Alkalinity generation by Fe(III) reduction versus sulfate reduction in wetlands constructed for acid mine drainage treatment

Despite the widespread use of wetlands for acid mine drainage (AMD) treatment, alkalinity generating mechanisms in wetlands and their abiotic and biotic controls are poorly understood. While both dissimilatory sulfate reduction and Fe(III) reduction are alkalinity-generating mechanisms, only the former has been considered as important in wetlands constructed for AMD treatment. This study was conducted to determine the extent to which Fe(III) reduction occurs and the extent to which sulfate reduction versus Fe(III) reduction contributes to alkalinity generation in 5 wetlands constructed with different organic substrates (Sphagnum peat with limestone and fertilizer, Sphagnum peat, sawdust, straw/ manure, mushroom compost) that had been exposed to the same quality and quantity of AMD for 18–22 months. These substrates had Fe oxyhydroxide concentrations of 250–810 μmol Fe g^?1 dry substrate. Flasks containing 100 g of wet substrate along with either 150 mL of wetland water or 130 mL of wetland water and 20 mL of 37 % formalin were incubated at 4 °C in January and 25 °C in May. On days 0, 2, 4, 8, 12 and 16, the slurry mixtures were analyzed for concentrations of H⁺, Fe²⁺ and SO₄ ^2?. The bulk of the evidence indicates that for all except the mushroom compost wetland, especially at 25 °C, biologically-mediated Fe(II) reduction occurred and generated alkalinity. However, in none of the wetlands, regardless of incubation temperature, was there evidence to support net biological sulfate reduction or its attendant alkalinity generation. Sulfate reduction and concurrent Fe(III) oxyhydroxide accumulation may be important in the initial stages of wetland treatment of AMD, both contributing to effective Fe retention. However, as Fe(III) oxyhydroxides accumulate over time, Fe(III) reduction could lead not only to decreased Fe retention, but also to the potential net release of Fe from the wetland.     

Restoring the capacity of spent mushroom compost to treat coal mine drainage by reducing the inflow rate: A microcosm experiment

Four volumes of spent mushroom compost were exposed to synthetic coal mine drainage (pH 3.5, 48 mg L^?1 Fe, 22 mg L^?1 Mn) under oxidizing conditions (Eh 300 to 400 mV) at a relatively high rate of flow. After 15 days, the compost lost its ability to elevate pH, to lower the redox potential, to lower outlet iron concentrations, and to lower manganese concentrations, with larger volumes retaining more Fe and H⁺, but less Mn. Estimated retention maxima per liter of spent mushroom compost were 281 μeq H⁺, 5.56 g Fe, and 0.15 g Mn. These values are similar to those reported elsewhere for peat. The ‘saturated’ compost was then mixed and exposed to mine water in order to eliminate ‘dead zones’ in the compost. Subsequently, the compost was re-exposed to synthetic mine water (pH 4.0, 60 mg L^?1 Fe, O mg L^?1 Mn) under a much lower flow rate and less oxidizing regime for a period of 114 days. Under the low flow regime, iron was first exported from the compost as reducing conditions were established, and then retained on a stable basis. In addition, Eh was lowered and pH was elevated by the compost. On a net basis, the capacity of the compost to retain iron was increased and apparently stable under the decreased flow conditions.     

Fe,Al, Mn,and S chemistry of Sphagnum peat in four peatlands with different metal and sulfur input

Comparisons among 4 peatland sites representing a gradient of increasing Fe, Al, Mn, and S loading revealed significant accumulation of total Fe, Al, and S, but not Mn, in surface (0 to 20 cm deep) peat along the gradient. Iron and Al accumulation were contributed mainly by organically bound fractions, with oxides contributing to a lesser extent. Although SO₄ ^2? and Fe sulfides showed significant increases in concentration along the gradient, most of the accumulation of total S was contributed by organic, rather than inorganic S. Laboratory studies of Fe²⁺ adsorption by peat indicated that increasing the pH of added Fe²⁺ solutions (pH values of 3, 4, 5, and 6) did not significantly affect Langmuir equation estimates of either maximum Fe²⁺ adsorption capacity or the affinity of peat for Fe²⁺. Regardless of the pH of the added Fe²⁺ solutions, final solution pH values were relatively uniform, averaging about 3.4, reflecting a considerable bufferring capacity of Sphagnum peat. Factors affecting the accumulation of metals and S in peat remain topics for further investigation.     

