Acids In Soil Solution Research Articles

Production of dissolved organic carbon and low-molecular weight organic acids in soil solution driven by recent tree photosynthate

Dissolved organic carbon (DOC) is an important component in the terrestrial carbon cycle. Yet, the relative importance of different inputs of DOC to the soil solution remains uncertain. Here, we used a large-scale forest girdling experiment to examine how the supply of recent photosynthate to tree roots and their mycorrhizal fungi affects DOC, in particular low-molecular weight organic acids (LMWOA). We also studied effects of tree girdling on non-structural carbohydrates in microorganism, and examined the effects of freezing of soil and the presence of roots in the soil samples on soil solution DOC and LMWOA in this experiment. The concentration of DOC was reduced by 40%, while citrate was reduced by up to 90% in the soil solution by the girdling treatment. Other LMWOA such as oxalate, succinate, formate and propionate were unaffected by the girdling. We also found that girdling reduced the concentrations of trehalose (by 50%), a typical fungal sugar, and of monosaccharides (by 40%) in microorganisms in root-free soil. The effect of freezing on DOC concentrations was marked in samples from control plots, but insignificant in samples from girdled plots. Release of DOC from cell lysis after freezing was attributed equally to roots and to microorganisms. Our observations suggest a direct link from tree photosynthesis through roots and their mycorrhizal fungi to soil solution chemistry. This direct link should impact solute transport and speciation, mineral weathering and C dynamics in the soil compartment. Importantly, our finding of a substantial photosynthate driven production of DOC challenges the paradigm that DOC is mainly the result of decomposition of organic matter.

Influence of organic acid and amino acid on cadmium and lead desorption from soil

A study was conducted to investigate the influence of organic acid (citric acid, oxalic acid) and amino acid (histidine) on the desorption of cadmium and lead from artificial contaminated soil in north-east China. Results showed that when the concentration of organic acid and amino acid in desorption solution was relatively low, the presence of organic ligands inhibited the desorption behaviour of Cd. When organic acid and amino acid concentrations were higher (>2 mmol/L), the presence of organic acid and amino acid obviously promoted Cd desorption. The increment of citric acid, oxalic acid, and histidine concentration significantly (P < 0.01) accelerated the desorption of Pb. The influence of organic acid and amino acid on the desorption behaviour of Cd and Pb followed the same sequence: citric acid > oxalic acid > histidine. This revealed that the organic acids had stronger affinity with heavy metals than that of amino acids. Functional group positions and types were important in determining if an organic acid or an amino acid would complex metals and increase their potential leaching. The results of this work implicated that amendment of organic acid and amino acid would enhance conditionally the bioavailability of heavy metals adsorbed by soils, relying on the type and concentration of organic acid and amino acid in soil solution.

Amino acid uptake in deciduous and coniferous taiga ecosystems

We measured in situ uptake of amino acids and ammonium across deciduous and coniferous taiga forest ecosystems in interior Alaska to examine the idea that late successional (coniferous) forests rely more heavily on dissolved organic nitrogen (DON), than do early successional (deciduous) ecosystems. We traced 15N-NH4+ and 13C-15N-amino acids from the soil solution into plant roots and soil pools over a 24 h period in stands of early successional willow and late successional black spruce. Late successional soils have much higher concentrations of amino acid in soil solution and a greater ratio of DON to dissolved inorganic N (DIN) (ammonium plus nitrate) than do early successional soils. Moreover, late successional coniferous forests exhibit higher rates of soil proteolytic activity, but lower rates of inorganic N turnover. Differences in ammonium and amino acid uptake by early successional willow stands were insignificant. By contrast, the in situ uptake of amino acid by late successional black spruce forests were approximately 4-fold greater than ammonium uptake. The relative difference in uptake of ammonium and amino acids in these forests was approximately proportional to the relative difference of these N forms in the soil solution. Thus, we suggest that differences in uptake of different N forms across succession in these boreal forests largely reflect edaphic variation in available soil N (composition), rather than any apparent physiological specialization to absorb particular forms of N. These finding are relevant to our understanding of how taiga ecosystems may respond to increases in temperature, fire frequency, N deposition, and other potential consequences of global change.

