PyOM characterisation in fire-prone Mediterranean pine forests

Fire is a major disturbance to vegetation and soil and a driving factor of ecosystems in the Mediterranean basin, as up to 90% of all forest fires in the EU occur in Mediterranean countries (De la Rosa et al., 2008). The aboveground vegetation and the soil organic matter (SOM), which is the fuel in soil, are affected in terms of both abundance and composition. Fire impact on soil organic matter is usually confined to the organic horizon and a few top cm of the underlying mineral soil (Certini et al., 2011). A major outcome of wildfires is the production of charcoal – the result of incomplete combustion of plant biomass and litter ‐ and its release to soil (Knicker, 2011). In terms of composition, fire increases the heterogeneity of soil organic matter (SOM) because apparently no original component is totally removed, while new thermally‐condensed aromatic compounds form as a consequence of chemical reactions driven by the high temperatures (González‐Pérez et al., 2004). These fire‐derived compounds collectively form the so‐called pyrogenic organic matter (PyOM). Some PyOM is usually reported to have longer residence times in soil compared to the parent materials, enhancing the potential of soil to store carbon for long time, offsetting the release of human induced CO2 to the atmosphere (Schmidt and Noack, 2000). However, PyOM structure and composition is not yet well‐understood. PyOM is not so inert as formerly believed (Farrell et al., 2013) and can play crucial roles in soil biological processes (Zackrisson et al., 1996) and on SOM dynamics (Wardle et al., 2008). Consequently, there is gathering interest in collecting reliable information on structure and properties of PyOM and understanding its behaviour in soil. Mediterranean soils, in spite of being particularly affected by wildfires, were poorly investigated compared to soils of other environments. The present work deals with some wildfires occurred in Mediterranean pine forests of Tuscany, central Italy, and it is aimed at disentangling the composition and structure of PyOM produced during wildfire. To overcome the complexity of the investigated matter, we used various techniques and we fractionated charcoal according to particles size, starting from the assumption that those fractions have different composition and reactivity, which is plausible on the basis of previous studies (Rumpel et al., 2007; Nocentini et al., 2010; Francioso et al., 2011). By thermal (thermogravimetry, differential scanning calorimetry) and spectroscopic (FT‐IR, Raman, NMR) techniques, we observed that fire caused an accumulation of recalcitrant and refractory compounds at the expense of the SOM fraction most labile to thermal degradation. Charring, on the other hand, was not complete, as part of the burnt material still contained a fraction thermally degradable under 500 °C. As a consequence, the partly charred materials accumulated on the surface and into the first layer of mineral soil are prone to be oxidised by further fire events. The thermally recalcitrant pool was enriched in N poliaromatic molecules whose formation provides evidence that PyOM formed at temperatures not exceeding 400 °C. The size fractions we separated the macroscopic charcoal, actually showed different elemental composition, lignin signature and reactivity, as verified by dichromate chemical oxidation. This induces to hypothesise different precursor plant materials from which these fractions were originated and/or different fire conditions under which they were produced. Spectroscopic analyses (FT‐IR, normal Raman, surface‐enhanced fluorescence and surface‐enhanced Raman scattering, NMR) support such a hypothesis, clearly showing that the charcoal found on the ground immediately after a wildfire has a different chemical structure depending on size of particles, type of burnt biomass and fire conditions under which charcoal was produced. In particular, fine charcoal is generally less aromatic but richer in N‐bearing aromatic structures, aliphatic chains and carbonyl groups; it is also more reactive, suggesting a relatively fast mineralization in soil. On the contrary, the coarse fractions are less reactive, mainly comprising condensed aromatic material apparently more crystallised, being probably composed of larger graphitic clusters.