A multi-proxy study of Umbric Ferralsols genesis in a high-mountain environment in Rio de Janeiro state

The formation of strongly weathered soils of the tropics (Ferralsols) with high carbon stocks (thick A-horizon/umbric horizon) is still not understood. In order to contribute to the understanding of the formation of Umbric Ferralsols, we studied these soils in the highlands of Rio de Janeiro state (Brazil) from various perspectives. We analysed soil morphology, micromorphology, physical and chemical soil attributes, soil phytolith assemblages, and delta C-13 isotopic composition in a multi-proxy study. The profiles presented ferralic horizons with SiO2/Al2O3 ratio (k(i) index) < 1.0, low silt/clay ratio values, predominance of kaolinite and presence of gibbsite, and low cation exchange capacity. These characteristics indicate a high degree of weathering and ferralitization. The isotopic and phytolithic variations indicate climatic modifications during the Late Holocene (approximately 2300 years ago), with a predominance of mixed vegetation composed of plants of the C3 and C4 photosynthetic cycles. This condition favoured the accumulation of organic matter in the Umbric Ferralsols. The accumulation of organic matter in subsurface B horizons occurred under drier past climatic conditions in a savanna environment, which may have favoured the occurrence of fires, thereby explaining the charcoal fragments identified in the micromorphological analyses. In contrast, the development of the umbric horizon in the profiles was favoured under wetter environmental conditions than the present environmental conditions, and favoured the development of vegetation and intense accumulation of organic material on the surface and at depths close to 100 cm. The pedogenesis of umbric horizons in Ferralsols reinforces the importance of bioturbation in the incorporation of organic matter, intensifying the humification and resulting in a strong darkening of the soil (melanization).