Genesis of dark subsurface horizons of soils in area of Pelotas Batholith, RS.

Horizontes subsuperficiais escuros com caracter??sticas morfol??gicas similares ao s??mbrico t??m sido encontrados na regi??o Sul do Brasil, particularmente nas ??reas mais elevadas do Bat??lito Pelotas, situado na por????o leste do Escudo Sul-rio-grandense. O s??mbrico, inicialmente identificado por ped??logos belgas que trabalharam no Congo, constitui um horizonte diagn??stico nos sistemas de classifica????o de solos da FAO e da Soil Taxonomy, mas n??o no sistema brasileiro. A concep????o de horizonte s??mbrico nestes sistemas de classifica????o ?? a de um horizonte subsuperficial de drenagem livre, contendo h??mus iluvial n??o associado ao alum??nio, nem dispers??vel pelo s??dio, com valor e/ou croma menores do que o horizonte sobrejacente. No campo pode ser diferenciado do horizonte A enterrado pelo acompanhamento das varia????es laterais ou no laborat??rio por an??lise em sec????es delgadas. Os mecanismos respons??veis pela g??nese do horizonte s??mbrico s??o ainda pouco compreendidos, apesar da maioria dos autores concordarem com a origem do h??mus iluvial e de constituir horizonte diagn??stico. Diante da necessidade de caracterizar a ocorr??ncia desse tipo de horizonte na regi??o e do entendimento da g??nese destes objetivou-se neste trabalho descrever a ocorr??ncia de solos com horizontes de morfologia similar ?? do s??mbrico na regi??o do Bat??lito Pelotas e investigar o tipo de mat??ria org??nica envolvido no processo. Para atingir este objetivo foram realizadas an??lises f??sicas, qu??micas, morfol??gicas, mineral??gicas e isot??picas em dez perfis de solos da regi??o com horizontes subsuperficiais escuros, sendo dois deles sem as caracter??sticas morfol??gicas de s??mbrico e outro com ind??cios morfol??gicos de horizonte A enterrado, al??m de duas ??reas degradadas. A an??lise dos resultados permitiu verificar que: a) a posi????o destes horizontes coincide com os horizontes A2, AB, BA, topo do horizonte B ou ainda AC/CR; (b) os horizontes com morfologia similar a do s??mbrico n??o constituem horizontes enterrados, conforme indicado pela an??lise granulom??trica, mineralogia, morfologia e varia????es laterais destes horizontes no campo; c) o fracionamento das subst??ncias h??micas indicou haver um processo de iluvia????o de mat??ria org??nica, principalmente de ??cidos h??micos, para todos os perfis estudados, mostrando que a iluvia????o de material org??nico n??o ocorre somente nos perfis com horizontes ―s??mbricos‖; d) a aus??ncia do horizonte E, associado aos crit??rios de Al + ?? Feo, densidade ??tica do extrato de oxalato, fracionamento qu??mico e presen??a significativa de cutans de iluvia????o em subsuperf??cie indicam que o processo de podzoliza????o ?? apenas incipiente nos perfis; e) a composi????o isot??pica do carbono indica que uma parte dos horizontes subsuperficiais escuros possui mat??ria org??nica oriunda de plantas do tipo C4 (gram??neas), com varia????o isot??pica em rela????o ?? vegeta????o atual. Em outros, h?? mistura de plantas C3 (florestas) e C4 com pouca ou nenhuma varia????o em rela????o ?? vegeta????o atual, sugerindo que a g??nese destes horizontes n??o esteja relacionada somente a mudan??as na vegeta????o; f) parte significativa dos solos do Bat??lito Pelotas apresenta problemas de enquadramento na atual classifica????o brasileira de solos; g) a ado????o do horizonte ou car??ter s??mbrico e mudan??as na subordem dos Argissolos Bruno-Acinzentados em edi????es futuras do Sistema Brasileiro de Classifica????o de Solos enquadraria melhor uma significativa parte dos perfis da ??rea de estudo.Dark subsurface horizons with morphological characteristics similar to sombric horizonts have been found in southern Brazil, particularly in the higher areas of the Pelotas Batholith, located in the eastern portion of the Escudo Sul-rio-grandense. The sombric horizon, initially identified by soil scientists who worked in the Belgian Congo, constitutes a diagnostic horizon in soil classification systems of FAO and Soil Taxonomy, but not in the Brazilian system. The conception of sombric horizon in these classification systems is of a free draining subsurface horizon, containing iluvial humus not associated with aluminum or dispersible by sodium, with value and/or lower chroma than the overlying horizon. In the field it can be differentiated from the buried A horizon by monitoring lateral variations and in the laboratory by analyzing thin sections. The mechanisms responsible for the genesis of the sombric horizon are still poorly understood, although most authors agree on the illuvial humus origin and that it constitutes a diagnostic horizon. Considering the need to characterize the occurrence of such horizons in the region and the possible reasons for its genesis this work aimed to describe the occurrence of soil profiles exhibiting dark subsurface horizons with similar morphology to the sombric horizon in the region of Pelotas Batholith and to investigate the organic matter involved in the process. To achieve this goal physical, chemical, morphological, mineralogical and isotopic studies were performed in ten soil profiles with dark subsurface horizons, two of them without the sombric morphological characteristics and another one with morphological evidence of buried A horizon, and two degraded areas. The results showed that: a) the position of the dark subsurface horizons coincide with the A2, AB, BA horizons, top of B horizon or AC/CR; b) the horizons with morphology similar to sombric are not buried horizons, as indicated by particle size analysis, mineralogy, morphology and lateral variations of these horizons in the field, c) the fractionation of humic substances indicated a process of illuviation of organic matter, especially humic acids, for all soil samples, showing that the illuviation of organic material occurs not only in profiles with sombric horizons; d) the absence of the E horizon, associated with the Al + ?? Feo criteria, optical density of oxalate extract, chemical fractionation and significant presence of illuvial subsurface cutans indicate that the podzolization process is only incipient in the soil profiles; d) the C isotopic composition indicate that a part of the dark subsurface horizons have organic matter from C4-type plants (grasslands), differing from to the present. In other profiles, there is a mixture of C3 plants (forests) and C4 plants with little or no variation from the current vegetation, suggesting that the genesis of these horizons is not related only to changes in vegetation; f) a significant part of the soils located on the Pelotas Batholith present are not adequately classified in the Brazilian soil classification system, g) the adoption of the horizon or sombric character and changes in the Argissolos Bruno-Acinzentados suborder in future editions of the Brazilian System of Soil Classification would fit better a significant part of the soil profiles of the study area