Al-Spezies im Sickerwasser saurer Waldböden - Einfluß von Wasserbewegung und Löslichkeitsgleichgewichten

Aqueous Aluminum Species in Acidic Forest Soils - Influence of Water Pathways and Solubility Equilibria In the seepage of three typical Black Forest soils (Haplic Podzol, Dystric Cambisol, Dystric Planosol) the fractions ‘Labile-Monomeric Al’, ‘Stabile-Monomeric Al’ and ‘Acid-soluble Al’ were analyzed. Activities of aqueous Al species and saturation indices (SI) with respect to various Al-bearing minerals were calculated from ‘Labile-Monomeric Al’, using the computer program WATEQF. Al-mobilization/immobilization processes were evaluated by means of AI/CI molar ratios. With 1.5 mg/L in average, the Al_total concentrations are relatively low in all studied soils. In the O-horizon leachates, 70 to 80% of aqueous Al occur as ‘Stabile-Monomeric’ and ‘Acid-soluble’ forms mainly consisting of organo-complexes. This portion decreases in the mineral soil to 35% in the podzol and the planosol as well as to 10% in the Cambisol. Simultaneously, Al³⁺ increases to 40% (planosol), 50 (podzol), and 70% (cambisol). In all horizons, 5 to 15% of Al_total are covered by Al-fluoride-complexes, whereas Al-sulfate-complexes are insignificant. With 5 to 10% monomeric Al-OH-ions play a role only in the subsoil. Aluminum is strongly mobilized in the upper mineral horizons of all studied soils. In the planosol and the cambisol, Al is immobilized in the subsoil. In the subsoil of the podzol, in contrast, Al reveals further mobilization due to a distinct internal production of HNO₃ and H₂SO₄ as a consequence of mineralization of organic matter. In the podzol, rapid percolation in macropores is crucial for Al dynamics, whereas in the planosol the temporal variation of the perched water table. Leachates from all O-horizons and upper mineral horizons as well as from the planosol subsoil are undersaturated with respect to the solubility of all mineral phases considered. With SI > O imogolite appears to be a permanently stable mineral in the subsoils of both podzol and cambisol. There is evidence for the Al(OH)₃ interlayer of Al-chlorites controlling Al dynamics in the subsoil of the podzol. Al(OH)SO₄ type minerals are not likely to regulate aqueous Al activities in any of the studied soils.     

Solution chemistry in acid forest soils: Are the BC : Al ratios as critical as expected in Switzerland?

The molar ratio of base nutrient cations to total dissolved aluminum (BC : Al_tot) in the soil solution was measured at six forest sites in Switzerland in acid mineral soils to determine whether the ratio measured in the field was lower than the critical value of 1, as predicted by the mapping of exceedances of critical loads of acidity. The soil chemistry was then related to the soil solution composition to characterize the typical effective base saturation (BS) and BC : Al ratio in soil leading to critical BC : Al_tot in the soil solution. The median BC : Al_tot ratio in the soil solution never reached the critical value in the root zone at any sites for the whole observation period (1999–2002), suggesting that the BC : Al_tot ratios measured in the field might be higher than those modeled for the determination of critical loads of acidity. The gibbsite model usually applied for the calculation of critical loads was a poor predictor of the Al³⁺ activity at the study sites. A curvilinear pH‐pAl³⁺ relationship was found over the whole range of pH (3.8–6.5). Above a pH of 5.5, the slope of the pH‐pAl³⁺ relation was close to 3, suggesting equilibrium with Al(OH)₃. It decreased to values smaller than 1.3 below a pH of 5.5, indicating complexation reactions with soil organic matter. The BS and the BC : Al ratios in the soils were significantly correlated to the BC : Al_tot ratios in the soil solution. The soil solutions with the lowest BC : Al_tot ratios (≤ 2) were typically found in mineral soils with a BS below 10 % and a BC : Al ratio in the soil lower than 0.2. In acid pseudogleyed horizons overlying a calcareous substrate, the soil solution chemistry was strongly influenced by the composition of the underlying soil layers. The soil solutions at 80 cm had pH values and BC : Al_tot ratios much higher than expected. This situation should be taken into account for the calculations of critical loads of acidity.     

