Concentration and distribution of chromium and nickel fractions along a serpentinitic toposequence

Lithiogenic Cr and Ni enrichments are part of the crucial fertility limitations in serpentinitic soils. However, our understanding of the bioavailability and chemical fractions of Cr and Ni in the serpentinitic soils on different landscape positions is limited. For this study, four soil pedons on the summit (Entisol), shoulder (Vertisol), backslope (Alfisol), and footslope (Ultisol) positions along a serpentinitic toposequence in eastern Taiwan were selected and their Cr and Ni concentrations assessed by single extraction and selected sequential extraction (SSE). The results indicate that the total Cr and Ni of the soil pedons follow the following order: summit >= shoulder > backslope >= footslope on the landscape. The clay mineral composition reflected the weathering sequence of the soils. The soils on the summit and shoulder positions contained considerably more smectite and serpentine than the other soils, whereas vermiculite was increased in the soils on the backslope and footslope. AM four SSE fractions in the soils on the summit and shoulder were significantly higher than on the backslope and footslope. The greatest Cr and Ni concentrations occurred in the residual fraction, followed to a much lesser extent in the reducible, oxidizable and acid-extractable fractions. The Cr-extractable contents with diethylenetriaminepentaacetic acid and NH4OAc and acid-extractable fractions of Cr in the SSE for an the soils were generally below 5.0 mg/kg, indicating that the serpentinitic soils along the toposequence always displayed low levels of Cr in bioavailability and mobility. However, the bioavailability and mobility of Ni significantly exceeded those of Cr in the serpentinitic soils assessed with the above extractions, particularly for the summit and shoulder soils.