Development of QSAR's in soil ecotoxicology: Earthworm toxicity and soil sorption of chlorophenols,chlorobenzenes and chloroanilines

Soil adsorption and the toxicity of four chloroanilines for earthworms were investigated in two soil types. The toxicity tests were carried out with two earthworm species, Eisenia andrei and Lumbricus rubellus. LC₅₀ values in mg kg^?1 dry soil were recalculated towards molar concentrations in pore water using data from soil adsorption experiments. An attempt has been made to develop Quantitative Structure Activity Relationships (QSAR's) using these results and data on five chlorophenols and dichloroaniline in four soils and five chlorobenzenes in two soils published previously (Van Gestel and Ma, 1988, 1990; Van Gestel et al., 1991). Significant QSAR relationships were obtained between 1) adsorption coefficients (log K _om ) and the octanol/water partition coefficient (log k _ow ), and 2) LC₅₀ values (in itμmol L^?1 soil pore water) and log K _ow . It can be concluded that both earthworm species tested are equally sensitive to chlorobenzenes and chloroanilines, E. andrei is more sensitive than L. rubellus to chlorophenols.     

Cu and Ni Mobility and Bioavailability in Sequentially Conditioned Soils

The potential ecological hazard of metals in soils may be measured directly using a combination of chemical and biological techniques or estimated using appropriate ecological models. Terrestrial ecotoxicity testing has gained scientific credibility and growing regulatory interest; however, toxicity of metals has often been tested in freshly amended soils. Such an approach may lead to derivation of erroneous toxicity values (EC₅₀) and thresholds. In this study, the impact of metal amendments on soil ecotoxicity testing within a context of ion competition was investigated. Four coarse-textured soils were amended with copper (Cu) and nickel (Ni), incubated for 16 weeks and conditioned by a series of total pore water replacements. Rhizon^TM extracted pore water Cu, Ni, pH and dissolved organic carbon (DOC) concentrations were measured after each replacement. Changes in ecotoxicity of soil solutions were also monitored using a lux-based biosensor (Escherichia coli HB101 pUCD607) and linked to variations in soil solution metal and DOC concentrations, pH and selected characteristics of the experimental soils (exchangeable calcium (Ca) and magnesium (Mg)). Prior to conditioning of soils, strong proton competition produced relatively high EC₅₀ values (low toxicity) for both, Cu and Ni. The successive replacement of pore waters lead to a decline of labile pools of metals, DOC and alleviated the ecotoxicological protective effect of amendment impacted soil solution chemistry. Consequently, derived ecotoxicity values and toxicity thresholds were more reflective of genuine environmental conditions and the relationships observed more consistent with trends reported in historically contaminated soils.     

Copper in the terrestrial environment: Verification of a laboratory-derived terrestrial biotic ligand model to predict earthworm mortality with toxicity observed in field soils

This study was set up for validation of a regression model to predict mortality in the terrestrial earthworm Aporrectodea caliginosa following exposure to copper. This model was derived from a terrestrial biotic ligand model and incorporates the protective effects of H⁺ and Na⁺ on copper toxicity.Three soil sets were used for the experiments, all of which had a different copper contamination history over more than 20 years and were considered to be aged field soils. The soils were characterized by analysis of various copper pools in the solid phase and in the pore water, analysis of physical and chemical soil properties and by regression analysis. Measured and calculated copper activities (expressed as pCu) correlated reasonably well. Measured copper activities correlated with the total copper concentration in the pore water and the pH. The organic matter in the solid phase had no influence on the pCu in these soil sets.Earthworms were exposed to the soils for 28 days, after which survival was scored. Observed earthworm mortality after 28-days exposure was plotted as a function of the log-transformed difference between predicted (log₁₀ transformed) LC50-values and measured values of pCu for validation of the regression model. The results obtained were in agreement with the assumption that mortality is to be observed in those soils where the predicted LC50 exceeds the measured pCu. However, a structural underestimation of toxicity was apparent, which is most likely due to mixture effects related to the presence of additional substances in field soils. Nevertheless, the trend of the results in the validation tests demonstrates that the newly developed toxicity model is a useful tool in predicting lethality of copper contamination to earthworms in field soils.     

