Bayreuther Institut für Terrestrische Ökosystemforschung
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Matschonat, G; Vogt, R: Significance of the total cation concentration in acid forest soils for the solution composition and the saturation of exchange sites, Geoderma, 84, 289-307 (1998)
To assess the ecological quality of acid forest soils, the base saturation and the solution pH, Al concentration and base cationrAl ratios are of interest. Relationships between these parameters would facilitate this assessment. Examples are empirical relationships between pH and base saturation, and between the Al concentration and the ionic strength of the soil solution. From a mechanistic point of view, the theory of cation exchange implies that the total cation concentration SCCssum of cationic charge in a key factor in soil–solution interactions. Our aim was to demonstrate that relationships with the SCC to be expected from the theory can be observed in the field. Such relationships have special relevance in view of a decrease in acid deposition in Europe, which will lower the SCC. We studied the effects of the SCC on the soil solution composition and the saturation of exchange sites in simulated batch experiments based on the Gaines and Thomas equation. The results were compared to field data from a range of soils under the influence of highEurope.and negligibleNew Zealand.atmospheric deposition. Equilibrium calculations predict an increase in the fraction of trivalent cations in solution with SCC, while the fraction of monovalent cations decreases. This pattern with SCC was observed in part of the lysimeters on a site with high deposition rates. The variation in SCC among soil solutions from New Zealand also produced the pattern, whereas this was not the case for a broad range of European soils. We hypothesize that this difference be due to the impact of acid deposition on the European soils. The most pronounced expression of the pattern was observed for lysimeter solutions from a European experimental siteSolling.under ‘clean rain’ treatment, where a large variation in SCC occurred. To estimate the development of base saturation in the European soils under conditions of decreasing deposition, we calculated exchange equilibria for European soils on the basis of soil solutions from New Zealand. The results indicated that an increase in base saturation is not to be expected, as a low total cation concentration tends to lower the base saturation. We concluded that the SCC carries important information on the composition of the soil solution and probably also on the saturation of exchange sites. The patterns created by cation exchange can be observed in natural soil solutions on different spatial scales. q1998 Elsevier Science B.V. All rights reserved. Keywords: acid deposition; aluminium; cation exchange; forest soils; ionic strength; soil solution
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