Changes in cadmium and zinc phytoavailability in agricultural soil after amendment with papermill sludge and biosolids

Abstract

The co‐disposal of papermill sludge with biosolids is seen as an alternative soil amendment to papermill sludge and inorganic fertilizer. The objectives of this study were to assess the suitability of co‐disposal of papermill sludge and biosolids by measuring changes in the soil physicochemical properties and the phytoavailability of cadmium (Cd) and zinc (Zn). Biosolids were applied with papermill sludge as an alternative source of N to inorganic fertilizers at rates calculated on the basis of C:N ratios of the amendments and common papermill sludge disposal practices. Perennial ryegrass (Lolium perenne L.) was grown on amended soils for 6 months under glasshouse conditions. The papermill sludge amendment alone increased soil pH and the rate of carbon degradation compared to the control (no amendment) and biosolid co‐disposal amendment. There was no difference in dry matter yield per pot of ryegrass between the treatments. Cadmium concentrations in plant tissue increased through the trial with the application of biosolids and papermill sludge. These findings were correlated well with the sorption properties of the soils for Cd as derived from isotherms. However, Zn uptake was unaffected by the application of the papermill sludge and biosolids.     

Measuring reactive metal in soil: a comparison of multi‐element isotopic dilution and chemical extraction

The isotopically exchangeable metal pool (E‐value) of zinc (Zn), cadmium (Cd) and lead (Pb) were simultaneously measured, using stable isotope dilution, in soils contaminated by Pb/Zn mining activities and varying in properties likely to affect metal reactivity, including pH, organic matter content, metal concentration and land use. E‐values were compared with single and sequential extraction schemes. Results showed a wide range of metal reactivity (approximately 1–100% of total) depending on the extent of contamination and on the prevailing soil conditions. Across the range of soils, the E‐values showed no consistent correspondence to any single chemical extraction procedure (EDTA, DTPA and HNO₃) although there was reasonable agreement with the extractants 0.05 m EDTA and 0.43 m HNO₃ in acidic organic soils. Extraction with 0.005 m DTPA substantially under‐estimated the isotopically exchangeable metal content. E‐values corresponded reasonably well with the exchangeable metal (fraction 1 (F1) of the sequential extraction procedure) in calcareous soils but relatively poorly and inconsistently with F1–F2, F1–F3 or F1–F4 in acidic‐neutral soils. Operational aspects associated with determination of multi‐element E‐values are discussed.     

Initial results from long-term field studies at three sites on the effects of heavy metal-amended liquid sludges on soil microbial activity

In a long‐term study of the effects on soil fertility and microbial activity of heavy metals contained in sewage sludges, metal‐amended liquid sludges each with elevated Zn, Cu or Cd concentrations were applied over a 3‐year period (1995–1997) to three sites in England. The experiments were sited adjacent to experimental plots receiving metal‐rich sludge cakes enabling comparisons to be made between the effects of heavy metal additions in metal‐amended liquid sludges and sludge cakes. The liquid sludge additions were regarded as ‘worst case’ treatments in terms of likely metal availability, akin to a long‐term situation following sewage sludge additions where organic matter levels had declined and stabilised. The aim was to establish individual Zn (50–425 mg kg^?1), Cu (15–195 mg kg^?1) and Cd (0.3–4.0 mg kg^?1) metal dose–response treatments at each site, but with significantly smaller levels of organic matter addition than the corresponding sludge cake experiments. There were no differences (P > 0.05) in soil respiration rates, biomass carbon concentrations or most probable numbers of clover Rhizobium between the treatments at any of the sites at the end of the liquid sludge application programme. Soil heavy metal extractability differed between the metal‐amended liquid sludge and metal‐rich sludge cake treatments; Zn and Cd extractabilities were higher from the liquid sludge additions, whereas Cu extractability was higher from the sludge cake application. These differences in metal extractability in the treated soil samples reflected the contrasting NH₄NO₃ extractable metal contents of the metal‐amended liquid sludges and sludge cakes that were originally applied.     

