The long-term sustainability of forest soils may be affected by the retention of exchangeable nutrient cations such as Ca 2þ and the availability of potentially toxic cations such as Al 3þ . Many of our current concepts of cation exchange and base cation saturation are largely unchanged since the beginnings of soil chemistry over a century ago. Many of the same methods are still in use even though they were developed in a period when exchangeable aluminium (Al) and variable charge were not generally recognized. These concepts and methods are not easily applicable to acid, highly organic forest soils. The source of charge in these soils is primarily derived from organic matter (OM) but the retention of cations, especially Al species, cannot be described by simple exchange phenomena. In this review, we trace the development of modern cation exchange definitions and procedures, and focus on how these are challenged by recent research on the behaviour of acid forest soils. Although the effective cation exchange capacity (CECe) in an individual forest soil sample can be easily shown to vary with the addition of strong base or acid, it is difficult to find a pH effect in a population of different acid forest soil samples. In the very acidic pH range below ca 4.5, soils will generally have smaller concentrations of adsorbed Al 3þ . This can be ascribed to a reduced availability of weatherable Al-containing minerals and a large amount of weak, organic acidity. Base cation saturation calculations in this pH range do not provide a useful metric and, in fact, pH is modelled better if Al 3þ is considered to be a base cation. Measurement of exchangeable Al 3þ with a neutral salt represents an ill-defined but repeatable portion of organically complexed Al, affected by the pH of the extractant. Cation exchange in these soils can be modelled if assumptions are made as to the proportion of individual cations that are non-specifically bound by soil OM. Future research should recognize these challenges and focus on redefining our concepts of cation retention in these important soils.
Reco6ery of soil pH, cation-exchange capacity and the saturation of exchange sites from stemflow-induced soil acidification in three Swedish beech (Fagus syl6atica L.) forests.Stemflow water acidifies the soil in beech stands impacted by atmospheric deposition. To investigate whether the soil recovers from acidification, stemflow was experimentally removed. A horizon material was sampled at a distance of 10 -250 cm from the stems. Before the onset of the experiment, there were stemflow-induced gradients in the saturation of exchange sites with K + , H + and Na + that were larger near the stems, while the pH KCl , the cation-exchange capacity, and the saturation with Ca 2 + , Mg 2 + and Mn 2 + were smaller. After 8 yrs of recovery, the pH KCl and the saturation with Ca 2 + and Mg 2 + had increased close to the stems, while the saturation with Na + , H + , Mn 2 + and Fe 2 + and the C/N ratio had decreased. With some exceptions, e.g. base saturation, the recovery was not complete after 8 yrs. Soil far from stems had also changed similarly, probably because of the ongoing decrease in overall deposition in southern Sweden.
ZusammenfassungFourteen European forest soils from the boreal to the mediterranean climate on different parent materials were investigated with respect to their ability to store NH4+ in exchangeable form, using sorption isotherms. Distribution coeflicients for NH4' sorption per unit weight of soil were in the range of 0.02 to 0.77. NH4+ sorption coemcients were usually highest in the forest floor of a given soil.NH4' sorption behaviour of mineral soil horizons was correlated to soil parameters that are determined during routine soil analysis. A combination of CEC and base saturation explained up to 95% of the variability of NH4+ sorption. In the forest floors, variability in NH4+ sorption could not be explained quantitatively from independent soil parameters. The affinity of the sorption sites for NH,' was the most important factor for explanation of the variability in NH4+ sorption in the forest floors but was of low importance in mineral soil horizons.As NH4' exchanges predominantly base cations, susceptibility of NH4+ to transport through the soil profile increases with low base saturation of a soil as well as with low CEC values.
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