Differences of stabilized organic carbon fractions and microbiological activity along Mediterranean Vertisols and Alfisols profiles

This study examined the chemical and structural properties of humic substances and microbiological activity in order to verify differences in carbon dynamics along soil profiles in two Vertisols (Typic Haploxerert and Xeric Epiaquert) and two Alfisols (Mollic Haploxeralf and Ultic Haploxeralf) developed under Mediterranean climate in Italy. Humification parameters, thermal methods, including differential scanning calorimetry (DSC) and thermogravimetry (TG), together with Fourier transform infrared (FT-IR) and fluorescence spectroscopies were used to characterize humic acids (HA). Microbiological activity of soils was assessed by basal respiration, metabolic quotient (qCO2) and Cmic:Corg ratio. FT-IR spectra and thermal analysis DSC/TG of HA extracted from the upper horizons showed a higher aliphatic character, whereas HA extracted from the lower horizons had a higher content of aromatic structures and polysaccharides. Moreover, the fluorescence index of the HA (HIXflu) showed a higher degree of aromatic polycondensation in the subsoils. The Cmic:Corg ratio was negatively correlated with the HIXflu of HA (P < 0.05), while qCO2 was positively correlated with the labile components of HA (Exo1/(Exo2+Exo3)) (P < 0.05). These results suggest reduced C availability in deep horizons where HA structures resulted in complex and thermally more stable molecules. Following discriminant function analysis all the chemical and microbiological properties with the exception of labile thermal fraction of HA and soil pH largely varied between B and the upper horizons. In deep horizons of both Vertisols and Alfisols the microbial biomass was nutritionally stressed with a low efficiency in C turnover. No correlation was found between specific soil processes, such as argilliturbation or clay illuviation and humic substances properties or soil microbial biomass and activity.