Determination of labile Cu in soils and isotopic exchangeability of colloidal Cu complexes

Measurement of labile (isotopically exchangeable) pools of metals (E values) in soil is required to assess the size of metal pools potentially available to soil organisms, from both a micronutrient deficiency and metal toxicity viewpoint. In this paper, E values of soil Cu were measured by an isotope dilution technique using different solution extracts – water with and without resin purification, water coupled with Donnan dialysis (free ion determination) and 0.01 m CaCl₂. Using these techniques, the isotopic exchangeability of Cu species in water extracts was investigated. The results showed that the specific activity of ⁶⁴Cu in the water‐soluble fraction was less than in the free metal ion fraction or in the fraction that adsorbed to resin. The isotopically non‐exchangeable Cu in water extracts ranged between 4% and 40% of water‐soluble Cu (16% on average), and appeared to be associated with dispersed colloids. The existence of isotopically non‐exchangeable Cu in water extracts led to overestimation (17.7% on average) of isotopically exchangeable Cu in soils when based on the specific activity in water extracts. The method of isotope dilution coupled with resin extraction is recommended for the determination of isotopically exchangeable Cu in soils when water extracts are used.     

Chemical pools of cadmium, nickel and zinc in polluted soils and some preliminary indications of their availability to plants

This study was conducted to determine the chemical distribution and plant availability of Cd, Zn and Ni in eight metal-polluted soils in southern Ontario, Canada. There were altogether 30 different soil samples because two of the soils had received various sewage sludge treatments. The soils were sequentially extracted with 1 m ammonium acetate to remove soluble plus exchangeable metals, with 0.125 m Cu(II) acetate to remove complexed metals, and with 1 m HNO₃ to dissolve chemisorbed or occluded metals and precipitates such as oxides and carbonates. Expressed as a percentage of the metal so extracted, exchangeable Cd and Zn and Ni; complexed Cd and Zn>Ni and Ni>Zn>Cd in the acid-soluble pool. With a few exceptions (soils with high organic matter content or low pH) at least 50 per cent of the extracted metal was in the acid-soluble pool. The percentage of metal complexed was significantly correlated with organic matter content. The percentage of metal in the acid-soluble fraction was significantly correlated with soil pH. Preliminary findings based on the results with two soils suggested that for Cd and Zn plant availability was correlated with the concentrations of exchangeable, complexed or acid-soluble pools of Cd and Zn.     

Modelling the effects of ageing on Cd,Zn, Ni and Cu solubility in soils using an assemblage model

Ageing reactions can reduce trace metal solubility and can explain natural attenuation of contaminated soils. We modelled ageing reactions in soil with an assemblage model that considers slow reactions in Fe‐oxyhydroxides and reversible sorption on organic matter and clay minerals. Metal adsorption kinetics on Fe‐oxyhydroxides was obtained from data with synthetic oxyhydroxides. Metal solubility and isotopic exchangeability data were obtained from 28 soils amended with Ni, Zn, Cu and Cd metal salts and monitored for 850 days. The assemblage model was constructed in WHAM 6.0 and used soil properties and dissolved organic matter as input data. The model was first validated to predict dissolved metal concentrations, based on the concentration of isotopic exchangeable metals. The model overestimated metal solubility without parameter adjustment by mean factors of 4–7, and successful fits were obtained by increasing the specific surface area of Fe‐oxyhydroxides from measured values of synthetic systems to a value of 600 m² g^?1 recommended by other authors. The effect of ageing on the isotopic exchangeable metal fraction was subsequently modelled starting from the predicted fraction of metals present on Fe‐oxyhydroxides immediately after soil spiking. The observed isotopic exchangeable metal fractions of Ni, Zn and Cd agreed reasonably well with predicted values. The model predicts that ageing reactions are more pronounced at higher pH because metal sorption is increasingly directed to oxyhydroxide surfaces with increasing soil pH. Modelling fixation of Cu requires more information on fixation of that metal in organic matter.     

