Is the OECD acute worm toxicity test environmentally relevant? The effect of mineral form on calculated lead toxicity

In a series of experiments the toxicity of lead to worms in soil was determined following the draft OECD earthworm reproduction toxicity protocol except that lead was added as solid lead nitrate, carbonate and sulphide rather than as lead nitrate solution as would normally be the case. The compounds were added to the test soil to give lead concentrations of 625-12 500 microg Pb g(-1) of soil. Calculated toxicities of the lead decreased in the order nitrate> carbonate> sulphide, the same order as the decrease in the solubility of the metal compounds used. The 7-day LC50 (lethal concentration when 50% of the population is killed) for the nitrate was 5321+/-275 microg Pb g(-1) of soil and this did not change with time. The LC50 values for carbonate and sulphide could not be determined at the concentration ranges used. The only parameter sensitive enough to distinguish the toxicities of the three compounds was cocoon (egg) production. The EC50s for cocoon production (the concentration to produce a 50% reduction in cocoon production) were 993, 8604 and 10246 pg Pb g(-1) of soil for lead nitrate, carbonate and sulphide, respectively. Standard toxicity tests need to take into account the form in which the contaminant is present in the soil to be of environmental relevance.     

A simple and fast screening test to detect soils polluted by lead

Lead pollution is an environmental priority. The evaluation of contaminated soils was often based on the results of the toxicity characteristic leaching procedure (TCLP) or the synthetic precipitation leaching procedure (SPLP). This paper presents a simple and fast screening test to detect soil contaminated by lead. The test is based on the chemistry of the stomach (Cl- concentration, pH 2, T = 37 degrees C) and simulates the incidental oral ingestion of soil by young children. The gastric juice simulation test (GJST) and the TCLP were applied to six size fractions from five soils. The GJST solubilized more Pb (up to 169 mg/l) than the TCLP especially for the smallest size fraction. Particle size had less effect on the TCLP. The percentage of lead released with the GJST, was most significant for the <63 microm size fraction and varied from 18 to 74% of the total lead content. Lead leached during the TCLP as a function of the total lead content showed poor linear regression coefficient (R) values for soils <250 microm. R values were significant for all size fractions with the GJST. The pH of approximately 5 in the TCLP limits the solubilization of lead in the small size fractions. The five soils exceeded the toxicity threshold of 10 microgPb/dl of blood for a significant fraction of children between 0 and 36 months using the EPA's IEUBK model (Integrated Exposure Uptake Biokinetic). But the TCLP did not detect lead contamination in two of these five soils. The GJST proved to be a better estimator of lead bioaccessibility in the gastrointestinal tract.     

Metallothionein-like proteins in the freshwater oligochaete Limnodrilus udekemianus and their role as a homeostatic mechanism against cadmium toxicity

The purpose of this study was to determine if metallothioneins are present in the aquatic oligochaete Limnodrilus udekemianus and to determine the interplay between the presence of these proteins, cadmium (Cd) exposure, and Cd toxicity. The latter was geared specifically towards evaluating the role of metallothionein as a homeostatic mechanism against Cd toxicity. These issues are important for evaluating the usefulness of the quantification of metallothioneins as a biomonitoring tool. Worms in sediment were exposed to Cd under static conditions, with Cd initially added to the aqueous phase. Survival was monitored while respiration (as a measure of sublethal Cd effects) was determined immediately following exposure. Metallothioneins were separated from the cytosol by gel permeation high performance liquid chromatography (HPLC) while Cd levels were quantified in whole worms, cytosol and cytosolic fractions. Also, a Cd-saturation assay was used to determine the amounts of Cd bound to metallothionein and the total Cd-binding capacity of the metallothionein. Limnodrilus udekemianus has a metallothionein-like protein (an inducible cytosolic protein with an apparent molecular weight of approximately 15 kD that binds high levels of Cd and shows a red shift upon Cd binding). Sediment Cd levels above 60 microg/g were lethal to the worms (in 8-day exposures). Respiration rates at 13 and 41 microg/g Cd were not significantly different from controls, though cytosolic Cd levels were substantially increased in the 41 microg/g exposure. In this latter cytosol, Cd levels were significantly elevated in the low molecular weight pool (which includes metallothioneins) but not in the other pools, while the Cd-saturation assay also showed that worms in this group had significantly elevated levels of metallothionein-bound Cd. However, in all treatments the metallothionein was far from saturated by Cd. These observations indicate that no 'spill-over' of Cd was evident as lethal levels of Cd were approached. The overall cytosolic Cd distribution, and the degree of metallothionein saturation in Limnodrilus udekemianus thus do not appear to be good predictors of Cd toxicity in this species.     

