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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]

Soil carbonate accumulation plays an important role in C sequestration, accounting for approximately 40% of the C stored in soils worldwide. Under humid tropical climate these carbonates are rapidly dissolved, intensifying CO 2 emissions. Despite this, little is known about the weathering of carbonates in areas with complex geology. This study aimed to evaluate the influence of the parent material on soil attributes and weathering processes under humid tropical climate conditions along toposequences formed by dolomitic marbles and calc-silicate rocks interspersed with garnet-biotite gneiss. Samples and soils profiles were collected on sites of both parental materials and at different points of a toposequence, namely: upper third, middle third and lower third of a slope. The profiles were analyzed for their morphological, physical, chemical and mineralogical attributes. The attributes of soils formed from metamorphic sedimentary rocks were more influenced by geological processes than pedological ones. It was found that the nature of the parent material was the main responsible for the attributes of the profiles, especially the strongly alkaline pH, being greater than 8.0 in most soil horizons, and the high contents of exchangeable Ca2+ and CaCO 3 equivalent, with maximum values of 40.6 cmol c kg−1 and 763 g kg−1, respectively. As for mineralogical composition, it was verified the occurrence of minerals such as talc, vermiculite and Fe and Ti oxides, with metamorphic processes and the addition of hydrothermal fluids being the main responsible for the presence of these minerals. The supply of high contents of basic cations, with maximum values of 61.4 cmol c kg−1, high Si/Al ratio (reaching values close to 5.0) and the high contents of CaCO 3 conditioning the strongly alkaline pH hinder the dissolution of silicates, even those with low resistance to weathering, despite the hot and humid climate conditions. The action of weathering processes in humid tropical climate is more marked in acidic materials, being responsible for the in situ formation of kaolinite from the alteration of mica and feldspar. • Soil mineralogy is influciated for metamorphic processes and hydrothermal fluids. • Despite the tropical climate, the decarbonation process is slow. • Weathering processes in humid tropical climate is more marked in acidic materials. • The parent material affects soil attributes more than relief. [ABSTRACT FROM AUTHOR]

Carbonate rocks occur in Brazil under diverse climatic and environmental conditions, and therefore are of great importance for pedological studies. The northwest region of the State of Rio de Janeiro, Brazil, is composed of calc-silicate rocks and marbles that occur under tropical climate with a long period of water deficit, which provides conditions for peculiar pedogenesis. The objective of this study was to characterize the attributes of soils formed from carbonate materials and to understand the role of the main factors and processes in pedogenesis, as well as to provide information to the WRB (World Reference Base for Soil Resources) to improve carbonate soils classification. A toposequence was selected in the municipality of Italva, RJ, Brazil, with six profiles being described and sampled. All profiles presented median CaCO 3 equivalent contents, high base saturation with the predominance of Ca+2 and Mg+2, and high activity clay. The absence of effervescence and low CaO/MgO ratio obtained by acid digestion suggest the dolomitic nature of the carbonates. Pedogenic carbonates were amorphous, since no XRD peaks were found. The high Mg+2 concentration provided by the parent material increases the dispersion of clays and favors clay illuviation, which is manifested in the micromorphology through a large amount of clay infillings and coatings. Four profiles on the highest part of the landscape were classified as Kastanozems (Mollisols), the footslope profile as Fluvisol (Inceptisol), and the toeslope profile as Vertisol. The mineralogy of the Vertisol's clay fraction was predominantly composed of kaolinite interstratified with 2:1 phyllosilicates, mainly illite. Acid digestion can complete the analysis of the CaCO3 equivalent content by quantifying CaO and MgO concentration in fine earth samples, presenting the potential to distinguish the Dolomitic/Calcaric qualifiers. The recognition of Dolomitic/Calcaric qualifiers for the Kastanozem reference soil groups is suggested, considering that the carbonate present in the calcic horizon may be diverse. • In Brazil, soils formed from carbonate material occur in the most diverse environmental conditions. • In the Northwest region of the state of Rio de Janeiro, they are formed from Italva Group marbles. • The absent of effervescence and low values of CaO/MgO ratio suggest the dolomitic nature of the carbonates. • Variation of most soil attributes are controlled by the relief factor. [ABSTRACT FROM AUTHOR]