Influence of rice cultivation on the abundance and fractionation of Fe, Mn, Zn, Cu, and Al in acid sulfate paddy soils in the Pearl River Delta

Many of the areas dominated by acid sulfate (AS) soils in southern China have been reclaimed for rice cultivation and represent an important source of agricultural production and food security for the local population. In this study, we examined the abundance and partitioning of Fe, Mn, Zn, Cu, and Al in both rice paddy fields and uncultivated fields by using a six-step sequential extraction procedure. Rice cultivation significantly reduced acidity levels at soil depths of 0-100 cm, but acidity at depths of 100-140 cm increased significantly. A substantial loss of Mn in the oxide and acidified soil layer (0-100 cm) compared to the underlying parent material was observed, but there were few changes in the concentrations of Fe and Zn throughout the soil profile. However, both Cu and Al were substantially enriched in the superficial soil (0-40 cm) compared to the underlying subsoil (40-140 cm). In general, leaching of Fe, Mn, and Al was higher in the paddy fields because of intensified irrigation and flood conditions, whereas accumulation of Cu, mainly in the form of “acid soluble” and “crystalline iron oxide” Cu, was higher in the paddy fields than in the uncultivated fields. The most marked declines in metal concentrations occurred in the “labile” and “acid soluble” fractions, as a result of rice cultivation, which increased leaching of the readily mobilized “labile” and “acid soluble” fractions of heavy metals in these soils.