CARBON LOSS FROM MINERAL SOILS DUE TO APPLIED HEAT FROM ABOVE-GROUND FIRES

An average of 200 million hectares of land area is subject to biomass burning globally each year (Levine, 1999). The potential loss of carbon directly from the mineral (non-flammable) soil horizons beneath these surface-fires has been largely overlooked as a potential source of greenhouse gasses to the atmosphere. Here, we present the results of a laboratory-based study on two mineral dominated soil types (an Alfisol and an Inceptisol) common to forest environments elevated to temperatures traditionally seen beneath the surface of a forest fire. Our results show striking increases in carbon loss (up to 100%) between 200 and 400 ˚C common to all horizons of both soils. These measurements, taken in conjunction with our experimental fire temperature profiles, allowed for the calculation of total potential carbon release from mineral soil horizons during a fire event: 0.036 and 0.108 Gt C / (100 km)² for the Inceptisol and the Alfisol (respectively). Our study reveals essentially no difference in the C responses between any of the epipedon or subsurface horizons. This indicates that wide variations in C_org and clay content between soil horizons appear to play no major role in defining the loss of C_org at temperatures above 100˚C. Changes in δ¹³C_org values were also recorded in each soil horizon at each respective temperature revealing an average increase of 3‰ after exposure to temperatures ranging from 200 to 400˚C. In summary, our work suggests that considerable C can be lost from soils at temperatures well below what are commonly assumed to be required for ecosystem burning and that substantial losses of C can occur in the absence flaming combustion within the mineral soil horizons.