Relationships between extractable Ni, Co, and other metals and some microbiological characteristics of different ultramafic soils from New Caledonia

New Caledonian ultramafic soils have relatively high metal content, which normally induces toxicity in plants and microflora. The relationships between extractable metal concentrations, especially Ni and Co, and different microbiological factors were analysed by statistical methods. For this purpose, some physico-chemical and microbiological characteristics of 40 ultramafic soil samples were estimated: organic carbon, pH, extractable metals (Ni, Co, Fe, Mn and Mg), microbial density, total microbial activity and percentage of microorganisms tolerant to Ni. Total, partial, and multiple correlation analyses are reported. The release of metals (measured by their extractablity) into these soils appeared to be related to microbial activity; the link with pH was not clear. A laboratory experiment confirmed the role of the microflora in the release of Ni and Co, which were not previously known to be released by microbial processes in soil. The percentage of microorganisms tolerant to Ni was highly correlated to the extractable concentration of this metal in the soils.