Metal-contaminated Sewage Sludge Research Articles

Treatment of heavy metal-polluted sewage sludge using biochar amendments and vermistabilization.

Heavy metal contamination of sewage sludge hampers its recycling. Contaminated sewage sludge was amended with different proportions of biochar and vermistabilized. Biochar produced from wheat straw was added at four proportions (0%, 2%, 4%, and 6%). Ten earthworms Eisenia fetida were added, and the sludge was vermistabilized for 60days. Heavy metal and nutrient concentrations and the accumulation of metals to E. fetida were measured. The treatment with 4% biochar was the most efficient in reducing the concentrations of heavy metals. The concentration of Cd decreased 55%, Cr 28%, Cu 30%, and Pb 21%. The concentrations of plant nutrients increased: total N 43%, total P 92%, and total K 60%. E. fetida accumulated all heavy metals in their internal tissues. The survival and reproductive rate of E. fetida improved during the vermistabilization process. We interpret that the biochar alone did not improve the decomposition process, but the main actors were the earthworms E. fetida. The most efficient proportion of biochar was 4%, not the highest tested (6%). We recommend 4% biochar to be used in the vermistabilization of heavy metal-contaminated municipal sewage sludge. The study benefits both the management of heavy metal-contaminated sewage sludge and agriculture where the final vermistabilization product can be used to improve crop production.

Effects of co-pyrolysis of rice husk and sewage sludge on the bioavailability and environmental risks of Pb and Cd

ABSTRACT In this study, biochars were produced by co-pyrolysis of rice husk and sewage sludge, the environmental risk of heavy metal (Pd and Cd) in the biochars was assessed. Co-pyrolysis resulted in a lower yield but a higher C content compared with sewage sludge pyrolysis alone, the relative contents of Pb and Cd in biochars were declined. Co-pyrolysis process transformed the bioavailable heavy metals into stable speciation. The environmental risk assessment codes of Pb and Cd were reduced by 1–2 grades. The co-pyrolysis technology provides a feasible method for the safe disposal of heavy metal-contaminated sewage sludge.

Cu-Doped Porous Carbon Derived from Heavy Metal-Contaminated Sewage Sludge for High-Performance Supercapacitor Electrode Materials.

In this paper, we report a complete solution for enhanced sludge treatment involving the removal of toxic metal (Cu(II)) from waste waters, subsequent pyrolytic conversion of these sludge to Cu-doped porous carbon, and their application in energy storage systems. The morphology, composition, and pore structure of the resultant Cu-doped porous carbon could be readily modulated by varying the flocculation capacity of Cu(II). The results demonstrated that it exhibited outstanding performance for supercapacitor electrode applications. The Cu(II) removal efficiency has been evaluated and compared to the possible energy benefits. The flocculant dosage up to 200 mg·L−1 was an equilibrium point existing between environmental impact and energy, at which more than 99% Cu(II) removal efficiency was achieved, while the resulting annealed product showed a high specific capacity (389.9·F·g−1 at 1·A·g−1) and good cycling stability (4% loss after 2500 cycles) as an electrode material for supercapacitors.

Cumulative effects of bamboo sawdust addition on pyrolysis of sewage sludge: Biochar properties and environmental risk from metals

A novel type of biochar was produced by mixing bamboo sawdust with sewage sludge (1:1, w/w) via a co-pyrolysis process at 400–600°C. Changes in physico-chemical properties and the intrinsic speciation of metals were investigated before and after pyrolysis. Co-pyrolysis resulted in a lower biochar yield but a higher C content in the end product compared with use of sludge alone as the raw material. FT-IR analysis indicates that phosphine derivatives containing PH bonds were formed in the co-pyrolyzed biochars. In addition, co-pyrolysis of sludge with bamboo sawdust transformed the potentially toxic metals in the sludge into more stable fractions, leading to a considerable decrease in their direct toxicity and bioavailability in the co-pyrolyzed biochar. In conclusion, the co-pyrolysis technology provides a feasible method for the safe disposal of metal-contaminated sewage sludge in an attempt to minimize the environmental risk from potentially toxic metals after land application.

