Response of Cu partitioning to flooding: A δ65Cu approach in a carbonatic alluvial soil

The mobility and bioavailability of Cu in soils depend on the chemical Cu forms and are strongly influenced by water-induced changes in soil redox conditions. We chose an experimental approach by flooding a carbonatic floodplain soil and incubating it in a glovebox for 35 days. We used a five-step sequential chemical extraction to characterize the changes in chemical partitioning (F1–F5; NH4NO3-extractable, NaOAc-extractable, NH4Ox-extractable, hot H2O2/NH4OAc-extractable and residual fractions, respectively) of Cu and δ65Cu values in the soil during the 35 days. After flooding, Cu concentrations decreased in F1–F3 and increased in F4–F5. Overall, 73% of the total Cu was redistributed among the five studied fractions. Before flooding (Day 0), δ65Cu values in F1–F4 followed the estimated bonding strengths of Cu in the respective fractions, indicating equilibrium distribution of Cu at the beginning of the experiment. The total variation in δ65Cu values among F1–F5 changed strongly from 0.83 ± 0.18‰ on Day 0 to a maximum of 2.18 ± 0.17‰ on Day 7. This change indicates the reduction of Cu2 + to Cu+ or Cu0 after flooding. The strongest variations in δ65Cu values occurred in F3 (0.09 ± 0.07‰ to 1.43 ± 0.13‰) and F4 (− 0.24 ± 0.07‰ to 0.55 ± 0.07‰), while flooding had no or small effects on the δ65Cu values of F1, F2 and F5. Our results suggest a direct transfer of Cu from F3 to F4 because both concentration changes and changes in δ65Cu values were balanced between the two fractions. The responses of Cu partitioning and δ65Cu values to flooding are in line with the formation of CuxS or other reduced Cu species and reduction of Cu associated to Fe (oxyhydr)oxides. Our results emphasize the strong influence of redox conditions on Cu partitioning in soils and indicate a reduction in Cu availability under anoxic redox conditions