Analysis of the variable charge of two organic soils by means of the NICA-Donnan model

We have tested to see if the generic set of NICA‐Donnan model parameters, used to describe isolated humic substances, can also describe soil humic substances in situ. A potentiometric back‐titration technique was used to determine the variable surface charge of two organic peat soils at three different ionic strengths. The non‐ideal, competitive‐adsorption NICA‐Donnan model was used to simulate the surface charge, by assuming a bimodal distribution of H⁺ affinity on the soil solid phase. The model provided an excellent fit to the experimental data. The Donnan volume, V_D, varied slightly with ionic strength, although the variation was less than for humic substances in solution. The values obtained for the parameters that define the affinity distributions, the intrinsic proton binding constant (log K_i^int) and the heterogeneity of the site (m_i), were similar to those observed for isolated soil humic acids. The abundance of carboxylic groups in the whole soil represented 30% of the typical value for isolated soil humic acids. The composition of the organic matter of the whole soils, obtained by ¹³C CPMAS NMR, was comparable to the characteristic composition of soil humic acids.     

Zinc Sorption in Selected Soils

Abstract

The adsorption of nutrient elements is one of the most important solid‐ and liquid‐phase interactions determining the retention and release of applied plant nutrients and the efficiency of fertilization. The study showed that the soils with high cation exchange capacity (CEC), CaCO₃, organic matter contents, and heavy texture adsorbed more zinc (Zn). The alkaline soils from Pakistan adsorbed more Zn than English acidic soils. Langmuir and Freundlich isotherm fit was excellent, and r² values for the Langmuir isotherm were highly significant (r²=0.84 to 0.99). The Langmuir b values, representing the adsorptive capacity of a soil, increased as the texture fineness increased in the soil, with increases in the concentration of adsorptive material (such as organic matter and CaCO₃) and with increases in CEC and pH. The alkaline soils from Pakistan had higher bonding energy constant and higher log Kf values than the acidic English soils. Sequential extraction of Zn in these soils showed that most of the Zn was held in CaCO₃ pool in the alkaline soils, whereas in acidic soils adsorbed Zn was in exchangeable form.     

Increasing the Knowledge on the Management of Cu‐Contaminated Agricultural Soils by Cropping Tomato (Solanum Lycopersicum L.)

This paper increases the knowledge on the potential use of Cu‐contaminated agricultural soils with tomato (Solanum lycopersicum L.). The effect of Cu and its interaction with soil properties on plant biomass production and on the accumulation of this metal in plant tissues were evaluated by conducting biomass assays in four representative Mediterranean agricultural soils contaminated by Cu. Copper toxicity on plant biomass production, evaluated through the effective concentrations of Cu added to soil that reduce the biomass production by 50% (EC₅₀) and by 10% (EC₁₀), was higher in soils having less soil organic matter and clay content and even in soils with favourable properties but having salinity. For the cases in which tomato was collected, Cu concentrations in them were similar for all soils and doses and never exceed the maximum Cu concentration allowed by the Codex Alimentarius Commission Regulation (10 mg_Cu kg⁻¹ in fresh weight basis). According to our results, tomato could be cropped in Cu‐contaminated Mediterranean agricultural soils when concentrations of Cu determined in them rely between their respective EC₅₀ and EC₁₀, because production and quality of fruits, the latter understood as the Cu concentration in them, would not be compromised. For the soils assayed, these values would range between 32·9 and 1696·5 mg kg⁻¹, depending on soil properties. Because the baseline value considered is similar to those established in other parts of the European Mediterranean region, these results can be used as guidance for this region to establish adequate phytoremediation strategies and prevent land degradation processes. Copyright © 2014 John Wiley & Sons, Ltd.     

Decomposition of Organic Matter with Previous Cadmium Adsorption in Soils

Decomposition of organic matter with previous Cd adsorption (thereafter referred to as OMACd) in soils and in water was studied in order to clarify the mechanism of Cd-induced inhibition of organic matter decomposition in soil. Two types of organic materials (sludge, rice straw) with or without previous Cd adsorption were mixed with a Gley soil or a Light-colored Andosol in a proportion of 1%. In the soils amended with the Cd-free organic materials, a CdCl₂ solution was added to the soils. The decomposition of the organic matter was examined by measuring the CO₂ evolution for 4 weeks at 28°C. Although the same amount of Cd was added to the soils, the decomposition of OMACd was inhibited to a greater extent than that in the soils to which a CdCl₂ solution had been added.

