The humic substances that compose the soil organic matter participate actively in the pedogenetic soil processes, particularly in the Histic (Histic), Mollic (Chernozemic), Umbric (Humic) and Spodic (Spodic) diagnostic horizons. This study aimed to determine organic C amounts in the soil humic substances and evaluate their applicability for diagnostic horizons recognition. Fifty-six diagnostic soil horizons from different Brazilian regions were used, 52 from surface horizons or epipedons (H histic, A mollic, A proeminent, A humic and A moderate) and four subsurface (B spodic). Soil chemical and physical properties and the organic C amounts in the fulvic acid fraction (C-FAF), humic acid fraction (C-HAF) and humin (C-HUM) were determined by well-established methods. The C-HAF/C-FAF ratio and C-AE/C-HUM ratio (C-AE = C-FAF + C-HAF) and the percentage of each fraction in the total organic C (TOC) were calculated and all data were submitted to multivariate analysis. In the organic horizon, the amounts of H histic were highest and C-HUM and C-HAF amounts similar. C-HUM was predominant in the major mineral horizons, followed by C-HAF in the A mollic and A humic horizons, and by C-FAF in the A proeminent and A moderate horizons. A greater proportion of either C-FAF or C-HAF were observed in B spodic horizons. Based on canonical analysis, we identified the variables that could be used to separate diagnostic horizon types. The horizons with low influence of organic matter were distinguished from those with greater influence by cluster analysis. With the discriminatory analysis, satisfactory results were obtained for the classification of diagnostic horizons based on humic substance variables, compared to the standard classification. Multivariate analysis indicates that the distribution of C in the humic substances can be used to separate C-rich diagnostic horizon types of tropical soils in Brazil.
Chemical fractionating; diagnostic horizons; multivariate analysis
