Large Losses of Pyrogenic Carbon (Biochar) and Native Soil Carbon During a 15-Month Field Study in North Florida, USA

Although the application of biochar to soils has been proposed as a method of carbon sequestration for climate mitigation while improving crop yields, losses of biochar carbon (BC) through mineralization may reduce these benefits. However, few field studies have been conducted that control for biochar migration so that the rates and processes of BC remineralization from soils, as well as the effects of biochar on native soil organic carbon, can be accurately determined. Here, biochar made from different biomass types (oak, pine wood, and grass) and temperatures (lightly charred at 250 °C and pyrolyzed at 400 and 650 °C) were added to fine sandy Entisol in an open agricultural field and Spodosol in a shaded forested site in North Central Florida. After 15 months, BC losses, determined by the Kurth–Mackenzie–Deluca chemical–thermal oxidation method, ranged from 17.5 to 93.3% y −1 (14.0–51.5% y −1 for the 650 °C biochar). These losses were correlated with but much greater than the 0.4–3% y −1 BC losses recorded in a one-year laboratory study using the same biochars and those of several previous field studies (1–14% y −1 ). The losses of non-BC native carbon stocks (i.e., positive priming) also varied with biochar and soil type and ranged from 1.5 to 15.8% y −1 . The high BC losses observed in this study may be attributed to the subtropical and temporally variable climate conditions at the study site. Greater efforts should be made to base BC long-term stability estimates on field studies that monitor or control for biochar migration rather than reliance only upon laboratory incubations.