Furthermore the decomposition of sludge with previous Cd adsorption (thereafter referred to as SACd) in water after inoculation of soil microorganisms was investigated. Although the control sludge without Cd was markedly decomposed at 30°C during 4 weeks, SACd was not appreciably decomposed. These results suggest that OMACd cannot be readily decomposed by microorganisms.     

Acetochlor as a soil pollutant

The agricultural use of pesticides leads to diffuse pollution whereby the various contaminants of the soil infiltrate into the groundwater reaching lakes and drinking water aquifers. Due to the extensive application of these chemicals, their leaching presents a high environmental risk. Since the adsorption coefficient (K) characterizes the soil / water partitioning [1] and is also representative for leaching, the first step in understanding of the environmental fate of a pollutant is to study its adsorption properties. Weak binding to the soil constituents (low K) leads to groundwater pollution, while a strong binding (high K) results in surface water pollution through the erosion of the soil. Acetochlor is a widely used herbicide all over the world. Similar to other organic pollutants, the environmental fate of this chemical is strongly related to its adsorption properties. Static adsorption equilibrium measurements were carried out at 25°C on different types of Hungarian soils (chernozem, brown forest and sandy soil) characterized by varying amounts of organic matter and pH values. Acetochlor solutions were prepared in the presence of nitrate and phosphate ions (0.1 mol/L sodium nitrate and 0.1 mol/L phosphate uffer, pH=7) which are constituents of fertilizers occurring in high concentrations in the environment. In order to appreciate their effect, adsorption studies were also performed in pure aqueous medium. The equilibrated liquid was analyzed after centrifugation by two different methods (Total Organic Carbon measurement, High Performance Liquid Chromatography). Isotherms obtained under different conditions, as well as on various soils, exhibit a similar shape, thus indicating a two-step adsorption process. The plots cannot be interpreted according to the classes of isotherms suggested by Giles (H-, L- and C-type, [2]). The adsorption coefficients were estimated from the initial slope of the curves. These values were determined not only by the type of the soil, but also by the composition of the aqueous media. Due to the low value of the adsorption coefficients, the acetochlor is a rather mobile pollutant of the soil posing a potential danger to the aquatic environment. The organic matter adsorption coefficients (K_om) [3] were also calculated and they were approximately identical for soils of high organic matter. For the chernozem and brown forest soils, the values of the K and K_om parameters are increasing in the order from water < phosphate buffer < sodium nitrate. For soils of low organic content, the similarity of the K_om values cannot be expected (due to the hyperbolic nature of the equation) as the data really indicate it for the adsorption behavior of the sandy soil. Here, the organic matter plays a less important role and the adsorption is controlled by the solute / inorganic substance interactions. This conclusion is nicely proved by the adsorption of the acetochlor on quartz resulting thereby in a similar plot being obtained for the soils. According to the hypothesis presented here, the first step of the isotherms is controlled by the solute / surface interactions while the solute / adsorbed solute interactions are operating in the second step of the isotherm. The role of the organic matter in this region of the isotherm is probably negligible. The comparison of the adsorption coefficients leads to the conclusion that the presence of nitrate and phosphate ions enhances the adsorption of acetochlor on the soils containing a high percentage of organic matter. As these ions are present in the environment due to the extensive use of fertilizers, they may increase the acetochlor pollution of water by erosion. This conclusion corroborates those earlier observations that reported frequent acetochlor contamination of the surface waters [4–5]. As the organic matter content of the soils plays an important role in the acetochlor adsorption, humic substances must have a strong influence on the transport of this compound. Experiments to obtain adsorption isotherms of further pesticides and the development of a quantitative model are in progress.     