Characterization of zinc in contaminated soils: complementary insights from isotopic exchange,batch extractions and XAFS spectroscopy

Isotopic exchange (IE) of trace metals is an established method for characterizing metal reactivity in soils, but it is still unclear which metal species are isotopically exchangeable. In this study, we used IE to quantify ‘labile’ zinc (Zn) in 51 contaminated soils that were previously studied by Zn K‐edge X‐ray absorption fine structure (XAFS) spectroscopy and sequential extraction (SE). All soils had been contaminated by runoff water from 17‐ to 74‐year‐old galvanized power‐line towers. They covered a wide range in pH (4.0–7.7), organic carbon (0.9–10.2%), clay (3.8–45.1%) and Zn concentrations (251–30 090 mg kg^?1). Isotopic exchange was also performed on selected Zn minerals used as references for linear combination fitting of XAFS spectra. The isotopically exchangeable fraction (%E) of Zn generally decreased with increasing pH, but small %E values were also observed for acidic soils with a large fraction of Zn in hydroxy‐interlayered minerals (Zn‐HIM). The fraction of Zn identified by XAFS spectroscopy as (tetrahedrally and octahedrally coordinated) ‘sorbed Zn’ agreed reasonably well with the isotopically exchangeable fraction but was in many cases larger than the %E, indicating that some ‘sorbed Zn’ may be isotopically non‐exchangeable, such as Zn sorbed in micropores of Fe oxyhydroxides. Zinc identified by XAFS spectroscopy as Zn precipitates (Zn phyllosilicates, Zn‐layered double hydroxide (Zn‐LDH) or hydrozincite) or as Zn‐HIM was largely isotopically non‐exchangeable (‘non‐labile’). Comparison between IE and extraction results suggested that the isotopically exchangeable Zn was mainly extracted in the first two fractions of the SE. However, non‐labile Zn was also extracted in these first two fractions for some soils, including a hydrozincite‐containing soil. Despite the presence of Zn‐LDH and/or Zn phyllosilicates in almost all soils, the Zn concentrations in solution and labile Zn increased with increasing soil total Zn at a given pH, which contradicts the concept of precipitation control by a single phase. Solution Zn was well predicted from the labile Zn following a sorption model.     

Heavy Metals in Soil Treated with Sewage Sludge Composting, their Effect on Yield and Uptake of Broad Bean Seeds (Vicia faba L.)

The final use that may be given to biosolids that result from the treatment of residual municipal waters depends on their physicochemical and microbiological characteristics. Their organic matter content and wealth of essential elements may allow their use for agricultural fertilization purposes. The objective of this research project was to evaluate the physicochemical interactions between soil treated with biosolids and compost from municipal residual waters, and the nutritional parameters of broad bean seeds (Vicia faba L.). The studied area is located in the central region of the Mexican Republic. The biosolids were treated with aerated static pile composting. The experimental work was performed in the area surrounding the East Toluca Macroplant, where nine 2 × 3 m plots were defined and distributed in a Latin square; 3 plots were used as controls (without conditioning), 3 were conditioned with 4.5 Mg ha^?1 of biosolids on a dry base, and 3 were conditioned with the same amount of compost. The parameters determined for biosolids, compost, and soil were: pH, electrical conductivity (EC), organic matter (OM), total nitrogen, available phosphorus, cation exchange capacity (CEC), exchangeable cations (Ca, Mg, Na and K), total and available heavy metals (Cd, Cr, Cu, Ni and Zn); for the plant: height reached, green seeds productivity and yield per treatment (ton ha^?1); for the seeds: humidity, ashes, fiber, fats, protein, starch and total and available heavy metals (Cd, Cr, Cu, Ni and Zn). pH was slightly acid in soil treated with biosolids (6.71). OM and CEC did not represent significant differences. Total concentrations of Cr, Zn, Ni and Cu in soil presented significant differences (p < 0.05) between treated soil and the control, Cd was not detected. Cu was the most available metal in soil treated with compost (15.31%), Cd and Cr were not detected. The plants had higher growth rates with biosolids (112.22 cm) and compost (103.73 cm); higher green broad bean productivity and higher seed yield, especially in plots containing biosolids, which had rates three times higher than the control. In regards to broad bean seeds, content of ashes, fiber, fats, protein, starch and heavy metals (Cu, Ni and Zn), there were no significant differences between the treatments. Cd and Cr were not detected. In conclusion, it has been proven that the use of biosolids and compost studied in this broad bean crop do not involve an environmental risk, and thus give way to a solution to the problem of final disposition of biosolids in the region.     

