Response of copper concentrations and stable isotope ratios to artificial drainage in a French Retisol

Kusonwiriyawong, Charirat; Bigalke, Moritz; Cornu, Sophie; Montagne, David; Fekiacova, Zuzana; Lazarov, Marina; Wilcke, Wolfgang (2017). Response of copper concentrations and stable isotope ratios to artificial drainage in a French Retisol. Geoderma, 300, pp. 44-54. Elsevier Science 10.1016/j.geoderma.2017.04.003

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Copper is a redox-sensitive trace element, which can be both, an essential micronutrient and a pollutant. We therefore analyzed Cu concentrations and stable isotope ratios (δ65Cu values) in a drained Retisol to trace the response of Cu to a changing hydrological regime and enhanced clay eluviation. The study soil was artificially drained 16 years before sampling resulting in macroscopically visible pedogenetic changes and is thus a suitable site to investigate the influence of pedogenetic processes on the fate of Cu. Samples were collected from all horizons along a trench at four distances from the drain: 0.6 m, 1.1 m, 2.1 m and 4.0 m. In the E&Bt horizon, four different soil volumes (ochre, pale brown, white-grey and black) were sampled at all four distances from the drain. Furthermore, we analyzed soil solutions sampled with piezometer, porous cups, and at the drain outlet. The Cu concentrations were lowest in the surface (Ap) horizons (6.5–8.5 μg g− 1) and increased with depth to the clay-rich Bt horizons (10.5–12 μg g− 1), because of clay eluviation and associated Cu transport. The δ65Cu values significantly decreased from the surface (Ap = − 0.25 ± 0.07‰) to the deeper horizons, but showed no significant variation among the deeper horizons (− 0.41 ± 0.28‰) and no correlation with the clay content, indicating that clay eluviation did not significantly affect δ65Cu values. The isotopically heavier δ65Cu values in the Ap horizons can probably be explained by agricultural management practices like sludge application and fertilization. Close to the drain (position 0.6 m), Cu concentrations were depleted and the lighter Cu isotope was enriched (− 0.91 ± 0.15‰) in the uppermost part of the E&Bt horizon. We attribute this to the changing redox conditions, caused by the lowering of the water level close to the drain. Copper concentrations in black and ochre volumes were significantly higher than in pale-brown and white-grey volumes. The black volume had significantly higher δ65Cu values than the ochre volume indicating preferential sorption/occlusion of the heavy Cu isotope by Fe oxides. Enhanced clay eluviation in bulk soil close to the drain and in specific soil volumes did not affect δ65Cu values. Cu concentrations (2.1–14 μg L− 1) and δ65Cu (0.04–0.42‰) values in water samples showed no clear relation with redox changes along the trench perpendicular to the drain. The enrichment of the heavy Cu isotope in the solution samples (Δ65Cu(soil-solution) = − 0.61 ± 0.41) indicates that reductive Cu mobilization is not the main driver of Cu leaching, because this would preferentially mobilize isotopically light Cu. We conclude that the eluviation of the < 2 μm fraction, strongly controlled Cu concentrations, but had no discernible effect on δ65Cu values. The changing redox conditions did not seem to control Cu concentrations and the stable isotope distribution in most of the bulk soil, soil volumes and soil water. Instead, weathering, complexation of leached Cu, Cu application with fertilizers and sorption processes within the soil controlled its δ65Cu values.

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