Salts and acids as determinants of the solution composition of acidic forest soils — anion effects

The study aimed at evaluating whether salt-induced mobilization of acidity may be modified by the type of anion. For this purpose, the effects of different neutral salts on the solution composition of acid soils were investigated. The results were compared with those of the addition of acids. Two topsoil (E and A) and two subsoil horizons (Bs and Bw) were treated with NaCl, Na₂SO₄, MgCl₂, MgSO₄, HCl, and H₂SO₄ at concentrations ranging from 0 to 10 mmol dm^?3. With increasing inputs of Cl^? the pH of the equilibrium soil solution dropped, the concentrations of Al and Ca increased, and the molar Ca/(Al³⁺ + AlOH²⁺ + Al(OH)₂⁺) ratios decreased. These effects were the least pronounced when NaCl was added and the most at the HCl treatments. According to the release of acidity, the topsoils were more sensitive for salt-induced soil solution acidification whereas on base of the molar Ca/(Al³⁺ + AlOH²⁺ + Al(OH)₂⁺) ratios, the salt effect seems to be more important for the subsoils. Addition of S0₄^2? salts and H₂SO₄ induced higher pH and lower Al concentrations than the corresponding Cl^? treatments due to the SO₄^2? sorption, especially in the subsoils. The Ca/(Al³⁺ + AlOH²⁺ + Al(OH)₂⁺) ratios were higher than those of the corresponding Cl^? treatments. In subsoils even after H₂SO₄ additions these ratios were not higher than those of the NaCl treatments. The results indicate (I) that speculation about the effects of episodic salt concentrations enhancement on soil solution acidification not only need to consider the ionic strength and the cation type but also the anion type, (II) that salt-induced soil solution composition may be more crucial in subsoils than in topsoils, and (III) that in acid soils ongoing input of HNO₃ due to the precipitation load may induce an even more acidic soil solution than the inputs of H₂SO₄ of the last decade.     

Aluminium chemistry in two contrasted acid forest soils and headwater streams impacted by acid deposition,Vosges mountains,N.E. France

Al chemistry was studied in two acidic watersheds, one with a podzol, the other with an acid brown soil, in the Vosges mountains (N.E. France), by analysing both leaching and centrifugation soil solutions and spring waters over 3 yr. In the podzol, Al was mobilized in the eluvial horizons under the predominant influence of organic acidity, then leached down the profile as organic and F-bound Al. Strong undersaturation with respect to proto-imogolite and imogolite showed that the proto-imogolite theory of podzolization could not apply. Al was transferred from the soil to spring water mostly as Al³⁺ and Al-F. Al³⁺, as well as additional minor species (AlOH²⁺, AlSO₄ ⁺), originated from the redissolution of the top of the spodic horizons under the influence of both soil solution acidity and the occurrence of mobile anions derived from atmospheric deposition. Conversely, in the acid brown soil, Al mobilization was regulated by nitrate and occurred mainly as Al³⁺. Most of Al was retained in the deep soil and only traces of monomeric Al reached spring water. In the podzol eluvial horizons, soil solutions were undersaturated with respect to all relevant mineral phases and their chemical composition agree with the concept of a mobilization of Al from the solid soil organic Al and a control of Al³⁺ activity by complexation reaction with the solid and soluble soil organic matter and F. In the acid brown soil, soil solutions were found to be in equilibrium with natural alunite, and the formation of this mineral, if confirmed, would account for the occurrence of 'open' vermiculites instead of the expected hydroxy-Al interlayered vermiculites. Al solubility control in surface water of both watersheds remains unclear. The Al-F species in both watersheds and the likely control of Al solubility by alunite in the acid brown soil emphasize the influence of acid deposition on Al chemistry in acid watersheds.     