Effect of biosolid application to Mollisol Chilean soils on the bioavailability of heavy metals (Cu,Cr, Ni,and Zn) as assessed by bioassays with sunflower (Helianthus annuus) and DGT measurements

Purpose

This study assessed the effect of biosolid application on the bioavailable fraction of some trace elements (Cu, Cr, Ni, and Zn) using a bioassay with sunflower (Helianthus annuus) and a chemical assay, diffusion gradient in thin films (DGT).

Materials and methods

Five surface soil samples (0–20 cm) were collected from an agricultural zone in Central Chile where biosolids are likely to be applied. Municipal biosolids were mixed with the soil at concentrations of 0, 30, 90, and 200 Mg ha^?1. The experiment to determine the bioavailability of metals in the soil using the bioassay was performed using sunflower. The DGT technique and Community Bureau of Reference (BCR) sequential extraction were used to determine the bioavailable fractions of the metals.

Results and discussion

The application of biosolids increased the phytoavailability of Zn, Ni, and Cr in most of the soils, as indicated by the increasing concentrations in sunflower plants as the biosolid application rate increased. In two of the soils, Codigua and Pelvín, this increase peaked at an application rate of 90 Mg ha^?1. Decreases in the bioavailable fractions of Zn, Ni, and Cr were observed with higher biosolid application rates. The bioavailability of metals was estimated through multiple linear regression models between the metals in the sunflower plants and the different chemical fractions of metals in the soils treated with different biosolid rates, which displayed a positive contribution of the labile (water soluble, carbonate, and exchangeable), oxide, and organic metal forms in the soil, particularly with respect to Ni and Zn at application rates of 30 and 90 Mg ha^?1. The bioavailable fraction of metals was determined in soils using the DGT technique. The effective concentration (C _E) results were compared with those in sunflower plants. The DGT technique could effectively predict the bioavailable fractions of Cr, Ni, and Zn in the Taqueral soil but only that of Zn in the Polpaico soil.

Conclusions

The application of biosolids significantly increased the labile fraction of most of the metals in the studied soils, particularly at the highest biosolid application rate. C _E increased as the concentration of biosolids increased for most of the metals. The effectiveness of the DGT technique for predicting the bioavailability of metals was dependent on the soil type and the metal. However, the C _E for soil Cu was not related to plant Cu for all soils studied.     

施用碱稳定固体的酸性土壤的Cu和Zn的形态分布

Fractionation of metals in a granite-derived acid sandy loam soil amended with alkaline-stabilised sewagesIudge biosolids was conducted in order to assess metal bioavailability and environmental mobility soil solution was extracted by a centrifugation and filtration technique. Metal speciation in the soil solution wasdetermined by a cation exchange resin method. Acetic acid and EDTA extracting solutions were used forextraction of metals in soil solid surfaces. Metal distribution in different fractions of soil solid phase was determined using a three-step sequential extraction scheme. The results show that the metals in the soilsolution existed in different fractions with variable lability and metals in the soil solid phase were also presentin various chemical forms with potentially different bioavail ability and environmental mobility Alkaline-stabilised biosolids could elevate solubility of Cu and proportion of Cu in organically complexed fractionsboth in soil liquid and solid phases, and may therefore increase Cu mobility. In contrast, the biosolids lowered the concentrations of water-soluble Zn (labile fraction) and exchangeable Zn and may hence decrease bioavailability and mobility of Zn. However, Fe and Mn oxides bound and organic matter bound fractions are likely to be Zn pools in the sludge-amended soil. These consequences possibly result from the liming effect and metal speciation of the sludge product and the difference in the chemistry between the metals in soil.     

