The Changes With Depth of Humic and Fulvic Acids Extracted From the Fine Earth and Rock Fragments of a Forest Soil

Although much is known about the transformation of humic substances in the fine earth fraction of surface horizons, little is known about these processes in the coarse fragments (>2 mm) or about the changes with depth of humic and fulvic acids extracted from an entire soil profile. For each horizon of a forest soil developed on sandstone, the humic acids (HAs) and fulvic acids (FAs) were extracted from four fractions: fine earth, washing (i.e. fine material adhering to the clasts) and highly altered and slightly altered rock fragments. The HAs and FAs were also extracted from samples of the parent rock. The humic substances were then analyzed for their elemental composition and chemical structure by FT-IR and 13C NMR spectroscopy. In the A horizons (upper 20 cm of the profile), the HAs from all the fractions appeared highly aromatic, with a high degree of C substitution and an abundance of carboxyl groups; however, the HAs of the rock fragments were more aromatic and richer in carbohydrate and proteinaceous residues than those of the fine earth. The HAs showed a more condensed and unsubstituted aromatic pattern at depth than in surface horizons. In the Bw and BCb horizons the contribution of biological residues to the humic substances seems to be progressively replaced by the contribution of organic materials derived from the sedimentary parent rock. The presence of features ascribable to the HAs and FAs of the parent rock decreased following the order: slightly altered clasts > highly altered clasts > washing > fine earth. Conversely, FAs showed a high homogeneity throughout the profile and are considered mostly of pedogenic origin. The chemical characteristics of both HAs and FAs showed a break in the trend with depth between Bw and BCb horizons, confirming the presence of a pedogenic discontinuity between the upper modern soil (A-Bw2 horizons) and the buried paleosol (BCb horizons).