Chemometric evaluation of the field burn severity indexes

For temperate regions in forests and scrublands, a 5-level categorization was established to operationally define soil burn severity (SBS). This classification is important for estimating post-fire erosion risks and scheduling post-fire rehab activities [1]. This work describes the relationships between field SBS and molecular-level changes in the soil organic matter (SOM). Direct analytical pyrolysis [2] was used to study unburned (SBS-0) and burned soils (SBS 1 to 5) at two scenarios: 1) wildfire, and 2) burning laboratory experiment. The pyrolysis compounds were identified and plotted in modified 3D van Krevelen diagrams [3]. Moderate chemical changes occurred between stages SBS-0 to SBS-3, such as, dehydration, demethylation, depletion of polymethylene structures, carbohydrate rearrangements, lignin demethoxylation and triterpene preservation. Clear SOM transformations occurred at SBS-4 onwards with concentration of alkyl structures (triterpenes). Progressive changes in terms of SBS were not linear for all compounds. The subtraction surfaces suggest two major stages: SBS (0–1) generation of pyrolytic anhydrosugars; and SBS (3–4) generation of condensed compounds. Molecular-level changes in SOM after wildfire and laboratory experiment were similar. Nonetheless, the former showed incorporation of alkyl compounds and methoxyphenols, probably inputs of charred biomass from the vegetation.

Keywords: Soil Burn Severity; Statistical Analysis; Analytical Pyrolysis

References:
[1] Vega, JA. et al. 2013. Plant Soil 369, 73–91
[2] Jiménez-Morillo, NT. et al. 2016. Catena 145, 266–273
[3] Almendros, G. et al. 2018. J. Soils Sediments 18, 1303-1313

Acknowledgment:
Funding projects: MARKFIRE (EU-FEDER Andalusia; P20_01073), EROFIRE (FCT Portugal; PCIF-RPG-0079-2018). Contract: N.T.J.M. (FCT CEECIND/00711/2021 and RYC2021-031253-I). Technical assistance D. Monis and A. Carmona.