Fire-induced changes in soil organic matter stability along a catena on Mt. Etna, Sicily

Forest fires are a not negligible factor of pedogenesis in the Mediterranean areas, where they occur frequently. Soil organic matter (SOM) is particularly prone to change due to forest fire, both in terms of quantity and quality. We investigated a soil catena on Mount Etna, north-eastern side of Sicily, Italy, comprising six soils having different types of climate, from subtropical (500 m asl) to subalpine (1800 m asl). Along this altitudinal and climatic gradient there are two main vegetation systems: maquis at the lower sites and coniferous forest at the upper sites. A previous study in the same area revealed an increasing aromaticity of soil organic matter with decreasing altitude, which is probably correlated to more frequent fires at the low elevations, where climate regime is warmer and drier. We hypothesised that fire frequency is increasing with decreasing altitude and that this substantially affects organic matter characteristics and abundance. We consequently focused our work on the fractionation of soil organic matter from different depth into a labile and a stable fraction, carried out by using an H2O2 oxidation treatment. The bulk SOM and the stable fraction of SOM were quantified and characterised by diffuse reflectance Fourier transform infrared (DRIFT). The stable SOM fraction was also radiocarbon dated, so providing indications about its turnover time. Additionally, charcoal in the soil was collected, quantified, the plant species identified using a stereomicroscope and C-14 dated. We found a clear correlation between the aromatic content in the soil and the decreasing altitude confirming the initial hypothesis. The C and N concentration in the bulk soil increased with decreasing altitude, as well as the C stored in charcoal in the topsoil. The relative content of stable SOM is similar in the various soils of the catena, although it slightly decreases with altitude. The mean age of the stable organic fraction varies much in the different soils. In general, an age of a few hundred years is found at low altitudes, whereas at higher altitudes ages up to 8000 years could be measured. Overall, these findings suggest that the high fire frequency at low altitudes is a powerful rejuvenating factor for soil organic matter, removing part of the SOM and promoting plant recolonization.