Salinity effects on carbon mineralization in soils of varying texture

In salt-affected soils, soil organic carbon (SOC) levels are usually low as a result of poor plant growth; additionally, decomposition of soil organic matter (SOM) may be negatively affected. Soil organic carbon models, such as the Rothamsted Carbon Model (RothC), that are used to estimate carbon dioxide (CO₂) emission and SOC stocks at various spatial scales, do not consider the effect of salinity on CO₂ emissions and may therefore over-estimate CO₂ release from saline soils. Two laboratory incubation experiments were conducted to assess the effect of soil texture on the response of CO₂ release to salinity, and to calculate a rate modifier for salinity to be introduced into the RothC model. The soils used were a sandy loam (18.7% clay) and a sandy clay loam (22.5% clay) in one experiment and a loamy sand (6.3% clay) and a clay (42% clay) in another experiment. The water content was adjusted to 75%, 55%, 50% and 45% water holding capacity (WHC) for the loamy sand, sandy loam, sandy clay loam and the clay, respectively to ensure optimal soil moisture for decomposition. Sodium chloride (NaCl) was used to develop a range of salinities: electrical conductivity of the 1:5 soil: water extract (EC_1:5) 1, 2, 3, 4 and 5 dS m⁻¹. The soils were amended with 2% (w/w) wheat residues and CO₂ emission was measured over 4 months. Carbon dioxide release was also measured from five salt-affected soils from the field for model evaluation. In all soils, cumulative CO₂-C g⁻¹ soil significantly decreased with increasing EC_1:5 developed by addition of NaCl, but the relative decrease differed among the soils. In the salt-amended soils, the reduction in normalised cumulative respiration (in percentage for the control) at EC_1:5 > 1.0 dS m⁻¹ was most pronounced in the loamy sand. This is due to the differential water content of the soils, at the same EC_1:5; the salt concentration in the soil solution is higher in the coarser textured soils than in fine textured soils because in the former soils, the water content for optimal decomposition is lower. When salinity was expressed as osmotic potential, the decrease in normalised cumulative respiration with increasing salinity was less than with EC_1:5. The osmotic potential of the soil solution is a more appropriate parameter for estimating the salinity effect on microbial activity than the electrical conductivity (EC) because osmotic potential, unlike EC, takes account into salt concentration in the soil solution as a function of the water content. The decrease in particulate organic carbon (POC) was smaller in soils with low osmotic potential whereas total organic carbon, humus-C and charcoal-C did not change over time, and were not significantly affected by salinity. The modelling of cumulative respiration data using a two compartment model showed that the decomposition of labile carbon (C) pool is more sensitive to salinity than that of the slow C pool. The evaluation of RothC, modified to include the decomposition rate modifier for salinity developed from the salt-amended soils, against saline soils from the field, suggested that salinity had a greater effect on cumulative respiration in the salt-amended soils. The results of this study show (i) salinity needs to be taken into account when modelling CO₂ release and SOC turnover in salt-affected soils, and (ii) a decomposition rate modifier developed from salt-amended soils may overestimate the effect of salinity on CO₂ release.     

Selenium Distribution,Chemical Fractionation and Adsorption in some Egyptian Alluvial and Lacustrine Soils

Fifty-five soil samples representing Egyptian alluvial and lacustrine soils were chemically analyzed for total Se which was found to vary from 0.18 to 0.85 ppm with an average of 0.45 ppm. These levels are positively correlated with organic matter, total carbonate and clay content of the soils. Minimum variation of total Se with soil depth was found. The chemical fractionation of soil Se, expressed as percent of the total, indicates that on the average about 25.4 % exists in 0.2 M K₂SO₄-extractable form, 18.5 % is extracted with 0.5 N NH₄OH, 9.7 % as 6 N HCl-extractable form and 13.8 % as extractable with 9 N HNO₃. Amounts of K₂SO₄-Se and HNO₃-Se in soils correlated significantly with soil organic matter, total carbonate, free iron oxide and clay content. The NH₄OH-Se and HCl-Se fractions correlated significantly only with organic matter and clay content. There is also significant correlation between total Se and the studied Se fractions. Specific adsorption of Se by soils was low as expressed by the Langmiur adsorption maximum values. The high soil pH has a reducing effect on Se adsorption.     

