Spodosol formation on sandy ruins in a semi-arid climate in the Catimbau National Park, Northeast Brazil

The Catimbau National Park (CNP) is found in the Caatinga biome under a semi-arid climate. The soils of certain humid areas have horizons with morphology typical of Spodosols, and their occurrence in the CNP seems to be in discord with their formation model. This study aimed to investigate the genesis of Spodosols in the semi-arid climate of the CNP, testing the hypothesis that these soils are the result of local tectonic conditions associated with a past climate. As such, a sandy soils toposequence over sandstone of the Tacaratu Formation, under Caatinga vegetation and a semi-arid climate in northeast Brazil was studied. Sampling was based on the struc-tural model of the pedological coverage, whereby physical and chemical analyses, Fourier transform infrared spectroscopy, X-ray diffraction, and soil micromorphological analysis were carried out. The results indicate that paludization of the soils occurred in two more humid phases, being compounded by the occurrence of distended faults/fractures related to the geology of the CNP. Microbiological decomposition under hydromorphic condi-tions led to humification of the organic constituents, which acidified the environment and increased Al solubility, forming organometallic complexes with fulvic and humic acids. The FTIR spectra and the presence of charcoal confirm the condensed aromatic nature and the abundance of carboxylic groups in the soils, these being related to the pyrolysis of biotransformed plant residues. Podzolization occurred in pulses promoted by intense rains under a dry climate and Fe-poor parent material, which favored the translocation of organometallic compounds of Al. The dense packing of fine organomineral material in the Bh horizons and the open matrix in the adjacent horizons indicate that the narrowing of the porous environment may be an important factor for the immobili-zation of translocated material. With the return of the current drier conditions in Brazil's semi-arid, the podzolization process remains incipient.