Review of methodologies for extracting plant-available and amorphous Si from soils and aquatic sediments

There is a variety of methodologies used in the aquatic sciences and soil sciences for extracting different forms of Si from sediments and soils. However, a comparison of the published extraction techniques is lacking. Here we review the methodologies used to extract different Si fractions from soils and sediments. Methods were classified in those to assess plant-available Si and those to extract Si from amorphous silica and allophane. Plant-available Si is supposed to comprise silicic acid in soil solution and adsorbed to soil particles. Extraction techniques for plant-available Si include extractions with water, CaCl2, acetate, acetic acid, phosphate, H2SO3, H2SO4, and citrate. The extractants show different capabilites to desorb silicic acid, with H2SO3, H2SO4 and citrate having the greater extraction potential. The most common extractants to dissolve amorphous silica from soils and aquatic sediments are NaOH and Na2CO3, but both also dissolve crystalline silicates to varying degrees. In soils moreover Tiron is used to dissolve amorphous silica, while oxalate is used to dissolve allophanes and imogolite-type materials. Most techniques analyzing for biogenic silica in aquatic environments use a correction method to identify mineral derived Si. By contrast, in the soil sciences no correction methods are used although pedologists are well aware of the overestimation of amorphous silica by the NaOH extraction, which is most commonly used to extract silica from soils. It is recommended that soil scientists begin to use the techniques developed in the aquatic sciences, since it seems impossible to extract amorphous Si from soils completely without dissolving some of the crystalline silicates.

Optimization of RP‐HPLC Analysis of Low Molecular Weight Organic Acids in Soil

RP‐HPLC analysis for low molecular weight organic acids in soil solution has been optimized. An Atlantis™ C18 column was used for the analyses. An optimal determination for eleven organic acids in soil solution was found at room temperature (25°C) and 220 nm detection wavelength, with a mobile phase of 10 mM KH2PO4–CH3OH (95∶5, pH 2.7), a flow rate of 0.8 mL/min and 10 µL sample size. The detection limits ranged 3.2–619 ng/mL, the coefficients of variation ranged 1.3–4.6%, and the recoveries ranged 95.6–106.3% for soil solution with standard addition on the optimal conditions proposed.

Modelling the effect of climate change on recovery of acidified freshwaters: Relative sensitivity of individual processes in the MAGIC model

The MAGIC model was used to evaluate the relative sensitivity of several possible climate-induced effects on the recovery of soil and surface water from acidification. A common protocol was used at 14 intensively studied sites in Europe and eastern North America. The results show that several of the factors are of only minor importance (increase in pCO 2 in soil air and runoff, for example), several are important at only a few sites (seasalts at near-coastal sites, for example) and several are important at nearly all sites (increased concentrations of organic acids in soil solution and runoff, for example). In addition changes in forest growth and decomposition of soil organic matter are important at forested sites and sites at risk of nitrogen saturation. The trials suggest that in future modelling of recovery from acidification should take into account possible concurrent climate changes and focus specially on the climate-induced changes in organic acids and nitrogen retention.

Implications of the evolution of organic acid moieties for basalt weathering over geological time

Concentrations of organic acids in prebiotic soils were presumably low, given limitations in abiotic synthesis and the limited lifetimes of organic molecules before the ultraviolet shield developed on early Earth. Prokaryotes, the first land-colonizing organisms, commonly secrete aliphatic carboxylic acids, and, less extensively, secrete aromatic compounds as siderophores and antibiotics. In contrast, secretion of aromatic acids is considerable for fungi, lichens, and vascular plants. Aromatic acids are also produced by degradation of high-molecular-weight compounds from lignin and tannin, both abundant in vascular plants. The proportion of aromatic carboxylic acids in soil solutions therefore probably increased with the evolution of higher order organisms. As biomass of organisms increased over geological time, concentrations of organic acids in soil solutions and, in turn, the extent of ligand-promoted dissolution of minerals probably increased. To elucidate the contribution of ligands during weathering on early Earth, Columbia River basalt was dissolved under oxic and anoxic conditions in the presence (0.001 or 0.01 M) and absence of several organic ligands in batch experiments at pH 6. Release of all elements including Si was enhanced considerably in the presence of organic ligands. Citrate (tridentate) and gallate (tetradentate) increased element release to the greatest extent among the aliphatic and aromatic ligands, respectively. The extent of element mobilization observed for the aliphatic ligands decreased in the order: citrate > oxalate ≈ malonate, and for the aromatic ligands: gallate > salicylate ≈ phthalate. The effects of the ligands generally followed trends in cation-ligand stability constants, but aromatic ligands were less effective in element mobilization than aliphatic ligands. One exception was gallate, an aromatic ligand, which significantly enhanced Cu release. Ligand-promoted mobilization of Cu may therefore have increased over geological time with the increase in the proportion of aromatic ligands. In the presence of organic ligands, Fe was mobilized from basalt considerably more than Al even under oxic conditions. Complexation of Fe with organic ligands may have mobilized Fe in Precambrian paleosols where little Al mobility is observed. Extent of P and Y release was minor in ligand-free experiments and considerable with ligands regardless of P_O₂. Release of Cu was considerable under oxic conditions, especially with ligands, and minor under anoxic conditions. Mobility patterns of P and Y could thus possibly serve as “organomarkers” (indicative of prevalence of organic ligands in soil solutions) and mobility patterns of Cu could possibly serve as “oxymarkers” (indicative of the presence of molecular oxygen), respectively, in ancient soils.