Laboratory mesocosm studies of Fe,Al, Mn,Ca, and Mg dynamics in wetlands exposed to synthetic acid coal mine drainage

To evaluate the potential for constructed wetlands to treat acid coal mine drainage, six model wetland mesocosms (each 2.4 m × 15 cm) were filled with Sphagnum peat (15 cm deep), planted either with cattails (Typha latifolia) and living Sphagnum, living Sphagnum only, or left as bare peat (2 mesocosms per treatment). The model wetlands were exposed to synthetic acid coal mine drainage (pH 3.5, concentrations of Fe²⁺, Al³⁺, Mn^2*, Ca²⁺, and Mg²⁺ of 78.8, 10.0, 5.2, 12.0, and 4.5 mg L^?1, respectively) at a rate of 90 mL min^?1, 6 hr d^?1, 5 d wk^?1, over a 16 week period. Chemical analysis of peat at periodic intervals indicated that the model wetlands were net sources of Al³⁺, Mn²⁺ Ca²⁺ and Mg²⁺, but net sinks for Fe²⁺. Type of vegetation had no significant effect on Fe²⁺ retention; of the 204 g of Fe²⁺ added to the model wetland systems, 162 g were retained. Formation of Fe oxides accounted for 73 to 86% of the Fee' retention, with exchangeable Fe contributing 0.2 to 1.2%, organically bound Fe contributing 4 to 19%, and residual Fe contributing 7 to 15% of total Fee' retention. Fe retention was greatest at the inflow ends of the model wetlands where Fe retention appeared to reach saturation at a final Fe concentration in the peat of 235 mg g^?1. At the rate of application of the synthetic acid mine drainage, we estimated that the model wetland systems would have reached complete Fe saturation after 157 days. We suggest that the mesocosm approach could be useful in generating site-specific data that can be applied to the formulation of cost-benefit analyses that can compare a proposed wetland treatment system with alternative conventional chemical methods for treating acid mine drainage.     

The effects of drying and re-wetting and increased temperature on sulphate release from upland and wetland material

In central Ontario, elevated SO₄ concentrations and export have been measured in both upland and wetland-draining catchments following summer droughts, although the source of excess SO₄ is unclear. The objective of this study was to determine the effects of drying and re-wetting and temperature, respectively, on the release of SO₄ from the primary S pools in wetlands (Sphagnum and peat) and uplands (forest floor and mineral soil), using material collected from the PC1 catchment in Haliburton County, and from catchment S50 in the Turkey Lakes Watershed. Peat exhibited the most marked response to drying of the four materials considered, and within 24 h of re-wetting dried peat from both catchments released 3-4 times more SO₄ (50-67 mg kg⁻¹ S-SO₄) than continuously moist peat (16 mg kg⁻¹ S-SO₄), although temperature had only a marginal effect on SO₄ concentrations. There was no immediate response of Sphagnum to either drying or temperature, although S-SO₄ concentrations in Sphagnum tended to increase over the 30-day (d) incubation. There was a small but immediate increase in S-SO₄ concentrations in forest floor material (LFH) from both catchments within the first 24 h of incubation, which was greatest in treatments that were dried and/or incubated at a higher temperature. In contrast, neither temperature nor drying appeared to affect SO₄ release from mineral soil collected from either site. Results of laboratory incubations suggest that increases in SO₄ concentration that have been reported in wetland-draining streams immediately following summer dry periods may be quantitatively explained by drying and re-wetting of peat rather than increased mineralization in Sphagnum. Similarly, the higher SO₄ concentrations that have been measured in upland streams following summer droughts may in part be due to enhanced SO₄ release from the forest floor following drying and re-wetting. In contrast, while the mineral soil constitutes a large pool of total S, it does not appear to be responsive to changes in moisture or temperature in the short-term (<30 d) and therefore likely does not contribute to reported climate-related temporal variations in stream SO₄.     

