Investigating sulfate sorption and desorption of acid forest soils with special consideration of soil structure

When investigating the reversibility of soil and water acidification due to a reduction of SO₄^2? deposition, the size and stability of the reversibly bound SO₄^2? fraction in soils are important parameters. The desorption behaviour of SO₄^2? in three acid forest soils was investigated using columns with undisturbed and disturbed (< 5 mm sieved) soil material. The results were compared to batch experiments. A comparison of the undisturbed and the disturbed soil samples showed that the soil structure had no effect on the chemistry of the soil solution, the S-mineralisation rates or the SO₄^2? desorption rates. A comparison of the batch and the column method showed only minor differences in desorption rates. However, fitting the measured desorption rates to a modified Langmuir equation showed a more distinct difference between both methods. It was concluded that the batch method was more suitable to establish SO₄^2? desorption isotherms. When investigating SO₄^2? dynamics of soils, the heterogeneity of the soils has to be considered because the spatial variability of isotherm parameters was found to be greater than differences between the investigated methods. Furthermore, SO₄^2? sorption showed a distinct hysteresis. While most of the sorbed SO₄^2? was desorbed at concentrations < 5–10 mg SO₄² ?1^?1, a sorption of SO₄^2? was observed only at concentrations > 20–30 mg SO₄^2? ·1^?1.     

Significance of DOM in the translocation of cations and acidity in acid forest soils

We estimated the contribution of dissolved organic matter (DOM) to cation leaching and the translocation of acidity in three acid forest soils. The analysis was based on monitored (2 years) concentrations of dissolved organic carbon (DOC) in the field, measured total acidities of DOM, and measured as well as predicted weighted mean dissociation constants of the organic acids. Although the forest floor solutions were strongly acidic (pH 3.47–4.10), a considerable proportion of the organic acids was dissociated and organic anions represented 22–40% of the total anions in the mineral soil input. The flux of DOM-associated exchangeable protons from the forest floor to the mineral soil ranged from 0.35 (Wülfersreuth) to 3.72 (Hohe Matzen) kmol ha^?1 yr^?1. In the subsoil, this organic acidity may be neutralized by microbial decomposition of the organic acids, but a part of the hydrogen ions may dissociate and contribute to acidification of the soil solution and to weathering processes. Due to the pronounced retention of DOM in the mineral subsoil horizons, the contribution of DOM to the output of cations and acidity from the soil is much lower than in the surface horizons but still significant.?     

Patterns of soil solution composition in acid forest soils: Differences between undisturbed and bulk samples

In acid forest soils pores draining under conditions of low water tension contain a solution usually differing in composition from the equilibrium solution of the bulk soil. For tree nutrition, the former was described as comparable to the bulk soil solution or less favourable. Our recent work on solution quality contradicted the universal validity of these findings. It turned out that the methods usually recommended tend to consistently reproduce the pattern of more advanced acidification of coarse pores. We applied our combination of methods to a range of soils to find out whether our previous finding of more favourable solution conditions in undisturbed soil samples as compared to the bulk soil is common or rather an exception. Undisturbed cores and bulk soil were sampled from four forested sites differing in pH and base saturation. We used an iterative procedure to adjust the equilibrium soil solution of disturbed soil and a water extraction with a soil:solution ratio of 1:2. To extract solution from undisturbed soil cores only small volumes of water were added, and we distinguished analytically between total (by ICP) and ionic (by CE) concentrations of cations. We calculated Ca/Al molar ratios and the fraction of (Al³⁺ + H⁺) on total cationic charge in solution as criteria for solution quality. Whereas with total concentrations, there always appeared to be less favourable conditions in the soil pore solution, free ionic concentrations allowed a differentiation between the soils. In view of plant nutrition, the quality of the soil solution from undisturbed samples was better, equal to, or worse than that of the bulk soil, suggesting that forest sites represent a continuum with respect to these chemical gradients. Even in soils where gradients are not observed it is not necessary to assume an equilibrium achieved by internal processes to exist between coarse pores and the bulk soil.     

