Sulfate Retention Research Articles

Sulfate adsorption and microbial immobilization in northern hardwood forests along an atmospheric deposition gradient

While a number of studies have investigated adsorption and microbial immobilization as sulfate (SO42−) retention mechanisms, few have investigated these processes under field-like conditions on a regional, ecosystem basis. Adsorption and microbial immobilization of SO42− were studied in four northern hardwood stands that span an atmospheric deposition gradient in the Lake States region (5 to 10 kg S•ha−1•year−1). Soil cores collected in spring, summer, and autumn were labeled with 35SO42− to trace the flux of S between physical and biological sinks, and to investigate seasonal variation in sink strength. Intact soil cores were injected with Na235SO4 and incubated for 8 d in the laboratory at field temperature to study rates of adsorption and microbial immobilization. The amount of 35S recovered within these pools was significantly different between surface and subsurface soil horizons. Microbial immobilization was the dominant S sink in the A + E horizon, whereas adsorption was the most important S sink in the B horizon. During the 8-d incubation, the proportion of 35S that was immobilized in the A horizon (49% of applied 35S) was equivalent to the proportion of 35S adsorbed in the B horizon (47% of applied 35S). Microbial immobilization sequestered an additional 25% of the applied 35S in the B horizon. Adsorption and microbial immobilization were not significantly different among sampling dates. Sulfur retention in forested ecosystems should be viewed as a combination of geochemical and microbially mediated processes. However, given current levels of S deposition at these sites, neither process seems to represent a significant mechanism for long-term S retention.

Sulphur mineralization and release of soluble organic sulphur from camp and non-camp soils of grazed pastures receiving long-term superphosphate applications

Topsoils (0–75 mm) from four soil types with different sulphate retention capacities were collected from stock camp and non-camp (main grazing area) sites of grazed pastures in New Zealand which had been annually fertilized with superphosphate for more than 15 years. These soils were analysed for different S fractions and incubated at 30°C for 10 weeks using an open incubation technique in order to assess the extent of S mineralization and the release of soluble soil organic S from camp and non-camp soils during incubation. The soils were preleached with 0.01 M KCl, followed by 0.04 M Ca(H2PO4)2 before being incubated. Pre-incubation leachates and weekly 0.01 M KCl leachates were analysed for mineralized S (i.e., hydriodic acid-reducible S) and total S. Soluble organic S was estimated as the difference between these two S fractions. Results obtained show higher cumulative amounts of all three S fractions in leachates over a 10-week incubation period in camp than in non-camp soils, suggesting that higher mineralization occurred in camp soils. Cumulative amounts of mineralized S from camp and non-camp soils showed a linear relationship with duration of incubation (R 2≥0.985***), while the cumulative release of soluble organic S followed a quadratic relationship (R 2≥0.975***). A significant proportion (14.6%–40.8%) of total S release in KCl leachates was soluble organic S, indcating that organic S should be taken into account when assessing S mineralization. Mineralized S and soluble organic S were best correlated with 0.01 M CaCl2-extractable soil inorganic S (R 2=0.767***) and 0.04 M Ca(H2PO4)2-extractable soil inorganic S(R 2=0.823***), respectively. Soil sulphate retention capacity was found to influence amounts of mineralized S and soluble organic S, and thus periodic leaching with KCl to remove mineralized S from soils may not adequately reflect the extent of soil S mineralization in high sulphate-retentive soils. In low (<10%) sulphateretentive soils, increasing the superphosphate applications from 188 to 376 kg ha−1 year−1 increased S mineralization but not amounts of C-bonded and hydriodic acid-reducible soil S fractions.

