixing models coupled to measurements of CO2 efflux and soil C remaining. We measured microbial abundance, community composition, and carbon use efficiency (CUE) using phospholipid fatty acid (PLFA) biomarkers and 13C labeled glucose additions. After 30 months, the biochar remaining was a consistent proportion (>90%) of the C added in each soil, regardless of biochar addition rate, suggesting that soil C sequestration from biochar addition will be proportional to biochar addition rate. Biochar decay rates changed as a function of SOC, with an interesting lag-time phenomenon in soils with higher SOC concentrations (i.e., > 1.5%), suggesting that microbes use biochar as a C source only after accessible SOC is exhausted. Microbial community composition was only altered at higher biochar addition rates (10 and 20%). Soil pH and C:N increased after biochar addition, and were correlated with soil microbial compositional changes. The highest CUE occurred at the 1% or 5% biochar addition rate, depending on soil type. This study suggests that in soils with continuous fresh soil C inputs, biochar decay will proceed very slowly and common biochar field application rates (20-100 ton ha-1 comparable to our 1-5% addition) will be beneficial for the soil microbial community.

Technical Abstract: Biochar addition to soil has been proposed as a management strategy to sequester a recalcitrant form of carbon (C). However, there is growing evidence that biochar can be degraded by soil microbes and modify their abundance, community composition and activity. Yet we lack an understanding of how microbial use of biochar affects its persistence in soils, how biochar additions alter microbial soil C utilization, and how these dynamics differ across soils with differing physicochemical properties and biochar addition rates. We present results from a 30-month laboratory incubation, where oak-derived biochar was added at 0, 1, 5, 10, and 20% rate by weight to four soils varying in texture and soil organic carbon (SOC) content. We determined biochar C loss using natural abundance 13C isotope mixing models coupled to measurements of CO2 efflux and soil C remaining. We measured microbial abundance, community composition, and carbon use efficiency (CUE) using phospholipid fatty acid (PLFA) biomarkers and 13C labeled glucose additions. After 30 months, the biochar remaining was a consistent proportion (>90%) of the C added in each soil, regardless of biochar addition rate, suggesting that soil C sequestration from biochar addition will be proportional to biochar addition rate. Biochar decay rates changed as a function of SOC, with an interesting lag-time phenomenon in soils with higher SOC concentrations (i.e., > 1.5%), suggesting that microbes use biochar as a C source only after accessible SOC is exhausted. Microbial community composition was only altered at higher biochar addition rates (10 and 20%). Soil pH and C:N increased after biochar addition, and were correlated with soil microbial compositional changes. The highest CUE occurred at the 1% or 5% biochar addition rate, depending on soil type. This study suggests that in soils with continuous fresh soil C inputs, biochar decay will proceed very slowly and common biochar field application rates (20-100 ton ha-1 comparable to our 1-5% addition) will be beneficial for the soil microbial community.