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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]

Sodium-affected soils occur predominantly in arid and semi-arid regions. Sodification and solodization are the two main pedogenetic processes in the formation and evolution of these types of soils, in the landscape. In this study, we used soil morphological, physical, and hydraulic analyses to test the hypothesis that sodification and solodization are natural processes of soil degradation, constituting a form of regressive pedogenesis. Sodium-affected Solonetz, Vertisol, and Planosol profiles were sampled and subjected to morphological, physical, and hydraulic analyses. Under the influence of sodification, Solonetz and Planosol showed signs of regressive pedogenesis, including the formation of a surface horizon with sandy texture, weak structure, reduced available water capacity, and evident characteristics of natural erosion. The morphological, physical, and hydraulic properties of Vertisol were not influenced by sodification. Solodization (sodium leaching and topsoil acidification) had a positive effect on the structural characteristics of Planosol, increasing the degree of flocculation (55–100%); thus, solodization was not associated with natural degradation. Sodification and solodization are antagonistic processes. Whereas sodification can be regarded as a form of regressive pedogenesis leading to natural degradation of sodium-affected soils, solodization is a progressive process of profile evolution. Degradation is a natural phenomenon caused by regressive evolution factors during the formation of Solonetz and Planosol profiles. Our results may guide the development of land management strategies for sodium-affected soils in semi-arid regions according to their use capability. • Sodification can be considered a regressive pedogenic process. • Soil attributes and hydraulic dynamics are not influenced by sodification in Vertisol. • Solodization contribute to the increase in degree of flocculation and clay illuviation. • Solodization can be considered a progressive pedogenic process. [ABSTRACT FROM AUTHOR]

The extraction of oil from onshore wells and the production of biodiesel from castor bean crops have grown in recent years, resulting in the production of large volumes of drill cuttings from these wells and castor pie, which can be harmful to the environment. The objective of this work was to evaluate the influence of a combination of different doses of drill cuttings and castor pie on the chemical attributes of a Planosol and on the dry mass production of the aerial part (DMAP) and dry mass of roots (DMR) of castor bean crops (Ricinus communis L.), cultivar BRS-149 Nordestina. The experimental design was a randomized block with a factorial 6 × 6, consisting of five castor pie doses (2, 4, 8, 16, and 32 Mg ha−1); five drill cuttings doses (5, 15, 30, 45, and 60 Mg ha−1) and an experimental control using only soil samples, with three repetitions. The soil used as the main substrate was collected at 0.0–0.2 m depth in a Planosol. The mixture of the topsoil layer with the experimental doses was performed using a mixer for 5 min, and the samples were distributed in 8 dm3 pots; each pot corresponded to an experimental unit. The mixture of cuttings from drill wells and castor pie with a 30: 16 Mg ha−1 ratio, provided favorable chemical conditions for castor bean crop development; however, drill cuttings doses >30 Mg ha−1 can cause soil salinization, and negatively affect the development of castor bean crops. Unlabelled Image • Recycling is a way to reduce the environmental liability of industrial activities. • Drill cuttings plus castor pie can be used as agricultural inputs. • Improper disposal of drill cuttings salinizes the soil. • Degraded areas can be recovered with drill cuttings associated with castor pie. • Castor bean is a good plant to evaluate drill cuttings as agricultural inputs. [ABSTRACT FROM AUTHOR]