Immobilization of heavy metals in soils using inorganic amendments in a greenhouse study

The effect of red mud (10 g kg^–1), a by‐product of the alumina industry, zeolite (20 g kg^–1), a naturally‐occurring hydrous aluminosilicate, and lime (3 g kg^–1) on metal lability in soil and uptake by fescue (Festuca rubra L.) (FEST) and amaranthus (Amaranthus hybridus L.) (AMA) was investigated in four different soils from Austria. The soil collection locations were Untertiefenbach (UNT), Weyersdorf (WEY), Reisenberg (REI), and Arnoldstein (ARN). The latter was collected in the vicinity of a former Pb‐Zn smelter and was highly polluted with Pb (12300 mg kg^–1), Zn (2713 mg kg^–1), and Cd (19.7 mg kg^–1) by long‐term deposition. The other soils were spiked with Zn (700 mg kg^–1), Cu (250 mg kg^–1), Ni (100 mg kg^–1), V (100 mg kg^–1), and Cd (7 mg kg^–1) salts in 1987. The two plant species were cultivated for 15 months. Ammonium nitrate (1 M) extraction was used in a soil : solution ratio of 1:2.5 to assess the influence of the amendments on the labile metal pools. The reduction of metal extractability due to red mud was 70 % (Cd), 89 % (Zn), and 74 % (Ni) in the sandy soil (WEY). Plant uptake in this treatment was reduced by 38 to 87 % (Cd), 50 to 81 % (Zn), and 66 to 87 % (Ni) when compared to the control. Sequential extraction revealed relative enrichments of Fe‐oxide‐associated metal fractions at the expense of exchangeable metal fractions. Red mud was the only amendment that decreased lability in soil and plant uptake of Zn, Cd, and Ni consistently. Possible drawbacks of red mud application (e.g., As and Cr concentration) remain to be evaluated.     

Effects of sewage sludge stabilization on organic‐N mineralization in two soils

Sewage sludge is a valuable source of organic matter, N, P and certain micronutrients that have beneficial effects on plant growth and biomass production. However, sanitary regulations often require the stabilization of sewage materials prior to applying them to soils as biosolids. Environmental regulations also demand appropriate management of biosolid‐N to avoid groundwater contamination. Because stabilization processes usually make sewage sludge less putrescible, we hypothesized that the mineralization rates of organic‐N from stabilized biosolids would be affected. Therefore, this study aimed to evaluate the mineralization of five biosolids in two soils – a sandy Spodosol and a clayey Oxisol. Digested sludge, composted sludge, limed sludge, heat‐dried sludge and solar‐irradiated sludge were mixed with soil samples at a concentration of 32.6 mg N/kg soil (1.0 dry t/ha of digested sludge) and incubated at 25 °C in a humidity chamber for 23 weeks. Results showed that the stabilization processes generally slowed the release of mineral‐N in soils relative to the digested sludge from which the biosolids originated. However, increments in the levels of mineral‐N were more influenced by soil type than by the type of stabilization process applied to the sewage sludge. Mineralization rates were up to 5‐fold higher in the Oxisol than in the Spodosol soil, and as a result, organic‐N in biosolids mineralized 10–24% in Spodosol and 23–52% in Oxisol. Any appropriate plan for the management of biosolid‐N for plant use should consider the interaction between soil type and biosolid type.     

Effect of biosolids from municipal sewage sludge composted with rice husk on soil functionality

Two different biosolids were obtained composting anaerobic (A) and aerobic (B) municipal sewage sludge (SS) with rice husk. Higher amounts of SS (1:1 v/v) could be used in this composting process than in conventional ones. The two biosolids were characterized by chemical analysis and compared with a conventional green manure plus municipal solid waste and municipal SS compost. The effect of these products on soil functionality was studied in a 14-week incubation experiment by their addition to two different soils (silty clay—Ustic Endoaquert—and sandy loam—Aquic Xeropsamment). The total organic C ranged from 20 to 26 % and total N from 1.6 to 2.5 % in the two biosolids. The most relevant difference was due to dissolved organic C that was lower in the anaerobic biosolid (1 mg?C?kg^?1) than in the other products (5–6 mg?C?kg^?1). The total trace elements (Cd, Cr, Cu, Ni, Pb and Zn) contents were under the limits fixed by the European legislation for soil application of SS (EC Directive 86/278/EEC, 1986). The three biosolids did not show strong negative effects on soil functionality during the incubation experiment, although some significant differences were found. The aerobic biosolid B mainly increased cumulative N release, microbial activity, basal respiration rate, microbial biomass-C-to-total organic C ratio, β-glucosidase, alkaline phosphomonoesterase and aryl-sulphatase activities. The anaerobic one (B) decreased basal respiration rate, microbial biomass-C-to-total organic C ratio and aryl-sulphatase activity. DTPA soil bioavailable heavy metals were not affected by biosolids additions.     