Effects of fertilization with MgSO4 and (NH4)2SO4 on soil solution chemistry,mycorrhiza and nutrient content of fine roots in a norway spruce stand

This paper focuses on the short-term reaction of fine root and mycorrhiza on changes in soil solution chemistry following application of MgSO₄ (Kieserite) and (NH₄)₂SO₄ (ammonium sulfate). The experiments were conducted within the ARINUS Experimental Watershed Area near Schluchsee in the Black Forest (SW Germany). Yellowing of the older needles as related to Mg deficiency was the typical symptom observed within this 45 yr old Norway spruce stand. On the N treated plot the relative mycorrhiza frequency declined and the percentage of nonmycorrhizal root tips increased, whereas in the Mg fertilized plot these parameters did not differ from the control. The observed changes cannot be caused by Al, because elevated concentrations of potentially toxic Al species and extremely low Ca/A1 molar ratios appeared in the soil solution of both treatments and did not result in reduced growth of long roots as reported from solution culture experiments. Moreover, the Al content of fine roots did not increase. Therefore, it is concluded that the thresholds for Al toxicity derived from solution culture experiments with nonmycorrhizal seedlings cannot be transferred to forest stands. A direct toxic effect of elevated NH₄ ⁺ concentrations on mycorrhiza is unlikely, but cannot be excluded. Enhanced root growth due to a higher uptake of NH₄ ⁺ from soil solution may provide a more plausible explanation for the observed increase in the percentage of nonmycorrhizal root tips after N application. Even though the N content of fine roots did not increase, the diminished K content gives some indirect indication for NH₄ ⁺ uptake by the roots. This is also consistent with reduced Mg content due to NH ₄ ⁺ /Mg²⁺ antagonism. On the MgSO₄ treated plot, Mg contents of the fine roots increased thus reflecting Mg uptake by the deficient stand.     

ALUMINUM SPECIATION OF AMENDED ACID TROPICAL SOIL AND ITS EFFECTS ON PLANT ROOT GROWTH

Exchangeable and soluble soil aluminum (Al) is limiting plant growth in many soils worldwide. This study evaluated the effects of increasing rates of dolomite and magnesium carbonate (MgCO₃) on Al³⁺, pH, dissolved organic carbon, cations, anions, and Al speciation on oil palm Deli dura × AVROS pisifera root growth. Dolomite and MgCO₃ additions significantly raised linearly soil solution pH, magnesium (Mg²⁺), nitrate (NO₃ ^?) and chlorine (Cl^?) concentrations; exponentially decreased the activity of phytotoxic Al species [aluminum (Al³⁺), aluminum sulfate (Al₂SO₄), and aluminum fluoride (AlF₃)]; and reduced manganese (Mn) concentration and activity. High activity of those species exponentially reduced root dry weight. Optimum oil palm growth was achieved at: <50 μM monomeric Al, < 30 μM Mn, and <0.20 unit of the ratio Al+Mn to calcium (Ca)+Mg. High activity of Al species and Mn in acidic soil solution cause significant reduction of the root growth. Soil acidity alleviation either with dolomite or MgCO₃ mitigates the toxic effect of Al and Mn.     

Calcium alleviation of hydrogen and aluminum inhibition of soybean root extension from limed soil into acid subsurface solutions