某极度酸化的酸性硫酸盐土壤中可溶性和交换性酸的特征

An extremely acidified acid sulfate soil(ASS) was investigated to characterise its soluble and exchangeable acidity,The results showed that soluble acidity of a sample dtermined by titration with a KOH soulution was much significantly greater than that indicated by pH measured using a pH meter,paricularly for the extremely acidic soil samples,This is because the total soluble acidity of the extremely acidic soil samples was mainly composed of various soluble Al and Fe species,possibly in forms of Al sulfate complexes(e.g.,AlSO4^ ) and feerous Fe(Fe^2 )_,It is therefore suggested not to use pH alone as an indicator of soluble acidity in ASS,particularly for extremely acidic ASS,It is also likely that AlSO4^ actively participated in cation exchange reactions.It appears that the possible involvement of this Al sulfate cation in the cation adsorption has significant effect on increasing the amount of acidity being adsorbed by the soils.     

施用碱稳定固体减轻酸性土壤的酸度和铝毒

A pot experiment was catried out to study alleviation of soil acidity and Al toxicity by applying analkaline-stabilised sewage sludge product (biosolids) to an acid clay sandy loam (pH 5.7) and a strongly acidsandy loam (pH 4.5). Barley (Hondeum vulgare L. cv. Forrester) was used as a test crop and was grownin the sewage sludge-amended (33.5 t sludge DM ha^-1) and unamended soils. The results showed that thealka1ine biosloids increased soil pH from 5.7 to 6.9 for the clay sandy loam and from 4.5 to 6.0 for the sandyloam. The sludge product decreased KCl-extractable Al from 0.1 to 0.0 cmol kg^-1 for the former soil and from 4.0 to 0.1 cmol kg^-1 for the latter soil. As a result, barley plants grew much better and grain yield increased greatly in the amended treatments compared with the unamended controls. These observations indicate that alkaline-stabilised biosolids can be used as a liming material for remedying Al phytotoxicity instrongly acid soils by increasing soil pH and lowering Al bioavailability.     

Estimating water retention curves of forest soils from soil texture and bulk density

Forest soils differ significantly from the arable land in their distribution of the soil bulk density and humus content, but the water retention parameters are primarily derived from the data of agricultural soils. Thus, there is a need to relate physical parameters of forest soils with their water retention characteristics and compare them with those of agricultural soils. Using 1850 water retention curves from forest soils, we related the following soil physical parameters to soil texture, bulk density, and C content: air capacity (AC), available water capacity (AWC), and the permanent wilting point (PWP). The ACs of forest soils were significantly higher than those of agricultural soils which were related to the low bulk densities of the forest soils, whereas differences in AWCs were small. Therefore, for a proper evaluation of the water retention curves (WRCs) and the parameters derived from them, further subdivisions of the lowest (< 1.45 g cm^‐3) of the three bulk density classes was undertaken to the wide range of low soil densities in forest soils (giving a total of 5 bulk density classes). In Germany, 31 soil texture classes are used for the estimation of soil physical parameters. Such a detailed classification is not required because of insignificant differences in WRCs for a large number of these classes. Based on cluster analysis of AC, AWC, and PWP parameters, 10 texture collectives were obtained. Using 5 classes of bulk densities, we further calculated the ACs, AWCs, and the PWPs for these 10 classes. Furthermore, “van Genuchten parameters” (θ r, θ s, α, and n) were derived which described the average WRC for each designated class. In a second approach using multiple regression analysis, regression functions for AC, AWC, and PWP and for the van Genuchten parameter were calculated.     

Relationships between soil properties and feeding activity of soil fauna in acid forest soils

We investigated the relationships between soil chemical properties, humus form, and feeding activity in eight forest sites in Northrhine‐Westphalia, Germany. The study sites varied in forest type (oak, oak‐hornbeam, spruce, and pine). Three study sites were located under the same climatic conditions, and five study sites were distributed all over Northrhine‐Westphalia. We determined humus form, soil chemical properties, and feeding activity in three replication plots per site. We used the bait‐lamina test to determine feeding activity of soil fauna. Independent of forest type, all study sites were very acid with pH(CaCl₂) values in the Ah horizon between 2.8 and 4.0. The three study sites located under the same climatic conditions showed very homogenous soil chemical properties (pH in Ah: 2.9–3.0), whereas the five other sites varied significantly due to their soil chemical parameters (pH in Ah: 2.8–4.0). All single sites presented feeding activities with a very low spatial heterogeneity. Forest types and climatic conditions were not related to activity of soil fauna. Feeding activity and thickness of the O layer were strongly negatively correlated, whereas the activity was strongly positively correlated to soil pH in the five study sites with a wide range of soil chemical parameters. The three extremely acid forest sites presented significant differences in feeding activities that were not related to soil chemical properties.     

