The Reduction Rates of Fe and SO42- in Some Acid Sulphate Soils in Southern Vietnam.
Tran Kim Tinh, Department of Soil Science, Agronomy Faculty, Can Tho University, 3-2 Street,, Can Tho City, Vietnam, S. Ingvar Nilsson, Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7014, SE-750 07, Sweden, and Ingrid Öborn, Dept. of Soil Sciences, Swedish University of Agricultural Sciences (SLU), Box 7014, SE-750 07, Uppsala, Sweden.
The understanding of reduction/oxidation processes in acid sulphate soils (ASS) is of major importance for the design of proper land management, especially in relation to potential soil toxicity due to high concentrations of Fe (II), Al or H2S. To identify the key factors influencing (1) the reduction rate of iron, and (2) the interaction between Fe and sulphate reduction, a laboratory experiment was conducted. Samples from A (A samples) and B (B samples) horizons of four Sulfaquepts from the Mekong Delta of Vietnam were incubated for a total duration of 135 days at 28o C with either (1) limited oxygen supply (a 8 ppm O>2 at start) or (2) ‘no oxygen' supply. The soil solution was analysed for pH, redox potential (Eh), dissolved organic carbon (DOC) , dissolved Fe and SO2-4. In addition, exchangeable Fe, as well as Fe, which had precipitated after reoxidation at the soil solution-air interface, was analysed. The initial formation rate of dissolved Fe(II) was used as an estimate of the reduction rate. Under ‘no oxygen' conditions the Fe reduction rate of A samples was faster than under limited oxygen supply. Sulphate reduction started after 20 days (incubation under ‘no oxygen' supply), while no sulphate reduction was detected with limited oxygen supply after 75 days. B samples showed slow reduction rates compared to A samples and no sulphate reduction was detected. DOC was found to be the most important factor for the Fe reduction rate in the soil. DOC declined while soluble and exchangeable Fe increased, indicating that soluble organic matter was used as a substrate for Fe reducing microorganisms. The amount of Fe reduced during the incubation (water soluble + exchangeable + reprecipitated) was estimated for the B samples in the limited oxygen supply treatment. In one case more than 100 % of oxalate-extractable Fe was reduced, indicating that some crystalline Fe oxides (extractable with dithionite) were reduced as well. Fe diffusion to the water surface followed by Fe oxidation and H+ production delayed sulphate reduction. Our results indicate that thorough soil preparation such as harrowing and puddling may produce unfavourable conditions for rice cultivation in acid sulphate soils due to dense soil packing, resulting in a slow Fe diffusion rate. Management practices which increase the aeration of the soils are recommended including soil tillage as well as selection of rice varieties with a high radial oxygen loss in the rhizosphere. Key words: Mekong Delta, Vietnam, acid sulphate soil, reduction rate, sulphate, iron, dissolved organic carbon (DOC).