Contrasting the geochemistry of aluminum among peatlands

Aluminum concentrations were measured in surface waters, pore waters and surface peats of 15 wetlands in south-central Ontario. Wetlands were grouped floristically and chemically as mineralpoor, moderately-poor or mineral-rich fen. Mineral-poor fens were dominated bySphagnum, were low in alkalinity (0.31μeq L^?1) and pH (4.5–6.3). Moderately-poor fens had a mixture of vegetation (Sphagnum, sedges and grasses), mid-alkalinity (23–91μeq L^?1) and pH (5.8–6.4). Mineral-rich fens were dominated by sedges and grasses, had high alkalinity (104–181μeq L^?1) and circumneutral pH (6.2–6.3). Surface water Al concentrations were less in mineral-poor versus moderately-poor and mineral-rich fens (F=32.0; P<0.05). Pore water Al concentrations were lower in 4 of 5 mineral versus the mineral-rich fens (F=92.15; P<0.05). In all but two cases pore water Al (all species <0.2μm) were greater within the fen peats versus the overlying surface waters suggesting that peats could act as a source of Al to the overlying waters. In all wetlands, 70 and 30% of peat Al was recovered by a hydroxylamine hydrochloride/acetic extract (primarily inroganic) and an ammonium hydroxide extract (primarily organic), respectively. Differences in “extractable” Al recovered by the two reagents (i.e., inorganic+organic Al) among the 15 wetlands were independent of wetland type. Distribution coefficients, k _d , were different among the 3 types of wetlands (F=25.0; P<0.05) with theSphagnum dominated mineral-poor fens containing higher values versus the sedge and grass dominated mineral-rich fens. Lower surface and pore water concentrations of Al in mineralpoor versus mineral-rich fens may in part be a result of differences in the degree of minerotrophic influences between the two types of peatlands. As well, the greater binding capacity ofSphagnum peat as indicated by higher k _d 's in the mineral-poor fens, may have contributed to the observed lower pore water and surface water Al concentrations in mineral-poor versus mineral-rich fens. It has been postulated that anthropogenic acidification of peatlands will accelerate the transformation of a mineral-rich fen to that of a mineral-poor fen and ultimately to bog. Changes in Al geochemistry that may ensue as this transition occurs include decreases in pore and surface water Al concentrations with concurrent increases in peat bound Al.     

Nutrient‐rich compost versus nutrient‐poor vermicompost as growth media for ornamental‐plant production

A comparative study on the suitability of one compost and two vermicomposts, obtained from the same batch of tomato‐crop waste, as growth media for ornamental plant production was carried out. Each material was mixed with Sphagnum peat at 100 : 0, 75 : 25, 50 : 50, 25 : 75, and 0 : 100 (peat control) proportions by volume. Two ornamentals (Calendula officinalis, Viola cornuta) were sown and grown in the 13 substrates. Substrates were characterized physically and chemically. Seed germination, total leaf chlorophyll (SPAD units), plant growth, and plant nutrient concentrations were determined. The compost and the vermicomposts were markedly different from peat. Compost and the vermicomposts had greater bulk density and lower total porosity than peat. Compost had larger aeration and lower water‐holding capacity than vermicomposts and peat. Compost and vermicomposts were alkaline (pH = 8.8 on average) whilst peat was acidic (pH = 5.9). Electrical conductivity was low in peat (0.23 dS m^–1) and vermicomposts (0.65 dS m^–1), and high in compost (2.85 dS m^–1) due to the high concentrations of K⁺ and SO$ _4^{2-} $ . Mixing compost and vermicomposts with peat produced substrates with intermediate characteristics. Physical properties were within adequate range for all mixes except for the compost ones. pH was within adequate range only in pure peat, and salinity was extremely high in the compost mixes. Compost was phytotoxic, as shown by the strong reduction of seed germination, chlorophyll content, and plant growth of both ornamentals. Vermicomposts did not affect seed germination but reduced plant growth, though much less than compost. Mixing these materials with peat improved germination and growth. The diluted materials (compost at the 25 : 75 and vermicomposts at the 50 : 50 and 25 : 75 proportions) produced good‐quality plants.     