From agricultural use of sewage sludge to nutrient extraction: A soil science outlook

The composition of municipal wastewater and sewage sludge reflects the use and proliferation of elements and contaminants within society. In Sweden, official statistics show that concentrations of toxic metals in municipal sewage sludge have steadily decreased, by up to 90 %, since the 1970s, due to environmental programmes and statutory limits on metals in sludge and soil. Results from long-term field experiments show that reduced metal pollution during repeated sewage sludge application has reversed negative trends in soil biology. Despite this Swedish success story, organic waste recycling from Swedish towns and cities to arable land is still limited to only about 20 % of the total amount produced. Resistance among industries and consumers to products grown on land treated with sewage sludge may not always be scientifically grounded; however, there are rational obstacles to application of sewage sludge to land based on its inherent properties rather than its content of pollutants. We argue that application of urban organic wastes to soil is an efficient form of recycling for small municipalities, but that organic waste treatment from large cities requires other solutions. The large volumes of sewage sludge collected in towns and cities are not equitably distributed back to arable land because of the following: (i) The high water and low nutrient content in sewage sludge make long-distance transportation too expensive; and (ii) the low plant availability of nutrients in sewage sludge results in small yield increases even after many years of repeated sludge addition. Therefore, nutrient extraction from urban wastes instead of direct organic waste recycling is a possible way forward. The trend for increased combustion of urban wastes will make ash a key waste type in future. Combustion not only concentrates the nutrients in the ash but also leads to metal enrichment; hence, direct application of the ash to land is most often not possible. However, inorganic fertiliser (e.g. mono-ammonium phosphate fertiliser, MAP) can be produced from metal-contaminated sewage sludge ash in a process whereby the metals are removed. We argue that the view on organic waste recycling needs to be diversified in order to improve the urban–rural nutrient cycle, since only recycling urban organic wastes directly is not a viable option to close the urban–rural nutrient cycle. Recovery and recycling of nutrients from organic wastes are a possible solution. When organic waste recycling is complemented by nutrient extraction, some nutrient loops within society can be closed, enabling more sustainable agricultural production in future.

Effect of heavy metal contaminated sewage sludge on soil microbiological properties and growth of Indian mustard

A laboratory incubation and pot experiment were carried out to study the effect of amendment of uncontaminated and metal contaminated sewage sludge on soil microbial biomass carbon (C), soil enzyme activities and growth of Indian mustard in a sandy loam soil. The application of metal contaminated sewage sludge containing chromium (Cr) (2400 mg kg−1), nickel (Ni) (400 mg kg−1) and lead (Pb) (2400 mg kg−1) had a detrimental effect on soil microbial biomass C and soil enzyme activities at different levels. Metals present in sewage sludge inhibited the microbial biomass carbon by 5.6–19.0% at different levels after 90 days of incubation. Dry matter yield, uptake of nitrogen (N), phosphorus (P) and potassium (K) increased significantly with the increasing levels of uncontaminated sewage sludge, whereas, amendment of metal contaminated sewage sludge at the levels of 40 and 80 t ha−1 decreased the growth of Indian mustard. The accumulation of Cr, cadmium (Cd), Ni and Pb was 997, 36, 793 and 620 mg kg−1, respectively, in the shoots of Indian mustard. Sewage sludge contaminated with metals Cr, Ni, and Pb at double the EU limits showed adverse effect on soil microbiological activities and growth of Indian mustard.

Impact of heavy metal amended sewage sludge on forest soils as assessed by bacterial and fungal biosensors

Bacterial and fungal bioluminescence-based biosensors were used as indicators of potential heavy metal toxicity to microorganisms in the needle litter of a mature Pinus radiata forest under heavy metal contaminated sewage sludge. Sewage sludge was amended with increasing concentrations of Cu, Ni and Zn and applied to the surface of a mature P. radiata forest. The response of the bacterial and fungal biosensors to soluble Cu, Ni and Zn in needle litter extracts was investigated. The bioluminescence response of the bacterial biosensor Escherichia coli HB101 pUCD607 declined as water-soluble Zn concentrations increased. The effective concentrations that gave a 50% reduction in bioluminescence (EC50 values) for water-soluble Zn and total litter Zn were 1.3 mg l−1 and 3700 mg kg−1, respectively. The bioluminescence response of the fungal biosensor Armillaria mellea declined as soluble Cu concentrations increased. The EC50 values for water-soluble Cu and total litter Cu were 0.12 mg l−1 and 540 mg kg−1, respectively. No decline in bioluminescence was noted for either the bacterial or fungal biosensor on exposure to increasing concentrations of water-soluble Ni. The use of a combination of bacterial and fungal biosensors offers a rapid and sensitive tool for assessing toxicity of heavy metals to microorganisms and, thus, elucidating the environmental impact of contaminants in sewage sludge on litter dwelling microorganisms.

