The structural transformations undergone by lignocellulosic biomass (freeze-dried rye grass, Lolium rigidum) subjected to progressive isothermal heating (burning at 350 degrees C under oxidizing conditions for 30, 45, 60, 75, and 90 s) have been monitored by Curie-point pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). The pyrograms suggest that even charred residues after severe heating (carbon loss ca. 50%) still contain substantial concentrations of some resistant plant structural components. Several trends were observed when monitoring the relative concentrations of the different groups of pyrolysis compounds released during successive charring stages: (i) the tetrapyrrole moiety of chlorophylls is rapidly destroyed as indicated by the decreasing yields of pyrroles and pyrrolines, whereas the phytol backbone is comparatively more resistant, leading to phytadienes after dehydration and reduction; (ii) the increasing yields of imidazoles from progressively heated samples (maximum at 45 s stage) suggest accumulation of newly formed nitrogen-containing compounds that may survive natural fires; (iii) the lignin backbone shows a relative resistance, the yields of aromatic products pointing to progressive demethoxylation; and, (iv) a selective accumulation of recalcitrant alkyl material occurred, which is interpreted as the result of thermal condensation of hydrocarbons and fatty acids into macromolecular materials in the charred residue. In terms of the intensity of the isothermal heating, the yields of the different classes of alkyl compounds follow the order phytadienes < fatty acids < alkanes < wax esters < sterols.

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This study aims to identify the most significant, qualitative or quantitative indicators of the effect of forest fires. A series of analytical characteristics of the soils, in addition to the molecular composition of whole soil, humic acid (HA) and free lipid fractions, were described in replicated spatial samples from unaffected and fire-affected plots. A total of 385 analytical descriptors were examined in soil samples from two pine forests in Madrid (Central Spain) that had been subjected to medium to high intensity forest fires.The techniques used include both non-destructive (visible and Fourier-transformed infrared (FTIR) derivative spectroscopies, 13 C nuclear magnetic resonance (NMR)) and destructive approaches (analytical pyrolysis and wet chemical degradation with sodium perborate). Our results showed that a number of the variables analysed were highly susceptible to the direct or indirect effect of fire. The yield of benzenecarboxylic acids largely increased with increasing fire intensity. Increased aromaticity in post-fire organic matter was also reflected by visible and 13 C NMR spectroscopies. The response of other variables (i.e., cation exchange capacity (CEC), total extractable humus, ratio between diterpenoid and diterpene resin acids in the free lipid fraction or soil C mineralization rates) related to the amount of oxygen-containing functional groups. Overall, differences in the effect of the type of fire were mainly recognized in structural characteristics of the HAs (O/C ratio, polydispersity, occurrence of perylenequinonic chromophors, yield of phenols and naphthalenols after analytical pyrolysis, etc.).It was suggested that changes in the organic matter composition in the medium-intensity wildfire could be due to external inputs of charred plant material from the forest canopy. In the highintensity fire the results suggest both intense selective degradation of the most labile structures in addition to thermal condensation of newly-formed constituents. We conclude that the

Selective effects of forest fires on the structural domains of soil humic acids as shown by dipolar dephasing 13 C NMR and graphical-statistical analysis of pyrolysis compounds

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