Mineralogy, Micromorphology, and Genesis of Soils with Varying Drainage Along a Hillslope on Granitic Rocks of the Atlantic Forest Biome, Brazil

Abstract

Although the physical environment of the Atlantic Forest realm is well known, studies on the soil-landform relationships are fundamental to improve the management of soil resources to facilitate sustainable development. The purpose of this study was to evaluate a representative topossequence on the "Mares de Morros" landscape of deeply weathered regolith on leucocratic granite rocks and demi-orange convex slopes. The soils varied along the topossequence according to drainage and were classified as Acrudox, Pseudogleysol, and Epiaquent. The clay fraction was composed by kaolinite, in association with gibbsite, goethite, hematite, and traces of vermiculite and hydroxy-Al interlayered vermiculite (HIV). The kaolinite crystallinity index obtained by different methods showed high structural disorder throughout the sequence, indicating that long-term pre-weathering has produced a homogenous regolith with little differences in terms of mineralogy, despite the changes in drainage. On the other hand, micromorphological features showed a complete change from the typical, well-developed microaggregate structure of upland, well-drained soils, to a massive, poorly developed structure downslope, consistent with the morphological description. Changes in microstructure development and micropedological features occurred both vertically and laterally along the topossequence and indicate that mineralogy alone cannot account for the microaggregate structure of kaolinitic Latossolos (Oxisols) well-drained with low Fe contents. Soils from the "Mares de Morros" landscape of the Alegre river basin on leucocratic granitic rocks highlight an inheritance of a deep pre-weathered regolith, showing subtle chemical and mineralogical changes, but marked morphological and physical differences along the topossequence, basically controlled by soil drainage in the past or present.

Mineralogical, chemical and electrochemical attributes of soils

“…Higher clay content was found in P2 and in the subsurface diagnostic horizons of the P6 (Table 2). These results are in agreement with those found by other Brazilian authors (Matos et al, 2001;Mello et al, 2006;Pacheco et al, 2018).…”

“…The Fe o /Fe d ratios were higher in the Bi horizon of two Inceptisols (P19 and P20). These results were also in agreement with those reported by Pacheco et al (2018), in which the authors found a predominance of well crystallized Fe in more weathered soils, such as Oxisols. Highly weathered tropical soils have very low amounts of poorly crystallized Fe oxides (Fontes and Weed, 1991).…”

“…The presence of minerals was confirmed in the sand, silt and clay fraction via interplanar spacing (d). These minerals are in agreement with those observed by other authors for Brazilian soils (Fontes and Weed, 1991;Fontes and Carvalho Jr., 2005;Simas et al, 2005;Schaefer et al, 2008;Camêlo et al, 2017;Camêlo et al, 2018;Pacheco et al, 2018).…”

Morphology, chemical composition and origin of 2:1 phyllosilicates in Bw horizons of latosols of the Brazilian Central Plateau: contribution to the discussion of the microgranular structure origin

“…Bertolazi et al [2017] identified 2:1 clay minerals in the top 40 cm of latosols that appeared to be vermiculite or HIVs. Pacheco et al [2018] studied the mineralogy, micromorphology and genesis of soils developed on weathered precambrian migmatized paragneiss and found traces of vermiculite and HIVs in the soils studied, two of which were latosols. As part of their study of the soil properties governing phosphorous adsorption in soils of Southern Brazil, Oliviera et al [2020] found small amounts of 2:1 clay minerals by using XRD.…”

Allochthonous material originating from saprolite as a marker of termite activity in Ferralsols

“…Lastly, fine sand-size particles (50-500 µm long) were numerous in F1 and poorly present in F3 (Table 1). Thus, even if the elongated particles showing an internal structure of phyllosilicates varied in size, in abundance in the groundmass fabric of the submillimetric granular aggregates and in K 2 O content, they were present in the four ferralic B horizons studied and corresponded to the 2:1 clay minerals identified earlier using XR diffraction but without having the possibility to describe their morphology and to determine their chemical composition 29,[31][32][33][34][35][36][37] .…”