Xe NMR spectroscopy of adsorbed xenon as an approach for the characterisation of soil meso- and microporosity

¹²⁹Xe nuclear magnetic resonance (NMR) spectroscopy of adsorbed xenon was applied for the characterisation of soil meso- and microporosity. Model systems (Ca-montmorillonite, quartz sand) and three soil types (Luvisol Alh, Bt and Cv horizons; Gleysol Go horizon; Podzol Bvs horizon) were studied. For Ca-montmorillonite, the average intercrystallite pore size has been evaluated. In natural soils, ¹²⁹Xe resonance parameters were shown to be affected by different factors: pore heterogeneity, influence of organic functional groups, possible presence of paramagnetic compounds, occurrence of xenon exchange between inter- and intraparticle void spaces. The effect of those factors on the pattern of ¹²⁹Xe NMR spectra was tested. In the three soils studied, no micropores within the mineral phase available for xenon adsorption were found. The most probable reason is that such pores are occupied by small molecules of the soil organic matter (SOM). Variable extent of accessibility of mesopores within the mineral phase of the various soils was revealed. It was highest in the Podzol. Here, xenon exchange between different adsorption zones (i.e., pores of differing size, e.g., internal and external void spaces) was slow on an NMR time scale that allowed to detect separate signals, each characterising xenon behaviour in the respective adsorption zone. The pore system of the soil organic matter was shown to be not accessible for xenon, as it is accepted for N₂ and other nonpolar adsorbates. Based on analysis of the spectra, a model for the possible mutual location of organic matter and iron compounds in natural soils was suggested. According to this model, a certain part of organic matter species can form the layers above iron species, thus masking them and preventing ¹²⁹Xe NMR spectra from significant low-field shifts and signal broadening.     

Can a Single and Unique Cu Soil Quality Standard be Valid for Different Mediterranean Agricultural Soils under an Accumulator Crop?

The validity of the soil quality standard for copper (Cu) established by the Spanish legislation (Spanish Royal Decree 9/2005) is evaluated in representative agricultural Mediterranean soils under an accumulator crop (Lactuca sativa L. var. Romaine cv. Long Green), considering both the effect of the metal on crop growth (biomass production) and its accumulation in the edible part of the plant. For saline soils, such a soil quality standard seems not to be valid taking into account both of the aspects evaluated. For non-saline soils, the soil quality standard also seems not to be valid since, considering the metal accumulation in the edible part of the plant, the soil quality standard should be above such standard; but considering the productivity function of soil (biomass production), the standard should be much below, meaning that this function is being greatly affected by the presence of high concentrations of Cu. The soil quality standard for each soil considered should correspond to a value between its respective EC₅₀ and EC₁₀ values (effective concentrations of added Cu causing 50% and 10% inhibition on the biomass production), depending on the politicians and/or farmers' compromise with yield production and, therefore, with soil productivity. These threshold values were greater for the soil having more organic matter and clay content, showing that Cu toxicity also depends on these properties. Further research in other agricultural areas of the region would improve the basis for proposing adequate soil quality standards as highlighted by the European Thematic Strategy for Soil Protection.     

Short-term carbon mineralization in saline–sodic soils

Previous studies have shown that carbon (C) mineralization in saline or sodic soils is affected by various factors including organic C content, salt concentration and water content in saline soils and soil structure in sodic soils, but there is little information about which soil properties control carbon dioxide (CO₂) emission from saline-sodic soils. In this study, eight field-collected saline–sodic soils, varying in electrical conductivity (EC_e, a measure of salinity, ranging from 3 to 262 dS m⁻¹) and sodium adsorption ratio (SAR_e, a measure of sodicity, ranging from 11 to 62), were left unamended or amended with mature wheat or vetch residues (2% w/w). Carbon dioxide release was measured over 42 days at constant temperature and soil water content. Cumulative respiration expressed per gram SOC increased in the following order: unamended soil<soil amended with wheat residues (C/N ratio 122)<soil with vetch residue (C/N ratio 18). Cumulative respiration was significantly (p < 0.05) negatively correlated with EC_e but not with SAR_e. Our results show that the response to EC_e and SAR_e of the microbial community activated by addition of organic C does not differ from that of the less active microbial community in unamended soils and that salinity is the main influential factor for C mineralization in saline–sodic soils.     

