Soil-Landscape Relationship in a Toposequence over Granites in the Semi-Arid Region of Northeastern Brazil (Caatinga)

The Brazilian semi-arid region is composed of a landscape mosaic that reflects a complex and diversified soil cover. In this context, research that integrates geology and geomorphology can efficiently guide pedological investigations in heterogeneous environments. This research was aimed at evaluating the soil-landscape relationships and their correlation with soil attributes in a toposequence over granites in the semi-arid region (Caatinga) of Northeastern Brazil. A transect was delimited in the landscape, and in each slope segment, a soil profile was described and sampled, namely: a) summit (P1); b) shoulder (P2); (c) backslope (P3), and; footslope (P4). Physical and chemical attributes were evaluated. Mineralogical analyses of sand, silt, and clay fractions were conducted by X-ray diffraction, and the total contents of major elements were determined by X-ray fluorescence. An E horizon in the upper third confirms that argilluviation is an active process in this landscape segment. Elutriation and argillization (in situ clay formation) also form the textural gradient, including in the foothill segment, producing Lixisols with strong rubification in the Bt horizon. Expansive clays (e.g., montmorillonite) contribute to vertisolization and the higher exchangeable and total calcium and magnesium contents in the top and foothill segments (Vertisols). At the summit, the melanization of the Bw diagnostic horizon is also evident. The variation in morphological, physical, chemical, and mineralogical attributes under similar parent materials confirms the role of relief in the distribution of materials along the toposequence and in the genesis of soils.