Heavy metal distribution and electrical conductivity measurements in biosolid pellets

Purpose

Contamination of soils by potentially toxic elements (e.g. Cd, Ni, Cr, Pb) from amendments of biosolids is subject to strict controls within the European Union. Today, the use of biosolids to improve the nutrient content in a soil is a common practice. The present research was conducted to determine electrical conductivity in biosolid pellets (dry wastes) using an innovative methodology. On the other hand, the present study was designed to examine the partition of selected heavy metals in biosolid pellets and also to relate the distribution patterns of these metals.

Materials and methods

In this context, heavy metal concentrations were studied in biosolid pellets under different pressures. Electrical conductivity measurements were taken in biosolid pellets under pressures on the order of 50 to 150 MPa and with currents of 10^?15 A. Measurements of electrical conductivity and heavy metal content for different areas (H1, H2, and H3) were taken. Total content of metals was determined following microwave digestion and analysed by inductively coupled plasma mass spectrometry (ICP/MS). Triplicate portions were weighed in polycarbonate centrifuge tubes and sequentially extracted.

Results and discussion

The distribution of chemical forms of Cd, Ni, Cr, and Pb in the biosolids was studied using a sequential extraction procedure that fractionates the metal into soluble-exchangeable, specifically sorbed-carbonate-bound, oxidizable, reducible, and residual forms. The residual, reducible, and carbonate-sorbed forms were dominant. Higher Cr and Ni content were detected in pellets made with biosolids from the H3 horizon. The highest Cd and Ni values were detected in the H2 horizon.

Conclusions

The trends of the conductivity curves were similar for the sludge from the isolation surface horizon (H1) and for the horizon in the mesophilous area (H2). In the case of the horizon in the thermophilous area (H3), the electrical conductivity showed extremely high values. This behaviour was similar in the case of the Cr and Ni content. However, in the case of Cd and Pb, the highest values were detected in the H2 horizon. This experiment could be useful for establishing a general rule for taking measurements of electrical conductivity and heavy metals in biosolid pellets and other types of dry wastes.

     

Examining the integrity of soil metal bioavailability assays in the presence of organic amendments to metal‐spiked soils

This paper questions whether the presence of biosolids amendment in metal‐spiked soils alters the outcome of soil‐based assays of metal bioavailability. The effects of biosolids amendment on the efficacies of six soil metal bioavailability assays (total recoverable, EDTA, Ca(NO₃)₂, soil solution, diffusive gradient in thin films and free ion activity) were assessed against metal concentrations in wheat shoots (Triticum aestivum) germinated in three contrasting soils, each previously incubated for either 2 weeks or 6 months following treatment with Cd, Cu, Ni and Zn +/? biosolids amendment. Overall, Ca(NO₃)₂ was the most accurate method to predict Cd (r² = 0.62), Ni (r² = 0.73) and Zn (r² = 0.55) bioavailability in soils and therefore was used to compare variations in responses between biosolids and nonbiosolids‐amended soils. Comparisons between these two groups revealed no significant differences in linear relationships for all four metals and soil types assessed. These findings not only support Ca(NO₃)₂ as a robust and valid method for determining soil metal bioavailability across metal matrices and soil types, but also that the presence of biosolids does not compromise the predictive power of this assay or any of the others examined.     

