Biogeochemistry of an Amazonian podzol-ferralsol soil system with white kaolin

Abstract. The podzol-ferralsol soil systems, which cover great areas of Amazonia and other equatorial regions, are frequently associated with kaolin deposits and store and export large amounts of carbon. Although natural organic matter (NOM) plays a key role in their dynamics, little is known about their biogeochemistry. In order to assess the specific role of dissolved organic matter (DOM) on NOM storage in deep horizons and to determine possible relationships between kaolin formation and DOM properties, we studied the groundwater composition of a typical podzol-ferralsol soil catena from the Alto Rio Negro region, Brazil.

Groundwater was sampled using tension-free lysimeters placed according to soil morphology. DOC, E_H, pH, and dissolved Si, Al³⁺, Fe²⁺, and Fe³⁺ were analyzed for all samples and values are given in a database. Quantification of other dissolved ions, small carboxylic acids and SUVA₂₅₄ index and acid-base microtitration was achieved on selected samples.

Part of the DOM produced by the hydromorphic podzols is directly exported to the blackwater streams; another part percolates at greater depth, and more than 90% of it adsorbs in the Bh-Bhs horizons, allowing carbon storage at depth. Humic substances are preferentially adsorbed with regard to small carboxylic compounds.

With regard to kaolin genesis, kaolinite precipitation is favored by Al release from NOM mineralization within the Bh-Bhs and kaolin bleaching is ensured by iron reduction due to acidity and relatively low E_H. Fe²⁺ mobility can be related to small E_H variations and enhanced by the significant concentration of small carboxylic acids. The long-term result of these processes is the thickening of the kaolin, and it can be inferred that kaolin is likely to occur where active, giant podzols are close to a slope gradient sufficient enough to lower the deep water table.