Alleviation by calcium (Ca) of inhibition of soybean [Glycine max (L.) Merr. cv. ‘Ransom'] root elongation by hydrogen (H) and aluminum (Al) was evaluated in a vertical split‐root system. Roots extending from a limed and fertilized soil compartment grew for 12 days into a subsurface compartment containing nutrient solution with treatments consisting of factorial combinations of either pH (4.0, 4.6, and 5.5) and Ca (0.2, 2.0, 10, and 20 mM), Al (7.5, 15, and 30 μM) and Ca (2.0,10, and 20 mM) at pH 4.6, or Ca (2, 7, and 12 mM) levels and counter ions (SO₄ and Cl) at pH 4.6 and 15 μM Al. Length of tap roots and their laterals increased with solution Ca concentration and pH value, but decreased with increasing Al level. Length of both tap and lateral roots were greater when Ca was supplied as CaSO₄ than as CaCl₂, but increasing Ca concentration from 2 to 12 mM had a greater effect on alleviating Al toxicity than Ca source. In the absence of Al, relative root length (RRL) of tap and lateral roots among pH and Ca treatments was related to the Ca:H molar activity ratio of solutions (R²≥0.82). Tap and lateral RRL among solutions with variable concentrations of Al and Ca at pH 4.6 were related to both the sum of the predicted activities of monomeric Al (R²≥0.92) and a log‐transformed and valence‐weighted balance between activities of Ca and selected monomeric Al species (R²≥0.95). In solutions with 15 μM Al at pH 4.6, response of tap and lateral RRL to variable concentrations of CaSO₄ and CaCl₂ were related to predicted molar activity ratios of both Ca:Al³⁺ (R²≥0.89) and Ca:3 monomeric Al (R²≥0.90), provided that AISO₄ and AI(SO₄)₂ species were excluded from the latter index. In all experiments H and Al inhibited length of lateral roots more than tap roots, and a greater Ca:H or Ca:Al concentration ratio was required in solutions to achieve similar RRL values as tap roots.     

Speciation and Solubility Control of Aluminium in Soils Developed from Slates of the River Sor Watershed (Galicia,NW Spain)

The Al species in the soid and liquid phases were studied in eight soils developed from slates in a watershed subjected to acid deposition. From soil solution data the mechanisms possibly controlling Al solubility are also discussed. The soils are acidic, organic matter rich and with an exchange complex saturated with Al. In the solid phase, more than 75% of non-crystalline Al was organo-Al complexes, mostly highly stable. In the soil solutions, monomeric inorganic. Al forms were predominant and fluoro-Al complexes were the most abundant species, except in soil solutions of pH<4.8 and Al L/F ratio >3, in which Al³⁺ predominated and sulphato-Al complexes were relatively abundant. The most stable phases were kaolinite, gibbsite and non-crystalline Al hydroxides. In most samples, Al solubility was controlled by Al-hydroxides. Only in a few cases (solutions of pH 4-5, Al³⁺ activity >40 µmol L-¹ and SO₄ content >200 µmol L-¹), Al-sulphates such as jurbanite also could exert some control over Al solubility. In adition to these minerals, a possible role of organo-Al complexes or the influence of adsorption reactions of sulphate is considered, especially for samples with very low Al³⁺ content (<0.5 µmol L-¹).     

Influence of surface‐applied poultry manure on topsoil and subsoil acidity and salinity: A leaching column study

It has been suggested that surface applications of animal manure can ameliorate both top and subsoil acidity. For that reason, the effects of surface incorporation (0–5 cm) of a high rate of poultry manure to an acid soil on pH and exchangeable and soluble Al in the top‐ and subsoil were investigated in a leaching column study. During the experimental period of 108 d, columns received a total of 875 mm with leaching events occurring after 9, 37, 58, and 86 d. Incorporation of poultry manure into the surface 5 cm resulted in a large rise in pH measured in both 1M KCl and in soil solution. This liming effect was attributed primarily to the substantial CaCO₃ content of poultry manure. In the 15–45 cm layer, pH_KCl was not significantly different between poultry manure and control treatments but surprisingly, soil‐solution pH was substantially less in the poultry‐manure treatments. Exchangeable Al was significantly less in poultry manure than in control in all soil layers although the effect was most marked in the 0–5 cm layer. However, although concentrations and activities of monomeric Al (Al_mono), and the proportion of total Al present as Al_mono, in soil solution were lower under poultry manure than in control in the 0–5 cm layer, the reverse was, in fact, the case in lower soil horizons. This was attributed to a soluble‐salt effect, originating from the large cation content of poultry manure, displacing exchangeable Al³⁺ and H⁺ back into soil solution. Indeed, electrical conductivity and concentrations of Ca²⁺, Mg²⁺_, K⁺, and Na⁺ in soil solution were substantially higher in the poultry‐manure than in the control treatments at all soil depths. Poultry‐manure applications also resulted in substantial increases in the concentrations of Ca²⁺_, Mg²⁺_, K⁺, Na⁺, Al_mono, NH , and NO in leachates, particularly at the fourth leaching. It was concluded that although surface application of poultry manure can raise soil pH in the topsoil, increases in soluble‐salt concentrations in soil solution can greatly modify this effect in the subsoil.     

