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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

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