Assessment of inorganic lead species and total organo-alkyllead in some Egyptian agricultural soils

This study was carried out to assess the amounts of (i) total Pb in soil, (ii) inorganic Pb species: exchangeable (EXCH), carbonate (CARB), easily reducible (EASR), moderately reducible (MODR), organic matter and sulfides (ORGS), and residual (RESD) bound Pb, and (iii) total organo-lead as alkyllead, in alluvial and lacustrine soils of the Nile delta, Egypt. Wide ranges of soil Pb were found in the alluvial (18.2-1850 microg g(-1)) and the lacustrine (39-1985 microg g(-1)) soils. The topsoil was highly enriched with Pb relative to the subsurface soils, especially in highly contaminated soils. There was no significant relationship between soil type and Pb content. Amounts of soil Pb greater than the background level (14 microg g(-1)) are due to Pb deposited from various anthropogenic activities. The partitioning of soil Pb into different species varied according to the intensity of contamination. It followed the sequence: RESD > ORGS > CARB > MODR > EASR in the slightly contaminated alluvial as well as lacustrine soils. In the highly contaminated soils, it followed the sequence: ORGS > MODR > CARB > EASR > RESD in the alluvial soils, and the sequence: ORGS > CARB > MODR > EASR > RESD in the lacustrine soils. There is high binding capacity of organic matter and sulfides to Pb, especially in the highly contaminated soils. The concentrations of total alkyllead in soils varied markedly and were related to both intensity of contamination and depth in the soil. The subsurface soil (15-30 cm) was highly enriched by alkyllead (means 224 and 353 ng g(-1) in the alluvial and lacustrine soils, respectively) relative to the surface and deeper soils. The proportion of total alkyllead as a percentage of total Pb in the soil was generally very low. It did not exceed 1.6% in the slightly contaminated soils, and 0.6% in the highly contaminated ones.     

Effect of cropping and tillage on the dissipation of PAH contamination in soil

This study examined the effect of regular tillage and cropping on the dissipation rate of PAHs in contaminated soil. Lysimeters were placed under natural climatic conditions for 2 years and designed to measure the concentration of PAHs in soil and leachates and their toxicity. The soil initially contained 2077 microg PAHs g(-1). The largest decrease in PAHs concentration occurred during the first 6 months. No further significant decrease was observed after this time. The surface soil layer always contained significantly less PAHs than the deeper layer, regardless of the treatments. Less than 8.4 x 10(-8)% of the PAH initially present in the soil (e.g. less or equal to 33 microg PAHs per lysimeter) were leached from the soils during the experiment and the leachates presented no toxicity (as measured by the Microtox test). The toxicity of the soils decreased with time and was significantly lower on the cropped soil compared to the other treatments, despite the residual concentration of PAHs being the highest in this soil. This study demonstrated that the dissipation rates of PAHs were slow after using natural attenuation even when tillage and cropping were performed at the soil surface.     

Fractionation of heavy metals and distribution of organic carbon in two contaminated soils amended with humic acids

