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Despite its great soil diversity, the Amazon Biome still lacks pedological studies that would also contributes to the Brazilian Soil Classification System. Based on this, this study aimed to characterize and classify the soils from an area in western Pará state. Six soil profiles were described and collected, characterized both physically and chemically, and then classified according to the Brazilian Soil Classification System. There were soils with anthropic influence with thick superficial horizons, with high levels of organic carbon, Ca2+, Mg2+ and P, differing from the observed pattern concerning non-anthropized soils. A great variability of texture was also observed due to the nature of parental material. A high weathering degree was found in the Entisols located in the floodplain area, with low activity clays and low base saturation. Due to observing a superficial diagnostic anthropic horizon A in "Cambissolos Háplicos" and "Húmicos" profiles, it is suggested to create the "antrópico" (anthropic) subgroup for these soil classes in the Brazilian Soil Classification System. [ABSTRACT FROM AUTHOR]

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• Parent material is the main factor of the soil formation in the Cariri Valley. • Values of Ki (SiO 2 /Al 2 O 3) and Fed/Fet ratio indicate a very low degree of weathering. • Absorption features associated Mg-silicate were identified by spectroscopy (MIR). • Clay translocation in Luvisol indicates incipiente pedogenesis in the Cariri Valley. The Araripe Basin is the largest sedimentary basin in the countryside of Northeast Brazil. Despite the wide diversity of lithostratigraphic, geomorphological, and paleontological studies, little is known about the pedogenesis and weathering. This study aimed to characterize the morphological, physical and chemical attributes, and the spectral behavior of soils in a toposequence over Jurassic formations of the Araripe Basin so as to identify the main pedogenetic processes, as well as their degree of weathering. A toposequence composed of four soil profiles was selected, described, and sampled in a semi-arid depression that is geologically composed of lacustrine (mudstones) and fluvial (sandstones) deposits. The variability of soil attributes such as color, texture, base saturation, clay activity, and spectral behavior (vis-NIR and MIR) reflects the predominant influence of the parent material on the relief factor. The values of Ki index (SiO 2 /Al 2 O 3) and Fed/Fet ratio indicate a very low degree of weathering, conditioned by a dry climate. From the MIR spectra, absorption features associated with calcite, Mg-silicate, illite, and smectite-illite interstratifications were identified. These minerals were interpreted to have originated authigenically, formed from evaporation processes in lacustrine environments. Despite the smaller influence of the relief and the low action of pedogenetic processes, incipient evidence of clay translocation is observed in the upper third of the landscape. [ABSTRACT FROM AUTHOR]

The Global Soil Partnership with the Food and Agriculture Organization of the United Nations (FAO) defines Chernozem, Kastanozem, and Phaeozems as black soils characterized by high natural fertility and dark surface horizons enriched in organic matter. Chernozems and Kastanozems are also characterized by carbonate accumulation. In the Araripe basin located in northeastern Brazil, black soils formed from limestone are present in the scarps of the Araripe Plateau. Despite their importance, particularly in dry climatic regions, there are no studies characterizing the soils of this region. Thus, our study aimed characterizing the macro and micromorphological, physical, and chemical attributes of three soil profiles (Chernozem, Kastanozem, and Phaeozem, respectively in the upper, middle, and lower third) in a toposequence of the Araripe region. In addition, we identified the main mechanisms of organic and inorganic C accumulation in these profiles. Our results showed that the combination of high mineral-associated organic matter (MAOM) content, high MAOM/soil organic C ratio, and low C/N ratio indicates a high degree of transformation and stabilization the organic matter in these soils. The high 2:1 clays and exchangeable Ca2+ benefits the stabilization of organic C by means of organic–mineral interactions. Additionally, the dominance of carbonates of geogenic origin (limestone particles in the sand fraction) is indicative of the low dissolution of these carbonates. Through micromorphological evaluation, pedogenic carbonates were observed at the top of the B horizon of these soils, where the carbon dioxide (CO 2) pressure is lower. The variation in relief demonstrated an ostensible influence on inorganic C constrained to the Phaeozem (lower third). This favored high humidity and root activity at depth, increasing the CO 2 pressure and hindering the re-precipitation of carbonates despite the high pH and abundance of exchangeable Ca2+. [Display omitted] • Black Soils in the Araripe basin are formed from limestones of the Santana Formation. • Organo-mineral interaction is the main mechanism of organic C accumulation. • High 2:1 clay content and high exchangeable Ca2+ favour organo-mineral interactions. • Limestone fragments in the sand fraction indicate low dissolution of carbonates. • The relief factor affects the dynamics of carbonate re-precipitation in the soil. [ABSTRACT FROM AUTHOR]