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    Molecular descriptors of the effect of fire on soils under pine forest in two continental Mediterranean soils

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    En la versión que se adjunta consta de 44 páginas, 9 figuras, 14 tablas.This study aimed at identifying the most significant, qualitative or quantitative indicators of the effect of forest fires. A series of analytical characteristics of 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 included non-destructive [visible and Fourier-transformed infrared (FTIR) derivative spectroscopy, 13C nuclear magnetic resonance spectroscopy (NMR)] and destructive approaches (pyrolysis and chemical degradation with sodium perborate). A number of variables were highly susceptible to the direct or indirect effect of fire. The yield of benzene carboxylic acids largely increased with fire intensity. Enhanced aromaticity in post-fire organic matter was also reflected in visible and 13C NMR spectroscopy. The response of other variables mainly depending on the amount of O-containing functional groups [i.e., cation exchange capacity (CEC), total extractable humus, ratio of diterpenoid to diterpene resin acids or soil C mineralization rate] related to the type of fire. Overall, differences in the effect of the type of fire were mainly recognized via structural characteristics of the HAs (O/C ratio, polydispersity, occurrence of perylenequinone chromophores, phenol and naphthalenol yield from pyrolysis, etc.). It is suggested that changes in organic matter (OM) composition in the medium intensity wildfire could be due to external input of charred plant material from the forest canopy. The results of high intensity fire suggest both intense selective degradation of the most labile structures in addition to thermal condensation of newly formed constituents. We conclude that simultaneous study of analytical variables, mainly those concerning aromaticity and solubility, is needed for monitoring the effect of fire on soil OM.Peer reviewe
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