Impact of the vegetation on the lignin pyrolytic signature of soil humic acids from Mediterranean soils

Humic acids (HAs) from 16 soils in Continental Mediterranean areas under potential vegetation consisting of sclerophyllic, mesophyllic or conifer forests in Madrid (Spain) were studied by Curie-Point pyrolysis. Statistical analyses based on the absolute and the relative abundances of the whole compound assemblages released by pyrolysis suggested that a considerable portion of the variance (inertia) associated with the vegetation was accounted for the lignin-derived compounds. In the studied samples, a set of 12 index-methoxyphenols was considered to have a discriminating potential comparable to that of the classical index phenots released from sedimentary organic matter by wet CuO alkaline oxidation. The ecological features most accurately reflected by the methoxyphenolic patterns consisted of: (i) the expected lack of syringol derivatives in HAs formed in soils under Gymnosperm plants. (ii) the diagnostic patterns of guaiacols in the HAs from soils under different species of Angiosperms (Quercus spp., Fraxinus, Castanea, grasses): (iii) the responsivity of the methoxyphenols to the extent of some types of soil perturbations: the highest environmental impact on the HA structure was found in soil after severe burning, not in the soil affected by medium-intensity fire. (C) 2002 Elsevier Science B.V. All rights reserved.