Quantitative imaging of the 3-D distribution of cation adsorption sites in undisturbed soil

Hannes Keck, Bjarne W. Strobel, Jon Petter Gustafsson, John Koestel
2017 SOIL Discussions  
Several studies have shown that the distribution of cation adsorption sites (CAS) is patchy at a millimeter to centimeter scale. Often, larger concentrations of CAS in biopores or aggregate coatings have been reported in the literature. This heterogeneity has implications on the accessibility of CAS and may influence the performance of soil system models that assume a spatially homogeneous distribution of CAS. In this study, we present a new method to quantify the abundance and 3-D distribution
more » ... of CAS in undisturbed soil that allows for investigating CAS densities with distance to the soil macropores. We used X-ray imaging with Ba<sup>2+</sup> as a contrast agent. Ba<sup>2+</sup> has a high adsorption affinity to CAS and is widely used as an index cation to measure the cation exchange capacity (CEC). Eight soil cores (approx. 10&amp;thinsp;cm<sup>3</sup>) were sampled from three locations with contrasting texture and organic matter contents. The CAS of our samples were saturated with Ba<sup>2+</sup> in the laboratory using BaCl<sub>2</sub> (0.3&amp;thinsp;mol&amp;thinsp;L<sup>&amp;minus;1</sup>). Afterwards, KCl (0.1&amp;thinsp;mol&amp;thinsp;L&amp;thinsp;<sup>&amp;minus;1</sup>) was used to rinse out Ba<sup>2+</sup> ions that were not bound to CAS. Before and after this process the samples were scanned using an industrial X-ray scanner Ba<sup>2+</sup> bound to CAS was then visualized in 3-D by the difference image technique. The resulting difference images were interpreted as depicting the Ba<sup>2+</sup> bound to CAS only. The X-ray image-derived CEC correlated significantly with results of the commonly used ammonium acetate method to determine CEC in well-mixed samples. The CEC of organic matter rich samples seemed to be systematically overestimated and in the case of the clay rich samples with less organic matter the CEC seemed to be systematically underestimated. The results showed that the distribution of the CAS varied spatially within most of our samples down to a millimeter scale. There was no systematic relation between the location of CAS and the soil macropore structure. We are convinced that the here proposed approach will strongly aid the development of more realistic soil system models.
doi:10.5194/soil-2017-11 fatcat:fwt4fege6rhuvotunqovxc5fzy