Fire simulation effects on the transformation of iron minerals in alpine soils

Heat-induced changes in topsoil iron (Fe) species triggered by the relatively low temperatures (Ts) produced by wildfires (200–300 °C) have not been fully elucidated. Changes in the chemistry of Fe minerals can produce cascading impacts on the environment, and the resulting aftermath may be exacerbated in erosion-prone alpine soils. The aim of this study was to determine - at environmentally realistic conditions - changes in the composition and crystallinity of Fe species, and the dispersible organic matter (OM) pool (evaluated by the pyrophosphate extraction), as a function of rising Ts, native Fe phases and OM in alpine soils. Multiple techniques were employed, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), magnetic measurements and Fe K-edge X-ray absorption spectroscopy (XAS). The results demonstrated the dominance of poorly crystalline Fe phases at 300 °C. At the same T, we observed the partial conversion of maghemite to hematite (supporting the involvement of oxidative processes) and, in the presence of high organic carbon (OC) contents, an enrichment in magnetic minerals (suggesting the onset of reducing conditions). The heat-induced modifications in Fe species and organic compounds did not promote the stabilization of the remaining OM, highlighting the weak nature of soil organo-mineral associations in an after-fire scenario. These phenomena can be further aggravated in case of the steep terrain that characterize alpine soils.