Temporal changes in magnetic signal of burnt soils - A compelling three years pilot study.

Wildfires strongly affect soils, including iron biogeochemical cycling and carbon storage. Thus, it is important to reveal the dynamics of iron oxide synthesis and transformations during and after a wildfire. This study investigates the temporal stability of strongly magnetic minerals appearing after a wildfire. Following a designed experimental fire, samples from vegetation ash and mineral soil were taken immediately after and at progressively longer time intervals. The magnetic susceptibility monitoring of samples during three years period demonstrates an initial increase in magnetic signal of ash-rich material taken immediately after the fire followed by a gradual decrease over time. The behavior of samples collected later after the fire showed only a moderate decrease. It is suggested that the magnetic susceptibility rise during the laboratory storage could be due to the increased availability of nutrients and microbial activity immediately after the fire and related intense redox reactions involving iron oxide particles. The decreasing trend in magnetic susceptibility is ascribed to the oxidation of ultrafine magnetite particles with time. All mineral soil samples from the deeper level showed an initial susceptibility increase, assigned to a similar process. Magnetic susceptibility monitoring was also carried out on samples gathered shortly after natural wildfires. The soil samples affected by strong and long wildfires show a decrease in magnetic susceptibility with time. This effect is more pronounced in the surface black-colored ash layer. The ash material from a site disturbed by a strong short wildfire demonstrates behavior similar to the ash from the experimental fire. It is supposed that the temporal evolution of magnetic susceptibility of the mineral alteration products of wildfires is influenced by the grain size of the produced iron oxide particles, their redox reactions mediated by the heterotrophic bacteria, as well as the amplifying role of pyrogenic carbon for intensification of the redox reactions.