Modelling of aluminium chemistry in soil solution of untreated and dolomite treated podzolic soil

Soil solutions were taken from untreated and lime treated Norway spruce sites in the south of Sweden. At the limed sites 3.45 t ha −1 (L) or 8.75 t ha −1 (H) dolomite were applied 15 years before sampling. The mean concentration of inorganic Al (Al QR) in the untreated control plots (C) was 37 μM and the concentration of Al QR decreased when the dosages of dolomite treatments increased. For the H plots, with high doses of dolomite treatment the mean concentration of Al QR was 17 μM. The total Al (Al tot) also decreased by dolomite treatment, from 72 μM to 56 μM. Propionate (7–268 μM) and malonate (2–34 μM) were the low molecular weight organic acids (LMWOAs) found in the highest concentrations at Hasslöv. The differences in concentration of most of the determined LMWOAs were significantly increased due to treatment. The citrate average concentration for the different plots varied between 2.8 μM and 5.1 μM and the concentration of oxalate had concentrations between 1.7 μM and 2.6 μM. The experimental concentration of Al bound to LMWOAs obtained by ultrafiltration (<1000 Da) of the soil solution was in the mor layer on average 12% for the C plot and the L plots and 4% in the H plots. In the AE horizon the figures were 15% for the C plot and about 7% for the treated L and H plots, and in the Bhs horizon 9% for both the C, L and H plots. Two chemical equilibrium models were evaluated: Winderemere Humic Aqueous Model (WHAM) and a model treating the high molecular weight organic acids as monoprotic. The concentrations of inorganic Al, Al bound to identified LMWOAs and high molecular weights acids in soil solution (mor, AE and Bhs) from untreated and lime treated soil were calculated. The modelled inorganic Al values of the control plot were similar to the concentrations found in the soil solution (Al QR). For the dolomite treated plots the concentration of Al QR (0–40 μM) was higher in soil solution than that modelled by WHAM and the monoprotic model. The experimental concentration of Al bound to LMWOAs obtained by ultrafiltration (<1000 Da) of the soil solution was comparable but always higher than the modelled values. The modelling confirms that an appreciable part of the Al is strongly complexed with LMWOAs in the organic and eluvial horizons of the podzolized soil, suggesting that LMWOAs are efficient weathering agents and important for the formation of the eluvial horizon in podzols. Saturation indices (SI) for two solid Al phases were also calculated, including crystalline gibbsite and proto-imogolite (PI). The SIs for both the C and the treated plots (L and H) showed very broad ranges for the different horizons investigated, indicating undersaturation as well as oversaturation with respect to gibbsite and proto-imogolite.

(Un)certainty of overall binding constants of Al with dissolved organic matter determined by the Scatchard approach

