Characterization of Clay- and Silt-Sized Fractions and Corresponding Humic Acids Along a Terra Rossa Soil Profile

This study focuses on the relationship between soil mineralogy and soil organic matter stabilization along a Terra Rossa soil profile in Southern Italy. This soil profile is characterized by three horizons: Namely Ap, 2AB, and 2Bt, where the clay, silt, and sand size fractions were isolated and their mineralogical composition, and the different forms of iron, silicon, and aluminum were determined. Humic acids (HAs) were isolated from clay-size (C-HA) and silt-size (S-HA) fractions, and characterized by chemical and physico-chemical techniques. The clay fraction had the largest amount of amorphous and crystalline oxides, kaolinite, and chlorite. In all fractions, the 2AB horizon differed from the other two horizons showing a very high content of iron and aluminum bound to organic matter (Fep and Alp). Chemical and spectroscopic analyses of humic acids indicated the presence of macromolecules with larger size and polycondensation degree in S-HAs, apparently less susceptible to degradation than simpler hydrophilic compounds occurring in the clay-size HAs. A good correlation was found between Fep and Alp contents and HA organic carbon. These properties are expected to slow down organic matter dynamics in Terra Rossa Mediterranean soils even in the presence of a fast turnover of aggregates.