Status and distribution of soil sulphur fractions, total nitrogen and organic carbon in camp and non-camp soils of grazed pastures supplied with long-term superphosphate

Topsoils (0–75 mm) from four different soil types were collected from stock camp and non-camp (main grazing area) areas of grazed pastures in New Zealand, which had been fertilised annually with superphosphate for more than 15 years, in order to assess the effects of grazing animals on the status and distribution of soil S fractions and organic matter. These soils were analysed for organic C, total N, total S, C-bonded S, hydriodic acid-reducible S, 0.01 M CaCl2, and 0.04 M Ca(H2PO4)2-extractable S fractions, and soil pH. Soil inorganic and organic S fractions extracted by NaHCO3 and NaOH extractants were also determined. The results obtained showed that camp soils contain higher soil pH, organic C, total N, total S, organic (C-bonded S and hydriodic acid-reducible S) and inorganic S fractions, NaHCO3-and NaOH-extractable soil S fractions but a lower anion retention capacity than non-camp soils, attributed to a higher return of plant litter and animal excreta to camp soils. In both soils, total S, organic S, C-bonded S, and hydriodic acid-reducible S were significantly correlated with organic C (r≥0.90***, ***P≤0.001) and total N (r≥0.95***), suggesting that C, N, and S are integral components of soil organic matter. However, C: N : S ratios tended to be lower in camp (60: 5.6: 1–103: 7.2: 1) than in non-camp soils (60:6.1:1–117:8.3:1). Most (>95%) of the total soil S in camp and non-camp soils is present as organic S, while the remainder is readily soluble and adsorbed S (i.e. Ca(H2PO4)2-extractable S). C-bonded S and hydriodic acid-reducible S constituted 55%–74% and 26%–45% of total S, respectively, reflecting a regular return of plant litter and animal excreta to the grazed pastures. NaHCO3, and especially NaOH, extracted significantly higher amounts of total soil S (13%–22% and 49%–75%, respectively) than Ca(H2PO4)2 or CaCl2 ( 45%) contain higher levels of C-bonded and hydriodic acid-reducible S fractions than those of low sulphate retention soils (<10%). Long-term annual superphosphate applications significantly increased the accumulation of soil organic and inorganic S fractions, and organic C and total N in the topsoil, although this accumulation did not occur when the superphosphate application rates were increased from 188 to 376 kg ha-1 year-1.

Retention of Dissolved Organic Carbon and Sulfate in Aggregated Acid Forest Soils

AbstractAdsorption of dissolved organic carbon (DOC) and sulfate was examined in mineral A and B horizons of aggregated acid forest soils from declining (Oberwarmensteinach) and healthy (Wülfersreuth) Norway spruce [Picae abies (L.) Karst.] stands in northeastern Bavaria, Germany. Adsorption studies were conducted in the laboratory on undisturbed, aggregated soil samples under nonequilibrium conditions using a miscible displacement method. Initial mass relationships suggest a net release of DOC in the A horizons of both stands at all DOC concentration levels added. In contrast, the B horizons retained DOC from solutions containing more than 4 to 5 g C m−3 in the Oberwarmensteinach B horizon, and 2 to 3 g m−3 in the Wülfersreuth B horizon, respectively. These concentrations correspond to mean annual DOC concentrations in the B horizon solutions of both sites. Addition of sulfate (1.60 molc SO4 m−3) significantly reduced DOC retention, suggesting competition between SO4 and parts of DOC for adsorption sites. Retention of sulfate from solution occurred concentrations with more than 0.38 to 0.42 molc SO4 m−3 in Oberwarmensteinach, and 0.25 to 0.31 molc SO4 m−3 in Wülfersreuth, respectively. Compared with batch experiments, the miscible displacement method resulted in lower affinities of the soils for DOC. Reduction of DOC and sulfate retention by the use of the flow‐generated technique is due to the consideration of sorption kinetics, which are influenced by soil structure and texture. Although this method underestimates the affinity of a soil for DOC to a certain extent, we suggest that it reflects the situation in the field quite well. Flow‐generated experiments showed that the decline phenomena of spruce in Oberwarmensteinach correspond with a reduced potential for DOC retention in the upper B horizon, as well as with lower sulfate retention throughout the profile.

