Chemical and spectroscopic characterization of the humic substances from sandstone-derived rock fragments

The characterization of soil organic matter, and of humic substances in particular, has always been made on fine earth, i.e., the <2-mm fraction. The fraction larger than 2 mm, known as rock fragments or skeleton, is commonly discarded. We have extracted the humic acids (HAs) and fulvic acids (FAs) from fine earth, rock fragments, and rock fragments washing (i.e. the fine material adhering to the rock fragments) of the upper forest soil horizons, A1 and A2. The substances were characterized using wet-chemical analyses, FT-IR, and liquid state C-13 NMR spectroscopies. HAs of the rock fragments have higher N and H content, paraffinic chains, aliphatic-OH, and highly substituted aromatic groups than those of the fine earth. These features indicate that carbohydrates, lipids, and proteinaceous residues are incorporated in the humic acids of the rock fragments. The fresh biological material residues present in the structure of the skeleton HAs may be attributable to: (i) the selective preservation from microbial and chemical attack offered by the skeletal environment; (ii) the more rapid cycling of the organic matter inside the rock fragments compared with that of the fine earth. The FAs show greater homogeneity than the HAs, probably because of their mobility among the different soil compartments. However, the FAs of the rock fragments have more carboxyl and acidic-OH groups than found in the fine earth. Greater differences are also observed between the two horizons. The FAs extracted from the A2 horizon show less mineralization than those from the A1 horizon. The humic substances extracted from the rock fragments washing show characteristics more similar to those of the skeleton than to those of the fine earth. This suggests that most rock fragments washing originate from the weathering of the rock fragments and, therefore, may be regarded as an intermediate phase between the skeleton and the fine earth.