Use of pyrolysis/GC–MS combined with thermal analysis to monitor C and N changes in soil organic matter from a Mediterranean fire affected forest

In Mediterranean ecosystems, forest fires are a common phenomenon. They involve the transformation of vegetation and litter, leaving charred residues and so influencing the carbon cycle by changing (a) the amounts of soil organic matter and (b) the proportions within it of pools with differing stability. In addition to affecting C cycles, fires also affect the amounts of N within soil organic matter, and its availability.Because of the difficulties associated with solvent-based methods for the investigation of char and other highly stable carbon forms within soil, physical methods based on pyrolysis have several advantages. We have used thermal analysis coupled with on-line analysis of evolved gas species to determine the proportions of soil organic matter of differing stability and the pattern of evolution of gaseous nitrogen-bearing species during heating. Burnt soil clearly has a lower content of labile organic matter than unburnt soil, and nitrogen is closely associated with more stable forms of soil organic matter.Using the decomposition temperatures observed from TG, triple shot pyrolysis–GC/MS shows clear differences between the molecular composition of burnt and unburnt samples. At 340 °C, the unburnt soil has a diverse content of polysaccharide-derived and aliphatic compounds, while the burnt soil yields only two furan compounds. At 520 °C, alkyl and aromatic compounds are observed, including PAHs from the burnt soil. The 600 °C shot is dominated by aromatic compounds, with no differences between burnt and unburnt samples.Taken together, the results of elemental analysis, TG–DSC–QMS and pyrolysis–GC/MS are a very helpful tools in the deciphering of changes in the soil organic matter properties, specifically the main changes observed are (a) to increase the proportion of thermally-stable components, and (b) to increase the proportion of nitrogen relative to C through the development of thermally stable heterocyclic N.