Hydrology and alkalinity regulation of soft Florida waters: An integrated assessment

Natural waters in ridge provinces of Florida and southeast Georgia were classified geographically, by degrees of cultural disturbance, and according to the dominant hydrologic and biogeochemical processes controlling chemistry. The ionic composition of lakes, upland streams, and surficial aquifer (water table) springs in relatively undeveloped catchments reflects the geographic variations in bulk deposition corrected for evapotranspiration (Na, Cl), plus a slight gain (net watershed mobilization) of Mg, and partial to nearly complete losses (net retention) of nitrate, sulfate, Ca, and K. Recharge to the Floridan aquifer in infertile, forested, sandy ridge provinces of northern Florida contains 360–580 μmol CO2. On the basis of indirect geochemical evidence, sulfate retention appears less important in lake sediments than in the region's highly weathered, ferruginous, kaolinitic, sand soils. Silica concentrations in upland streams and water table springs closely reflect the predicted equilibrium between kaolinite and gibbsite. Along with other evidence, the Si concentrations in ridge lakes indicate that seepage inflow is much more important than assumed in Baker et al.'s (1988) regional model. Lakes and streams are acidified either by humic acids or nonmarine sulfate but rarely by both, as reflected by the significant inverse correlation between these two components. Contrary to previous reports, there is no significant difference in alkalinity for culturally undisturbed lakes in the northern Trail versus southern Highlands Ridge areas.

Effect of changes in pH on sulphate and phosphate retention in soils

Abstract Soils with differing capacities to sorb anions were incubated with lime, each to a range of pH values (4.5-6.8), and characterised for their ability to sorb phosphorus (P) and sulphur (S) by sorption isotherms, and “Quick Test” sulphate retention, and phosphate retention methods. Although both phosphate and sulphate sorption changed with pH, the effects of lime were much more significant for sulphate than for phosphate. Sulphate sorption was reduced in the soils studied as pH was increased, and this was also reflected in sulphate retention Quick Test results. In contrast, phosphate retention values showed little dependence on pH. These findings have implications for the mobility of sulphate in soils, the correlation between P retention and S retention values, and the methodology of sulphate retention tests.

SULFATE RETENTION AND RELEASE IN SOILS AT PANOLA MOUNTAIN, GEORGIA

Inorganic sulfate pools, sulfate sorption characteristics, and Fe and Al fractions were determined on soils at Panola Mountain, a 41-ha forested watershed in the Georgia piedmont. Sulfate adsorption was measured in batch solutions that bracketed the range of ambient sulfate concentrations and soil solution acidity. The slope and intercept of the initial mass (IM) isotherms, formed from plots of sulfate retained against sulfate added, were used to compare sorption behavior among soils. The reversibility of sulfate adsorption was determined by measuring desorption of soluble sulfate from soil before and after equilibration with a concentrated sulfate solution. Sulfate sorption properties of soils at Panola Mountain fall along a continuum between two end members. The “low-adsorbing” end member comprises shallow soils (0–10 cm), with high water-soluble sulfate (Sw), low phosphate-extractable sulfate (Sp-w), high organic matter, low sulfate retention ability (IM isotherm slope near 0.0), and high sulfate adsorption reversibility. The “high-adsorbing” end member comprises deeper soils (>10 cm), with higher total native sulfate (mostly as Sp-w), low organic matter, high sulfate retention ability (isotherm slope near 1.0), and low sulfate adsorption reversibility. Sulfate retention was only weakly related to Fe and Al fractions, possibly because of inhibition of adsorption by organic matter. Sulfate concentrations in surface waters reflect the spatial distribution of soil sulfate retention properties; baseflow, representing water which has equilibrated with the mineral subsoil, has sulfate concentrations near 10 μeq/L, whereas stormflow, which is dominated by water flowing through shallow horizons, has sulfate concentrations near 100 μeq/L.

