Epsilon Archive for Student Projects

Abstract

Most soil system models assume that the cation adsorption sites in soils are accessible and homogeneously distributed in space. This assumption is likely to be invalid, which may have significant consequences on the performance of these models. The cation exchange capacity (CEC) as a measure of the abundance of cation adsorption sites is commonly positively correlated with the soils' clay and organic matter content. Several studies have given evidence of spatially varying contents of soil organic matter and clay in the sheaths of biopores or as illuvial clay accumulation. This suggests that cation adsorption sites are not homogeneously distributed and not equally accessible. In order to investigate the 3D distribution of the cation adsorption sites in undisturbed soils, eight small soil cores (approx. 30 cm³) were sampled from different locations. Their cation adsorption sites were saturated with Ba²⁺ in the laboratory. Ba²⁺ is widely used as index cation for standard procedures to measure the CEC and as a contrast agent in X-ray scanning. Before and after Ba²⁺ saturation the samples were scanned using an industrial X-ray scanner. 3D difference images of the soil cores were obtained by subtracting the first from the second image. These were interpreted as depicting the Ba²⁺ bound to cation adsorption sites only. The results show that the cation adsorption sites are heterogeneously distributed in space. One sample showed an increase in cation adsorption sites in biopore sheaths as compared to the soil matrix, others did not show a clear relation of the location of cation adsorption sites to pore structure. Our method correlates significantly with results of the commonly used ammonium acetate method to determine CEC and with a geochemical model describing the CEC for each sample. For the first time we were able to visualize the heterogeneous distribution of cation adsorption sites in undisturbed soil.