The effects of humic acids (HAs) extracted from two different organic materials on the distribution of heavy metals and on organic-C mineralisation in two contaminated soils were studied in incubation experiments. Humic acids isolated from a mature compost (HAC) and a commercial Spaghnum peat (HAP) were added to an acid soil (pH 3.4; 966 mg kg(-1) Zn and 9,229 mg kg(-1) Pb as main contaminants) and to a calcareous soil (pH 7.7; 2,602 mg kg(-1) Zn and 1,572 mg kg(-1) Pb as main contaminants) at a rate of 1.1g organic-C added per 100g soil. The mineralisation of organic-C was determined by the CO(2) released during the experiment. After 2, 8 and 28 weeks of incubation the heavy metals of the soils were fractionated by a sequential extraction procedure. After 28 weeks of incubation, the mineralisation of the organic-C added was rather low in the soils studied (<8% of TOC in the acid soil; <10% of TOC in the calcareous soil). Both humic acids caused significant Zn and Pb immobilisation (increased proportion of the residual fraction, extractable only with aqua regia) in the acid soil, while Cu and Fe were slightly mobilised (increased concentrations extractable with 0.1M CaCl(2) and/or 0.5M NaOH). In the calcareous soil there were lesser effects, and at the end of the experiment only the fraction mainly related to carbonates (EDTA-extractable) was significantly increased for Zn and decreased for Fe in the humic acids treated samples. However, HA-metal interactions provoked the flocculation of these substances, as suggested by the association of the humic acids with the sand fraction of the soil. These results indicate that humic acid-rich materials can be useful amendments for soil remediation involving stabilisation, although a concomitant slight mobilisation of Zn, Pb and Cu can be provoked in acid soils.     

Heavy metal contamination from mining sites in South Morocco: 1. Use of a biotest to assess metal toxicity of tailings and soils

Our work was conducted to investigate the heavy metal toxicity of tailings and soils collected from five metal mines located in the south of Morocco. We used the MetPAD biotest Kit which detects the toxicity specifically due to the heavy metals in environmental samples. This biotest initially developed to assess the toxicity of aquatic samples was adapted to the heterogeneous physico-chemical conditions of anthropogenic soils. Contrasted industrial soils were collected from four abandoned mines (A, B, C and E) and one mine (D) still active. The toxicity test was run concurrently with chemical analyses on the aqueous extracts of tailings materials and soils in order to assess the potential availability of heavy metals. Soil pH was variable, ranging from very acidic (pH 2.6) to alkaline values (pH 8.0-8.8). The tailings from polymetallic mines (B and D) contained very high concentrations of Zn (38,000-108,000 mg kg(-1)), Pb (20,412-30,100 mg kg(-1)), Cu (2,019-8,635 mg kg(-1)) and Cd (148-228 mg kg(-1)). Water-extractable metal concentrations (i.e., soil extracts) were much lower but were highly toxic as shown by the MetPAD test, except for soils from mines A, E and site C3 from mine C. The soil extracts from mine D were the most toxic amongst all the soils tested. On this site, the toxicity of soil water extracts was mainly due to high concentrations of Zn (785-1,753 mg l(-1)), Cu (1.8-82 mg l(-1)) and Cd (2.0-2.7 mg l(-1)). The general trend observed was an increase in metal toxicity measured by the biotest with increasing available metal contents in tailings materials and soils. Therefore, the MetPAD test can be used as a rapid and sensitive predictive tool to assess the heavy metal availability in soils highly contaminated by mining activities.     

Soil lead levels in a small town environment: a case study from Mt Pleasant, Michigan

Few studies are made on the potential soil Pb burden for a small city in rural environment. Data obtained by atomic absorption spectrophotometry suggest a somewhat weak significant positive relationship (r=0.27) between increased traffic volume and roadside soil Pb content. Median soil Pb levels along the most heavily travelled roads are 320 microg g(-1) while background concentrations are 200 microg g(-1). No significant relationship is found between predominant wind direction and soil Pb content. Zones where cars idle have only slightly elevated Pb levels. Older homes have soil Pb values exceeding 1000 microg g(-1); a significant positive relationship (r=0.59) exists between increasing soil Pb and home age. Schools, which are mainly located away from heavily travelled roads and typically of brick construction, have soil Pb concentrations at background levels. In general, the small city Pb burden is lower than in major urban areas. However, soils around older homes and in special locales, such as salvage yards, have Pb levels comparable to major urban areas.     

