Chemometric approaches applied to data from destructive (analytical pyrolysis) and non--‐ destructive (nuclear magnetic resonance, C--‐13 NMR) techniques were explored to characterize soil organic matter fire--‐induced alterations. Direct determination of pyrogenic organic matter (e.g., black carbon or polycyclic (>3--‐ring) compounds) by the above techniques, is limited, either because these structures remain ‘invisible’ under the usual spectral acquisition conditions, or to non--‐stoichiometric pyrolytic yields. Here, both approaches were used to find surrogates for pyrogenic structures defined as those which are in excess compared to fire--‐unaffected soils. Humic acids from two forest soils under Pinus halepensis or Pinus sylvestris were studied in unaltered or burned sites affected by medium--‐intensity or severe fires. Dipolar dephasing (DD) C--‐13 NMR spectra displayed significant differences as compared with conventional CPMAS C--‐13 NMR, mainly in signals ascribed to rigid and mobile alkyl structures, those for tannins or carbohydrates. An index of fire--‐induced transformation was calculated as the extent to which the signal intensity in the non--‐substituted aromatic region (110–140 ppm) remains unchanged irrespective to proton decoupling conditions. Concerning analytical pyrolysis, the compounds were represented in 3D plots after sorting out their atomic ratios in chemical spaces, i.e., the classical van Krevelen’s method showing the positioning of various classes of molecules onto 2 axes representing H/C and O/C atomic ratios and z axis the compound yield. Comparisons of these yields as 3D surfaces, directly reveal fire--‐induced changes in molecular assemblages with similar stoichiometry (i.e., ‘clusters’ of saturated and unsaturated hydrocarbons, methoxyphenols, alkylbenzenes, polycyclic aromatics…). Best correlations between pyrolytic and DD C--‐13 NMR results can be used for quantifying damage levels based on the fire effects on soil HA properties.

Poster presentado en el 5th International Conference of Fire Effects on Soil Properties (FESP5) University College Dublin 14th-17th July (2015)

Peer Reviewed