Low molecular weight organic acid adsorption in forest soils: effects on soil solution concentrations and biodegradation rates

Low molecular weight (LMW) organic acids are believed to play a key role in many rhizosphere and pedogenic processes; However, their efficiency is likely to depend on their susceptibility to sorption and biodegradation. The sorption characteristics of three organic acids (citrate, oxalate and acetate) and phosphate were examined over the concentration range 0-1000 μM in three coniferous forest soil profiles. Sorption to the soil's solid phase could be adequately described by the Langmuir equation with sorption capacity following the horizon series: B>C>E>O. The strength of anion sorption followed the series: phosphate>oxalate≥citrate?acetate. Calculations indicated that between 50 and 95% (O and E horizons) and >93% (B horizons) of these LMW organic acids entering the soil will become sorbed to the solid phase. The amount of organic acids predicted to be present on the solid phase at typical soil solution concentrations ranged from <1 to 1100 nmol g⁻¹ yielding adsorbed-to-solution ratios (adsorption coefficients) of between <0.1 and 3100. In the case of citrate, sorption to the solid phase significantly reduced its biodegradation potential by 35-99% depending upon the degree and type of sorption surface. The findings of this work are discussed in the context of the quantitative effects of adsorption on organic acids, their ecological functions and role in soil forming processes.     

Water-soluble organic matter and soil solution acidity in the main soil types of the central forest state biosphere reserve

The dynamics of the soluble organic carbon (C_WSO) and pH in soil solutions of the main soil types of the Central Forest State Biosphere Reserve were described. Possible reasons for the pH variation were considered. It was found that different soils are characterized by specific levels of pH and C_WSO. The structure of the soil solution acidity and its changes with depth were analyzed, and the relationship between the acidity and the content of water-soluble organics substances (WSOs) was determined. The predominantly hydrophilic nature and polydispersity of WSOs in soil solutions were revealed, as well as a decrease in the proportion of high-molecular soil fractions with depth. It was found that the mobility of metals in soils depends on the amphiphilic properties of WSOs to which they are bound, the metal features, and the soil type. The possibility of selective absorption of metals bound to the hydrophobic WSO fraction was shown.     

Mineralization of dissolved organic carbon in mineral soil solution of two forest soils

Dissolved organic carbon (DOC) constitutes an important carbon input flux to forested mineral soils. Seepage from mineral subsoils contains only small amounts of DOC because of mineralization, sorption or the formation of particulate organic matter (POM). However, the relation between these processes is largely unknown. Therefore, the objective of this study was to quantify the mineralization of DOC from different depths of forest soils, and to determine degradation rate constants for rapidly and slowly degradable DOC pools. Mineralization of DOC and formation of POM in mineral soil solution from two forested sites in northern Bavaria (Germany) were quantified in a 97 days laboratory incubation experiment. Furthermore, spectroscopic properties such as specific UV absorption and a humification index derived from fluorescence emission spectrometry were measured before and after incubation. DOC in all samples turned out to belong mainly to the stable DOC pool (> 95 %) with half‐lives ranging from years to decades. Spectroscopic properties were not suitable to predict the mineralization of DOC from mineral soils. However, together with data on DOC from the forest floor and long‐term data on DOC concentrations in the field they helped to identify the processes involved in C sequestration in mineral subsoils. Mineralization, formation of POM, and probably sorption seem all to be responsible for maintaining low concentrations of DOC in the upper mineral soil. DOC below the upper mineral soil is highly resistant to mineralization, and thus the further decrease of DOC concentrations in the subsoil as observed under field conditions cannot be attributed to mineralization. Our results suggest that sorption and to some minor extent the formation of POM may be responsible for C sequestration in the subsoil.     

Effect of land developmental process on soil solution chemistry in acid sulfate soils distributed in the Mekong Delta,Vietnam