Thermal Immobilization of Lead Contaminants in Soils Treated in a Fixed- and Fluidized-Bed Incinerator at Moderate Temperatures

Abstract

Artificially lead-contaminated soils with different lead contents (200, 450, 600, and 900 ppm) were thermally immobilized in both fixed-bed and fluidized-bed modes at moderate treating temperatures (less than 500 °C) for various retention times. Cement powder and brick powder were added to the artificially contaminated soils to enhance lead immobilization. Results indicate that increasing treating temperature and time increases the extent of lead immobilization, as determined by the U.S. Environmental Protection Agency's (U.S. EPA) Toxicity Characteristics Leachability Procedure (TCLP). The percentage of lead leached from the soil/ cement mixtures was in the range of less than 0.251%, compared with the range between 13.6% and 40.7% for the corresponding soil/brick mixtures. As the amount of brick dust added to the Pb-doped soil was increased, the specific Pb immobilization effectiveness increased from 0.0675 to 0.149 mg Pb/g brick (for the 20- and 50-gram brick addition, respectively). An increase in air flow rate from 2 to 40 L/min caused a slight decrease in the Pb leaching percentage from 14.96% to 11.59%. Both the Freundlich and Langmuir isotherms give a satisfactory fit (r = 0.945 ~ 0.998) for the data derived from a TCLP test of the thermally-treated soil samples (105 °C and 500 °C) that contained lead contaminants. Sorption of lead contaminants in soil and sorbent matrices was the primary type of chemisorption. The kinetic results indicated that the Pb-doped soil system was too complicated to be described by a simple calculation.     

Factors influencing lead sorption-desorption at variable added metal concentrations in Rhodoxeralfs

The response of ten soils of the lithic Rhodoxeralf type to the supply of lead at concentrations of 500, 1000, 2000, 3000, 4000, 5000 and 6000 mg kg-1 was examined in batch sorption-desorption tests. Lead availability in the soils was found to depend on its partitioning between the soil solution and the solid phase as reflected in adsorption isotherms. The isotherms, of the H type, were consistent with a high affinity of the sorbent for the metal, with which it forms stable inner-sphere complexes on the soil surface. Sorption-desorption tests revealed that some properties of the soils such as their pH (mean=8) and high contents in clays (particularly in kaolinite) and crystalline iron oxides significantly influence Pb sorption, the effect being especially marked at high added metal concentrations. Added lead is largely retained by crystalline iron oxides and the soil clay fraction; the pH of the soil favours the release of variably-charged sites from both. The extent of Pb desorption was small, particularly at the lowest added levels (500 and 1000 mg kg-1). Desorption increased with increasing added Pb concentration and exceeded 50% at 5000 and 6000 mg kg-1; this suggests that Pb is present not only as inner-sphere complexes, but also as outer-sphere complexes and, partly, as precipitates. The desorption isotherms consist of three segments that exhibit significant differences depending on the added Pb concentration, namely: 500-1000, 2000-4000 and 5000-6000 mg kg-1.     

Effects of lead contamination on soil microbial activity and rice physiological indices in soil-Pb-rice (Oryza sativa L.) system

The effect of lead (Pb) treatment on the soil microbial activities (soil microbial biomass and soil basal respiration) and rice physiological indices were studied by greenhouse pot experiment. Pb was applied as lead acetate at six different levels in two different paddy soils, namely 0 (control), 100, 300, 500, 700, 900 mg kg-1 soil. The results showed that the application of Pb at lower level (<300 mg kg-1) as lead acetate resulted in a slight increase in soil microbial activities compared with the control, and had an inhibitory influence at high concentration (>500 mg Pb kg-1 soil), which might be the critical concentration of Pb causing a significant decline in the soil microbial activities. However, the degree of influence on soil microbial activities by Pb was related to the clay and organic matter contents of the soils. On the other hand, when the level of Pb treatments increased to 500 mg kg-1, there was ecological risk for both soil microbial activities and plants. The results also revealed that there was a consistent trend that the chlorophyll contents increased initially, and then decreased gradually with increase in Pb concentration. Pb was effective in inducing proline accumulation and its toxicity causes oxidative stress in rice plants. In a word, soil microbial activities and rice physiological indices, therefore, may be sensitive indicators reflecting environmental stress in soil-Pb-rice system.     