Influence of the Soil Solution Composition on Retention and Release of Sulfate in Acid Forest Soils

The potential for sulfate retention is an important soil feature for buffering of atmospheric acid deposition. We studied the effects of increasing additions of different neutral salts and acids on mobilization and retention of SO₄ ²- in acid forest soils. Soils containing up to 11 mmol SO₄ ²- kg^-1 were equilibrated with H₂O, NaCl, MgCl₂, and HCl. Release of SO₄ ²- was highest with H₂O and NaCl additions and lowest when HCl was used. Increasing the ionic strength of the added solutions caused decreasing SO₄ ²- concentrations in equilibrium solution. Decreasing pH in equilibrium solution was found to be the reason for the decrease in release. Even when the pH was < 4, the SO₄ ²- release decreased. We assume that this finding resulted from the fact that in the soils studied the SO₄ ²- sorption was controlled by the high contents of Fe oxides/hydroxides. Experiments with Na₂SO₄, MgSO₄, and H₂SO₄ demonstrated that the B horizons already containing high amounts of SO₄ ²- were still able to retain SO₄ ²-. Sulfate retention increased in the order Na₂SO₄ < MgSO₄ < H₂SO₄, which corresponds to increasing H⁺ availability. The higher SO₄ ²- retention along with MgSO₄ compared to Na₂SO₄ may be caused by higher potential of Mg to mobilize soil acidity compared to Na.     

Biogeochemistry of aluminum in a forest catchment in the Czech Republic impacted by atmospheric inputs of strong acids

The Lysina catchment in the Czech Republic was studied to investigate the biogeochemical response of Al to high loadings of acidic deposition. The catchment supports Norway spruce plantations and is underlain by granite and podzolic soil. Atmospheric deposition to the site was characterized by high H⁺ and SO₄ ^2– fluxes in throughfall. The volume-weighted average concentration of total Al (Al_t) was 28 mol L^–1 in the O horizon soil solution. About 50% of Al_t in the O horizon was in the form of potentially-toxic inorganic monomeric Al (Al_i). In the E horizon, Al_t increased to 71 mol L^–1, and Al_i comprised 80% of Al_t. The concentration of Al_t (120 mol L^–1) and the fraction of Al_i (85%) increased in the lower mineral soil due to increases in Al_i and decreases in organic monomeric Al (Al_o). Shallow ground water was less acidic and had lower Al_t concentration (29 mol L^–1). The volume-weighted average concentration of Al_t was extremely high in stream water (60 mol L^–1) with Al_i accounting for about 60% of Al_t. The major species of Al_i in stream water were fluorocomplexes (Al-F) and aquo Al³⁺. Soil solutions in the root zone were undersaturated with respect to all Al-bearing mineral phases. However, stream water exhibited Al_i concentrations close to solubility with jurbanite. Acidic waters and elevated Al concentrations reflected the limited supply of basic cations on the soil exchange complex and slow weathering, which was unable to neutralize atmospheric inputs of strong acids.     

