Properties of humic acids in Mediterranean forest soils (Southern Italy): influence of different plant covering

The chemical and spectroscopic properties of humic acids (HAs) isolated from four litters and their corresponding underlying soils at three depths in a protected forest area in Southern Italy were investigated as a function of four different plant coverings: Quercus ilex L., mixed Carpinus betulus L. and C. orientalis Mill., Pinus halepensis L., and mixed Quercus trojana Webb. and Q. ilex L.. The forested site is a part of a calcareous plateau, characterized by homogeneous soils classified as Eutric Cambisols associated with Calcic Luvisols. The changes in the composition of HAs with soil depth have been evaluated on the basis of chemical (elemental and COOH groups) and spectroscopic analyses (E4/E6 ratio and FT IR spectra), and lignin-derived CuO oxidation products. A different distribution of the main elements was found in the various HAs which is apparently related to the type of humic acid-precursor biomolecules in the parent litters. The HAs isolated from soils under Q. ilex and mixed Carpinus species showed a slight increase in the C/H ratio and COOH content downward the soil profile, suggesting increasing aromatic polycondensation and humification degree with depth. On the contrary, no trend was observed for HAs from soils under Pinus halepensis L. and mixed Quercus species, indicating a partial incorporation of residues deriving from litter degradation into these HAs. Further, the content of lignin-derived phenols was higher in Pinus halepensis L. and mixed Quercus species layers, with the same trend measured for the corresponding HAs, thus confirming a lignin contribution related to the lignin type of plant covering.