One of the best approaches to date to obtain overall binding constants ( K o) for Al and dissolved organic matter (DOM) from acidic soil solutions is to collect ‘free’ Al data with diffusive gradients in thin films (DGT) and to infer the K o values by fitting a continuous distribution model based on Scatchard plots. Although there is clear established literature demonstrating the usefulness of the Scatchard approach, relatively little attention has been given to a realistic assessment of the uncertainties associated with the final fitted K o values. In this study we present an uncertainty analysis of the fitted K o values using a synthetic dataset with different levels of random noise and a real data set using DGT data from an acidic soil solution. The parameters in the continuous distribution model and their corresponding upper and lower 95% uncertainty bounds were determined using the Shuffled Complex Evolution Metropolis (SCEM) algorithm. Although reasonable fits of the distribution model to the experimental data were obtained in all cases, an appreciable uncertainty in the resulting K o values was found due to three main reasons. Firstly, obtaining ‘free’ Al data even with the DGT method is relatively difficult, leading to uncertainty in the data. Secondly, before Scatchard plots can be constructed, the maximum binding capacity (MBC) must be estimated. Any uncertainty in this MBC propagates into uncertainty associated with the final plots. Thirdly, as the final fitted K o values are largely based on extrapolation, a small uncertainty in the fit of the binding data results in an appreciable uncertainty in the obtained K o. Therefore, while trends in K o for Al and DOM could easily be discerned and compared, the uncertainty in the K o values hinders the application in quantitative speciation calculation. More comprehensive speciation models that avoid the use of K o seem to fit better for this purpose.

Organic complexation of Al and Fe in acidic soil solutions: Comparison of diffusive gradients in thin films analyses with Models V and VI predictions

The last decade has seen important development of analytical and modeling tools to investigate the interactions in solution between metals and dissolved organic matter. We evaluated the internal consistency of the analytical method of diffusive gradients in thin films (DGT) and the speciation Models V and VI for investigating organic complexation of Al and Fe in acidic sandy soil solutions. For this purpose we used our dataset obtained by DGT of the speciation of Al and Fe(III) between dissolved inorganic metal and soluble metal-dissolved organic matter (DOM) complexes in acidic soil solutions at pH=3.5, 4.0 and 4.5 and different imposed metal/organic carbon (M/C) ratios in solution. We modeled this data-set with Models V and VI, using the default parameter values but optimizing the fraction of DOM that is inert with respect to metal binding (24%). The calculated speciation of Al by both Models V and VI was generally in good agreement with the DGT results. However, at low Al/C ratios the Al fraction measured by DGT was higher than the dissolved inorganic fraction calculated by the models, most likely due to the detection of small (<1000 Da), labile Al–DOM complexes by DGT. This discrepancy did not occur with Fe(III) probably because its complexes with DOM are not labile enough to be detected by DGT. The dissolved, inorganic Fe fractions calculated by Model V were in good agreement with the DGT results, especially considering the fact that extra uncertainty was introduced by the analytically determined Fe(II)/Fe(III) speciation in solution. Model VI calculated the trends in Fe binding well, but generally estimated a higher dissolved, inorganic Fe fraction than Model V and DGT. The Fe(III) binding constant in Model VI was recently modified to reflect stronger binding to DOM. Since both Models V and VI predicted the inorganic Fe in solution to consist mainly of Fe(II), we propose a similar critical evaluation of the Fe(II) binding constant in Model VI.

Impact of ectomycorrhizas on the concentration and biodegradation of simple organic acids in a forest soil

SummarySimple organic acids seem to be important in many soil processes including nutrient uptake in the rhizosphere and long‐term pedogenic processes such as podzolization. The factors regulating the concentrations of these acids in soil, however, remain poorly understood. We have investigated one set of factors, namely the impact of ectomycorrhizal (Paxillus involutus) and non‐mycorrhizal Picea abies seedlings and humic acid on the concentration and dynamics of organic acids in soil solution. We did so over 10 months in laboratory columns containing soil from the E horizon of a sandy forest soil. Several organic acids were identified in the solution extracted from the root zone including oxalic, citric, malonic, succinic, acetic, formic and lactic acids at concentrations ranging from &lt;0.1 to 2.3 µm. Both plants and ectomycorrhizas had significant effects on the concentration of organic acids in soil solution. In contrast, omitting P from the irrigation water appeared to have little effect on the concentrations. The microbial mineralization kinetics of oxalate conformed well to a single Michaelis–Menten equation. Further, the soil with the mycorrhizas had a significantly faster mineralization of oxalic acid over a wide concentration range than did the soil without ectomycorrhizas and without plants. We conclude that the oxalate flux through the soil with both trees and mycorrhizas is much faster than is evident from measurements of solution concentration at steady state. Humic acid had little effect on the concentrations of organic acids or dynamics in the soil solution. Oxalic acid concentrations in the soil solution were correlated with hyphal length, rate of microbial mineralization, soil respiration, and shoot to root weight ratio. We conclude that both mycorrhizas and plants have a large impact on organic acid cycling in forest soils.

