Organic carbon loss and delta-13C changes in mineral soils due to above-ground fires

All soils contain mineral horizons contain up to 20% organic carbon (Corg). In fact, most soils are dominated by mineral horizons and do not contain the thick organic and potentially combustible horizon associated with their more-celebrated organic counterparts. The potential for carbon release from mineral horizons beneath aboveground fires has been largely overlooked despite the large reservoir of this soil organic carbon relative to standing terrestrial biomass. We subjected the thin organic horizons and mineral horizons of an Alfisol and an Inceptisol, two soils containing widely different soil properties, to elevated temperatures ranging from 150--500°C in 50°C increments using new samples at each temperature level. These two soils represent a chronosequence in which one soil has been developing a substantially longer time than the other and consequently represent end-members of pedogenic development. We compared the resultant carbon content and carbon isotope composition from the laboratory heating to the original (pre-incineration) values to reveal striking increases in carbon loss (up to 100%) and enrichment in 13 C (up to 3.0 ‰) between 200 and 400°C common to all horizons of both soils. Our results implied the release of significant amounts of 13C-depleted compounds at relatively low temperatures. These laboratory measurements, taken in conjunction with temperature profiles gained during field-based burning, allowed for a calculation of the maximum carbon loss levels from each soil horizon at elevated temperatures. This work indicates mineral soils begin to release carbon at temperatures as low as 150°C and can release up to 80% of their original carbon content to the atmosphere at temperatures below those required for flaming combustion (325°C). Most soil horizons reached their maximum levels of carbon loss at 400°C indicating that a complete loss of organic material is possible from mineral soils at temperatures commonly realized during above ground fires in forested environments. This work will provide new insights into the study of fire effects on mineral soils both immediately following a fire and the study of paleosols at archeological sites.