Humic substances along the profile of two Typic Haploxerert.

In Vertisols, organic matter contributes to soil colour by formation of organo-mineral complexes and affects morphological, physico-chemical, biological and biochemical properties. Turbation may affect the chemical and structural composition of the most stabilised fractions of soil organic matter (SOM), i.e., humic substances (HS). The objectives of this study were to: (1) characterise SOM in two Vertisols (V1 and V2) developed under Mediterranean climate in Italy, using some HS characteristics as indicators of SOM turnover in Vertisols, and (2) explore possible differences related to the pedomorphologic conditions of the area under which two soils have formed. SOM evolution along the soil profile was investigated by applying different techniques to chemically extracted HS. Humification parameters, thermal methods, such as Differential Scanning Calorimetry (DSC) and Thermogravimetry (TG) together with isoelectric focusing (IEF) were used. The results showed a significant difference in the chemical characteristics of SOM in the considered Vertisols. A higher amount of total and humified carbon was found in pedon V2. In particular, the results indicated an accumulation of humified compounds in deeper horizons of V2: this effect is particularly significant since both soils were under the same agricultural management and the same climatic conditions, but were different in physiografic position, slope and colour. The higher amount of not humified SOM in the pedon V1 could explain the differences found in the shape and size of soil structure between the two pedons. The different SOM evolution occurring in the two Vertisols apparently influenced the structure and composition of the HS fraction. Thermal analysis and isoelectric focusing data provided evidence of a different level of stabilisation of HS from the two Vertisols. The HS from pedon V2 have a higher molecular weight structure, which would imply a higher level of SOM stabilisation. Conversely, HS in the pedon V1 appeared to be richer in aliphatic and phenolic structures, which may imply the presence of less stabilised HS fractions. These findings agree well with the different water dynamics occurring in the two sites, which confirmed the influence exerted by water on chemical and structural composition of HS and, consequently, their effect in determining Vertisols colour. In conclusion, thermal and isoelectric focusing characterisation of HS appear to shed some light in understanding the structural complexity of Vertisols. The results obtained in the present study, if confirmed on a larger number of Vertisols, suggest the possibility of using HS as indicators of SOM turnover in relation to pedomorphology, use and management of soils.