Partition of geogenic nickel in paddy soils derived from serpentinites

Serpentinitic soils contain high concentrations of geogenic Ni. During serpentinitic mineral weathering, the Ni released from soils into ecosystems could be a source of non-anthropogenic metal contamination. In this study, soil samples were collected from two pedons in paddy fields in Taiwan and Japan; these samples were used to explore the profile distribution of total and labile Ni, demonstrating the contribution of Ni-bearing Fe and Mn oxides to the Ni partition in these soils. Serpentine and chlorite were the dominant primary minerals; thus, the soils reflected serpentinitic characteristics and exhibited high background concentrations of Ni. The total Ni content ranged from 240 to 520 mg kg^?1. Repeated redox and leaching cycles caused the redistribution of Ni in the paddy soils. The diethylenetriamine pentaacetate (DTPA)-extractable Ni, an availability index of Ni, increased as the soil depth decreased in the two pedons. An additional pool of labile Ni was present in the soils because the Ni concentration determined using a 0.1 N HCl extraction was much higher than was that determined using the DTPA extraction. Fe and Mn oxides were closely related to the labile Ni in these paddy soils. However, Ni was predominantly retained by amorphous and crystalline Fe oxides rather than Mn oxides. Shortening the flooded duration of paddy field is required to reduce the solubility of Ni because that the labile Ni and redox-sensitive Fe oxides can affect both the paddy soils and environment when Ni is released into the soil solution and becomes bioavailable under reducing conditions.