ABSTRACT The occurrence of dark subsurface horizons rich in organic matter (OM) associated with polychrome in the B horizon (yellowish over reddish hue) is common in soils from Southern Brazil. The formation of these horizons and the combination with such morphological attributes has not been properly documented, and neither has the cause effect relationship. Four soil profiles with such sombric-like horizons with a yellowish color at the upper part of the B horizon over red subsoil were studied in Southern Brazil. Results from micromorphology, extractable sesquioxide minerals, clay mineralogy and isomorphic substitution of Fe by Al in iron minerals showed that melanization, xanthization, bioturbation, moderate shrinking/swelling and moderate ferralitization were the most evident pedogenetic processes in role. Xanthization is closely related to the sombric-like horizon formation. In the studied area the findings demonstrated that no clay and OM illuviation had taken place. Therefore, the classification of these soils was revisited, so as to take into account the processes that underlie their genesis with emphasis on xanthization, clay illuviation and soil aggregation. The results suggest that the sombric horizon may need redefinition, unless profiles can be found in which illuviation of clay and/or OM can be proven.

A ocorrência de Latossolos com horizonte A húmico hiperdesenvolvido (> 100 cm de espessura) nos trópicos e subtrópicos úmidos é um paradoxo, pois os processos de decomposição da matéria orgânica nesses ambientes são bastante eficientes. Apesar disso, esses solos têm sido documentados nas regiões Sudeste e Sul do Brasil, com algumas ocorrências na região Nordeste. Aspectos da gênese e do significado paleoambiental desses Latossolos aguardam por melhor entendimento. Os processos que levaram o horizonte A a ser tão espesso e escuro e contribuíram para preservação de C orgânico (CO) em profundidades consideráveis nesses pedons são de especial interesse. Neste trabalho, oito Latossolos com horizonte A húmico hiperdesenvolvido (Lh) sob diferentes fitofisionomias (Floresta Ombrófila, Floresta Estacional e Savana Arborizada) foram amostrados e os seus atributos macromorfológicos, físicos, químicos e mineralógicos estudados, com o objetivo de relacioná-los com a preservação de CO em profundidade. Esses Lh são argilosos a muito argilosos, fortemente ácidos, distróficos, álicos e muitos possuem fragmentos de carvão vegetal dispersos na matriz do solo. Caulinita é o principal mineral da fração argila nos horizontes A e B, seguido em abundância por gibbsita e por vermiculita com hidroxi entrecamadas. Este último argilomineral é identificado apenas nos Lh derivados de rochas basálticas da região Sul. Os teores de C total (CT) variam de 5 a 101 g kg-1 no horizonte A húmico. Carbono oxidado por via úmida com dicromato de potássio constitui, em média, 75 % do CT dos horizontes A húmicos, enquanto o C não oxidável com dicromato (C-res), que inclui formas de C recalcitrantes (como carvões), contribui para os restantes 25 % do CT. Os conteúdos de C foram independentes da maioria das variáveis inorgânicas do solo, exceto do Al extraído com oxalato de amônio ácido (OAA), que individualmente explica 69 % (P < 0,001) da variabilidade de CT do horizonte A húmico. Os conteúdos de argila não foram estimadores eficazes do CT e de outras formas de C estudadas, como também não foram: densidade do solo, Al3+ trocável, saturação por Al, capacidade de troca de cátions efetiva e outros parâmetros obtidos por extrações seletivas com OAA e ditionito-citrato-bicarbonato. Interações entre materiais orgânicos e minerais pobremente cristalinos, como indicado pelo Al extraído com OAA, apresentam-se como um dos mecanismos possíveis para proteção da matéria orgânica nesses solos.

The occurrence of Umbric Ferralsols with thick umbric epipedons (> 100 cm thickness) in humid Tropical and Subtropical areas is a paradox since the processes of organic matter decomposition in these environments are very efficient. Nevertheless, this soil type has been reported in areas in the Southeast and South of Brazil, and at some places in the Northeast. Aspects of the genesis and paleoenvironmental significance of these Ferralsols still need a better understanding. The processes that made the umbric horizons so thick and dark and contributed to the preservation of organic carbon (OC) at considerable depths in these soils are of special interest. In this study, eight Ferralsols with a thick umbric horizon (UF) under different vegetation types were sampled (tropical rain forest, tropical seasonal forest and savanna woodland) and their macromorphological, physical, chemical and mineralogical properties studied to detect soil characteristics that could explain the preservation of high carbon amounts at considerable depths. The studied UF are clayey to very clayey, strongly acidic, dystrophic, and Al-saturated and charcoal fragments are often scattered in the soil matrix. Kaolinites are the main clay minerals in the A and B horizons, followed by abundant gibbsite and hydroxyl-interlayered vermiculite. The latter was only found in UFs derived from basalt rock in the South of the country. Total carbon (TC) ranged from 5 to 101 g kg-1 in the umbric epipedon. Dichromate-oxidizable organic carbon represented nearly 75 % of TC in the thick A horizons, while non-oxidizable C, which includes recalcitrant C (e.g., charcoal), contributed to the remaining 25 % of TC. Carbon contents were not related to most of the inorganic soil variables studied, except for oxalate-extractable Al, which individually explained 69 % (P < 0.001) of the variability of TC in the umbric epipedon. Clay content was not suited as predictor of TC or of the other studied C forms. Bulk density, exchangeable Al3+, Al saturation, ECEC and other parameters obtained by selective extraction were not suitable as predictors of TC and other C forms. Interactions between organic matter and poorly crystalline minerals, as indicated by oxalate-extractable Al, appear to be one of the possible organic matter protection mechanisms of these soils.