Abstract

Water-soluble ionic substances in acid sulfate soils are likely to be strong controls for crop production and to have impacts on aquatic ecosystems. In dry seasons, in particular, oxidation of the soil surface followed by acidification probably produces lots of acids and soluble metals. To estimate acid and metal loads from acid sulfate soils to aquatic environments, we determined the composition of water-soluble ions from soils distributed in the Mekong Delta, Vietnam. At the end of the dry season, soils were taken from each soil horizon in two soil profiles on the delta under different land developmental processes. Water-soluble ions were extracted using both distilled water and artificially synthesized irrigation water (pH 6.3) adjusted to the same ionic strength and pH as the field canal water. The relationship between extracted basic cations in both extracts showed high linear correlation, indicating a similar extraction mechanism between both extractants. Higher ionic strength in the artificial irrigation water may not have any advantages for extraction by ion exchange and, thus, properties of extracts are likely to depend on the soil properties. The older the soil parent material, the larger the rates of soluble Al and Fe and the lower the pH. Progressive weathering of the soil on the older delta has already discharged greater amounts of bases, probably for compensation of acids, and the soil has started to release exchangeable Al sorbed onto negative-charge colloids and Fe from decomposed oxides. The soil profile of the older delta released relatively greater concentrations of Al with a lower content of base cations, where the annual averaged rice yield was half that of another site. Soluble metals and acids at both sites appear to accumulate in the upper horizons above the low permeable layer, which is probably widely distributed in the Mekong Delta.     

Long-term changes in the acidity of forest soils in North-East Scotland

Abstract. Fifteen soil profiles in the Alltcailleach Forest in NE Scotland have been resampled after almost 40 years. The pH, in 0.01 M CaCl₂, of the soil has decreased by 0.07 to 1.28 units in 80% of the surface organic horizons and by 0.16 to 0.54 units in 73% of the mineral horizons below 40 cm. The key factors governing increases and decreases in soil pH are changes in ground vegetation and tree canopy, although some effects of acid deposition cannot be ruled out.     

Occurrence of polymerized silicic acid and aluminum species in two forest soil solutions with different acidity

Silica species were determined in soil solutions from different soil horizons of acidic Dystric Cambisols in Austria and France. In general, more acid pH and elevated concentrations of Al, Si, and DOC were found in solutions of the upper soil horizons and in particular in the Austrian soil solutions. Aqueous Al speciation was dominated by complexes with respect to DOC and sulphate and by free Al³⁺, whereas dissolved Si was dominated by monosilicic acid. The average proportion of low molecular weight polymerized silicic acid (PS) is about 10 mol% of total dissolved Si. However, PS proportions sporadically reached up to 23 mol%. The origin of polymers is most likely due to weathering of soil silicates and polymerization of silicic acid by freezing or evaporation events.

We found a positive relationship between the occurrence of PS and the saturation degree with respect to secondary Al-silicates. Thus, we suggest that PS may even indicate precursors for the formation of secondary Al-silicates. In accordance with this concept, our results show that PS preferentially occurred in the upper B1 horizons, where low pH and high Al content may prevent its destruction.     

在实验酸性硫酸盐条件和酸性硫酸盐土壤上可溶性铝的控制

The controls of soluble Al concentration were examined in three situations of acid sulfate conditions:1) experimental acid sulfate conditions by addition of varying amounts of Al(OH)3(gibbsite) into a sequence of H2SO4 solutions;2)experimental acid sulfate conditions by addition of the same sequence of H2SO4 solutions into two non-cid sulfacte soil samples with known amounts of acid oxalate extractable Al; and 3) actual acid sulfate soil conditions.The experiment using gibbsite as an Al-bearing mineral showed that increase in the concentration of H2SO4 solution increased the soluble Al concentration,accompanied by a decrease i the solution pH, Increasing amount of gibbsite added to the H2SO4 solutions also increased soluble Al concentration,but resulted in an increase in solution pH.Within the H2SO4 concentration range of 0.0005-0.5mol L^-1 and the Al(OH)3 range of 0.01-0.5g(in 25 mL of H2SO4 solutions),the input of H2SO4 had the major control on soluble Al Concentration and pH .The availability of Al(OH)3，however,was responsible for the spread fo the various sample points,with a tendency that the samples containing more gibbsite had a higher soluble Al concentration than those containing less gibbsite at equivalent pH levels.The experimental results from treatment of soil samples with H2SO4 solutions and the analytical results of acid sulfate soils also showed the similar trend.     

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

The 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 > 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.     

