Genesis and micropedology of soils at Serra do Divisor and Moa river floodplain, northwestern Acre, Brazilian Amazonia

ABSTRACT The northwestern part of the Acre State (Brazil) possesses singular soils in Brazilian Amazonia, but have been very little studied. This study aimed to discuss the genesis and some micropedological aspects of the soils from Serra do Divisor and adjacent floodplain soils of the Moa river, to enhance the knowledge on their formation. A toposequence of soils ranging from the uppermost part of sub-Andean Serra do Divisor to the Alluvial soils of Moa river floodplain was studied, regarding chemical, physical, mineralogical, and micromorphological attributes. The parent material of the Serra do Divisor is basically quartzose sandstone, and the soils along the toposequence were classified as Typic Haplorthods (P1), Spodic Quartzipsamment (P2), Lithic Quartzipsamment (P3), and Lithic Quartzipsamment (P4). Along the Moa river floodplain, we also identified and collected, Typic Udifluvent (P5), Typic Kandiudult (P6), Typic Kandiudalf (P7), and Arenic Plinthic Kandiudult (P8). The Serra do Divisor soils have very low fertility, high acidity, and low cation exchange capacities, presenting a coarse sandy texture, even shallow pedons. The X-ray diffraction analysis of these soils indicates the predominance of kaolinite, with traces of quartz and gibbsite. The shallow mountain Podzols on sandstone have an expressive accumulation of organic material in surface horizons, with evidence of ferrihydrite and imogolite in the subsurface. At the Moa river floodplain, all soils are originated from recent sediments (Cenozoic), which have a geological source upstream. Varying sedimentary layers are key aspects influencing soil genesis. Those soils have evidence of 2:1 clays with hydroxyl-Al interlayers in subsurface horizons. The Serra do Divisor steep landforms and the coarse texture of the soils promote good drainage and favor leaching and chemical impoverishment. Kaolinite and gibbsite were formed by severe leaching and there are evidences of in situ neoformation of gibbsite by extreme Si losses. All studied soils have some peculiarities such as high accumulation of organic material or 2:1 clay minerals. Most investigated soils were affected by colluvial, reworking, mass movements or a strong variation on sedimentation.

摘要：巴西阿克里州西北部拥有巴西亚马逊地区独特的土壤类型，但相关研究却极少。本研究旨在探讨迪维索尔山脉土壤及莫阿河邻近泛滥平原土壤的发生学机制与部分微土壤学特征，以深化对其成土过程的认知。本研究针对从亚安第斯迪维索尔山脉最高处延伸至莫阿河泛滥平原冲积土的土壤地形序列展开研究，分析其化学、物理、矿物学及微形态学属性。
迪维索尔山脉的土壤母质以石英砂岩为主，该地形序列上的土壤分别被划分为典型灰土（Typic Haplorthods，P1）、腐殖质淀积砂质新成土（Spodic Quartzipsamment，P2）、石质砂质新成土（Lithic Quartzipsamment，P3）与石质砂质新成土（Lithic Quartzipsamment，P4）。在莫阿河泛滥平原区域，研究团队还鉴定并采集了典型冲积新成土（Typic Udifluvent，P5）、典型高岭湿润老成土（Typic Kandiudult，P6）、典型高岭湿润淋溶土（Typic Kandiudalf，P7）以及砂质铁磐高岭湿润老成土（Arenic Plinthic Kandiudult，P8）。
迪维索尔山脉的土壤肥力极低、酸度极高且阳离子交换量偏低，质地以粗砂质为主，甚至存在浅位土壤单体。对这些土壤的X射线衍射（X-ray diffraction）分析结果显示，其矿物组成以高岭石为主，伴随少量石英与三水铝石。发育于砂岩之上的山地浅位灰壤，其表层有机质累积显著，亚表层则检测到水铁矿与伊毛缟石的存在迹象。
莫阿河泛滥平原的所有土壤均形成于新生代近代沉积物，其物源来自上游地质体。不同类型的沉积层是影响成土作用的关键因素。该区域土壤的亚表层中存在带羟基铝夹层的2:1型黏土矿物迹象。
迪维索尔山脉的陡峭地貌与土壤粗质地促进了良好的排水性，同时加速了淋溶作用与化学贫瘠化过程。高岭石与三水铝石均通过强烈淋溶作用形成，且有证据表明，极端硅流失过程可原位生成三水铝石。所有供试土壤均具备独特特征，例如有机质大量累积或富含2:1型黏土矿物。多数供试土壤受到坡积物再沉积、块体运动或沉积作用强烈波动的影响。