Solubility of lead, zinc and copper added to mineral soils

Elevated levels of heavy metals in soils are a result of industrial activities, atmospheric deposition, and the land application of sewage sludges and industrial by-products. Their persistence in the soil environment has created interest in the possible changes in solubility. In this study, total dissolved concentrations of Pb, Zn, and Cu were monitored in seven metal-amended soils (a calcareous and six acid mineral soils). Single metal solutions were added to soils and equilibrated (aged) for 40 days. During the 40 days the soil was allowed to air-dry and was rewetted in cycles of about 5 days. At the end of this reaction period, metal solubility was measured (by atomic absorption spectrometry and direct current plasma spectrometry) at the initial soil pH and at decreased pH values which were induced by addition of small aliquots of acid. As expected, solubility of added Pb, Zn, and Cu increased with a decrease in pH. Furthermore, the results showed that the solubility relationship with pH was similar in all non-calcareous soils. This suggests that metal solubility may be controlled by similar soil components, presumably involving soil characteristics such as pH, organic matter content, and soil mineralogy. For each metal, an approximate pH value was found at which solubility deviated from the solubility of metals when they occur in soils at typical (natural) values. This pH was about (pH+/-0.2): 5.2 for Pb, 6.2 for Zn, and 5.5 for Cu. Thus, pH values below these thresholds may enhance metal mobility, biological availability and toxicity in soils. Metals dissolved at higher pH in the calcareous soil (18.8 g kg(-1) inorganic carbon, initial pH 8.2). In a calcareous soil, a significant fraction of these metals react with carbonates, and decreased pH results in much higher metal dissolution. Yet, metal solubility in soils is not determined by the formation and dissolution of single metal compounds.     

Influence of lead acetate on soil microbial biomass and community structure in two different soils with the growth of Chinese cabbage (Brassica chinensis)

A greenhouse pot experiment was conducted to evaluate the impact of different concentrations of lead acetate on soil microbial biomass and community structure during growth of Chinese cabbage (Brassica chinensis) in two different soils. The field soils were used for a small pot, short-term 60-day growth chamber study. The soils were amended with different Pb concentrations, ranging from 0 to 900mgkg(-1) soil. The experimental design was a 2 soilx2 vegetation/non-vegetationx6 treatments (Pb)x3 replicate factorial experiment. At 60 days the study was terminated and soils were analyzed for microbial parameters, namely, microbial biomass, basal respiration and PLFAs. The results indicated that the application of Pb at lower concentrations (100 and 300mgkg(-1)) as lead acetate resulted in a slight increase in soil microbial biomass, whereas Pb concentrations >500mgkg(-1) caused an immediate gradual significant decline in biomass. However, the degree of impact on soil microbial biomass and basal respiration by Pb was related to management (plant vegetation) or the contents of clay and organic matter in soils. The profiles of 21 phospholipid fatty acids (PLFAs) were used to assess whether observed changes in functional microbial parameters were accompanied by changes in the composition of the microbial communities after Pb application at 0, 300 and 900mg Pbkg(-1) soil. The results of principal component analyses (PCA) indicated that there were significant increases in fungi biomarkers of 18:3omega6c, 18:1omega9c and a decrease in cy17:0, which is an indicator of gram-negative bacteria for the high levels of Pb treatments In a word, soil microbial biomass and community structure, therefore, may be sensitive indicators reflecting environmental stress in soil-Pb-plant system. However, further studies will be needed to better understand how these changes in microbial community structure might actually impact soil microbial community function.     

Lead concentrations in soil, sediment and clam samples from the Pungo River peatland area of North Carolina, USA

Lead (Pb) concentrations were measured in samples of peat soils, sediments and clams (Rangia cuneata) collected from the Pungo River region of coastal North Carolina. In peat soils, mean Pb concentrations (dry weight +/- 1 SD) were significantly higher (p<0.05) in surface samples (12.8microg g(-1)+/-7.6) than in samples from depths of 20 cm (2.7microg g(-1)+/-2.7) or 1 m (3.6microg g(-1)+/-3.6). Mean Pb concentrations in surface sediments from canals draining peatlands and from the Pungo River which receives this drainage ranged from 0.1microg g(-1)+/-0.1 to 7.0microg g(-1)+/-0.6. These Pb concentrations are similar to values reported in other studies for areas considered uncontaminated. Fractionation analysis revealed that the majority of the Pb in the peat and sediment samples was associated with the residual fraction, with lesser amounts in the organically-bound fraction, and generally negligible amounts in the water-soluble fraction. These results indicate that the bulk of the Pb in the soils and sediments of this area is relatively immobile and non-bioavailabe. This is supported by the relatively low concentrations of Pb (0.2-0.5 microg g(-1), dry weight) observed in soft tissues of R. cuneata collected from the Pungo River.     

