Aluminium solubility mechanisms in moderately acid Bs horizons of podzolized soils

Abstract

Summary The processes controlling the retention and release of aluminium in acid forest soils are still subject to controversy, and therefore a universal hypothesis as to what mechanisms are operating has not been firmly established. By studying the Bs horizons of Swedish and Swiss podzolized soils, and by analysing data in the literature, we have found that aluminium hydroxide, and in some cases also poorly ordered imogolite, may control Al solubility in moderately acid (pH > 4.2–4.3) Bs horizons. The strongest evidence in support of the presence of a quickly reacting Al(OH)3 pool came from the temperature dependence of Al solubility in a Bs horizon, which was consistent with the reaction enthalpy of an Al(OH)3 phase such as gibbsite, and from the observation that the ion activity product for Al(OH)3 was the same regardless of whether equilibrium was reached from over‐ or undersaturation. The pool of Al(OH)3 is commonly small and may be completely dissolved after large additions of acid. This may be explained by the continuing redissolution of reactive Al(OH)3 to form less soluble imogolite‐type phases. By using the same methods it was found that soil suspensions did not reach equilibrium with poorly ordered imogolite even after 17 days. Thus, imogolite probably does not control Al solubility in the short term in many soils despite the common occurrence of this mineral. This is due to the relatively slow kinetics of imogolite formation and dissolution, especially at low temperatures and at small solution H4SiO4 concentrations.

From soil water to surface water – how the riparian zone controls element transport from a boreal forest to a stream

“…The difference between the stream water and the uphill soil water is remarkable given that this Podzol profile should be reasonably representative for the vast majority of soils throughout the catchment. For example, rapid degradation of TOC and subsequent precipitation of Fe, Al and associated elements are typical processes in the development of Podzols (Gustafsson et al, 2001;Sauer et al, 2007;Sundman et al, 2014). This illustrates that the riparian zone has a profound impact also on the stream water chemistry for a wide range of elements, particularly those with a strong affinity for organic matter (Fig.…”

“…Hence, the pH gradient experienced by the substances transported from the uphill Podzol to the riparian zone is approximately in the order of 0.5-1 pH unit. This shift in pH may, however, have palpable consequences for the speciation of many metals and the solubility of solid phases such as Al(OH) 3 and various Fe precipitates (Gustafsson et al, 2001;Sauer et al, 2007;Sjostedt et al, 2010), especially when combined with the increase in TOC (Fig. 2).…”

“…ilar patterns were, however, absent in S22, so the mobility of Al in these inorganic soils was evidently controlled by other processes. For instance, it has been shown that the mobility of Al in the Bs horizon of Podzol soils can be limited by the precipitation of Al(OH) 3 and in some cases imogolite (Gustafsson et al, 2001). Hence, while the large differences in TOC concentrations between the uphill mineral soils and the riparian zone were responsible for the elevated levels of many elements in the riparian soil water, this does not necessarily imply that it also controls their mobility in each of the investigated soil layers.…”

Increasing silicon concentrations in Bohemian Forest lakes

“…The effect of a temperature increase of ~0.7 °C on the solubility of amorphous silica and phytoliths suggests an increase in Si of only about 2% for standard heats of dissolution, DH o r = 15 (amorphous silica) and 22 (phytoliths) kJ mol 1 (Farmer et al, 2005;Gustafsson et al, 2001). Increased temperature also enhances evapotranspiration and vegetative cycling of Si.…”

Interaction of Brilliant Blue dye solution with soil and its effect on mobility of compounds around the zones of preferenial flows at spruce stand

“…By "anorganic" podzolization theory (Anderson et al 1982;Farmer & Lumsdon 2001) inorganic Al, Si and Fe are translocated via (proto)imogolite sols from the eluvial to the illuvial horizons, where its precipitation in the form of immogolite, causes the development of Bh and Bs horizons. Gustafsson et al (2001) propose immobilization of Al in B horizons in the form of inorganic precipitates at the higher pH values (> 4.2), however, they suggest that precipitation is induced by the equilibrium of dissolved Al with Al(OH) 3 (s). Lundström et al (2000), in contrast with the classical fulvate theory propose that complexation of Al and Fe to low-molecular weight (LMW) organic acids plays a main role in the mobilization of Al, Fe and organic matter from podzol E horizons and microbial degradation of the LMW organic in the soil causes precipitation of inorganic Al and Fe complexes.…”