Journal cover Journal topic
SOIL An interactive open-access journal of the European Geosciences Union
doi:10.5194/soil-2017-11
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Original research article
03 Apr 2017
Review status
This discussion paper is under review for the journal SOIL (SOIL).
Quantitative imaging of the 3-D distribution of cation adsorption sites in undisturbed soil
Hannes Keck1,2, Bjarne W. Strobel2, Jon Petter Gustafsson1, and John Koestel1 1Department of Soil and Environment, Swedish University of Agricultural Sciences, P.O. Box 7014, 750 07 Uppsala, Sweden
2Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark
Abstract. 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 of CAS in undisturbed soil that allows for investigating CAS densities with distance to the soil macropores. We used X-ray imaging with Ba2+ as a contrast agent. Ba2+ 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 cm3) were sampled from three locations with contrasting texture and organic matter contents. The CAS of our samples were saturated with Ba2+ in the laboratory using BaCl2 (0.3 mol L−1). Afterwards, KCl (0.1 mol L −1) was used to rinse out Ba2+ ions that were not bound to CAS. Before and after this process the samples were scanned using an industrial X-ray scanner Ba2+ bound to CAS was then visualized in 3-D by the difference image technique. The resulting difference images were interpreted as depicting the Ba2+ 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.

Citation: Keck, H., Strobel, B. W., Gustafsson, J. P., and Koestel, J.: Quantitative imaging of the 3-D distribution of cation adsorption sites in undisturbed soil, SOIL Discuss., doi:10.5194/soil-2017-11, in review, 2017.
Hannes Keck et al.
Hannes Keck et al.
Hannes Keck et al.

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Short summary
Several studies have shown that the cation adsorption sites in soils are heterogeneously distributed in space. However, in many soil system models this knowledge is not included yet. In our study we proposed a new method to map the 3-D distribution of cation adsorption sites in undisturbed. The method is based on three dimensional X-ray scanning with a contrast agent and image analysis. We are convinced that this approach will strongly aid the development of more realistic soil system models.
Several studies have shown that the cation adsorption sites in soils are heterogeneously...
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