APPLYING BATCH-DETERMINED RETENTION PROPERTIES TO PREDICT SULFATE TRANSPORT THROUGH SOIL COLUMNS

Equilibrium properties of sulfate retention by soil under different experimental conditions were determined and evaluated with regard to their impact on subsequent transport simulations. The sulfate retention characteristics of a silty clay soil (Typic Hapludult) were examined in sulfate solutions at 3 initial pHs, and with or without phosphate in solution. Langmuir and Freundlich adsorption isotherms were fit to the data with low standard errors. An increase in pH from values ranging between 4.28 and 4.55 to values ranging between 5.96 and 6.47 decreased the estimated maximum retention from 4.7 to 3.4 mmol/kg soil. The addition of a phosphate solution further decreased estimated maximum sulfate retention from 3.4 to 2.5 mmol/kg. Independently determined transport parameters combined with batch-determined retention properties accurately predicted sulfate transport through a small laboratory column when column effluent pH and batch experiment pH were similar. Retention properties were not transferable from laboratory tube to small column when batch and column pH differed or when solutions contained phosphate during batch experiments but not during the column experiment. Relative concentration of the simulated breakthrough curves for a 0.1 m M sulfate solution equalled 0.5 after 8.5 and 34 pore volumes for sulfate retention characteristics determined at pH 5.96–6.47 and 4.28–4.55, respectively. The results indicate that reliable predictions of transport from batch-determined retention properties are possible, but only if batch conditions closely match conditions for specific transport situations.

Kinetics of Sulfate Retention on Soil as Affected by Solution pH and Concentration

AbstractKnowledge of the time dependence of chemical retention by soil is crucial to a full understanding of how retention is affected by solution variables and to accurately simulate the fate of soil‐applied chemicals. The kinetics of SO2‐4 retention by and release from soil were examined in a continuously stirred reactor tank. The concentration‐time histories were processed with a number of commonly used kinetic equations. Of those tested, an equation that was first order in available retention sites consistently gave the best fit to the data. Sulfate‐retention kinetics were strongly influenced by SO2‐4 concentration and the presence of PO3‐4 in solution, but unaffected by a change in pH from 3 to 4. For example, when SO2‐4 concentration was doubled from 0.313 to 0.625 mM in the retention phase of the experiment, the average first‐order forward rate constant increased from 0.07 to 0.24 min−1. The addition of PO3‐4 to the solution increased the forward rate constant to 0.13 min−1 and decreased equilibrium retention from 8.1 mmol/kg when PO3‐4 was not present to 5.2 mmol/kg. In the release phase of the experiment, the presence of PO3‐4 increased the estimated reverse rate constants by an order of magnitude, and an even greater increase in the reverse rate constant was estimated when the initial sulfate concentration was increased from 0.313 to 0.625 mM. Although the data was closely approximated by first‐order kinetics, changes in the rate coefficients for the different influent solutions indicate that commonly determined kinetic parameters have limited applicability beyond the particular conditions of the experiment.

Stream chemistry in the eastern United States: 1. Synoptic survey design, acid‐base status, and regional patterns

To assess the regional acid‐base status of streams in the mid‐Atlantic and southeastern United States, spring base flow chemistry was surveyed in a probability sample of 500 stream reaches representing a population of 64,300 reaches (224,000 km). Approximately half of the streams had acid‐neutralizing capacity (ANC) ≤ 200 μeq L−1. Acidic (ANC ≤ 0) streams were located in the highlands of the Mid‐Atlantic region (southern New York to southern Virginia, 2330 km), in coastal lowlands of the Mid‐Atlantic (2600 km), and in Florida (462 km). Acidic streams were rare (less than 1%) in the highlands of the Southeast. Inorganic monomeric aluminum (Alim) concentrations were highest in acidic streams of the Mid‐Atlantic Highlands where over 70% of the acidic streams had Alim greater than 100 μg L−l, a concentration above which deleterious biological effects have frequently been reported. Dissolved organic carbon concentrations were much higher in lowland coastal streams, compared with inland streams. Our data support a hypothesis that atmospheric sources and watershed retention control regional patterns in streamwater sulfate concentrations. Most stream watersheds retain the vast majority of the total nitrogen loading from wet deposition. The data suggest, however, that some deposition nitrogen may be reaching streams in the Northern Appalachians. These results show that acidic surface waters are found outside the glaciated northeastern portions of the United States and that watershed sulfate retention is not sufficient to prevent acidic conditions in some Mid‐Atlantic Highlands streams.

