Toxic levels of metals in Ferralsols under natural vegetation and crops in New Caledonia

The chemistry of soil solutions and the potential toxicity of trace metals (Co, Cr, Cu, Ni and Mn) were investigated on soils formed on ultramafic rocks. Soil solutions were collected along a soil toposequence under natural vegetation and under a cropped field. In the latter, metal speciation and species activity were computed with the WHAM 6 model. Total element concentrations varied with the soil topographic position. Upslope, in well-drained soils, they were relatively small with mean concentrations of < 0.2 mu mol l(-1) for Co and Cr and < 2 mu mol l(-1) for Ni and Mn. Downslope, in temporarily waterlogged soils, concentrations reached 37 (Mn), 5.6 (Ni), 1.9 (Co) and 0.1 (Cr) mu mol l(-1). Under crops, Ni, Mn and Co concentrations were similar to those under natural vegetation, but Cr concentration averaged 5 mu mol l(-1). Cu concentration was close to 1 mu mol l(-1). Free-ion species amounted to 53-71% of all species for Co, Ni and Mn but only 5% for Cu. Cr was almost entirely in the Cr(VI) form (CrO42-, HCrO4-). The free-metal-ion activities were in the range 26-81% of the corresponding free-metal-ion concentration. Comparing our data with levels that are toxic to crops, Ni and Cr are potentially toxic in the well-drained and the poorly-drained soils. In the latter, Co and Mn are also potentially toxic. Both the large concentration of metals and the chemical species in which they occur in solution could limit the use of the land for agricultural purpose