Seasonal Variations in Vertical Redox Stratification and Potential Influence on Trace Metal Speciation in Minerotrophic Peat Sediments

Seasonal variations in pore water and solid phase geochemistry were investigated in urbanized minerotrophic peat sediments located in southwestern Michigan, USA. Sediment pore waters were collected anaerobically, using pore water equilibrators with dialysis membranes (“peepers”) and analyzed for pH, alkalinity, dissolved ΣPO₄ ^?3, ΣNH₄ ⁺, ΣS^?2, SO₄ ^?2, Fe⁺³, Fe⁺², and Mn⁺² at 1-2 cm intervals to a depth of 50 cm. Cores collected adjacent to the peepers during all four seasons were analyzed for reactive solid phase Fe according to extraction methods proposed by Kostka and Luther (1994). The association of Fe and trace metals (Mn, Pb, Zn, Cu, Cr, Co, Cd, U) with operationally defined solid phase fractions (carbonates, iron and manganese oxides, sulfides/organics and residual) was assessed for cores extracted during winter and spring using extraction methods proposed by Tessier et al. (1979, 1982). Pore water Fe and S data demonstrate a clear seasonal variation in redox stratification of these sediments. The redox stratification becomes more compressed in spring and summer, with relatively more reducing conditions closer to the sediment water interface (SWI), and less reducing conditions near the SWI in fall and winter. In the upper 10–15 cm of sediment, the pool of ascorbate extractable Fe, thought to be indicative of reactive Fe(III) oxides, diminishes during spring and summer, in agreement with seasonal changes in redox stratification indicated by the pore water data. Tessier extractions indicate that the total extractable quantity of all metals analyzed in this study decrease with depth, and that the majority of the non-residual Fe, Pb, Zn, Cu, Cr, Co, Cd, and U is typically associated with the sulfide/organic fraction of the sediments at all depths. Non-residual Mn, in contrast, is significantly associated with carbonates in the upper 15–25 cm of the sediment, and predominantly associated with the sulfide/organic fraction only in deeper sediments.     

Effect of Organic Amendment Derived from Co-Composting of Chicken Slurry and Rice Straw on Reducing Nitrogen Loss from Urea

Co-composting of chicken slurry and rice straw with clinoptilolite zeolite and urea as additives was conducted to determine the characteristics of a compost and their effects on controlling ammonium (NH₄⁺) and nitrate (NO₃^?) losses from urea. Quality of the compost was assessed based on temperature, moisture content, ash, pH, electrical conductivity, carbon/nitrogen (C/N) ratio, NH₄⁺, NO₃^?, macronutrients, heavy metals, humic acid, microbial population, germination index, and phytotoxicity test. Moisture content and C/N ratio of the compost were 43.83% and 15, respectively. Total N, humic acid, ash, NH₄⁺, NO₃^?, phosphorus (P), calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) increased after co-composting rice straw and chicken slurry. Copper, iron (Fe), manganese (Mn), zinc (Zn), and microbial biomass of the compost were low. The germination rate of Zea mays on distilled water and Spinacia oleracea growth on peat-based growing medium (PBGM) and compost were not significantly different. Urea amended with compost reduced N loss by retaining NH₄⁺ and NO₃^? in the soil.     