Barium Sulfate Bronchography*: Report of a Complication

Alveolarization of the barium sulfate and subsequent retention of barium sulfate for years was demonstrated in three patients in whom dilute suspension of barium sulfate in water was used for bronchography. Pathologic examination in one patient showed barium sulfate within macrophages in the alveolar spaces and walls and in the perivascular and peribronchial interstitium. Since the residual barium sulfate interferes with imaging procedures of the lungs, it represents an unwanted event in patients with pulmonary disease. High-resolution computed tomography is the preferred method of evaluating for bronchiectasis. If bronchogram is performed, it should be performed after bronchoscopy using oily propyliodone (Dionosil).

Sulfate retention and cation leaching of forest soils in response to acid additions

A vacuum extractor was used to examine the effects of increased SO42− deposition on net S retention and cation leaching in three Maine Spodosols (Berkshire, Dixfield, and Rawsonville series) and an Illinois Alfisol. Columns (leached daily for 30 days with either a simulated throughfall solution containing 80 μequiv. SO42−•L−1 (pH = 4.77) or a simulated throughfall solution plus 200 μequiv. H2SO4•L−1 (pH = 3.66)) were constructed using O horizons over upper B (Bh or Bhs) and lower B (Bhs or BC) horizons for the Spodosols and two depth increments of bulked soil (0–12 and 12–25 cm) for the Alfisol. Leachate concentrations of base cations were dominated by Ca2+ and were generally greater in the Alfisol than in the Spodosol leachates. Declining concentrations of base cations and NH4+ with time led to an increase in Al3+ concentrations and a decrease in pH for some of the Spodosol leachates (e.g., leachate Al3+ increased from 19 μequiv. Al3+ •L−1 (day 2) to 194 μtequiv. Al3+ •L−1 (day 30), and pH decreased from 5.53 to 4.41, respectively, for the Dixfield high-S treatment). Columns that received the high-S treatment retained a greater percentage of the added S than those that received the low-S treatment because of increased SO42− adsorption in the former. High rates of net organic S mineralization were found for all soil types (e.g., 169 μg organic S•g−1 over 30 days for the Rawsonville Bhl horizon, 15% of the total S); no treatment effects were found for the amount of S mineralized. Trends in net S retention across soil type (within the same treatment) reflected increases in soil SO42− from adsorption, as well as decreases in organic S from mineralization. When net S mineralization was removed from leaching losses of S, trends in net S retention closely reflected SO42− adsorption differences (50, 23, 8, and 1% of the added S retained by the Rawsonville, Dixfield, and Berkshire series, and the Alfisol, respectively, for the high-S treatment). Because of the low SO42− adsorption capacity of the Alfisol relative to the Spodosols, organic processes were more important in affecting net S retention for the Alfisol. Inorganic processes of S retention were more important for the three Spodosols studied.

Sulfate and nitrate retention in lakes

Analysis of literature data showed that both nitrate and sulfate removal should be modelled on an areal, not volumetric, basis. In addition, the retention of both ions is showed to be independent o...

