Fire effects in the molecular structure of soil organic matter fractions under Quercus suber cover

• SOM molecular alterations after a wildfire are studied using Py-GC/MS.
• Van Krevelen plots & Alkane CPI, S/L indicated about fire mediated transformations
• Fire exerted oxidation, chain cracking, polysaccharide removal and lignin preservation

Forest fires cause immediate and lasting environmental impacts in soil organic matter (SOM) by modifying existing chemical structures, forming new ones, or adding/removing materials such as fresh or charred biomass. In this paper, the information provided by analytical pyrolysis (Py-GC/MS) of whole soil samples and two particle-size fractions (coarse and fine) is analysed in a sandy soil from a typical Mediterranean oak forest close to Doñana National Park (Southwest Spain) that was affected by a severe wildfire. The pyrograms from unburnt whole soil samples show a prevalence of compounds derived from polysaccharides, proteins and lignin, whereas in pyrograms from burnt soils lignin markers prevail followed by proteins, alkylaromatic and polycyclic aromatic hydrocarbons. In fact, it seems clear that fire preferentially removes thermolabile biogenic materials with a concomitant selective preservation of lignin. Concerning particle size fractions, SOM in the unburnt coarse fraction had a major impact on vegetation, whereas the fine fraction consisted of comparatively humified SOM with lignin degradation and microbial-derived compounds. The pyrolysis products from the burnt soil coarse fraction revealed post-fire litter inputs from stressed vegetation, whereas the fine fraction shares chemical characteristics with those from the unburnt soil, with additional signs of fire alteration (less labile carbohydrate structures and higher relative abundance of PAHs). Alkane changes observed in the carbon preference index indicate that fire affected SOM mostly in the fine fraction, whereas the short/long chain ratio points to the cracking of long-chain alkanes. In addition, when plotting O/C and H/C values in a Van Krevelen diagram, additional information about the main chemical modifications produced by fire in SOM are described, including condensation (oxidation and hydrogenation paralleling) and dealkylation (associated with oxidation and dehydrogenation) processes. This is the first time that Py-GC/MS molecular information from soil samples is analysed using a Van Krevelen graphical approach and together with other classic geochemical proxies it proves to be a valuable and intuitive tool in facilitating the interpretation of complex geochemical data.