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Revista Brasileira de Ciência do Solo

versão On-line ISSN 1806-9657

Resumo

SILVA, A. C.  e  VIDAL-TORRADO, P.. Genesis of Humic Oxisols and its relationships with the evolution of the landscape of a cratonic area in the South of Minas Gerais state, Brazil. Rev. Bras. Ciênc. Solo [online]. 1999, vol.23, n.2, pp.329-341. ISSN 1806-9657.  http://dx.doi.org/10.1590/S0100-06831999000200017.

A study on the genesis of Humic Oxisols and its relationships with the evolution of the landscape was carried out in an area of 5,750 ha in the central segment of the South of Minas Gerais Brazil. Two morphopedological systems (SMI and SMII), representative of the regional geology and geomorphology were chosen. Pedological (chemical, physical, morphological, micromorphological) and geomorphological studies, as well as soil charcoal quantification and radiocarbon tating were performed in four profiles classified as: Humic Xantic Hapludox and Typic Hapludox in SMI, and Humic Rhodic Hapludox and Rhodic Hapludox in SMII. The highly developed umbric epipedons of the humic Oxisols are thought to be very old and have a continuous, progressive melanization with depth, likely deeply affected by charcoal fragment decomposition and its redistribution by soil fauna. Charcoal is more abundant in these Oxisols than in others found elsewhere in the landscape.. The addition, transformation and translocation of materials in the soil profile by the action of its fauna would provoke the formation of brown microaggregates, the darkening of the remaining microaggregates of nonbiological origin, and the thickening of the umbric horizon. In situ soil formation in SMII suggests a great stability of the plateau where humic Oxisols are found. This also suggests a higher incidence of fires at this landscape position during the Quaternary, which can explain the higher soil charcoal content in the Humic Oxisols. The occurrence of this kind of soils on the lower backslope and footslope of SMI indicates an important short-distance soil transport, probably as a consequence of substratum block tilting during Quaternary times. Fault mirrors in the saprolite and in the solum of one Oxisol, dramatic differences among charcoal datings between 125 and 150 cm of depth (from 6,850 to 40,340 years BP), as well as the seismic disturbances occurring at present in the area point to the role of resurgent tectonics in the evolution of the landscape. Thus, the humic Oxisols could be considered as relic paleosols and, as such, they would constitute a fundamental link to understand the dynamics of regional landscape.

Palavras-chave : melanization; humox; charcoal; quaternary fires; 14 C dating; soil fauna; tectonic resurgent; soil-landscape relationships.

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