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

Abstract

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 SOM with acid dichromate was 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 cm³. 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 cm³ except for (macro)charcoal that occurs in the lightest fraction (b 1.0 g cm³). 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. 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 SOM with acid dichromate was used to evaluate the contribution of pyrogenic organic carbon to total SOC. Furthermore, four SOM density fractions were sep...

Variability in the composition of charred litter generated by wildfire in different ecosystems

“…Because of the important role of litter as a source of SOM for mineral soil, accumulation of pyrogenic recalcitrant organic compounds determines the long-term SOM content, SOM properties and C-sequestration capacity of the soil (Knicker et al, 2012;Mastrolonardo et al, 2013). The data from elemental and thermal stability analyses confirm previous reports suggesting that the charred biomass generated in wildfire may not be stable in the long term (e.g.…”

“…Thus, Santín et al (2012) observed large differences in the abundance of charred biomass in rainforest and eucalypt forests. Mastrolonardo et al (2013) also attribute the amount and chemical nature of the burnt litter to changes in the vegetation. This variability may also explain differences in short-term microbial responses found in different ecosystems after burning (Bárcenas-Moreno et al, 2013).…”

“…Santín et al, 2012), soil run-off erosion (Cerdà and Doerr, 2008;Dlapa et al, 2013;Neris et al, 2013;Vega et al, 2005) and many ecological processes (Bárcenas-Moreno et al, 2013;Vega et al, 2013). Litter is one of the main precursors of soil organic matter (SOM), and many of the properties of mineral soils following wildfire may therefore be directly influenced by the amount and composition of the burnt litter (Bodí et al, 2014;Mastrolonardo et al, 2013).…”

Soil pyrogenic organic matter characterisation by spectroscopic analysis: a study on combustion and pyrolysis residues

“…plants with waxcoated leaves, which naturally induce water repellent properties to soil (Lozano et al 2013). As a consequence, PyOM formed here at relatively low temperatures may indeed have a large contribution from aliphatic compounds (Knicker et al 2005b;Mastrolonardo et al 2013). This could have many ecological implications.…”

“…In the last decades, the role of PyOM in biogeochemical cycles has received much attention since several forms of PyOM are recalcitrant to both biotic and abiotic degradation; thus, PyOM acts as an important inert carbon reservoir in the global carbon cycle (Schmidt and Noack 2000). PyOM was studied using several different methods, such as vibrational or magnetic spectroscopy (Almendros et al 2003;Knicker et al 2006;Francioso et al 2011;Mastrolonardo et al 2013), thermal analysis (De la Rosa et al 2008;Mastrolonardo et al 2014), visual microscopy (Stoffyn-Egli et al 1997;Pituello et al 2014) and chromatographic analyses (De la Rosa et al 2008;Nocentini et al 2010). Nevertheless, the nature of PyOM remains to be fully understood.…”