Weathering and clay formation in semi-arid calcareous soils from Northeastern Brazil

Despite the great number of studies exploring the inter-profile genetic relationships in a catena, the microrelief and microtopographic features have been overlooked. This study investigated the role of microrelief surfaces (convex, flat and concave) on the mineral formation and pedogenesis in soils derived from limestones in NE Brazil. Soil profiles were studied in different micro-landscape surfaces, and soil samples were taken by horizon for chemical (weathering indexes) and mineralogical analyses. Mineralogy was determined in samples from diagnostic subsurface horizons, which were subjected to the proper chemical treatment for identification (X-ray diffraction - XRD) and quantification (Newmod II and Thermogravimetry - TG) of the phyllosilicates. The XRD patterns showed similar composition among the parental material of the studied profiles. The chemical also showed similar dynamics along the surfaces; however, the concave surface had soils with a higher degree of weathering, when compared to others. The clay minerals identified on the convex and flat surfaces were vermiculite, illite, smectite and kaolinite; while on the concave surface, only illite and kaolinite were found. Based on the information generated by the Newmod II, there was an increase in the amount of kaolinite towards the concave surface, from 21%, on the convex surface, to > 50% of pure kaolinite crystals on the concave surface. These data were supported by the TG curves. Our results evidence that microrelief features, marked by low topographic amplitudes, are determinant for the formation and distribution of phyllosilicates in calcareous soils; and, thus, may be of key importance for predicting the occurrence of contrasting mineralogy in soils derived from calcareous rock.