Similar Soils but Different Soil-Forming Factors: Converging Evolution of Inceptisols in Brazil

Abstract

Soils result from the interaction of five independent formation factors. If one factor varies, while the others remain constant, different soils can be produced. Herein, we demonstrated an opposing trend, wherein two soils were similar, despite considerable differences in all factors of soil formation. We sampled two Inceptisols (Oxic Dystrudepts) formed on different parent materials (gneiss vs. mica schist), climate (tropical altimontane vs. warmer, drier plateau), topography (1 650 m, 45% slope vs. 1 000 m, 8% slope), time (rejuvenated vs. old, stable surface), and vegetation (rainforest vs. Cerrado savanna). The two soils had similar chemical properties, whereas the soil on mica schist had finer particle size distribution, lower porosity, and lower saturated hydraulic conductivity. These properties were related to a coarser blocky microstructure compared to the soil on gneiss. Both soils presented active mineral weathering and pronounced pedoplasmation, demonstrated by clay contents > 300 g kg−1, although only the Dystrudept on gneiss possessed coarse rock fragments. The C horizons of both soils presented fragmented clay coatings suggestive of argilluviation, likely relict, because they were not observed in the B horizons. The similarities in many properties of the two Dystrudepts, despite contrasting factors of soil formation, suggest converging evolution and that soil classification at the subgroup level was efficient in grouping similar formative processes in tropical conditions. Moreover, this work revealed that similar pedogenic processes acting on different factors of soil formation can result in similar soil properties, at least for Inceptisols where further soil development is hindered by topographic limitations.