Transformation of beech forest litter as a factor that triggers arsenic solubility in soils developed on historical mine dumps

Purpose

Soils that develop on the dumps in historical arsenic mining sites contain high concentrations of As thus constituting a serious environmental risk. This study was aimed to examine the changes in arsenic solubility in mine soils as induced by organic matter introduced with forest litter.

Materials and methods

Four large samples of initially developed soils were collected from the dumps remaining in former mining sites and were incubated for 90 days at various moistures: 80% of maximum water holding capacity and 100% (flooded conditions), with and without addition of beech forest litter (BL), 50 g/kg. Soils contained up to 5.0% As. Soil pore water was collected periodically with MacroRhizon suction samplers and examined on As, Mn, and Fe concentrations, pH, Eh, and dissolved organic carbon (DOC). The properties of dissolved organic matter were characterized by UV-VIS spectroscopic parameters A4/A6 and SUVA₂₅₄.

Results and discussion

Application of BL resulted in an intensive release of As from soils, particularly at 100% moisture. As concentrations in soil pore water increased strongly during the first 2 or 4 weeks of incubation and then started to decrease in all cases, except for one flooded soil. As was released particularly intensively from carbonate-containing soils. The mechanisms of As mobilization, including reductive dissolution of Mn and Fe oxides and the competition with DOC for sorption sites on the oxides, were discussed as related to soil properties. Pore water concentrations of DOC were increasing at the beginning of incubation and started to decrease after two or four weeks. Spectroscopic parameters of dissolved organic matter in ZS soils indicated increasing aromaticity and progress of humification.

Conclusions

Forest litter introduced to mine dump soils causes a mobilization of As into soil pore water. This effect, particularly strong in carbonate-rich soils, is apparently related to high concentrations of DOC and usually declines with time, which may be explained by the progress in humification. The relationships between DOC properties and As speciation in soil pore water should be dissected for better interpretation of experimental results.

     

Influence of soil factors on the soil enzyme inhibition by Cd

A comparative study was conducted on the toxicity of Cd to alkaline phosphatase activity (ALP) and dehydrogenase activity (DHA) in 18 top soils with contrasting soil properties representative of 14 major soil types in China. Soil pH and carbonate content, soil organic matter, and cation exchange capacity (CEC) largely affected the Cd toxicity on two enzyme activities; with the soil pH having only minor effect on the median ecological dose values based on total Cd concentrations (ED_{50 T}). The values of ED_{50 T}/ED_{50 W} (based on water-soluble Cd content) of alkaline phosphatase and dehydrogenase were strongly influenced by pH and CEC contents, which explained up to 71% of the variation for alkaline phosphatase, 82% of the variation for dehydrogenase, and also were significantly correlated with the parameter K_F derived from Freundlich adsorption isotherms. This study suggests that the values of ED_{50 T}/ED_{50 W} could be useful to evaluate the buffer capacity of soils which protects soil enzymes from harmful effects of heavy metal.     

ADSORPTION OF HERBICIDE-DERIVED p-CHLOROANILINE RESIDUES IN SOILS: A PREDICTIVE EQUATION

Relationships between the adsorption of p-chloroaniline and the original adsorbate concentration were investigated for five soils ranging in organic matter content from 1.7 to 8.1 per cent and in clay content from 0.5 to 21 per cent. Adsorption data were analyzed applying the linear form of the Freundlich equation. To evaluate the general relationship between adsorption of p-chloroaniline by soils and the solution concentration (C₀), values of partition coefficient (Kp), reflecting the magnitude of distribution of chemical at equilibrium between soil colloids and solution were calculated. The experiments showed that the regression parameters were significantly correlated with the soil organic matter content. A comparison of the experimental results obtained with other soils and the calculated values gave satisfactory agreement.     