Physical and Zeta Potential Characteristics of Erodible Lateritic Soils

Physical, mineralogical, and zeta potential studies were conducted on selected erodible lateritic soils from Belo Horizonte, Brazil. These soils are residual and highly weathered, and they have a very low amount of organic matter and an acid-soil solution natural pH in the range of 4.8-5.3. Engineers consider similar soils to cover 65% of the Brazilian surface. Soil sampling was performed at representative geological sites underlain by gneiss, dolomitic-itabiritic phyllites, and phyllites as parent materials. The soil erosion susceptibility was in the following decreasing order: MA-2 (soil developed from standard gneiss, more coarse materials); R-2 (soil developed from milonitic gneiss); VP-2 (soil developed from phyllite); BR-2 (developed from dolomitic-itabiritic phyllites and rich in hematite). Soil samples developed from gneiss, whether standard gneiss or milonitic gneiss (MA-2 and AR-2), are highly prone to erosion. They do not present a reverse surface charge, although they contain the same major mineralogical phases as the ones developed from phyllites, particularly kaolinite (probably less weathered) and hematite (probably in very small amounts). All the areas studied in this research presented slopes above 20% and sparse or no vegetation. Soil samples developed from phyllites, namely, VP-2 and BR-2 (dolomitic-itabiritic phyllites), are less erosion prone and present charge reversal, with iso-points at a soil solution pH of 4.8 and 4.4, respectively. These samples show a strong cation adsorption and a less repulsive force. In this sense, the colloidal activity may tend to diminish erosion.

Environmental fate and impacts of sulfometuron on watersheds in the southern United States.

Dissipation of sulfometuron (SM), methyl 2-[[[[(4,6-dimethyl-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl] benzoate, in streamflow, sediment, plant tissue, litter, and soil following operational forestry applications at the target rate of 0.42 kg a.i. ha(-1) was monitored. Streamflow samples were collected at a weir on the perimeter and 30, 60, and 150 m downstream from the perimeter of the application site. Sulfometuron was detected in streamflow at low levels up to 29 days after treatment (DAT) on the watershed treated with the 75% dispersible granule formulation (Oust; DuPont Chemical Company, Wilmington, DE) and less than 53 DAT on the watershed treated with the experimental formulation (1% pellet). Twenty-four-hour average SM concentration in water ranged from not detected to a maximum of 49.3 microg L(-1). Sulfometuron was not detected at quantifiable levels (1 microg L(-1)) 150 m downstream. Stream sediment, vegetation, litter, and soil were sampled periodically up to 180 DAT. All samples were analyzed for SM by high performance liquid chromatography. Sulfometuron dissipated from these watersheds with half-lives that ranged from 4 d in plant tissues to 33 d in soil. Acidic soil solution on these treated watersheds contributed to their rapid dissipation. Environmental impacts are discussed for these watersheds in the context of available toxicological data.

Seasonal changes in organic compounds in soil solutions obtained from volcanic ash soils under different land uses

We investigated the organic matter composition in soil solutions as influenced by land use. Undisturbed mineral soils were collected periodically from mineral layers of Andosols under coniferous and deciduous forest, grasslands and upland fields for 1 year using soil core samplers. Soil solutions were obtained by centrifugation under −1.55×10 6 Pa condition, which equals to 4.2 pF. The dissolved organic carbon (DOC), hexoses, pentoses, phenolic acids and amino acids in soil solutions were determined. The concentrations of DOC under forest and grassland stands were not influenced by precipitation, but were negatively correlated to the yield of soil solution ( P<0.01). Temporal variations in concentrations of hexoses were similar to those of DOC at all Fujisawa sites but not at the Tsukui sites. Except for one forest stand, the concentrations of hexoses increased from autumn to winter. Concentrations of pentoses and phenolic acids showed similar seasonal variation in the forests and grasslands, and peaked in autumn and spring. Variations in the concentrations of pentoses corresponded to litter fall and ground grass cutting in forests, drying of grass at grasslands and harvest at upland fields. Although the concentrations of phenolic acids varied with seasons, differences between land uses were not clear. The release of phenolic acids might depend on the variation of the temperature. Correlations between pentoses and phenolic acids and DOC were weak. The concentrations of amino acids were lower than those of other organic compounds at all sites. At the upland fields, the concentrations of amino acids were significantly ( P<0.01) correlated with those of hexoses. The composition of dissolved organic matter (DOM) varied throughout the year. Seasonal changes in organic matters in soil solutions depended on yields of soil solution, temperature variations and the timing of litter supply under different land use.

