R. 1. Wright et al. (Eds.), Plant-soil interactions at low pH, 15-24.
© 1991 KhtWer Academic Publishers. PLSO AS02
Solubility of aluminium and silicon in acidified spodosols: Evidence for soluble aluminosilicate
M.G. JOHNSON and M.B. McBRIDE
Department of Agronomy, Cornell University, Ithaca, NY 14853, USA
Key words: aluminium, imogolite, labile AI, method for AI, polarography, silicon, soil acidification, speciation, spodosols, 8-hydroxyquinoline
Abstract
The speciation of soluble AI, particularly estimation of the concentration of the highly phytotoxic A13+
ion, is problematic in acid mineral soils. This investigation examines the possibility that soluble Al may be complexed with silica in spodosols, creating an error in most methods for estimating free AI3+.
Spodosol samples from the Adirondack region of New York were equilibrated either in the natural state at field moisture capacity, or after the addition of a range of strong acid concentrations. Labile and total Al were determined in the soil solutions by the 8-hydroxyquinoline complexation method, while soluble Si was determined by a procedure which estimates monomeric Si(OH)4.
Addition of strong acid to the spodosol horizons caused total soluble Al to increase, decrease, or remain unchanged, depending upon the particular horizon investigated. Decreases in soluble Al in certain B-horizon solutions were associated with concomitant decreases in soluble Si and organic matter. A test of dialyzability revealed that labile Al and "monomeric" silica in these same B-horizon solutions contained a significant fraction of high molecular weight species. The results suggested the presence of imogolite-like soluble alumino silicate complexes, which may have co-precipitated with fulvic acids upon acidification.
Solubility diagrams showed many of the E-horizon solutions to be undersaturated with respect to imogolite or gibbsite, while the B-horizon solutions tended to be saturated or supersaturated. The apparent supersaturation is attributed to the overestimation of free AI3+ by 8-hydroxyquinoline in solutions containing aluminosilicate complexes. Thus, this chelating agent is sufficiently aggressive to decompose soluble aluminosilicate complexes, and to a lesser extent, soluble AI-organic complexes.
Introduction
Increased soluble aluminium is considered to be one of the more serious potential negative effects of acid precipitation in soils. This increase in Al is thought to occur as Al is mobilized from organic and inorganic phases in soils to ground water and eventually into stream and lake water (Cronan and Schofield, 1979). In natural water systems the speciation of Al is complex, compli- cated by the presence of organic and inorganic ligands that effectively compete with H20 for Al
ligand positions. The formation of these Al com- plexes can increase the total soluble Al in natural waters.
Fluoride, hydroxyl, and sulfate are considered to be the primary inorganic ligands of Al in natural systems (Lindsay, 1979). Generally, solu- ble Si has been ignored as a potential Al ligand.
Recently, Browne and Driscoll (1989) have ex- perimentally verified the existence of soluble aluminosilicate complexes in acidic solutions pre- pared from dilute solutions of monomeric Al and monosilicic acid. Consequently, as proposed ear-
16 Johnson and McBride
lier by Farmer (1981), dissolved silica may have a very significant effect on the speciation of Al in acid soil solutions.
Many attempts have been made to arrive at an Al speciation scheme that can accurately and consistently quantify the forms of Al in aqueous media. All schemes rely upon operational defini- tions and uncertain thermodynamic data for some of the Al species (Campbell et al., 1983;
James et al., 1983). The method of James et al.
uses 8-hydroxyquinoline complexation with aqueous Al to speciate AI. The Al that is com- plexed by 8-hydroxyquinoline during a 15 second reaction time is operationally defined as 'labile', and may include more than free AI3+. Total Al is determined on a second sample that has been wet and dry ashed. The difference between labile and total Al is called 'non-labile'. Although the individual non-labile species are not differen- tiated by this method, the non-labile fraction is thought to include Al complexed with F-, or- ganic matter, silica and OH-.
Recently, Johnson and McBride (1989) have reported the presence of imogolite-like materials in the B horizons of Adirondack spodosols.
Since it has been suggested that these reactive minerals could control the chemistry of Al and Si in soil solutions (Farmer and Fraser, 1982), we investigated the solution chemistry of Al and Si in spodosols known to contain imogolite-like ma- terials in two long term equilibration studies. In one we examined the effect of acid additions upon solution chemistry and in the other we tried to carefully speciate Al and Si in soil solu- tions equilibrated with the soil solids.
Materials and methods
All soil samples used in this study were refriger- ated in the field-moist condition prior to use, and the chemistry and mineralogy of these samples are described in detail elsewhere (Johnson and McBride, 1989).
Soil acidification experiment
Five lO-gram, field moist samples of the follow- ing soil horizons were placed in 125 mL erlen- meyer flasks: Croghan (E, Bs2, BC horizons),
Adams (E, Bs horizons), Berkshire (E, Bs horizons), Potsdam (E, Bs1, BC horizons). Fifty mL of distilled water were added to each flask.
Incremental aliquots of 0.1 N HCl were added to four of the five flasks. One flask received no acid addition. Enough acid was added to give approx- imately a unit pH difference between the soil suspension receiving no acid and the one receiv- ing the most. The flasks were capped and gently shaken on a wrist action shaker for 10 days at room temperature. After the equilibration period, the supernatants were decanted, cen- trifuged at 18,000 rpm for 30 minutes, and passed through a 0.45 p,m filter. The electrical conductivities (EC) of these filtrates were mea- sured using a YSI Model 31 conductivity bridge equipped with a 3403 conductivity cell. Solution pH was recorded using an Orion Model 601 pH meter and a combination electrode. Monomeric Si was determined colorimetrically with the molybdenum blue method of Weaver et al.
(1968). Labile and total Al were measured using the 8-hydroxyquinoline method of James et al.
(1983). Dissolved organic carbon (DOC) was determined using a modified Walkley-Black dich- romate oxidation method (Nelson and Sommers, 1982).
Soil solution chemistry
Field moist soil samples were carefully placed in the upper half of 2 piece, 250 mL, plastic Buch- ner funnels, which were then placed in large beakers containing enough distilled water so that the water and soil surface were nearly at the same level. The soils were allowed to wet at room temperature until fully saturated as indi- cated by water glistening at the soil surface.
Once saturated, the soils in the Buchner contain- ers were placed in large dessicator jars and equilibrated at 100% relative humidity. Gravita- tional water was allowed to freely drain from the Buchner vessels. Approximately 100 gram sam- ples of soil were needed; only soil solutions of the Croghan (E, Bs2, BC horizons), Adams (Bs horizon), Berkshire (Bs horizon), and Potsdam (Bs1 horizons) were investigated.
After an equilibration period of two months the soil solutions were removed by securing the Buchner vessel containing the soil between two
