Chemical and spectroscopic characteristics of humic acids and dissolved organic matter along two Alfisol profiles

The aim of this study was to elucidate the heterogeneous structural and functional composition of humic acids (HAs) and dissolved organic matter (DOM) isolated from two Alfisol profiles with different soil texture, in order to develop a better understanding of the organic matter dynamics. Soil samples were collected at different depths from three (Ap, 2AB and 2Bt) and eight (A1, A2, A3, E1, E2, 2Bt1, 2Bt2 and 2Bt3) soil horizons of two Alfisols located in the south (PR1) and north (PR2) of Italy, with a clay texture and a silt loam to loam ones, respectively. Chemical and spectroscopic methods were used to characterize the HAs and the DOM isolated from different soil horizons, including Ultraviolet–Visible (UV–Vis), Fourier Transform Infrared (FTIR), and Fluorescence spectroscopies. The HAs and the DOM isolated from the two Alfisols apparently showed significant differences in their compositional, structural and functional characteristics. In particular, the HAs isolated from the PR1 featured a higher degree of humification and molecular complexity with respect to those isolated from the PR2. On the contrary, the DOM samples isolated from the PR2 showed a more marked aromatic character and polycondensation degree. Both the HAs and the DOM obtained from the PR1 presented a greater qualitative homogeneity with respect to those obtained from the PR2. These results could be reasonably ascribed to the different texture and horizons of the two Alfisols, and to a greater pedogenesis occurred in the PR1.