Organic soils in Southeastern Brazilian highlands: formation and relations to vegetation history

Abstract

Well-drained organic soils form in distinctive highland environments in contrast to wetland organic soils. Similar to the concept of endemic soils, they are restricted to specific geographic areas due to a unique combination of soil-forming factors. In this contribution, we aim to improve the pedological knowledge of the Brazilian highlands, focusing on well-drained organic soils. The objective was to propose a soil formation scheme for the study area, combining traditional techniques and paleoenvironmental reconstruction (phytoliths, isotypes, dating). Organic soils were formed by litter accumulation in two pedoenvironments: (1) directly on bedrock in the high-elevation grasslands, by the processes of addition and transformation of the litter, which constitute the parent material from these soils; and (2) in upper-montane forests, with organic horizons formed above mineral horizons. In both conditions, the cold and humid climate and vegetation (organisms) are the main soil-forming factors, reducing organic matter decomposition, and influencing the physical and chemical soil properties. Phytolith records and isotopic composition of soil organic matter indicated four environmental phases: Phase I (before ∼18,9k yr BP). A cooler and drier climate than present, with predominance of subtropical grassland vegetation and fire events; Phase II (∼18,9k yr BP to ∼11,1k yr BP). A slightly increase of humidity, decrease of fire events, and expansion of forest formations, with rare presence of Araucaria angustifolia; Phase III (from ∼11,1k years BP) increase in humidity, the establishment of current climate conditions (cold and humid climate, typical of Southeastern Brazilian highlands), and significant presence of Araucaria in upper-montane forests; Phase IV (present) environmental changes related to increased fire events, reduction of Araucaria and increase of Bambusoid, Arecaceae and C4 plants, possibly resulted from human activities. This contribution enhances the understanding of well-drained organic soils in the Southeastern Brazilian highlands, showing their unique formation under cold and humid climates, influenced by vegetation history and environmental phases, as revealed through phytolith records, isotopic composition, and dating.