Effects of fire on soil organic matter quality along an altitudinal sequence on Mt. Etna, Sicily

Soil organic matter (SOM) can directly be affected by wildfires, both in terms of quantity and quality. The combustion is often incomplete, leading to the formation of pyrogenic organic matter (PyOM), a highly heterogeneous material with prevailing aromatic nature. Focusing our attention on PyOM, we investigated an altitudinal soil sequence on Mount Etna, Italy, ranging from 500 m to 1800 m asl, where the fire frequency is assumed to have been higher at lower elevations due to a warmer and drier climate. We evaluated the effect of fire on the chemical and physical characteristics of SOM along the altitudinal sequence. At two sites at a similar altitude but having a different recent fire history, we also examined the effect of fire frequency on SOM. Chemical oxidation of SOMwith acid dichromatewas used to evaluate the contribution of pyrogenic organic carbon to total SOC. Furthermore, four SOM density fractions were separated and characterised for principal composition, spectroscopic properties by Diffuse Reflectance Infrared Fourier Transform (DRIFT) and Nuclear Magnetic Resonance (NMR), and particle morphology by Scanning Electron Microscopy (SEM). The stocks of C and N as well as the chemical oxidation resistant carbon (COREC) increased with decreasing altitude, hence supporting the hypothesis of higher fire frequency at lower altitudes. The highest C concentrations were found in the density fraction 1.0–1.6 g cm−3. At the lower sites, a considerable amount of aromatics, and hence a significant part of COREC,was found in the density fraction 1.0–1.6 g cm−3 except for (macro)charcoal that occurs in the lightest fraction (<1.0 g cm−3). The properties of this latter density fraction seemed to be well related to vegetation characteristics. The dichromate oxidation procedure, however, has methodological restrictions. This procedure resulted here to be poorly specific for PyOM in the investigated soils, since it isolated a fraction rich in lipids. The lack of specificity for PyOM is most likely due to the rather low content of aromatics in SOM, although fire frequency was relatively high.