From Oxisols to Spodosols and Histosols: evolution of the soil mantles in the Rio Negro basin (Amazonia)

Abstract The Upper Rio Negro basin, under a constantly humid equatorial climate, is a low-altitude peneplain of more than 165,000 km 2 . Prevailing soils are Oxisols and Spodosols, and their distribution is usually related to the lithology of the parent materials; Spodosols being generally associated with sandy deposits. After exploratory surveys in this extensive region, six major soil-geomorphic units have been identified in which selected toposequences have been studied by means of micromorphological, chemical and mineralogical analyses. Detailed field analysis of the horizonation of the soil mantle has been carried out in three sequences consisting of Oxisols, Ultisols and Spodosols. Results show that sharp transitions, within distances of less than one hundred meters, separate the Oxisols from the Spodosols. However, the arrangement of the horizons in the soil mantle and similarities in micromorphological features, chemical composition and mineral components between the adjacent horizons are evidences that genetic relationships link contiguous profiles. The authors propose an alternative explanation for this soil distribution, based on the lateral transformation of Oxisols and Ultisols into Spodosols, and on the lateral evolution of the giant Spodosols into Histosols (peat) and waterlogged Ultisols. Interpretative models of landscape evolution as consequence of soil evolution are thus proposed.