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Field burn severity indexes and its relation with soil organic matter molecular changes described by graphical statistical analysis of pyrochromatographic data

AutorVega Hidalgo, José A.; Fernández Filgueira, Cristina; Fontúrbel Lliteras, Mª Teresa; Jiménez Morillo, N. T. ; Almendros Martín, Gonzalo ; González-Pérez, José Antonio
Fecha de publicación3-oct-2017
EditorSociedad Española de Cromatografía y Técnicas Afines
Citación15as Jornadas de Análisis Instrumental (15 JAI), 3–5 octubre 2017 Barcelona
ResumenTo operationally characterize soil burn severity (SBS) in the field, a 5-level classification was proposed for temperate climate areas in forests and scrublands, useful in assessing post-fire erosion risks and to plan post-fire rehab activities [1]. In fact, this SBS classification is an operational tool currently used by forest managers [2]. This work describes the relationships between field SBS and molecular-level changes in the soil organic matter (SOM). Direct analytical pyrolysis (Py-GC/MS, 400 ºC) [3] was used to study fireaffected soils under different SBS (1 to 5) at two scenarios: 1) burnt in the field by a wildfire, and 2) monoliths burnt experimentally in semi-natural conditions. Unburnt soils (SBS-0) were also studied. More than 100 pyrolysis compounds were identified and total abundances plotted in modified 3D van Krevelen diagrams (H/C vs. O/C vs relative abundance) [4]. Stepwise analysis of SOM changes was depicted in C-normalized subtraction surfaces: positive values indicate the extent to which SOM constituents are progressively destroyed or transformed by fire, whereas negative values point to net accumulation of newly-formed structures [5]. Moderate chemical changes occured between stages SBS-0 to SBS-3 with a progressive shifting to compounds with lower H/C and O/C ratios, suggesting dehydration and demethylation pathways and depletion of polymethylene structures, carbohydrate rearrangements, lignin demethoxylation and triterpene preservation. Clear SOM transformations occured at SBS-4 onwards with concentration of alkyl structures, mainly triterpenes that is interpreted as final distillates released from a condensed black carbon matrix. Progressive changes in terms of SBS were not linear for all compounds. The subtraction surfaces suggest two major stages with generation of pyrogenic compounds not existing at that concentration in the preceding stages: SBS (0¿1) generation of pyrolytic anhydrosugars, and a major compound-productive stage between SBS (3¿4) with generation of alkylated polycyclic hydrocarbons and phenols. Molecular-level changes in SOM after natural fire and heated soil monoliths were very similar. Nonetheless, the former showed incorporation of alkyl compounds and methoxyphenols, probably inputs of charred biomass from the standing vegetation.
DescripciónReferences [1] Vega, JA., Fontúrbel, MT., Merino, A., et al. Plant Soil 369 (2013) 73-91 [2] Fernández, C., Vega, JA. Earth Surf. Proc. Land. 41 (2015) 928-935 [3] Jiménez-Morillo, NT., de la Rosa, JM., Waggoner D. et al. Catena 145 (2016) 266-273 [4] Almendros, G., Tinoco, P., de la Rosa, JM., et al. J. Soils Sediments (2017) in press (doi:10.1007/s11368-016-1595) [5] Almendros, G., González-Vila FJ., Martin F., et al. Sci. Total Environ. 117/118 (1992) 63-74
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