Influence of organic matter on selenite sorption by Andosols

Abstract

The influence of soil organic matter on selenite sorption was investigated in the selenite adsorption capacity and the surface particle charge change by ligand exchange reaction using the hydrogen peroxide (H₂O₂) treatment and the ignition treatment of two Andosols. The removal of organic carbon (C) in soils accelerated selenite sorption, implying that organic matter of soils had negative influence on the selenite adsorption on the soils. Positive charge decrease on soil particles, concomitant proton consumption, and release of silicon (Si), sulfate (SO₄ ^2‐), and organic C were observed in selenite sorption by the soils. The development of surface particle negative charge with selenite sorption was smaller in the H₂O₂‐treated soil than in the original soils and was scarcely observed in the ignition‐treated soil. It can be assumed that the increase of negative charge by selenite sorption was attributed to new negative sites borne by released insoluble organic matter and negative charge development directly by selenite sorption was small.     

Characterization by isoelectric focusing of the organic matter of a regenerated soil

Abstract

Regeneration of a soil with a high degree of desertification was conducted by the addition of different doses of municipal solid waste organic matter (MSW). Five years after this treatment, humic substances were extracted from these soils and characterized by spectroscopy and isoelectric focusing. No significant differences between E4/E6 ratio and ?log K (Log A400 nm‐log A600 nm) were observed for humic substances extracted from treated and untreated soils. However, the isoelectric focusing (IEF) technique established differences between the humic substances from control and treated soils. The focusing pattern of the former showed a well defined band at pH 9.1 which nearly disappeared in the soils with high doses of MSW (1–2 %). However the organic matter which focused at pH 5.8 was present in all soils.     

Influence of low-molecular-weight organic acids on the solubilization of phosphates

A range of low-molecular-weight organic acids were identified in rhizosphere soil, leaf litter, and poultry manure compost. Laboratory and greenhouse experiments were carried out to examine the effects of seven low-molecular-weight organic acids on phosphate adsorption by soils, and the solubilization and plant uptake of P from soil pre-incubated with monocalcium phosphate and North Carolina phosphate rock. Acetic, formic, lactic (monocarboxylic), malic, tartaric, oxalic (dicarboxylic), and citric (tricarboxylic) acids were used in the study. The addition of organic acids decreased the adsorption of P by soils in the order tricarboxylic acid>dicarboxylic acid>monocarboxylic acid. The decreases in P adsorption with organic acid addition increased with an increase in the stability constant of the organic acid for Al (logK _Al). Organic acids extracted greater amounts of P from soils meubated with both monocalcium phosphate and phosphate rock than water did. Although more phosphate was extracted by the organic acids from monocalcium phosphate — than from phosphate rock — treated soils in absolute terms, when the results were expressed as a percentage of dissolved phosphate there was little difference between the two fertilizers. The amount of P extracted by the organic acids from both fertilizers increased with an increase in logK _Al values. The addition of oxalic and citric acids increased the dry matter yield of ryegrass and the uptake of P in soils treated with both fertilizers. The agronomic effectiveness of both fertilizers increased in the presence of organic acids and the increase was greater with the phosphate rock than with the monocalcium phosphate. The results indicated that organic acids increase the availability of P in soils mainly through both decreased adsorption of P and increased solubilization of P compounds.     

Effect of Water Extract of Compost on the Adsorption of Arsenate by Two Calcareous Soils

The increasing mobility of arsenate will increase its leachingpotential to groundwater and uptake by plants. The mobility ofarsenate in soils is related to the competitive adsorption with other substances. The effect of organic substances on the adsorption of arsenate by soils was evaluated using the water extract of compost (WEC) as a complex anion source in a batch experiment. Two calcareous slate alluvial soils, Chiwulan andShuipientou, with higher arsenic contents of 23.7 and 12.9 mg kg^-1, respectively, were used. The Langmuir equation has been used to describe successfully the As adsorption isotherm for the two soils. The maximum adsorption of As was 6.098 and 4.785 μmol g^-1 for Chiwulan and Shuipientou soils, respectively. There was competitive adsorption for binding siteson the soils between arsenate oxyanions and organic anions derived from the dissolved organic carbon (DOC) of WEC. Differentcritical pH values were for arsenate addition related to arsenateadsorption on both soils in the absence of DOC of WEC but not inthe presence of DOC of WEC. The soil properties related to arsenate adsorption by the two soils may govern the critical pH values.