Effects of H2SO4 and HNO3 on Soil Acidification and Aluminum Speciation in Variable and Constant Charge Soils

The difference in effect on acidification and species of aluminum speciation between HNO₃ and H₂SO₄for two contrasting types of soils in surface charge was investigated. The results show that the effect of H₂SO₄ on acidification of variable charge soils (Ferric Acrisol and Haplic Acrisol) of subtropical regions wasweaker than that of HNO₃, due to the specific adsorption of SO₄ ^2- and the accompanied release of OH^-. For two constant charge soils, Haplic Luvisol andEutric Cambisol, the difference in effect between the two acids is small. The concentrations of total inorganic monomeric aluminum, Al³⁺, Al-F complexes and Al-SO₄complexes in the extract from variable charge soils are alsolower in H₂SO₄ systems than those in HNO₃ systems, whereas the reverse is true for constant charge soils, except the concentration of Al³⁺. For variable charge soils, Al-F complexes are the major species of inorganic monomeric aluminum at high pH butAl³⁺ might contribute to a largepart at low pH, while for constant charge soils Al-F complexes contribute almost exclusively to the inorganic monomeric aluminum. The presence of a large amount of SO₄ ^2- in the extract from the constant charge soilsleads to a higher proportion of Al-SO₄ complexes in totalinorganic monomeric aluminum than that from variable charge soil, although the numerical value of proportion is small.     

Increases in pH and soluble salts influence the effect that additions of organic residues have on concentrations of exchangeable and soil solution aluminium

It has been suggested that additions of organic residues to acid soils can ameliorate Al toxicity. For this reason the effects of additions of four organic residues to an acid soil on pH and exchangeable and soil solution Al were investigated. The residues were grass, household compost, filter cake (a waste product from sugar mills) and poultry manure, and they were added at rates equivalent to 10 and 20 t ha^?1. Additions of residues increased soil pH measured in KCl (pH_(KCl)) and decreased exchangeable Al³⁺ in the order poultry manure > filter cake > household compost > grass. The mechanism responsible for the increase in pH differed for the different residues. Poultry manure treatment resulted in lower soil pH measured in water (pH_(water)) and larger concentrations of total (Al_T) and monomeric (Al_mono) Al in soil solution than did filter cake. This was attributed to a soluble salt effect, originating from the large cation content of poultry manure, displacing exchangeable Al³⁺ and H⁺ back into soil solution. The considerably larger concentrations of soluble C in soil solution originating from the poultry manure may also have maintained greater concentrations of Al in soluble complexed form. There was a significant negative correlation (r = ?0.94) between pH_(KCl) and exchangeable Al. Concentrations of Al_T and Al_mono in soil solution were not closely related with pH or exchangeable Al. The results suggest that although additions of organic residues can increase soil pH and decrease Al solubility, increases in soluble salt and soluble C concentrations in soil solution can substantially modify these effects.     

Aluminium speciation: Variations caused by the choice of analytical method and by sample storage

We have compared a new FIA method (M1) for aluminium speciation in natural waters and a manual one based on one of Driscoll's proposals (M2). In synthetic solutions, aluminium's fluoro complexes were measured as ‘labile monomeric AP (Al _i ) by M2, but not as ‘quickly reacting Al’ (Al _qr ) by M1. Aluminium's complexes with Nordic Reference Humic Acid were measured neither as Al _i nor as Al _qr , and the same result was obtained for Al's citrato complexes. After excluding aluminium's fluoro complexes from Al _i , the results of the two methods agreed well for soil leaching samples and fairly well for natural water samples. Detection limits: 10μg L^?1 (ca. 0.4 μM) for both methods. Sample throughputs: M1∶66 injections h^?1; M2∶5 samples h^?1. Repeatabilities (RSD) on natural water samples: M1∶0.6–5.8% and M2 0.7–4.6%. Sample storage studies on soil solutions (FIA method) indicated that storage effects were ‘sample type specific’. A sample with a low level of Al _qr and a high DOC level (P2A) was more sensitive to storage than one with a high [Al] _qr and a low [DOC] (P2B^*). The decrease in [Al] _qr was statistically significant after 10 h (P2A) and 24 h (P2B^*), respectively. After 3 days' storage, [Al] _qr had decreased by some 40% in P2A, while the decrease in P2B^* was less than 10% after 8 days. The results of this part of the study also emphasize the importance of careful method standardization in sample storage studies.     