Temporal variation in the surface-water chemistry of a blanket bog on Dartmoor, southwest England: analysis of 5 years' data

Blanket‐bog peats, mapped as the Winter Hill and Crowdy associations by the Soil Survey of England and Wales, are an oceanic manifestation of the ombrotrophic ‘raised‐bog’ (Hochmoor) peats that cover large tracts in the boreal zone of the northern hemisphere. This paper examines monthly analyses from 1992 to 1997 of major ions and other variables from an upland blanket bog in southwest England in relation to seasonality, rainfall, and the chemical composition of rainwater. Average ionic composition of surface water (and peat) integrates variable atmospheric solute inputs over the years. The dominant ions in the surface water, Na⁺ and Cl^–, showed only weak seasonality, but divalent cations a stronger seasonal pattern with a summer maximum. Mean pH ranged from c. 4.4 in February to c. 4.2 in August. Changes in concentration of different cations were closely interlinked by cation exchange. The anion deficit, accounted for by anionic groups on the dissolved organic matter, was strongly seasonal with a summer maximum, as was optical absorbance at 320 nm. Nitrate and NH₄⁺ were both at much smaller concentrations than in rain. Nitrate exceeded 1 μmol l^?1 only during cold periods in winter, mainly following drought in the summer of 1995; NH₄⁺ reached a few μmol l^?1 only in summer. There was evidence of net retention of S by the peat in wet sites and during wet periods, and of net release of SO₄^2– (and acidity) under dry conditions. The 1995 summer drought and ensuing dry year in 1996 had marked and persistent effects on pH, apparent ion deficit (DEF), SO₄^2–, the divalent cations and Fe.     

Release Kinetics of Some Nutrients from a Sandy Loam Soil Treated with Different Organic Amendments

ABSTRACT

An incubation study was undertaken to examine the periodic release of some macronutrients and micronutrients in a sandy loam treated with different organic amendments (farmyard manure, mushroom compost, poultry manure, vermicompost, biogas slurry, and biochar of Lantana weed) added @ 15 t ha^?1 for 120 d through entrapment of released nutrients on ion exchange resins. Among organic amendments, the highest total contents were recorded for Ca, Mg, and S in farmyard manure, for K, Fe, and Mn in mushroom compost, for P, Zn, and Cu in biogas slurry, for B in biochar. The highest average release was recorded for P, Zn, Mn, and B from poultry manure, for Cu from biogas slurry, for Fe from vermicompost, for Ca, Mg, and S from mushroom compost, and for K from farmyard manure. The kinetics of mineralization and release of these nutrients conformed well to the zero-order kinetics and also to a power function equation. The initial release amount and release rate coefficient estimated by the power function equations were correlated significantly to the general properties of organic amendments and also to the type of C species present in organic amendment. Organic amendments having relatively higher content of water soluble C or fulvic or humic acids are likely to release nutrients through an early mineralization/solubilization from soil reserve.     

Soil chemistry versus environmental controls on production of CH4 and CO2 in northern peatlands

Rates of organic carbon mineralization (to CO₂ and CH₄) vary widely in peat soil. We transplanted four peat soils with different chemical composition into six sites with different environmental conditions to help resolve the debate about control of organic carbon mineralization by resource availability (e.g. carbon and nutrient chemistry) versus environmental conditions (e.g. temperature, moisture, pH). The four peat soils were derived from Sphagnum (bog moss). Two transplant sites were in mid‐boreal Alberta, Canada, two were in low‐boreal Ontario, Canada, and two were in the temperate United States. After 3 years in the field, CH₄ production varied significantly as a function of peat type, transplant site, and the type–site interaction. All four peat soils had very small rates of CH₄ production (< 20 nmol g^?1 day^?1) after transplant into two sites, presumably caused by acid site conditions (pH < 4.0). One peat soil had small CH₄ production rates regardless of transplant site. A canonical discriminant analysis revealed that large rates of CH₄ production (4000 nmol g^?1 day^?1) correlated with large holocellulose content, a large concentration of p‐hydroxyl phenolic compounds in the Klason lignin, and small concentrations of N, Ca and Mn in peat. Significant variation in rates of CO₂ production correlated positively with holocellulose content and negatively with N concentrations, regardless of transplant site. The temperature response for CO₂ production varied as a function of climate, being greater for peat formed in a cold climate, but did not apply to transplanted peat. Although we succeeded in elucidating some aspects of peat chemistry controlling production of CH₄ and CO₂ in Sphagnum‐derived peat soils, we also revealed idiosyncratic combinations of peat chemistry and site conditions that will complicate forecasting rates of peat carbon mineralization into the future.     