Procedures of trophic chain samples preparation for determination of triazines by HPLC and metals by ICP-AES methods

The aim of this research was monitoring the distribution of atrazine and simazine as well as metals Pb, Cd, Zn, Al, Co, Ni, and V along with trophic chains: soil-vegetables and soil, carrot or grass and meat. Different techniques of herbicides extraction by means of many solvents were examined. Triazines were analysed by means of HPLC, metals by means of ICP-AES. Detection limits: LOD=0.2 microg ml(-1), determination limits: LOQ=0.73 microg ml(-1) for atrazine and LOD=0.3 microg ml(-1), LOQ=1.12 microg ml(-1) for simazine were obtained. The content (microg g(-1)) of simazine in soil was in range: 3.45-8.60, in vegetable roots: 6.62-38.15, in vegetable leaves: 2.45-31.71, in rabbit fat: 0.13-49.90. The content (microg g(-1)) of atrazine in soils was in range: 11.9-13.03, in vegetable roots: 13.61-92.90. In analysed material the particular metals after microwave or dry digestion were determined in range (microg g(-1)): Pb: 6.48-43.18; Cd: 0.11-0.57; Zn: 8.79-51.90; Al: 10.22-24.48; Co: 0.18-3.89; Ni: 0.37-6.36; V: 0.29-1.48.     

Hydrolytic and ligninolytic enzyme activities in the Pb contaminated soil inoculated with litter-decomposing fungi

The impact of Pb contamination was tested to five hydrolytic (beta-glucosidase, beta-xylosidase, beta-cellobiosidase, alpha-glucosidase and sulphatase) and two ligninolytic (manganese peroxidase, MnP and laccase) enzyme activities in the humus layer in the forest soil. The ability of eight selected litter-degrading fungi to grow and produce extracellular enzymes in the heavily Pb (40 g Pb of kg ww soil(-1)) contaminated and non-contaminated soil in the non-sterile conditions was also studied. The Pb content in the test soil was close to that of the shooting range at H?lv?l? (37 g Pb of kg ww soil(-1)) in Southern Finland. The fungi were Agaricus bisporus, Agrocybe praecox, Gymnopus peronatus, Gymnopilus sapineus, Mycena galericulata, Gymnopilus luteofolius, Stropharia aeruginosa and Stropharia rugosoannulata. The Pb contamination (40 g Pb of kg ww soil(-1)) was deleterious to all five studied hydrolytic enzyme activities after five weeks of incubation. All five hydrolytic enzyme activities were significantly higher in the soil than in the extract of the soil indicating that a considerable part of enzymes were particle bound in the soils. Hydrolytic enzyme activities were higher in the non-contaminated soil than in the Pb contaminated soil. Fungal inocula increased the hydrolytic enzyme activities beta-cellobiosidase and beta-glucosidase in non-contaminated soils. All five hydrolytic enzyme activities were similar with fungi and without fungi in the Pb contaminated soil. This was in line that Pb contamination (40 g Pb of kg ww soil(-1)) depressed the growth of all fungi compared to those grown without Pb in the soil. Laccase and MnP activities were low in both Pb contaminated and non-contaminated soil cultures. MnP activities were higher in soil cultures containing Pb than without Pb. Our results showed that Pb in the shooting ranges decreased fungal growth and microbial functioning in the soil.     