Selenium retention in the organic matter of Swedish forest soils

Fractions of selenium present in the soil profiles of three Swedish podzols were analysed using a sequential extraction scheme to characterize Se distribution among the organic and inorganic fractions. The process by which selenite deposited from the atmosphere is retained in a podzolic profile rich in organic matter was studied in a column experiment. Selenium present in organic fractions accounted for most of the Se extracted by Na₄P₂O₇/NaOH. All soil organic matter fractions, particularly those in the B horizons, were considerably enriched with Se as compared with plant biomass. The most enriched fraction was that containing hydrophobic fulvates which had C to Se ratios ranging from 33 000 to 80 000. The distribution of Se among the organic fractions differed markedly from that of sulphur. Selenite applied to columns continuously for 67 d was fixed very rapidly upon entering the forest floor layers, with 77% being recovered in the top 2 cm of the forest floor after the experiment. In column leachates from the surface layers, C to Se ratios decreased progressively following Se application. No effect specifically related to Se application was observed for leachates and soil horizons underlying Bs1. The mechanism responsible for the efficient and rapid Se immobilization by organic matter is unknown.     

Aluminium chemistry of the soil solution in an acid forest soil as influenced by percolation rate and soil structure

The predicted activity of Al in the soil solutions of acid forest soils often differs from that observed in the field. We have investigated the influence of soil structure and flow rate of the soil solution on the aluminum release to explain this divergence. Disturbed and undisturbed samples of soil were collected from the A and B horizons of a dystric cambisol at Waldstein (Fichtelgebirge, Germany). The samples were irrigated with solutions mixed according to field data on throughfall or soil solution composition with pH 3.5 with flow rates of 4 mm d^?1, 12 mm d^?1 and 36 mm d^?1. The percolates were analysed for major ions. Resulting relations between pH and pAl were compared with batch experiments. In neither the A horizon nor in the B horizon did soil structure influence the relation between pH and pAl. The apparent equilibrium between pH and pAl was described as the pK_app value with pK_app= pAl—a pH (where a is an empirical constant). It was found that the pK_app values for the column percolates were in the range of variation of those found in batch experiments. Flow rate had no influence on pK_app at 4 and 12 mm d^?1. At 36 mm d^?1 a significant increase of pK_app was observed. This relative undersaturation of Al was more pronounced in the A horizon than in the B horizon. When flow is fast Al release into the percolating soil solution might be limited by diffusion.     

Area of cultivated acid sulfate soils in Finland

Abstract. A database of 1065 fields in all parts of Finland, two soil profiles (augerhole borings) per field, was screened for acid sulfate (a.s.) soils. Each field represented 2100 14;ha of cultivated land. Soil pH and redox potential were determined in the field, at intervals of 10 14;cm, to a depth of 200 14;cm. Of the maximum of 124 profiles considered as a.s. soils according to the Soil Taxonomy and ILRI (International Institute of Land Reclamation and Improvement) systems, 46 profiles exhibited pH <3.5. These represented 48 14;000 14;ha of land. More than half of these severely acidic soils were associated with reduced subsoils and probably contained actively oxidizing sulfidic materials within 150 14;cm of the soil surface, while the remaining profiles were oxidized at least down to 150 14;cm. Using Soil Taxonomy criteria, the total area of cultivated a.s. soils was 67 14;000–130 14;000 14;ha. The minimum estimates exclude soils that may be leached or too low in sulfide to meet the criteria of a.s. soils. Application of the ILRI system produced an estimate of 61 14;000–130 14;000 14;ha. In the maximum estimate, 27% of the profiles were raw, 61% ripe and 12% potential a.s. soils. According to the FAO/UNESCO system, the area of cultivated a.s. soils (pH <3.5 or assumed sulfidic materials) is considerably less: 43 14;000–78 14;000 14;ha. All these estimates are only a fraction of the area considered to be covered by a.s. soils by established Finnish criteria. The choice of estimate has important economic implications for liming subsidies and planning regulations for the drainage of a.s. soils.     

Analysis of sulfur compounds in acid sulfate soils and other recent marine soils

Abstract

A refined scheme for the semi micro chemical analysis of sulfur fractions in soils is presented. Pyrite is analyzed, as iron, after extraction in HNO₃. Non‐pyrite iron is excluded by a pretreatment with HF/H₂SO₄. Water‐soluble sulfate and jarosite [KFe₃(SO₄)₂(OH)₆], the other dominant sulfur fractions in acid sulfate soils, are analyzed turbidimetrically, as sulfate, after successive extractions by EDTA.3Na (water soluble plus exchangeable SO₄) and by hot 4 M HCl (jarosite). These methods are simpler, less bulky and more specific than most existing procedures.

Introduction of elemental sulfur analysis permits estimation of organic sulfur fraction as well. Sums of individual sulfur fractions agree well with separate total sulfur determinations.

The proposed analysis of pyrite permits also distinction of the components Fe₂O₃, FeO and FeS₂ in soils and rocks².