Structural heterogeneity of vegetation fire ash

Any wildfire generates ash, the solid residue derived from burning biomass. Vegetation fire ash consists of charred organic material, charcoal, and inorganic mineral substances. Recent studies identified ash deposits as a dual system: soil and ash strata. The addition of ash to the soil profile alters soil properties and dynamics. A thorough analysis of ash–soil profile reveals a structural appearance of thin laminas. The laminas differ in a variety of physical properties and mineral composition. This research aims at assessing the unique properties of ash–soil profiles by performing an infrared spectroscopic study and statistical analysis. For that purpose, several laboratory experiments were conducted. The paper presents semiquantitative results of spectral analysis calculated by 4 statistical methods. The results indicate a well‐established laminar structure of the ash and evaluate the characteristics of each lamina. The proposed methodology was examined under real‐world conditions; a field experiment of 2‐m2 parcel with in situ O horizon was flamed and burned out. The field samples illustrated the formation of microlaminas, which proved to be similar to the laboratory samples. This detailed approach may promote a better understanding of the complex nature to the ash, the ash–soil interactions and its effect on the edaphic ecosystem. KEY W ORDS fire‐induced ash, heat‐map classification, midinfrared spectral features, physical properties, random forest, sequential feature selection, soil, spectral indices 1 | I N T RO D U CT I O N Wildfires represent an important agent of land degradation in Mediterranean ecosystems (Cerda, 1998). The postfire elimination of the vegetation, litter, and ground biomass, coupled with the modification in soil physicochemical properties, might have significant consequences for soil erosion and land degradation, particularly on hillslopes, where the hydrogeomorphological processes are more intense (Moody & Martin, 2009; Jordan et al., 2016). Additionally, recent studies stressed the role of the ash layer, commonly covering the soil following fires, as having a key function in soil wettability and the hydrological behavior of the soil immediately after a wildfire (Balfour & Woods, 2013; Bodí et al., 2014; Cerdà & Doerr, 2008). Field observations of postfire erosion found positive links between ash and accelerated erosion rates (Cannon, Kirkham, & Parise, 2001; Reneau, Katzman, Kuyumjian, Lavine, & Malmon, 2007; Prats, Wagenbrenner, Martins, Malvar, &