Caracterização química e espectroscópica da matéria orgânica em solos do Brasil

Abstract

The importance on the study of organic matter in pedology and soil classification studies in tropical regions can be substantiated by the differential attributes: organic carbon content, degree of organic matter decomposition, and fibers content. However, quantitative and qualitative variables such as chemical fractionating of organic matter and spectroscopic humic substance characterization are not contemplated in the Brazilian System of Soil Classification (SiBCS). This study had as objective to analyze chemically and spectroscopic ally the organic matter from some Brazilian soil orders and to correlate the results with chemical properties and texture. There were utilized 52 soil profiles of the following soil orders: Mollisols, Oxisols, Spodosols, Entisols, and Histosols, that total 87 diagnostics horizons, being 61 surface and 26 subsurface horizons. In these horizons they were evaluated for pH, H+, sum of bases (S value), T value (CEC), total organic carbon (COT), and texture. Also, the chemical fractionating of organic matter to obtain the organic carbon of fulvic acid fraction (C-FAF), humic acid fraction (C-HAF) and humin (C-HUM), and spectroscopic analyses on visible range for the alkaline extract (AE) and humic fraction, and on infrared region for the humic acid fraction. The results of chemical fractionating showed unique for each soil order, which were separated in three groups, with base on nature of parent material and soil drainage. In the group of mineral and well drained soils (Mollisols and Oxisols) the HUM was predominant in the studied horizons (average of 61.0%), and the values of C-AE/C-HUM and C-HAF/C-FAF ratios were, in general, smaller than 1.0. In the mineral and imperfect and poor drained soils (Spodosols and Entisols), the HUM prevailed only in the surface horizons; while, in the subsurface horizons the FAF and HAF had the highest participations. Within this group, for the subsurface horizons the values of C-AE/C-HUM ratio were higher than 1.5, indicating the movement of alkaline-soluble fractions along the soil profile. For the soils of organic constitution and very poor drained (Histosols), the HAF was predominant and the values of CHAF/ C-FAF ratio were higher than 2.0, showing the influence of hidromorphic conditions in the soil organic matter dynamics. The most significant correlations were observed among CHAF, C-HUM and TOC with pH, S value and CEC, an evidence of the importance of the SOM fractions and TOC in the fertility of these soils. The results of the spectroscopic analyses in the visible range of AE showed different behavior among the horizons, being the E4/E6 values influenced by the quantity of fulvic and humic acids; while, for the humic acid, the E4/E6 values showed high humification rate and/or big molecules size and molecular mass. The characteristics of the infrared spectra of humic acid allowed to differentiated five types, named as Type A (more aromatic); Type B; Type C (intermediate); Type D and Type E (more aliphatic). The distribution of the humic substances according to the chemical fractionating and the groupings showed by the variables in the analysis of principal components, associated to the characteristics of the diagnostic horizons spectra may contribute to structuring the SiBCS in the lower hierarchical levels (5th and 6th); mainly, in the mollic, humic, and moderate A horizons, and the spodic (B) and histic (H) horizons.