Effects of lime and ash treatments on DOC fractions and low molecular weight organic acids in soil solutions of acidified podzolic soils

Dissolved organic carbon (DOC) fractions and different low molecular weight organic acids (LMWOAs) were determined in soil solutions from two lime or ash treated Norway spruce sites in the south of Sweden. At Hasslov, 3.45 t ha-1 or 8.75 t ha-1 dolomite were applied 15 years before sampling. Horrod was treated with 4.28 t ha-1 ash and 3.25 t ha-1 dolomite and sampled four years later. Propionate (7–268 μM) and malonate (2–34 μM) were the LMWOAsfound in the highest concentrations at Hasslov. Two other LMWOAs dominated at Horrod, namely citrate (18–64 μM)and fumarate (5–31 μM). The differences in concentration of most of the determined LMWOAs at Hasslov were significantly increased due to treatment. The LMWOAs comprised between 1.1–6.3% of the DOC at Hasslov and 4.5–17.6% at Horrod. At Hasslov normally 3–10% of the total acidity (TA) was due to LMWOAs and the average specific buffer capacity was 74 ± 22 mmol mol-1C.The total DOC concentration in the mor layer solution was ∼16 mM for the dolomite treated plots compared to ∼10 mM at the untreated plot. A major part of the increase in DOC at the treated plots apparently had a hydrophobic character and was of high molecular weight corresponding to 3–10 kDa. The concentration of DOC < 1 kDa in the control and treated plots was similar.

Simple method to enable the high resolution determination of total free amino acids in soil solutions and soil extracts

Amino acids, either in a free or polymeric state, represent a significant addition of N to most ecosystems and therefore comprise a key pool in the soil nitrogen cycle. The ability of some plants to take up amino acids directly from soil may confer a competitive advantage particularly in N-limited environments. Whilst measurements of inorganic N concentrations (NH 4 + and NO 3 −) are routinely undertaken on either soil solutions or soil water/KCl extracts, there is a need for a supplementary procedure to determine free amino acids and enable determination of all plant-available pools. The aim of this study is to develop and test a procedure for the rapid and sensitive determination of total free amino acids in soil solutions and soil extracts (water or 2 M KCl) and to compare this with the standard ninhydrin method of Moore and Stein (1954). The spectrofluorometric procedure relies on the reaction of free amino acids with o-phthaldialdehyde and β-mercaptoethanol. The fluorometric method is much more sensitive (working range 0.1–50 μM) than typical spectrophotometric analysis procedures for free amino acids which employs the ninhydrin reagent (working range 10–500 μM). In addition, the method only requires small sample volumes (1–50 μl), is rapid (1 min sample −1), simple to perform, and linear over a concentration range of three orders of magnitude (0.1–100 μM). The method was used to determine free amino acid concentrations across a range of ecosystem types (upland and lowland grasslands, forest, heathland and coastal saltmarsh). Generally, the concentration of free amino acids in soil solution was low and relatively independent of soil type. The average soluble N concentrations across all sites ( n=7) were 24±8 μM for amino acids, 39±14 μM for NH 4 + and 67±42 μM for NO 3 −. Free amino acids typically constituted 10–40% of the total soluble N in the soil solution and therefore comprise a significant soluble N and plant available pool in soil.

Efecto del Pinus ponderosa Dougl. sobre la reacción del suelo en el Sudoeste del Neuquén, Argentina

Soil reaction and some properties related with Al activity were studied in 26 plots distributed through an approximately west-east transverse: 13 with native vegetation, Nothofagus forest and 13 with Pinus ponderosa Dougl. -the first plantation from 14 to 56 years- under equivalent site conditions. Soil was sampled in each plot at three different points, and a composite sample of three simple samples per centimeter in the first five was obtained. The pH (KCl) values were near five, indicating a relative dominance of Al in the soil solution. They were lower than pH (H2O), probably indicating the presence of a variable electronegative charge. Within the five first soil centimeters, all properties, except pH (H2O), were significantly different. The marked decrease of pH (KCl) under pines, to values lower than five, indicated an increase of exchangeable acidity and Al. Total exchangeable cations were lower under pines, suggesting that Al occupied exchangeable sites. The absence of changes in soil solution acidity and the increase of exchangeable acidity are related to the high soil buffering capacity of volcanic soils enhanced by Al activity, and the permanent addition of volcanic ashes. The significant increase of pH (NaF) under pines would indicate a trend to allophane formation, which in turn would contribute to increases in the buffering capacity of the soils.