Soil properties that affect sulphate adsorption by palexerults in western and central Spain

Abstract

The beneficial action of gypsum in suppressing aluminum (Al) toxicity in Bt horizons of Ultisols is related to the self‐liming effect of the adsorption of sulphate (SO₄ ^2‐) ion. The relationship between SO₄ ^2‐ adsorption by gypsum‐amended soils and some components and properties of 38 surface and subsurface horizons from seven Palexerults in western and central Spain was analyzed. The highest correlations of maximal SO₄ ^2‐ adsorption as determined from langmuir isotherms were with clay, free iron oxyhydroxides (Fe_dcb), and exchangeable Al contents, and pH. Liming reduces SO₄ ²’ ion adsorption; consequently, the joint application of limestone and gypsum to the surface of these soils results in increased availability of gypsum for the subsurface horizons.     

The solubility of aluminium in two Swedish acidified forest soils: An evaluation of lysimeter measurements using batch titration data

This paper presents aluminium (Al)-solubility data for two acid forest soils (Inceptisol and Spodosol), obtained in connection with lysimeter measurements (tension-cup and zero-tension lysimeters) and batch equilibrium experiments. The solubility of Al obtained in the batch experiments was used as a reference to test whether Al³⁺in soil solutions collected by the lysimeters was in equilibrium with secondary forms of solid-phase Al (Al(OH)₃or organically bound Al). The relation between pH and Al³⁺activity found for the zero-tension lysimeter solutions collected from the Inceptisol agreed well with that obtained in the batch experiment. This suggests that Al³⁺in the lysimeter solutions were in, or close to, equilibrium with the solid phase, whether this was organically bound Al (A horizon) or an Al(OH)₃phase (B horizon). For the tension-cup lysimeters, solutions obtained from the Inceptisol B and Spodosol Bs1 horizons were generally close to equilibrium with respect to secondary solid-phase Al (apparently Al(OH)₃; average ion activity product was 10^9.3and 10^8.8, respectively), whereas the Inceptisol A and Spodosol Bh solutions were not. The Al solubility in Inceptisol A and Spodosol Bh horizons was consistently higher than that obtained in the batch equilibrium experiment, indicating that the sampled solution partly originated from the underlying horizons. Thus, tension-cup lysimeters should be used with care in soils (or in parts of soil profiles) having steep solute concentration gradients because the soil volume from which the sample is drawn with this lysimeter type seems to be poorly defined.     

Chemical effects of pH 3 sulphuric acid on a soil profile

Cores of podzolic soil (monolith lysimeters) were treated for 4.8 yr with 1500 mm yr^?1 of either 0.5 mM H₂SO₄ at pH 3, equivalent to 24 g S m^?2 yr^?1 (acid treated) or distilled water (controls). The acid treatment was about 37 times greater than the average annual input of H₃O⁺ from rain at the site from which the monoliths were taken. Acid treatment acidified the litter (from pH(CaCl₂)3.4 to pH(CaCl₂)2.6) and the mineral soil to a depth of 80 cm (mean pH(CaCl₂) decrease of 0.2 unit). In the litter and upper A horizon, ion-exchange reactions provided the main neutralizing mechanism, resulting in a decrease in the reserves of extractable (in 2.5 % acetic acid) Ca, Mg, and Mn of about 70 to 80 %. Dissolution of solid phase Al from hydrous oxides provided most neutralization below this depth. Al3⁺ was the principal soluble Al species throughout the profile. In the litter and upper A horizon, some of the mobilized Al³⁺ was retained on cation exchange sites resulting in an increase in exchangeable Al. Deeper in the profile, where the exchange sites were effectively saturated with Al³⁺, no increase in exchangeable Al occurred, and Al³⁺ was, therefore, available for leaching. Some reversible adsorption of SO₄ ^2?, associated with hydrous Al oxides, occurred in the Bs and C horizons. The results are discussed in relation to possible effects of acid deposition over regions of Europe and N. America.     