pH and hydrogen ion deposition patterns in precipitation for the continental United States and Canada

Maps of both volume-weighted mean pH and mean H⁺ deposition in precipitation were developed for the continental United States and Canada using data from 12 precipitation chemistry monitoring networks. The maps were constructed using 1981–1982 laboratory pH data from approximately 130 monitoring sites. The area of greatest acidity (lowest pH/highest H⁺ deposition) is located in the northern Ohio Valley and southeastern Ontario, an area corresponding to high SO_x and NO_x emissions.     

Factors controlling the removal of sulfate and acidity from the waters of an acidified lake

Although Lake Anna, an impoundment in Central Virginia, receives acid mine drainage (AMD) from Contrary Creek, the effects of the AMD pollution on the lake are less severe than expected. Previous work at Lake Anna has shown that bacterial sulfate reduction in the lake sediments plays an important role in the recovery of the lake from the AMD inputs. Sulfate removal rates were measured in sediment microcosms under a variety of experimental conditions to determine the factors controlling the rate of sulfate and acidity removal from the lake water. Sulfate removal rates were not significantly different over the short term (3 weeks) in summer sediment microcosms incubated at 6 vs 26 °C. Winter sediment microcosms showed no significant sulfate removal during the 18 day experiment when incubated at either 6 or 28 °C. Thus there is a strong seasonal temperature effect in Lake Anna sediments but no significant short term effect. Simulated AMD, with and without Fe, was added to sediment microcosms collected from an unpolluted part of the lake. The microcosms with Fe had significantly higher rates of sulfate removal indicating that Fe plays an important role in transporting sulfate to the sediment and/or in preventing oxidation of the reduced sulfide. After 27 days, from 54 to 96% of the added sulfate in the simulated AMD was recovered as FeS or S⁰ in the top 4 cm of sediment. In a separate experiment, ³⁵S-SO _inf4 ^sup2? was found to attach to precipitating Fe oxyhydroxides (1.5 to 4.7 mol SO _inf4 ^sup2? mol^?1 Fe precipitate) upon mixing Contrary Creek (AMD) and Lake Anna waters. Results of this study suggest that sulfate removal may be more rapid in metal rich AMD systems thans in metal poor systems characteristic of those which receive acidic deposition.     

The effect of sulphur dioxide on extractable manganese in soils

Abstract

The treatment of soil with SO₂ caused rapid increases in concentrations of water soluble plus exchangeable (extractable) Mn. The extractable Mn increased with increasing pressure of SO₂ and time of exposure, and increased with increasing water contents up to field capacity but decreased beyond this point. The addition of peat to a sandy loam increased the effect of SO₂ on extractable Mn while the addition of lime decreased the effect. Reactive Mn oxides in soils may contribute significantly to SO₂ sorption by soils.     

凉水国家级自然保护区溪流水化学特征分析

 采用集水区对比分析方法,探讨凉水国家级自然保护区不同森林类型对溪流水化学特征的影响。结果表明:保护区内各集水区溪流水均呈弱酸性至中性;主要离子中阳离子均以Ca2+的质量浓度最高,Na+次之,阴离子均以HCO3-的质量浓度最高,SO2-4次之;不同集水区溪流水中TP月平均质量浓度为0.031～0.077mg/L,TN为0.682～0.942 mg/L,NO-3-N的质量浓度高于NH+4-N;Fe的月平均质量浓度为0.030～0.037mg/L,Mn为0.010～0.012 mg/L。溪流水化学季节变化规律表现为:除HCO-3、SO2-4、Ca2+、Fe和Mn外,其他元素在融雪（4、5月）含量均较高;大多数元素在雨季的质量浓度比9月份低,而TN、TP、Fe、Mn表现为9月份的质量浓度低于雨季。保护区内原始阔叶红松林集水区溪流水质为最优,其他研究集水区溪流中的溶解物质含量增高,但除TP和SO2-4（p<0.05）外,差异性均不显著。说明凉水国家级自然保护区内虽进行过森林采伐和人工造林等人为干扰,但对于溪流水质并没有显著影响。     

