Influence of extractable iron,aluminium, and manganese on P-sorption in flooded acid sulfate soils

We evaluated the effect of 1 N NH₄OAc and sodium-citrate dithionite extractable forms of soil Fe, Al, and Mn on P-sorption of a flooded acid sulfate soil (Sulfic Tropaquepts) and a non-acid sulfate soil (Typic Tropaquepts) under different soil oxidation-reduction and pH conditions. We used Maha-Phot soil (Sulfic Tropaquepts) and Bangkok soil (Typic Tropaquepts) from the Bangkok Plain, Thailand, and incubated them with 0.2% rice straw under aerobic (O₂ atmosphere) and anaerobic (N₂ atmosphere) conditions at three different levels of pH (4.0, 5.0, and 6.0) for 6 weeks in stirred soil suspensions with a soil to 0.01 M CaCl₂ solution ratio of 1:7. After the incubation period, the soil suspensions in the first treatment (control) were not washed or pretreated with any extractants. For the second treatment (II), the soil suspensions were treated with 1 N NH₄OAc (buffered to pH 4.0) to remove Fe, Al, and Mn in exchangeable form. In the third treatment (III), the soils suspensions were treated with sodium citrate dithionite solution (20%) to remove Fe, Al, and Mn in the form of free oxides. The soil residues were then equilibrated with KH₂PO₄ ranging from 0 to 500 mg P kg^-1 soil. Sorption isotherms were described by the classical Langmuir equation. The P-sorption parameters under study were standard P requirement (SPR), Langmuir maximum sorption capacity (X_m), Langmuir sorption constant (k), and buffering index (BI). Treating soils with 1 N NH₄OAc reduced X_m by 32–55%, SPR by 68–84%, and also decreased the differences in P-sorption due to the effects of pH and oxidation-reduction conditions. Significant correlations between the P-sorption parameters and the amount of free iron oxides indicated the primary role of iron oxides in P-sorption of acid sulfate soils. Aluminium oxides seemed to play a secondary role in P-sorption of these soils. Manganese also showed an important effect on P-sorption, but the mechanism is ambiguous.This is a contribution from the Wetland Biogeochemistry Institute, Louisiana State University, Baton Rouge, LA 70803-7511