FROM OXISOLS TO SPODOSOLS AND HISTOSOLS - EVOLUTION OF THE SOIL MANTLES IN THE RIO-NEGRO BASIN (AMAZONIA)

The Upper Rio Negro basin, under a constantly humid equatorial climate , is a low-altitude peneplain of more than 165,000 km(2). Prevailing s oils are Oxisols and Spodosols, and their distribution is usually rela ted to the lithology of the parent materials; Spodosols being generall y associated with sandy deposits. After exploratory surveys in this ex tensive region, six major soil-geomorphic units have been identified i n which selected toposequences have been studied by means of micromorp hological, chemical and mineralogical analyses. Detailed field analysi s 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 met ers, separate the Oxisols from the Spodosols. However, the arrangement of the horizons in the soil mantle and similarities in micromorpholog ical features, chemical composition and mineral components between the adjacent horizons are evidences that genetic relationships link conti guous profiles. The authors propose an alternative explanation for thi s soil distribution, based on the lateral transformation of Oxisols an d Ultisols into Spodosols, and on the lateral evolution of the giant S podosols into Histosols (peat) and waterlogged Ultisols. Interpretativ e models of landscape evolution as consequence of soil evolution are t hus proposed. (C) 1998 Elsevier Science B.V. All rights reserved.