Characterization of wildfire effects on soil organic matter using analytical pyrolysis

The effectiveness of pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) as a rapid analytical technique to get insight in wildfire-induced molecular alterations of the soil organic matter in Mediterranean Leptosols was examined. To this end, the topsoil of two slopes with adjacent patches of burnt and unburnt Maritime Pine (Pinus pinaster) and eucalypt plantations (Eucalyptus globulus) were sampled in the Serra de Lousã, central Portugal. Several differences were found between the neighbouring burnt and unburnt soils, both in thermal desorption and pyrolysis behaviour. Thermal desorption results showed large amounts of aliphatic compounds (both alkyl and carbohydrate-derived compounds), which may indicate the incorporation of fresh plant material or low wildfire severity. Pyrolysis at 500 °C revealed an increase of low molecular weight molecules for certain homologous series in fire-affected soils, suggesting the occurrence of thermal breakdown and cracking of long-chain components. In addition, the presence of several thermo-labile markers pointed to the low severity of the wildfire. Elemental analysis indicated marked fire-induced increases in TOC and TN for the pine stand as opposed to noticeable decreases for the eucalypt stands. Probably, this contrast between the two sites is not due to differences in direct fire effects (especially fire severity) but to indirect fire effects i.e. in particular needle/leaf fall from affected canopies.