The signs of past wildfires encoded in the magnetic properties of forest soils

Among the main environmental factors, which influence the functioning of the soil system are wildfires. Their impact on vegetation cover and mineral soil has long been studied in relation to carbon cycling and preservation of soil health. Information about the changes caused by past wildfires in two forest soils, Umbrisol and Albic Luvisol, were obtained by environmental magnetic studies, combined with diffuse reflectance spectroscopy, x-ray diffraction, and Fe and C selective chemical analyses. Maghemite was identified as a main ferrimagnetic carrier of both pedogenic and fire-induced signals. The presence of goethite in the fire-affected depths of the two profiles indicated that the maximum temperature reached in the mineral soil during wildfires was lower than the transformation temperature of goethite (250-300 °C). Therefore, we suggest that the strong magnetic enhancement of the burnt soils (upper 1-2 cm) was caused mainly by the magnetic fraction contained in the vegetation ashes. Higher iron content in coniferous species, probably, was responsible for the higher degree of fire-induced magnetic enhancement of the Umbrisol as compared to less iron incorporated in the broadleaf vegetation at Albic Luvisol. No relationship between the degree of fire-induced magnetic enhancement and time since fire was observed.