Influence of pH and metal/carbon ratios on soluble organic complexation of Fe(II), Fe(III) and Al(III) in soil solutions determined by diffusive gradients in thin films

Soluble complexation of Al(III), Fe(II) and Fe(III) with dissolved organic matter (DOM) greatly influences the bioavailability and mobility of the metals as well as the DOM itself in acidic forest soils. A scarcity of analytical tools to distinguish between ‘free’ Al(III), Fe(II) and Fe(III) and soluble organic complexes in acidic soil solutions has limited research in this area. We further tested diffusive gradients in thin films (DGT) for this purpose and used it to assess the influence of pH, redox potential and metal/organic carbon (M/C) ratios on soluble organic complexation of Al(III), Fe(II) and Fe(III). We used water extracts of an organic soil (H) horizon from a Fimic Anthrosol at pH 3.5, 4.0 and 4.5 to which we sequentially added the three metal species. DGT worked well for all but Fe(II) at pH 3.5 where a correction for proton competition was needed. For all three metal species, the ‘free’ fraction increased with increasing M/C ratios. The order of soluble complexation strength was Fe(III)>Al(III)>Fe(II). At low M/C ratios the ‘free’ fraction was highest at the lowest pH value due to more deprotonated functional groups, at high M/C ratios increased (organic) precipitation at higher pH reversed this effect. Both reduction of Fe(III) and oxidation of Fe(II) were found. Selective precipitation of Fe(III) complexes led to predominantly soluble Fe(II)–DOM complexes at higher M/C ratios. Therefore, in studies of the mobility and bioavailability of Fe and Al in acidic forest soils, both M/C ratios and Fe(II)/Fe(III) speciation in solution must be determined.

Modeling the distribution of aluminum speciation in acid soil solution equilibria with the mineral phase alunite

This paper reports the modeling of the distribution of aluminum (Al) speciation in acid soil solution equilibria with the mineral phase alunite. It is based on thermodynamic calculation using Turbo C++ language to write the computer program. Dependence of distribution of Al speciation on pH was investigated, including Al3+, Al(OH)2+, Al(OH)2+, Al(OH)4–, Al-F, Al-SO4, Al-Org, Al-PO4, and Al-ploy, as well as total Al concentrations. Influences of the various factors such as C*SO4, C*K, C*F, and DOC (dissolved organic carbon) on Al speciation were also inspected. Comparisons with gibbsite and other Al oxysulfate minerals such as jurbanite/basaluminite were performed. Some unique characteristics were discovered: for example, the SO42– concentration has a remarkable influence on the distribution of Al speciation, and the toxicity of Al is not a major factor in the soil solutions' equilibria with alunite. Results for application to the practical analysis of Al speciation in real soil solutions are reported.

Aluminium effects on organic acid mineralization in a Norway spruce forest soil

Organic acids such as malate, citrate and oxalate have been hypothesized to be involved in the long-term podzolization process and also to offer short-term protection to plants and microorganisms against aluminium under acid soil conditions. However, the reactions of organic acids in soil remain poorly understood. The aim of this study was to characterize the reactions of citrate and oxalate in Picea abies forest soils of contrasting Al status. 14C-labelled citrate and oxalate were added to soil at low levels close to steady state organic acid soil solution concentrations and their fate followed over a subsequent 24 h period. Organic acid decomposition was greatest in surface horizons ( t 1/2=0.5 h) compared to subsurface horizons ( t 1/2=8 h) and the long-term field addition of Al to soil did not influence the mineralization rate or biomass-C yield by the microbial community. In experiments where the levels of Al were manipulated in the laboratory, only at high Al concentrations (≥5 mM) was organic acid mineralization significantly affected. Similar experiments with non-Al complexing glucose indicated that this effect could be attributable to a direct Al complexation effect on the organic acids and partly due to an indirect toxic effect of Al on the microorganisms themselves. In conclusion, while the release of organic acids from plants may offer protection against rhizotoxic Al, the efficiency of this mechanism will be reduced by the rhizosphere microbial community.