Fe, Al and Si species and organic matter leached off a ferrallitic and podzolic soil system from Central Amazonia N. Patel-Sorrentino a, ⁎ , Y. Lucas a , F. Eyrolle b , A.J. Melfi c a Laboratoire des Processus de Transferts et d'Echanges dans l'Environnement (PROTEE), Université du Sud Toulon- Var, BP 20132, 83957 La Garde Cedex — France b CEA Cadarache, IRSN/DPSE, Bât. 153, 13108 Saint-Paul-Lez Durance — France c NUPEGEL, Universidade of São Paulo, ESALQ, CP9, Piracicaba, CEP 13418-900 (SP) Brasil Received 26 April 2006; received in revised form 1 September 2006; accepted 1 October 2006 Available online 13 November 2006 Abstract The geochemistry of soil formation in central Amazonia, Brasil, was investigated by studying the waters draining off small podzolic, ferrallitic or mixed catchment areas. Dissolved, colloidal and particulate fractions were obtained by cascade filtration and tangential-flow filtration. The organic carbon, Fe, Si, Al concentrations and the complexing capacity with regard to Cu 2+ were determined for each fraction. In the waters draining podzolic areas, bulk concentrations were in the range 25.0–38.1 mg L - 1 for organic carbon, 240–280 μgL - 1 for Fe, 130–630 μgL - 1 for Al and around 0.9 mg L - 1 for Si. Fe mainly migrates as organo-metallic complexes, while Al migrates roughly half as inorganics in the particulate fraction and half as small species likely inorganic in the dissolved fraction. The result is the leaching of all elements and the relative accumulation of residual quartz. In the waters draining ferrallitic areas, bulk concentrations were in the range 1.2–1.9 mg L - 1 for organic carbon, 45–55 μgL - 1 for Fe, 106–220 μgL - 1 for Al and around 1.9 mg L - 1 for Si, this later concentration remaining below saturation with quartz. Most elements were transported in the dissolved fraction, except 10% of Si which was in the particulate fraction, likely as quartz, and 40–45% of Al which was in the colloidal fractions, likely as Al-hydroxides. The result is a relative enrichment of the soil in Si with regard to Al. The soils strongly control the physico-chemical characteristics of the forest stream waters, and their transport capacity with regard to complexable metals. Moreover, our results showed that the behaviour of Al with regard to organic matter was different from the behaviour of Fe. © 2006 Elsevier B.V. All rights reserved. Keywords: Amazon basin; Ferrallitic soils; Leaching; Podzols; Soil organic matter; Soil water 1. Introduction Podzols, also called spodosols, broadly occur in cool humid regions (McKeague et al., 1983; Lundström et al., 2000; Sommer et al., 2001), or in tropical warm humid regions (Klinge, 1965; Brabant, 1987; Schwartz, 1988). In both regions, they are characterized by the following horizons: a surface A horizon with slightly decomposed vegetative material, an elu- viated E horizon mainly formed by relic quartz sand or poorly weatherable minerals, and spodic horizons enriched with humic compounds (Bhs), or with Fe and Al oxides or poorly crys- tallized alumino-silicates, allophane or imogolite (Bs). A specificity of the podzols is the complete leaching of all elements from the eluviated E horizon, including Al and Fe. Those elements are poorly soluble in the soil solution con- ditions, and the organic matter is supposed to play a key role in their transport. Many hypotheses and theories have been proposed to explain the formation of podzols (Deb, 1949; Stobbe and Wrigh, 1959; De Coninck, 1980; Anderson et al., 1982; Buurman, 1984; Wang et al., 1986; Ugolini and Dahlgren, 1987; Taylor, 1988). Most of them were developed from the geochemical properties of the solid phase. Only a few con- sidered the role of the soluble mobile phase (Ugolini and Dahlgren, 1987; Lucas et al., 1996; Lundström et al., 2000; Lucas, 2001). The currently used theories are the fulvate theory and the proto-imogolite theory. The fulvate theory involves the formation of soluble organo- metallic complexes, especially with fulvic acids (FA) (Ugolini Geoderma 137 (2007) 444 – 454 www.elsevier.com/locate/geoderma ⁎ Corresponding author. Tel.: +33 4 94142306; fax: +33 4 94142057. E-mail address: patel@univ-tln.fr (N. Patel-Sorrentino). 0016-7061/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2006.10.002