ABSTRACT
Serpentinite outcrops have been found in every continent except Antarctica; however, serpentine soils are derived from ultramafic rocks alone and also from serpentinites, which contain enriched geogenic nickel (Ni). Serpentinites contain up to 3600 mg/kg of Ni. Total content of Ni in serpentine soils worldwide ranged up over 70000 mg/kg, and thus release of Ni into ecosystems during serpentine weathering suggests that serpentine landscapes provide a source of non-anthropogenic Ni contamination. Nickel is commonly incorporated into the crystal lattices of clay, silt and sand-sized particles of the parent material and secondary minerals and retained by Fe/Mn oxides. As chemical weathering progresses, these minerals destabilize and release Ni into the ecosystem. The diethylenetriamine pentaacetate (DTPA)-extractable concentration of Ni has been recommended as a bioavailability index of Ni which might be used to estimate Ni uptake by plants grown in upland serpentine soils. However, the 0.1 N HCl-extractable Ni was the more suitable index for Ni bioavailability in the soil to paddy rice than that of DTPA-extractable Ni. Regarding forest serpentine ecosystems, organic acids from humus and root exudates lead to input/formation of protons and ligands which affect Ni release from the soils. However, Ni release from the forest serpentine soils is dependent on pH and ionic strength of organic acids and may be strongly accelerated by complex forming ligands; both ligands and protons corroborate to promote the release of Ni from serpentine soils into the environment.
