Tu Cong;Tu C

;College of Resources and Environment, Southwest Agricultural University, Chongqing 630716, China.

ph heavy metals;pollution calcium carbonate;cation exchange capacity;clay minerals;nickel distribution transformation soil types genetic;entisols china sichuan

The distribution and transformation of added and native Ni in purple soils [Entisols] was investigated with a sequential extraction procedure and an isotopic tracer technique. The distribution was shown to be dependent on soil properties. Low pH favoured the soluble plus exchangeable (EX) fraction, while higher pH favoured Fe/Mn oxides, clay-Fe/Mn oxide (OX) and residual (RES) fractions. Added Ni, however, did not reach distribution equilibrium during the 150 days of incubation. This was reflectedin the fact that the proportion of the EX fraction from added Ni was 150%-600% that of native Ni, while the RES fraction was only _80%. Applied soluble Ni was rapidly transformed in the soil into other fractions, but the organic complexed (OM) fraction of added Ni was relatively stable throughout incubation. The carbonate bound (CAB) fraction showed a progressive increase and attained a peak value after 14 d of incubation, thereafter decreasing gradually. The occurrence time of this peak advanced as initial soil pH increased, but the peak disappeared when initial soil pH was higher than 7.5 (or the soil contained free CaCO3). The transformation processes of the EX, OX and RES fractions were described by Elovich and two-constant rate equations, and therates were positively correlated with soil pH, CEC and clay content. These findings help explain differences in ecological and environmental effects of Ni in different soils and at various intervals.