THE ROLE OF BASIC ALUMINUM SULFATE MINERALS IN CONTROLLING SULFATE RETENTION IN THE MINERAL HORIZONS OF TWO SPODOSOLS

The authors collected soil solutions during the spring snowmelt at two depths using tension-free lysimeters in two Humic Cryorthods located in forested catchments. They also extracted soil solutions by centrifugation from the B and the C horizons of these soils. Although most data points cluster around the solubility line of natural gibbsite, the relationship between Al{sup 3+} activity and solution pH showed a marked departure from Al(OH){sub 3} solubility with a slope of {minus}1.91. Moreover, at pH values below 4.6, solutions were slightly undersaturated with respect to natural gibbsite. Results from thermodynamic calculations suggested that the Hermine B, and probably the Coniferous B, horizons have attained chemical conditions sufficient to enter the stability field of a basic aluminum sulfate mineral formed in the presence of Al(OH){sub 3}. Mineral stability relationships indicated that the solid phase of concern could be alunite (pK{sub s} = 85.4) and that the apparent saturation status of the soil solutions with respect to alunite increased with increasing reaction time. However, physical evidence for the presence of alunite in these horizons is still lacking. Solutions from the C horizons were all clearly undersaturated with respect to any Al{sub x}(OH){sub y}(SO{sub 4}){sub z} mineral. While the possible formationmore » of alunite could be affecting sulfate retention in these acid soils, the results do not exclude the pH-dependent sulfate adsorption mechanism proposed by a number of investigators.« less

Effects of vegetation type on the biogeochemistry of small catchments (Mont Lozere, France)

Input-output budgets are presented for three granitic catchments, with contrasting vegetation types (Spruce, Beech and grassland), for the Mont Lozere region of France. Budget results are compared with information for analogous catchments with varying atmospheric pollution loading. Comparisons show relatively small losses of cations and marked sulphate retention in the soils (particularly for the beech and grassland sites) of the Mont Lozere catchments. Retention of sulphate may be underestimated, particularly for the spruce site, owing to occult and dry deposition inputs which have not been measured. Specific effects of the vegetation type upon the soil dynamics and the biological functioning of the ecosystem are described. Losses of the base cations are least significant for the beech forest, and this is linked to low bicarbonate production coupled with high sulphate retention in the soil. The spruce afforestation programme has led to a marked cation-loss increase although the streams have yet to acidify. Deforestation leads to further cation losses in relation to increased nitrate leaching. The biogeochemical functioning of the Mont Lozere ecosystems, although slightly disturbed, is not in steady state; sulphate is still accumulating and the effects of afforestation and harvesting are marked.

Hydrological and hydrochemical fluxes through vegetation and soil in the Allt a' Mharcaidh, western Cairngorms, Scotland: Their effect on streamwater quality

A detailed investigation of the hydrochemical alteration of input water by vegetation and soils was undertaken in an upland catchment in the Cairngorm Mountain region of Scotland. The composition of catchment outflow water reflects the hydrological routing of water through different soil horizons and the importance of long residence time water. There is uptake of nitrogen and neutralization of incoming anthropogenic acidity by the vegetation, and sulphate adsorption in the mineral soils. Streamwater quality is dominated by the contribution of long residence time water, especially during base flow. Sulphate retention and cation release are the major neutralization mechanisms buffering outflow chemistry at this site.

Aliphatic Acids: Influence on Sulfate Mobility in a Forested Cecil Soil

AbstractDissolved organic substances derived from forest litter are believed to influence the retention and movement of SO2−4 in forest soils. A column study was conducted in which 35SO4 was surface applied to a soil and leached with either low‐molecular‐weight aliphatic acids (AA) or a forest‐litter extract. Oxalic, malonic, and succinic acids were used in the concentration range 8.0 × 10−3 to 1.0 × 10−5 mol L−1. Movement of 35SO4 was determined with column depth, as was the 35SO4 activity in the collected leachates. Labeled SO4 soil movement was found to increase with increasing AA concentration. Leachate 35SO4 activity was observed to increase in the order malonic &gt; succinic &gt; oxalic for acid treatments &gt; 1.0 × 10−4 mol L−1. The 1.0 × 10−5 mol L−1 acid treatments did not result in 35SO4 break‐through in the soil columns. Reduced exchangeable 35SO4 for organic acid ≥ 1.0 × 10−3 mol L−1 suggests that at high concentrations, aliphatic acids may activate sulfate retention sites in soil, while at lower concentrations these acids may accelerate the movement of 35SO4 in forest soils due to selective organic‐acid adsorption by soil components.

