Gênese e classificação de solos da Bacia Sedimentar do Araripe

Abstract

Despite several lithostratigraphic and paleontological studies, there are only few studies about the genesis of soils in the Araripe Sedimentary Basin and the main factors and processes at work, as well as the role of these soils in the storage of organic and inorganic carbon. In this study we evaluated (i) the action of pedogenesis and weathering of soils formed from shales and fine sandstones under Caatinga vegetation in the Vale do Cariri; (ii) the mechanisms of organic and inorganic carbon accumulation of black soils formed from limestone under Cerrado vegetation; and (iii) the main soil organic matter compartments of Umbric Ferralsols formed from coarse sandstones under Subcaducifolic Tropical Forest vegetation. The values of Ki index (SiO2/Al2O3) and Fed/Fet ratio indicate a low degree of weathering, conditioned mainly by a dry climate. From the mid-IR 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 profile located in the upper third of the landscape. In black soils formed from limestone, the combination of high mineral-associated organic carbon (MAOC) content, high MAOC/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 levels of 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 (CO2) pressure is lower. The variation in relief influences only on inorganic carbon of soils in the lower third of the landscape. This favored high humidity and root activity at depth, increasing the CO2 pressure and hindering the re-precipitation of carbonates, despite the high pH and abundance of exchangeable Ca2+. In the soils classified as Umbric Ferralsols of the Chapada do Araripe, charcoal fragments were observed in the coarse fractions, representing an important carbon pool due to the high recalcitrance. The high carbon contents of the free light fraction (C-FLF) suggest high carbon inputs by native vegetation. The high contents of residual particulate organic carbon (POCres) may have underestimated the contents of the light intraggregate fraction (C-ILF), given the low contents of C-ILF, despite the fact that all horizons have a strongly developed granular structure. Due to the relevance of the light fractions, land use changes can drastically reduce the carbon content in the Umbric Ferralsols of the Chapada do Araripe.