Abstract
Clay minerals and K feldspars were evaluated as sources of K in a Norway spruce stand (Picea abies (L.) Karst.) from the Skogaby experimental forest in southwest Sweden. The soil, developed in a Quaternary glacial till, has only 3–5% clay, and more than 95% of its K resides in feldspars. Ratios of K/Rb were assessed in interlayers of 2:1 clay minerals (extracted with hot (100 °C) 2 M HCl), biomass and the forest floor. These compartments had similarly low K/Rb ratios, whereas K feldspars were significantly poorer in Rb. A fractionation model indicated preferential retention of Rb in the biomass and forest floor, due to stronger adsorption of Rb than K in the humus, as well as preferential uptake of K from the exchange complex in the mineral soil. Preferential uptake of K may result from weaker adsorption of K by the cation exchanger, or preference for dissolved K over Rb by the roots. A quantitative mineralogical analysis revealed that loss from micas may account for half of the Holocene loss of K from the soil, which was approximately 22 Mg ha−1. Exceptionally low K/Rb ratios in HCl extracts of the upper 60 cm of the profile indicated extensive loss of K from mica in the parent material and re-fixation of K and Rb at lower ratios. The results indicate that fixation in and release from clay minerals may be prominent in the cycling of K, even in a soil that is poor in clay minerals.
Highlights
In the undisturbed forest ecosystem, nutrient supply is mainly achieved by continual recycling of elements
Forest ecosystems that are subject to harvesting of timber or biofuel, on the other hand, often show declining nutrition status, due to poor geochemistry and mineralogy that fail to compensate for the export of nutrients (Olsson et al 1993)
The mineralogical reserve of K in soils is essentially restricted to K present in feldspars and interlayer sites of micas and 2:1 clay minerals such as illite and mixed-layer minerals
Summary
In the undisturbed forest ecosystem, nutrient supply is mainly achieved by continual recycling of elements. Forest ecosystems that are subject to harvesting of timber or biofuel, on the other hand, often show declining nutrition status, due to poor geochemistry and mineralogy that fail to compensate for the export of nutrients (Olsson et al 1993). Such soils show a tendency to develop enhanced soil acidity when exposed to pollution. The mineralogical reserve of K in soils is essentially restricted to K present in feldspars and interlayer sites of micas (biotite, muscovite) and 2:1 clay minerals such as illite and mixed-layer minerals (illite/vermiculite, illite/ smectite). The presence of organic matter and hydroxide interlayers may decrease the capacity of clay minerals to fix both K? and geochemically similar cations like Rb? (Ross 1971; Chaudhuri et al 2007)
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