Initial results from a long-term, multi-site field study of the effects on soil fertility and microbial activity of sludge cakes containing heavy metals

In a long‐term study of the effects on soil fertility and microbial activity of heavy metals contained in sewage sludges, metal‐rich sludge cakes each with high Zn, Cu or Cd concentrations were applied annually for 4 years (1994–1997) to nine sites throughout Britain. These sites were selected to represent agricultural soils with a range of physical and chemical properties, typical of those likely to be amended with sewage sludge. The aim was to establish individual total Zn (approx. 60–450 mg kg^?1), total Cu (approx. 15–200 mg kg^?1) and total Cd (approx. 0.2–4 mg kg^?1) metal dose–response treatments at each site. Sludges with low metal concentrations were added to all treatments to achieve as constant an addition of organic matter as possible. Across the nine sites, soil pH was the single most important factor controlling Zn (P < 0.001; r² = 92%) and Cd extracted with 1 m NH₄NO₃ (P < 0.001; r² = 72%), and total iron content the most important factor controlling Cu extracted with 1 m NH₄NO₃ (P < 0.001; r² = 64%). There were also positive relationships (P < 0.001) between soil organic carbon (C) concentrations and soil biomass C and respiration rates across the nine sites. Oxidation of sludge C following land application resulted in approximately 45% of the digested sludge cake C and approximately 64% of the ‘raw’ sludge cake C being lost by the end of the 4‐year application period. The sludge cake applications generally increased soil microbial biomass C and soil respiration rates, whilst most probable numbers of clover Rhizobium were generally unchanged. Overall, there was no evidence that the metal applications were damaging soil microbial activity in the short term after the cessation of sludge cake addition.     

Bioavailability of Heavy Metals in Biosolids‐Amended Soil

Abstract

A single biosolids application was made to 1.5×2.3 m confined plots of a Davidson clay loam (Rhodic Kandiudult) in 1984 at 0, 42, 84, 126, 168, and 210 Mg ha^?1. The highest biosolids application supplied 750 and 600 kg ha^?1 of Cu and Zn, respectively. Corn (Zea mays L.), from 1984 to 2000, and radish (Raphanus sativus L.) and romaine lettuce (Lactuca sativa var. longifolia), from 2001 to 2004, were grown at the site to assess heavy‐metal bioavailability. Extractable (0.005 diethylenetriamine (DTPA) and Mehlich 1) Cu and Zn were determined on 0 to 15‐cm depth samples from each plot. Corn yield increased with biosolids rate each year until 1993 to 1997, when yield decreased with biosolids rate because of phytotoxicity induced by low (<5.0) soil pH. The corn yield reduction was reversed between 1998 and 2000 upon raising the soil pH to approximately 6.0 by limestone addition following the 1997 season. Between 2001 and 2004, radish and lettuce yields were either not affected or slightly increased with biosolids rate, even as soil pH declined to below 5.5. Plant‐tissue metal concentrations increased with biosolids rate and as pH declined but were always within the normal range of these crops. Mehlich 1 and DTPA extractable metals increased linearly with biosolids rate. Extractability of Cu and Zn decreased approximately 50% over the past 20 years despite a decrease in soil organic matter concentration and greater than 95% conservation of the metals.     

Effect of biosolid amendment on enzyme activities in earthworm (Lumbricus terrestris) casts

We studied the effect of amendment of sewage sludge biosolids on enzyme activity in soil and earthworm (Lumbricus terrestris) casts. Enzyme activities and contents of nutrients and organic matter of surrounding soil were compared with the corresponding properties of earthworm casts. This short time experiment was conducted at 20 ± 0.5 °C in the laboratory, simulating field conditions of biosolid treatments. In general, all of doses of biosolid treatments influenced the enzyme activity and contents of nutrients and organic matter of earthworm casts and surrounding soil. Enzyme activity such as urease (UA), alkaline phosphatase (APA), and arylsulfatase (ASA) and the contents of organic matter and nutrients N and P in earthworm casts and surrounding soil increased with increasing biosolid application. Without biosolid additions, enzyme activities in cast of L. terrestris exceeded those in the soil. In contrast, when biosolid was added, DHA in casts was lower than the soil. Activities of UA and APA were consistently higher in L. terrestris casts than in soil of all biosolid treatments. Biosolid amendments generally increased ASA at low doses, but at higher doses, ASA decreased. In general, organic matter and contents of N and P were higher in surface casts of L. terrestris and soils than in the control soil. Activities of UA, APA, the contents of organic carbon and nutrients N and P in soil and casts showed positive correlations. On the contrary, ASA and DHA were negatively correlated with the contents of organic matter and nutrients.     

