2012 年 55 巻 3 号 p. 449-457
Acid sulfate soils, which have low productivity due to strong acidity, cover approximately 1.6 million ha of the Mekong Delta in Vietnam. In this paper, we show that rice yields decrease with distance from the Bassac River, which is one of branches of the Mekong River. Core boring samples were collected to determine sulfur composition of the soils with depth. The pH (H2O) of the topsoil at a 10 km site (the closest site to the river) showed that the soil was weakly acid, whereas strong acidity (pH < 4.0) was observed in the topsoil at a 59 km site (the third nearest site to the river). The sedimentation rate at the 10 km site, estimated from the 14C age of the subsoil, was rapid compared with other sites. Alluvial deposition from the Bassac River affected the thickness of the upper horizons. Buried carbon in the subsoil accumulated at around 6.0 ky BP, when the sea level was at its highest. Pyrite, which was the dominant sulfur in the soil-sulfur compounds, was found to have accumulated in the subsoil, except at the 10 km site. Sites far from the river contained larger amounts of pyrite and this accumulated in thicker horizons. The vertical distribution and content of the pyrite is considered to reflect the supply of sulfur during the transgression and alluvium deposition that occurred with regression of the sea level. The change in rice productivity according to the distance from the river seems to be the result of the soil acidity, which is mainly produced through the oxidation process of the pyrite in the subsoil. This acidification is a symptom of land deterioration by land cover and land use changes.