Soil tests for aluminum toxicity in the presence of organic matter: Laboratory development and assessment

Abstract

Al toxicity in plants is related to the activity of Al³⁺ and Al‐hydroxy monomers in the soil solution, whereas Al complexed with ligands such as fluoride (F), sulphate (SO₄ ^2‐), and oxalate is not toxic. Estimation of toxic Al relies on measurement of “labile”; Al after short contact times with colorimetric reagents or cation‐exchange resins. However, shifts in equilibrium may result in non‐toxic forms of Al reacting with the complexing agent or resin.

A series of laboratory experiments tested the degree to which labile Al is related to Al³⁺ in simplified media and compared methods of estimating labile Al in the presence of organic ligands and in soils. Cation‐exchange resins extracted more than the theoretical concentration of Al³⁺ from solutions containing a range of concentrations of OH^‐ and SO₄ ^2‐. More Al was extracted in 15 s by 8‐hydroxy‐quinoline than by Chelex‐100 from solutions of Al‐humate at pH 4. In sands which had been spiked with Al and organic matter, the estimation of labile Al varied with both the method of measurement and type of extract. The cations present in commonly used soil‐extracting chloride solutions can decrease the proportion of organically complexed Al.     

Aluminum compounds in soil solutions and their migration in podzolic soils on two-layered deposits

The fractional composition of aluminum compounds was studied in soil solutions obtained using vacuum lysimeters from loamy podzolic soils on two-layered deposits. The concentration of aluminum was estimated in brooks and a river draining the area with a predominance of these soils. In soil solutions, the concentration of aluminum was experimentally determined in the following compounds: (1) organic and inorganic monomers, (2) stable complexes with HAs and FAs together with polymers, and (3) the most stable complexes with HAs and FAs together with fine-crystalline and colloidal compounds. The total Al concentration in soil solutions from forest litter was 0.111–0.175 mmol/l and decreased with depth to 0.05 mmol/l and lower in solutions from the IIBD horizons. More than 90% of the Al in the solutions was bound into complexes with organic ligands. Some amount of Al in solution could occur in aluminosilicate sols. The translocation of Al complexes from the litter through the AE horizon to the podzolic horizon was accompanied by an increase in the ratio between the Al concentration in fraction 2 and the C concentration in the solution. The concentrations of Al_tot in the surface waters varied in the range from 0.015 to 0.030 mmol/l. Most of the Al came to the surface waters from the litter and AE horizons and partially from the podzolic horizon due to the lateral runoff along the waterproof IIBD horizon. Approximate calculations showed that the recent annual removal of Al from the AE and E horizons with the lateral runoff was 7 to 560 mg (3–21 mmol) from 1 m².     

Micro-scale variation of solid-phase properties and soil solution chemistry in a forest podzol and its relation to soil horizons

In order to evaluate micro-scale heterogeneities 55 micro suction cups were placed in an array at 15 mm intervals in a profile face of a cambic podzol. The chemistry of soil solution (mineral anions, pH, UV absorption as a measure for DOC) was compared with solid-phase properties from soil samples (2 cm³ volume), which had surrounded the suction cups. Sequential extraction techniques (water, NF₄Cl, hydroxylamin-hydrochloride, citrate-bicarbonate, oxalate, dithionite-citrate-bicarbonate) and base titrations were applied to characterize the solid phase. Although the average soil solution concentrations between horizons often differed significantly, the spatial distributions of pH and SO₄^2? did not correlate with soil horizon borders. Even if concentration isolines and soil horizon borders were parallel, marked concentration gradients could be observed within individual soil horizons. The less intense the interaction between solute ion and soil matrix, the greater was the variation in solution concentration within a soil horizon. For the soil solid phase only a weak correlation of slow buffer reactions to soil horizons was found. The distribution of extractable Fe and Al was typical for a podzol profile, however, with very steep gradients within single soil horizons. Except for pH, which was related mainly to citrate-bicarbonate extractable aluminium, no solid-phase characteristic showed a clear correlation with soil solution chemistry.