The association of cobalt with easily reducible manganese in some acidic permanent grassland soils

The association of cobalt with manganese oxides was examined in some surface and sub-soils from areas of permanent grassland in England. Hydroquinone (0.2% in ammonium acetate at pH 7.0) was used to extract easily reducible Mn oxides from the soils. Two sequential extractions with this reagent removed significant proportions of the total Co along with the Mn; no significant amounts of iron were extracted. Mn-rich iron concretions were isolated from two of the soils and these contained from 230 to 880 μg g^?1 Co. Approximately 30% of the Co in the concretions was dissolved by two sequential extractions with hydroquinone with concurrent release of considerable amounts of Mn, but, as with the soils, not of Fe. The concretions contained 20–41 mg g^?1 Mn and 62–171 mg g^?1 Fe that were soluble in acidified H₂O₂. It was concluded that significant proportions of the Co in soils is associated with Mn oxides, and will show the same sensitivity to changes in acidity and redox potential as Mn.     

Influence of mist pH and acid anion on cation efflux from pinto bean foliage

The cation content of droplets collected from Phaseolus vulgaris (pinto bean) leaf surfaces during misting was more strongly influenced by mist pH (2.5, 4.0, or deionized water) than by source of acidity (HCI or H₂SO₄ + HNO₃). Concentrations of Ca²⁺, Mg²⁺, and K⁺ were highest in droplets from leaves treated with the pH 2.5 mists, but there were often no differences between the pH 4.0 and deionized water treatments. Cation content and pH of droplets from leaves treated with pH 2.5 mists increased across the three days of treatment, while those from leaves treated with less acidic mists decreased or did not change across the days of treatment. Source of acidity often affected foliar concentrations of Mg²⁺, K⁺, and Na⁺, but in inconsistent directions, and foliar concentrations of Mg²⁺ and K⁺ were unaffected by mist pH. Foliar Ca ²⁺ concentrations were often highest in leaves treated with pH 2.5 mists, in contrast to expectation, perhaps because of effects of acidic mist on foliar carbohydrate status. Despite the large efflux of cations from leaves treated with pH 2.5 mists, foliar cation concentrations in nonmisted foliage were sometimes lower than in misted foliage (Ca²⁺), but were higher in other cases (Na⁺) or indistinguishable in still others (K^?). While exposure of plants to highly acidic mists appeared to cause accelerated efflux of foliar cations, effects on foliar chemistry are probably dependent on soil nutrient status and on other aspects of plant vigor.     

Changes in chemical processes in soils caused by acid precipitation

In the absence of SO _infin4 ^sup= and NO _inf3 ^su? in precipitation, the pH of precipitation is primarily a function of CO₂?₂0 equilibria. Soil CO₂ and organic acids, acquired during descent through soil profiles, augment the dissolving capacity of the solutions which initially may have a pH of 4 or lower. The recent man-related increase of H₂SO₄ and HNO₃ in rainfall results in a significant lowering of pH in incident precipitation and an increase in corrosiveness of soil solutions. H₂SO₄ and HNO₃ may contribute some Eh buffering capacity. Particularly susceptible to these changes are clay minerals and redox sensitive elements such as Fe, Mn, Ni, and Co. The overall chemical weathering trends associated with increased acidity of rainfall will be de-stabilization and eventual solution of clay minerals (and the loss of their cation exchange capacity), increased rates of chemical denudation, and solution of illuvial Fe and Al oxides and hydroxides. The latter results in the loss of the adsorbed and coprecipitated metal trace elements associated with these highly reactive phases. The general result in soils developed on non-carbonate substrates is a tendency toward extensive podsolization, with associated decrease in clay minerals, loss of cation exchange capacity, and decrease in fertility.     

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.