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.摘要 尽管大西洋森林生物区的自然环境已被广泛认知，但开展土壤-地形关系研究，对于优化土壤资源管理、助力可持续发展而言仍具核心意义。本研究旨在评估发育于浅色花岗岩岩体之上、半弧形凸坡的"Mares de Morros"地貌中的典型地形序列（toposequence）。沿该地形序列的土壤随排水条件变化而分异，被划分为强发育湿润氧化土（Acrudox）、假潜育土（Pseudogleysol）以及表层滞水新成土（Epiaquent）。其黏粒组分以高岭石为主，伴生三水铝石、针铁矿、赤铁矿，以及微量蛭石和羟基铝夹层蛭石（HIV）。通过多种方法测得的高岭石结晶度指数显示，整个地形序列内的高岭石均表现出高度结构无序性，这表明长期的预风化作用形成了矿物学特征差异极小的均一残积层，尽管排水条件存在变化。另一方面，微形态特征（micromorphological features）显示，高地排水良好土壤典型且发育良好的微团聚体结构，在下坡处完全转变为块状且发育不良的结构，这与形态学描述一致。沿地形序列的垂直与横向方向均出现了微结构发育程度和微土壤学特征（micropedological features）的变化，这表明仅靠矿物学特征无法解释低铁含量、排水良好的高岭质Latossolo（氧化土，Oxisols）的微团聚体结构。发育于浅色花岗岩岩体之上、阿莱格雷河流域"Mares de Morros"地貌中的土壤，凸显了深度预风化残积层的继承性特征：其化学与矿物学特征变化微弱，但沿地形序列呈现出显著的形态学与物理学差异，这些差异基本由过去或当前的土壤排水条件所控制。