Copper solution geochemistry in arable soils: Field observations and model application

The solubility and vertical displacement of Cu in near neutral arable soils largely depends on the presence of dissolved organic carbon (DOC) in solution. In this paper the solubility of Cu in sandy soils is predicted by a previously calibrated model that describes Cu binding to both low and high molecular weight components of DOC. Soil solution samples were obtained by centrifugation of field moist soil. Dissolved Cu concentrations increased with DOC and total soil Cu content, but decreased with organic matter and pH; after lime application, both DOC and Cu solubility were strongly reduced in the CuSO₄-treated soils. No single multiple linear regression equation could be obtained, however, that was able to describe the dissolved Cu concentrations in the various treatments. In contrast to this, a two-species Langmuir model enabled us to describe the dissolved Cu concentrations in soil solution samples quite well in the pH range from 4.2 to 6 in both contaminated and noncontaminated soils (R² = 0.5-0.9 for various treatments). Measured Cu concentrations were generally underpredicted by the model below pH 4.2. In the manured soil, the predictive capacity of the model decreased with depth (R² = 0.6 in the topsoil to 0.3 in the deeper soil horizons), which is most likely related to changes in the nature of DOC. The use of a relatively simple model that requires only a few input parameters may prove useful for the prediction of the solubility of Cu under field conditions.