Copper and zinc adsorption by sewage sludge‐treated soil in southern Spain

Abstract

Heavy metal accumulation in soil due to the application of sewage sludge may induce changes in its ionic retention capacity. In this work, sludge application effects on copper (Cu) and zinc (Zn) adsorption by the surface horizon of a calcareous soil in Southern Spain have been studied. Sludge from the urban waste water treatment plant in Cordoba was applied at rates of 0,40, and 80 Mg sludge ha^‐1 for two consecutive years. Despite the low metal content of this sludge and the high amount of organic matter added, a decrease in the soil adsorption levels for both Cu and Zn was detected in the sludged samples. Differences were observed, though, in the behavior of each metal, relative to the main adsorption mechanism involved. Copper adsorption was related to carbonate surfaces in soil. Values up to 400 mmol adsorbed Cu kg^‐1 soil were obtained for the three treatments, in equilibrium with 13.6, 16.7, and 17.4 mmol Cu m^‐3 in solution for 0, 40, and 80 Mg sludge ha^‐1, respectively, the first year, and 13.9, 21.2, and 52.2 mmol Cu m^‐3 in solution the second year. However, an increase of soil Cu availability is not likely to arise during or after continuous application of these sludge doses, due to the high calcium carbonate content of this soil. In the case of Zn, adsorption took place in CEC sites, with values up to 40 mmol adsorbed Zn kg^‐1 soil in the 0 and 80 Mg sludge ha^‐1 samples, in equilibrium with 158.3 and 164.5 mmol Zn m³ in solution, respectively the first year, and 196.1 and 505.8 mmol Zn m³ in solution the second year, whereas in the 40 Mg sludge ha^‐1 samples adsorption was below 25 mmol Zn kg^‐1 soil, in equilibrium with 171.1 mmol Zn m³ in solution the first year, and 231.0 mmol Zn m^‐3 the second year. As soil CEC values and its organic matter content are positively related, Zn availability in this soil could be expected to increase after the cessation of sludge applications, due to the mineralization of the organic matter added.     

Relationship Between Extractable Metals in Acid Soils and Metals Taken Up by Tea Plants

Abstract

The accumulation of heavy metals in plants is related to concentrations andchemical fractions of the metals in soils. Understanding chemical fractions and availabilities of the metals in soils is necessary for management of the soils. In this study, the concentrations of copper (Cu), cadmium (Cd), lead (Pb), and zinc (Zn) in tea leaves were compared with the total and extractable contents of these heavy metals in 32 surface soil samples collected from different tea plantations in Zhejiang province, China. The five chemical fractions (exchangeable, carbonate‐bound, organic matter‐bound, oxides‐bound, and residual forms) of the metals in the soils were characterized. Five different extraction methods were also used to extract soil labile metals. Total heavy metal contents of the soils ranged from 17.0 to 84.0 mgCukg^?1, 0.03 to 1.09 mg Cd kg^?1, 3.43 to 31.2 mg Pb kg^?1, and 31.0 to 132.0 mg Zn kg^?1. The concentrations of exchangeable and carbonate‐bound fractions of the metals depended mainly on the pH, and those of organic matter‐bound, oxides‐bound, and residual forms of the metals were clearly controlled by their total concentrations in the soils. Extractable fractions may be preferable to total metal content as a predictor of bioconcentrations of the metals in both old and mature tea leaves. The metals in the tea leaves appeared to be mostly from the exchangeable fractions. The amount of available metals extracted by 0.01 mol L^?1 CaCl₂, NH₄OAc, and DTPA‐TEA is appropriate extractants for the prediction of metals uptake into tea plants. The results indicate that long‐term plantation of tea can cause sol acidification and elevated concentrations of bioavailable heavy metals in the soil and, hence, aggravate the risk of heavy metals to tea plants.