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ABSTRACT: Ultramafic (UM) rocks are defined as igneous rocks that contain more than 90% of mafic minerals. Soils derived from ultramafic rock are generally nutrient-deficient and have concomitant high concentrations of potentially phytotoxic trace elements (Ni, Cr, Co, Mn). Consequently, to assess the dynamics of nutrients and metals in the ultramafic complex of Niquelândia (Brazil), soil solutions have been sampled in soils characterized by high Cr(VI) availability. The metal contents in surficial water have also been analyzed to investigate the metals’ leaching and mobility. Soil solutions featured low nutrient contents, a large Ca:Mg imbalance, and high Ni and Cr concentrations. Chromium was present in its toxic dissolved form (Cr(VI)) in the soil and surficial solutions. Metals concentrations were often above the toxic limit for biota and were therefore able to affect soil functioning. Ni behavior in the topsoil appeared to be primarily controlled by organic matter, while Cr was more likely to be released from Fe-oxides by anionic exchange. This result agreed with the Cr(VI) lability assessed using isotopic exchange kinetics in a companion study. In these serpentinic tropical soils, the highly leached Fe-oxide horizons appear to play a large role in the sequestration and diffuse leaching of labile Cr(VI) and Ni, respectively, in the deeper part of soil profiles and the topsoil. At the catchment scale, surficial solutions results suggest that metals may be exported to surrounding ecosystems that are not adapted to these metals.

The Oxisols of the Cerrado region have a simple mineralogy, and are composed mainly of kaolinite and iron and aluminium oxides. The aim of this work was to perform a sequential extraction of iron and aluminium and to relate them to texture, mineralogy and organic carbon in Oxisols of the Cerrado region. The soil samples, 35 in total, were taken from the 0–20 cm layer in Oxisols under pasture located in the Brazilian Cerrado Region. The levels of iron and aluminum extracted by full attack, dithionite-citrate-bicarbonate, ammonium oxalate and sodium pyrophosphate were analyzed according to the textural classes using the Kruskal-Wallis nonparametric test. Then these data were related to the mineralogy and organic carbon through a principal component analysis. Whatever the textures, iron oxides of high degree of crystallinity were the main form of iron and are positively correlated with the iron total content. The aluminum oxides have a high affinity to organic carbon than iron oxides. The levels of iron and aluminum extracted by sodium pyrophosphate were closely related to soil pH.Os Latossolos da região do Cerrado apresentam mineralogia da fração argila relativamente simples, sendo constituídos principalmente por caulinita e óxidos de ferro e de alumínio. O objetivo deste trabalho foi realizar extrações sequênciais de ferro e de alumínio e relacioná-los à mineralogia, textura e teores de carbono orgânico em Latossolos da região do Cerrado. Coletaram-se 35 amostras na camada de 0-20 cm de Latossolos sob pastagem da Região do Cerrado. Os teores de ferro e de alumínio extraídos pelo ataque total, ditionito-citrato-bicarbonato de sódio, oxalato ácido de amônia e pirofosfato de sódio foram analisados de acordo com a classe textural através do teste não paramétrico de Kruskal-Wallis. Em seguida estes dados foram relacionados com a mineralogia e o carbono orgânico por meio de uma análise de componentes principais. Independentemente da classe textural, os óxidos de ferro de alto grau de cristalinidade representaram a principal forma de ferro e se correlacionam positivamente com os teores totais de ferro. Os óxidos de alumínio apresentaram maior afinidade ao carbono orgânico do que os óxidos de ferro. Os teores de ferro e alumínio extraídos pelo pirofosfato foram estreitamente relacionados ao pH dos solos.

Este trabalho teve como objetivo descrever os teores totais e disponíveis de cobre, ferro, manganês e zinco e suas relações com a acidez, fertilidade, textura e mineralogia de latossolos sob pastagens na Região do Cerrado. Os solos estudados foram amostrados em três regiões: às margens das rodovias GO-060 e BR-154, entre os municípios de Goiânia, GO, e Barra do Garças, MT; no Distrito Federal, e nos municípios de Unaí e Paracatu, noroeste de Minas Gerais. Coletaram-se 54 amostras na camada de 0-20 cm. Relacionaram-se as quantidades disponíveis de Cu, Fe, Mn e Zn (extraídos com DTPA) com seus teores totais, textura, mineralogia e variáveis químicas dos solos, por meio da análise de componentes principais. Os teores totais e disponíveis desses elementos foram extremamente variáveis. A disponibilidade de Fe, Cu e Mn se relacionou principalmente com os teores de ferro cristalino e com o pH em água. A disponibilidade de Zn se relacionou principalmente com a capacidade de troca catiônica do solo. O Zn é, entre os micronutrientes avaliados, o mais freqüentemente limitante ao desenvolvimento das pastagens na Região do Cerrado.The objective of this work was to describe the total and available Cu, Fe, Mn and Zn contents, and their relation with the oxisols acidity, fertility, texture and mineralogy under Cerrado pastures. The soil samples were taken from three regions: along the highways GO-060 and BR-154, between the towns of Goiânia, GO, and Barra do Garças, MT; in the northeast of the State of Minas Gerais, in the towns of Unaí and Paracatu, MG; and in the Federal District. Fifty-four soil samples were collected from the 0-20 cm layer. Available quantities of Cu, Fe, Mn and Zn (DTPA extracted) were related to: total contents, chemical variables, soils texture and mineralogy, through the analysis of the principal components. The available and total Cu, Fe, Mn and Zn content varied greatly. The availability of Fe, Cu and Mn related mainly to the crystalline iron and pH in water. The availability of Zn was mainly related to the soil exchange capacity. Zn was considered the most frequent limiting micronutrient for Cerrado pasture development.