Water-soluble ionic substances in acid sulfate soils are likely to be strong controls for crop production and to have impacts on aquatic ecosystems. In dry seasons, in particular, oxidation of the soil surface followed by acidification probably produces lots of acids and soluble metals. To estimate acid and metal loads from acid sulfate soils to aquatic environments, we determined the composition of water-soluble ions from soils distributed in the Mekong Delta, Vietnam. At the end of the dry season, soils were taken from each soil horizon in two soil profiles on the delta under different land developmental processes. Water-soluble ions were extracted using both distilled water and artificially synthesized irrigation water (pH 6.3) adjusted to the same ionic strength and pH as the field canal water. The relationship between extracted basic cations in both extracts showed high linear correlation, indicating a similar extraction mechanism between both extractants. Higher ionic strength in the artificial irrigation water may not have any advantages for extraction by ion exchange and, thus, properties of extracts are likely to depend on the soil properties. The older the soil parent material, the larger the rates of soluble Al and Fe and the lower the pH. Progressive weathering of the soil on the older delta has already discharged greater amounts of bases, probably for compensation of acids, and the soil has started to release exchangeable Al sorbed onto negative-charge colloids and Fe from decomposed oxides. The soil profile of the older delta released relatively greater concentrations of Al with a lower content of base cations, where the annual averaged rice yield was half that of another site. Soluble metals and acids at both sites appear to accumulate in the upper horizons above the low permeable layer, which is probably widely distributed in the Mekong Delta.

Acids and soluble metal loads resulting from strong acidification preceded by severe drying of acid sulfate soils are fatal soil conditions for agricultural productivity and aquaculture environments. A greater concentration of dissolved Al and Fe after seasonal drought was confirmed in our previous paper using acid sulfate soils in the Mekong Delta. Furthermore, moisture desiccation resulting from land-use change and subsoil excavation for drainage probably promotes soil oxidation followed by acidification. We prepared air-dried soil moisture condition actual acid sulfate soils in the Mekong Delta to evaluate acidification and the release of acids and metals with drying. The pH values of the deepest horizon in the Hydroaquentic Sulfaquepts drastically dropped and greater concentrations of Al and Fe were released with drying. It was a common change from the sulfidic horizon to the sulfuric horizon. A large increase in Al release without a drop in pH after drying was observed in the sulfuric horizons of the Typic Sulfaquept, which developed on the oldest land in the Mekong Delta. Acidification by oxidation of reduced sulfur compounds might be compensated mainly by exchangeable Al and/or by the dissolution of Fe and Al oxides. In contrast, the concentration of Al increased slightly by drying in the sulfuric horizons in the Hydroaquentic Sulfaquept, which developed in the newer delta. The exchangeable basic cations in these soil horizons might still be effective in compensating for the protons produced by the oxidation of pyrite. The difference in the Al released by drying between the two sulfuric horizons might be attributed to the ripeness of the acid sulfate soils. The ripeness of the soil on the older delta might be higher than that on the younger delta. In this study, drying of acid sulfate soils, even in the sulfuric horizon, can promote metal release due to the exchange by protons derived from the oxidation of reduced sulfur compounds. However, further acidification might be compensated by a substantial amount of exchangeable cation and oxide dissolution.

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