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

The characterization of soil organic matter, and of humic substances in par ticular, 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 su bstituted aromatic groups than those of the fine earth. These features indi cate that carbohydrates, lipids, and proteinaceous residues are incorporate d in the humic acids of the rock fragments. The fresh biological material r esidues 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 ma tter inside the rock fragments compared with that of the fine earth. The FAs show greater homogeneity than the HAs, probably because of their mo bility among the different soil compartments. However, the FAs of the rock fragments have more carboxyl and acidic-OH groups than found in the fine ea rth. Greater differences are also observed between the two horizons. The FA s extracted from the A2 horizon show less mineralization than those from th e A1 horizon. The humic substances extracted from the rock fragments washing show charact eristics more similar to those of the skeleton than to those of the fine ea rth. This suggests that most rock fragments washing originate from the weat hering of the rock fragments and, therefore, may be regarded as an intermed iate phase between the skeleton and the fine earth.