Shifts in soil organic matter pools exerted by wildfires as seen by the combined use of analytical pyrolysis and advanced thermogravimetric techniques

Abstract

In Mediterranean ecosystems, forest fires are a recurrent phenomenon. They involve the transformation of vegetation and litter, leaving charred residues and so influencing the global carbon cycle by (a) affecting the amount of soil organic matter (SOM) and (b) exerting shifts within SOM pools differing in stability [1]. In addition fires also affects other biogeochemical cycles like N cycle, the amount and chemical forms of N within SOM are modified, and therefore its availability [2]. Thermal analysis methods, specifically thermogravimetry-differential scanning calorimetry (TG-DSC), have been used to characterise chemical changes in the organic matter fractions of soils and sediments, degraded plant tissue, and compost [3]. TG-DSC has also been used to compare the proportions of reactive and more stable components in organic matter fractions under contrasting conditions [4] and recently to study SOM pools including the most refractory C forms like black carbon [5]. Extending the capability of thermal analysis, the coupling of an isotope ratio mass spectrometer (IRMS) to a thermal analysis system (TG-DSC-IRMS) allows measurements of C and N stable isotope ratios to be made on the gas evolved from organic carbon within a sample, quantifying each fraction from the measured weight loss as it is combusted experimentally. With regards to its thermal stability, SOM can be divided into three main groups in terms of the proportions of labile (Exo 1, 200-380 ºC), recalcitrant (Exo 2, 380-475 ºC) and refractory (Exo 3, 475-650 ºC) components [4]. Using thermal analysis (TG-DSC) coupled with on-line analysis of evolved gas (IRM-QMS) species, we found that in burnt soils there is a conspicuous decrease in labile organic matter and that nitrogen is closely associated with more stable forms of soil organic matter. Based in the differential decomposition temperatures as deduced from TG techniques, we have developed a three steps-pyrolysis-GC/MS, which shows clear differences between the molecular composition of burnt and un-burnt samples. At the temperature where cellulose decomposes corresponding to Exo 1 (280 ºC) (Labile organic matter) the pyrograms in the un-burnt soil are dominated by aliphatic C and carbohydrates, while the burnt soil yields only two furan compounds. At temperature corresponding to Exo 2 (460 ºC), aromatic-rich components (methoxyphenols) are detected in both soils with varying amount that corresponds to lignin decomposition. At 550 °C (corresponding to Exo 3) alkyl and aromatic compounds are observed, including polycondensed aromatic carbon (PAHs) from the burnt soil.