Adsorption of dissolved organic carbon and sulfate by acid forest soils in the Fichtelgebirge, FRG

AbstractThe adsorption of dissolved organic carbon (DOC) and sulfate was examined in mineral horizons of acid soils from damaged (Oberwarmensteinach) and healthy (Wülfersreuth) Norway spruce forested sites in the Fichtelgebirge (NE‐Bavaria). The A horizons of both sites desorbed DOC at all levels added, whereas the B horizons (Bs and Bv) retained added DOC at levels &gt; 5 mmol C kg−1. An initial mass isotherm used on the B horizon data indicated that these soils have a greater affinity for DOC than B horizons from Spodosols in the northeastern U.S. Sulfate was only retained at high solution levels, and retention was pH dependent. Nitrate and sulfate additions (1000 μeq L−1 anion) had minor effects on DOC adsorption. Overall, there was little difference in DOC or sulfate retention at the two sites, indicating atmospheric deposition inputs have not affected these processes.

Distribution of aluminum in black spruce saplings growing on two New Brunswick forest soils with contrasting acid sulfate sorption

Two adjacent soils with contrasting sulfate sorption were examined in terms of (i) water-soluble and ion-exchangeable Al, Fe, Ca, Mg, K, Mn and Zn, (ii), water- and bicarbonate-extractable sulfate, (iii) Truog-extractable P, (iv) dithionite-extractable Al, Mn and Fe and (v) treatment response to irrigation with simulated acid precipitation. The biomass of 8 year old black spruce saplings growing on the soils, and the distributions of Al, Fe, Ca, Mg, K, Mn, P and Zn within these plants, were also examined. The soils were well to moderately-well drained, with the mineral soil exposed by site preparation prior to planting. The exposed soil underneath individual saplings was treated with acid sulfate solutions (75 mm containing 2 to 50 mg L−1 H2SO4) applied during each of three consecutive growing seasons. The results indicate that Al, much like Fe, Ca, Mn and Zn, accumulated with time in the foliage, but K, Mg and P were highest in young plant tissues. Much of Al and Fe taken up remained in the fine roots. Aluminum uptake increased with the amount of dithionite-extractable Al (free Al oxide) in the soil. Growth of the black spruce saplings was not visibly affected by readily accessed Al in each soil, or by acid irrigation.Instead, growth was restricted by factors other than soil Al and acid irrigation in spite of (i) low soil pH, (ii) high levels of exchangeable Al, and (iii) high levels of Al in fine roots. Sulfate retention across and within the two soils was positively correlated with free Al oxide. The two soils responded to acid irrigation by accelerated silicate weathering and enhanced ion leaching. Sulfate sorption reduced these effects.

Acid rain, cation dissolution, and sulphate retention in three tropical soils

SUMMARYSurface and subsurface samples of three tropical soils were examined with respect to their interaction with dilute solutions of sulphuric acid of pH 3. In calcareous clayey samples with a large cation exchange capacity the H+ was replaced by an equivalent concentration of metal cations which remained in solution along with SO2‐4 as counterion. In a coarse‐textured neutral soil with small cation exchange capacity, there was less chemical interaction and a major proportion of the H2SO4 remained unchanged in the equilibrium solution. Another soil exhibited considerable ability to remove SO2‐4 from solution and, therefore, the total ionic concentration was greatly reduced. Other samples showed behaviour which was intermediate to these three types.The ability to adsorb SO2‐4 is one of the most important factors which determines the nature of the interaction of soil with dilute sulphuric acid. This ability was shown to be affected by the content of hydrous sesquioxides and organic matter in these soils.