Copper migration and isotope fractionation in a typical paddy soil profile of the Yangtze Delta

To decipher Cu migration in paddy soils, which is important for understanding Cu supply in rice cultivation, Cu concentrations and isotope compositions were measured in a paddy soil profile in Suzhou, Eastern China, in the central Yangtze Delta. The results show that the variations in delta Cu-65 values and Cu concentrations are not coupled along the profile. From top to bottom, the delta Cu-65 values show small variations (0.07 +/- 0.0396o to 0.25 +/- 0.0196 parts per thousand) in the upper layers (Ap-Br1), with a decrease in the subsurface Br2 layer (from 0.16 +/- 0.0496 parts per thousand to -0.19 +/- 0.0296 parts per thousand), are almost homogeneous in the transitional Br3-BCrg layers (-0.01 +/- 0.0196 parts per thousand to -0.10 +/- 0.0296 parts per thousand), and further decrease to -0.33 +/- 0.0196 parts per thousand in the permanently submerged G1 and G2 layers. Copper concentrations in the Ap layer show some fluctua-tions (25.8 to 29.0 mu g/g), increase in the Br2 and Br3 layers (23.9 mu g/g to 31.9 mu g/g), and then decrease to 15.1 mu g/g in the lower layers. The lack of coupling between delta Cu-65 values and Cu concentrations may be ascribed to various physicochemical conditions in different layers. In the upper layers, Cu(I) enriched in light isotopes migrates downward with soil solutions under flooded conditions, leaving the soils of the Ap and Br1 layers enriched in heavy Cu isotopes. In the Br2 layer, the readsorption of light Cu isotopes on clay minerals results in decreased delta Cu-65 values and increased Cu concentrations. In the Br3-BCrg layers, Cu(I) can be oxidized to Cu(II). The homogeneous Cu isotopes in these layers may mainly result from equilibrium adsorption of Cu on clay minerals. The decreased delta Cu-65 values and Cu concentrations in the G layer are mainly attributed to groundwater transport in this layer. This study represents the Cu isotope variations in a paddy soil profile and the possible mechanism of Cu isotope fractionation.