Partitioning and potential mobilization of aluminum, arsenic, iron, and heavy metals in tropical active and post-active acid sulfate soils: Influence of long-term paddy rice cultivation.

Drainage of potential acid sulfate soils (PASS) for paddy rice cultivation results in the formation of active acid sulfate soils (AASS) and subsequently post-active acid sulfate soils (PAASS). The drainage of PASS causes severe environmental problems including acidification and metal contamination of soil and water resources. This study examined the vertical distribution and partitioning of Al, As, Co, Cu, Fe, Mn, Ni, Pb, and Zn in six tropical acid sulfate soils representing AASS and PAASS under long-term paddy rice cultivation (>145 years). The bulk soil samples were analyzed for total concentrations of Al, As, Co, Cu, Fe, Mn, Ni, Pb, and Zn. The partitioning of these elements was examined by a sequential extraction procedure. Labile Al is higher in ASS which is associated with low soil pH. During drainage, mobilization of As, Cu, and Pb is limited by coprecipitation with (poorly) crystalline Fe oxides minerals in the topsoil and partly oxidized layer of both soil types. These elements are associated with iron (mono) sulfides in unoxidized layer. When PASS are exposed to air, Co, Mn, Ni, and Zn are leached from the soils and are dominantly associated with iron sulfides in the unoxidized sediments. Labile Mn, Ni, and Zn are elevated in the unoxidized layer of PAASS because these elements are leached from the partly oxidized layers and adsorbed onto soil constituents. Cobalt is probably precipitated or adsorbed onto (poorly) crystalline minerals.