Mineralogy and biogeochemistry of potassium in the Skogaby experimental forest, southwest Sweden: pools, fluxes and K/Rb ratios in soil and biomass

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.

Base cations in the soil bank. Non-exchangeable pools may sustain centuries of net loss to forestry and leaching

“…Ca from Ca-oxalate; Dauer et al 2014), secondary minerals (e.g. K from clay interlayers; Simonsson et al, 2016) and relatively labile or easily weathered primary minerals (e.g. Ca from apatite, Mg from biotite; Blum et al, 2002;Lucash et al, 2012).…”

“…Given the large size and potential to buffer against leaching and many forest harvest rotations of net base cation loss, understanding the chemical nature and availability of the HCl-extractable pool is important to forming management 25 recommendations. Extractions with strong acids at moderate (0.05 M -1 M) concentrations, as was performed in this study, have been used to selectively extract calcium oxalate (Dauer and Perakis, 2014;Cromack et al, 1979), apatite (Blum et al, 2002;Nezat et al, 2007), nonexchangeable clay interlayer K (Simonsson et al, 2016;Li et al, 2015), and Fe-and Mn-oxides (Krasnodębska-Ostręga et al, 2001) from soils.…”

“…in particular, the amount of minerals which are likely reservoirs of interlayer K (Simard et al, 1989). Indeed, Mengel and Rahmatullah (1994) observed that the clay interlayer K pool was both large and increasingly important for plant uptake in coarser soil mixtures as clay content decreased, due to the relative reduction in exchangeable K, and Simonsson et al (2016) showed that clay interlayer K may constitute a very large pool of K ( > 20 X exchangeable K) even when clay contents were quite low (3-5%). We observed that HCl-extractable K in the E horizon was larger than that of the B horizon and similar in size to the C horizon in of the 3 soils (upslope and midslope).…”

“…However, more research is needed to determine what controls the availability of acid-extractable non-exchangeable pools and the rate of flux into exchangeable pools. Simonsson et al, (2016) used K/Rb ratios in different soil extracts to examine the flux rates between HClextractable putatively clay-interlayer K and exchangeable K, and concluded that there was active and continuous exchange between the exchangeable and the "nonexchangeable" HCl-extractable K pools during a single forest rotation. A number of studies have used stable or radio-isotope tracers in plant-soil systems to examine the relative contributions of different soil pools to plant uptake.…”

Base cations in the soil bank: non-exchangeable pools may sustain centuries of net loss to forestry and leaching

“…Aqua regia, HF, or lithium metaborate fusion extract the total or near total reserves of elements, which may then be used to (i) estimate the relative distribution of minerals in the soil (Posch and Kurz, 2007) and (ii) estimate the mineral weathering flux based on assumptions of weathering kinetics derived from laboratory dissolution experiments (Warfvinge and Sverdrup, 1992). Moderately concentrated (defined here as 0.1 M-1 M) strong acid extractions (primarily HCl, but also HNO 3 ) have been used to extract nutrient elements from soil organic matter (e.g., Ca from Ca oxalate; Dauer and Perakis, 2014), secondary minerals (e.g., K from clay interlayers; Simonsson et al, 2016) and relatively labile or easily weathered primary minerals (e.g., Ca from apatite, Mg from biotite; Blum et al, 2002;Lucash et al, 2012). However, these extractions are non-specific and will dissolve a range of secondary minerals, likely releasing microbial nutrient pools and partially dissolving non-target granite-derived tectosilicate minerals.…”