Increases in Ca/Mg ratios caused the increases in the mobile fractions of Cr and Ni in serpentinite-derived soils in humid Asia

Serpentinite-derived soils are characterized by low Ca/Mg ratio and abnormally high Cr and Ni concentrations due to the geogenic sources. The breakdown of parent materials during pedogenesis may lead to the changes in both Ca/Mg ratios and fractions of Cr and Ni. This study hypothesized that total Ca/Mg (Ca-t/Mg-t) and exchangeable Ca/Mg (Ca-e/Mg-e) ratios increase with soil development, associated with the increases in potentially mobile fractions (PMFs) of Cr and Ni. Thirty-five soil horizon samples of eight pedons from Japan, Taiwan, Philippines, and Vietnam were collected, and the exchangeable concentrations of Ca and Mg were measured. The fractions of Cr and Ni were further determined by the BCR sequential extraction, and the sum of the acid soluble, reducible, and oxidizable fractions was regarded as the PMFs of Cr and Ni. Based on the elemental and mineral analyses, the studied samples from different countries varied in the parent materials. The elemental mappings with electron probe microanalysis elucidated that the sources of Cr and Mg were spinels and silicates, such as chromite, serpentine, and chlorite, while those of Ni were the silicates. The Ca-e/Mg-e ratio was a potential index of weathering degree of the soils. The PMF of Cr mainly associated with the organic matter and Fe/Mn oxides increased during pedogenesis, corresponding to the significant (P < 0.05) increase in Ca-e/Mg-e ratio. Furthermore, the PMF of Cr was predicted by a function of Ca-t/Mg-t and Ca-e/Mg-e ratios, clay, and organic carbon using multivariate linear regression analysis. Although the PMF of Ni was poorly correlated with the Ca-e/Mg-e ratio, it was predicted by Ca-t/Mg-t and Ca-e/Mg-e ratios, total Ni, and dithionite-citrate-bicarbonate extractable Fe. These results supported that the breakdown of serpentinitic framework led to loss of Mg, increase of Ca/Mg ratios, and increase of Cr and Ni in PMFs during pedogenesis.