Effects of metals on life cycle parameters of the earthworm Eisenia fetida exposed to field-contaminated, metal-polluted soils

Two control and eight field-contaminated, metal-polluted soils were inoculated with Eisenia fetida (Savigny, 1826). Three, 7, 14, 21, 28 and 42 days after inoculation, earthworm survival, body weight, cocoon production and hatching rate were measured. Seventeen metals were analysed in E. fetida tissue, bulk soil and soil solution. Soil organic carbon content, texture, pH and cation exchange capacity were also measured. Cocoon production and hatching rate were more sensitive to adverse conditions than survival or weight change. Soil properties other than metal concentration impacted toxicity. The most toxic soils were organic-poor (1-10 g C kg(-1)), sandy soils (c. 74% sand), with intermediate metal concentrations (e.g. 7150-13,100 mg Pb kg(-1), 2970-53,400 mg Zn kg(-1)). Significant relationships between soil properties and the life cycle parameters were determined. The best coefficients of correlation were generally found for texture, pH, Ag, Cd, Mg, Pb, Tl, and Zn both singularly and in multivariate regressions. Studies that use metal-amended artificial soils are not useful to predict toxicity of field multi-contaminated soils.     

Predicting molybdenum toxicity to higher plants: Estimation of toxicity threshold values

Four plant species (oilseed rape, Brassica napus L.; red clover, Trifolium pratense L.; ryegrass, Lolium perenne L.; and tomato, Lycopersicon esculentum L.) were tested on ten soils varying widely in soil properties to assess molybdenum (Mo) toxicity. A larger range (66-fold-609-fold) of added Mo concentrations resulting in 50% inhibition of yield (ED₅₀) was found among soils than among plant species (2-fold-38-fold), which illustrated that the soils differed widely in the expression of Mo toxicity. Toxicity thresholds based on soil solution Mo narrowed the variation among soils compared to thresholds based on added Mo concentrations. We conclude that plant bioavailability of Mo in soil depends on Mo solubility, but this alone did not decrease the variability in observed toxicity enough to be used in risk assessment and that other soil properties influencing Mo toxicity to plants need to be considered.     

Small town lead levels: a case study from the homes of pre-schoolers in Mt. Pleasant, Michigan

This study evaluates the relationship between household Pb levels and four variables (home age, distance to road, traffic volume adjacent to the home, and the amount of exposed soil) for 42 homes in a small city. As a whole, Pb levels for the Mt. Pleasant sample were very low compared to large cities. Home age appeared to have the greatest impact on Pb levels as determined by atomic absorption soil = 65 microg g(-1), vacuum dust = 620 microg g(-1), window sill = 291 microg m(-1), indoor play area = 22 microg m(-1), and home entrance = 291 microg m(-1). The correlation coefficient for increasing home age and soil Pb level was r=0.63 (p<0.000). An inverse relationship (r=-0.45, p=0.003) occurred between soil Pb levels and distance from the road. Household Pb levels generally increased both with higher traffic volumes and greater amounts of exposed soil although both trends were not statistically significant. Study participants kept their home in a good to excellent state of repair and resided on lightly trafficked streets; as such, Pb deposition through the weathering of Pb-based paint and the former combustion of leaded gasoline was minimized.     

Short-term alleviation of aluminum phytotoxicity by urea application in acid soils from south China

A laboratory experiment was conducted to study effects of urea fertilizer on the chemical composition of soil solutions over time, and to determine Al toxicity as a function of rates of urea application. The experiment revealed that addition of urea fertilizer to soils caused drastic changes in soil pH during the hydrolysis and nitrification stages of urea transformation in the experiment. These pH changes, depending on the N rate of urea application and time courses, had variable effects on soil exchangeable Al, extracted with artificial solutions containing 1 mol l(-1) KCl. The Al mobilization rate could be resolved into two phases: A declining phase for Al was attributed to the urea-induced hydrolysis while a second rising phase was dependent with the nitrification of added N fertilizer. The decreases in exchangeable Al reached the greatest in 4-7 days after fertilization, consistent with soil pH increase. Decreased Al availability had been observed as a consequence of increasing urea addition and soil pH when using Root elongation of maize seedlings as the estimators. Results from the present study demonstrate that urea fertilizer to the surface of soils may lead to a temporary immobilization of Al and, therefore, alleviated Al toxicity to plant seedlings.