Leaching of heavy metals (Cu, Ni and Zn) and organic matter after sewage sludge application to Mediterranean forest soils

In Mediterranean dry and semiarid areas, soil organic matter is often depleted due to ancient and intensive human activity. Under these conditions the use of sewage sludge as a land reclamation technique may be a means to revert desertification processes and to enhance soil function and nutrient cycling. However, applications of heavy metal-contaminated sewage sludges can significantly increase potentially toxic metal concentrations in soils and metal transfer to freshwater and plants. The aims of this study are 1) to investigate the leaching of Cu, Zn and Ni from three contrasted Mediterranean forest soils (a basic loam, a basic clay, and an acid loam) treated with sewage sludges and 2) to explore the relationships between metal mobility and soil properties and with the leaching of organic matter. The selected soils were incubated in columns (5 replicates × 3 soils × 3 treatments). Treatments were (a) soil application of low metal content sewage sludge (LMS), (b) soil application of metal-enriched sewage sludge (MES), and (c) control. The sewage sludge application represented a dose of 6 kg dry weight m − 2 . Soil columns were incubated at room temperature for 110 days and were irrigated weekly with deionised water to make a total of a 1130 mm. Leachates were collected and analysed for pH, EC, organic carbon Cu, Ni and Zn concentrations. The concentration of metals and organic matter in the leachates depended on the soil characteristics and on the type of sewage sludge added to the soil. Basic soils with a high amount of clay showed the highest metal retention capacity, while acid soils with low clay content showed the lowest. Of the three metals studied, Ni exhibited the greatest mobility. Zn mobility was also rather high, particularly in the acid soil. Despite the fact that basic soils showed greater OM content than the acid soil, organic carbon in leachates after sludge addition was of the same order of magnitude in all studied soils. OM mobility may enhance the leaching of metals while the OM bound to soil particles may enhance the retention of metals. The rate of leaching per unit of metal in the soil, for Cu in all soils and for Zn in the basic soils, did not increase even in soils amended with metal-enriched sewage sludge. However, the total amount of Cu, Zn, and Ni leached through the 30 cm columns of the metal-enriched sewage sludge increased in comparison to the control soils. In spite of this fact, metal concentrations were in most cases well below limit for drinking water quality criteria of Spanish legislation. Thus, after one single addition of metal-enriched sewage sludge, for the studied Mediterranean soils, the risk of heavy metal pollution of groundwater appears to be low.

Bio-indicators to assess impacts of heavy metals in land-applied sewage sludge

Soils from a pastoral farm that had received large amounts of heavy metal contaminated sewage sludge 6–10 years previously were investigated to determine the impact of heavy metals on Rhizobium. The 8 ha application area was originally divided into five different-sized blocks (blocks 1–5), which received sludge at different times between 1991 and 1994. The response of a lux biosensor based on R. leguminosarum bv. trifolii (Rhizotox-C), was compared with more traditional techniques for measuring the presence of effective strains of Rhizobium (MPN) and nitrogen fixation ( δ 15N natural abundance). Although population size (MPNs), nitrogen fixation and biosensor response varied between treatment blocks, linear regression analysis determined that this block effect could not be directly linked to soil heavy metal concentrations, but was probably due to biological, physical, chemical and environmental compounding factors at the site. In this type of uncontrolled field application, the lux bioassay may provide the most useful information as it measures toxicity to any microorganism exposed to the soil solution, for example, the free living rhizobia.

The use of bioassays for evaluating the toxicity of sewage sludge and sewage sludge-amended soil

In soils, the most commonly mentioned hazardous substances are metals. One of the sources of its accumulation is the application of sewage sludge. However, little information is available regarding the estimation of the toxicity of sewage sludge or soil treated with sewage sludge, even by means of a battery of bioassays. In this study an evaluation of a battery of bioassays was carried out for toxicity assessment of sewage sludge and sewage sludge-treated soil. The objectives of this study were a) to compare the sensitivity of the different bioassays for the toxicity determination of sewage sludge contaminated with metals and soil treated with this sewage sludge, b) to elaborate a procedure for the attribution of sewage sludge samples to hazard classes based on the ecotoxicological data, and c) to evaluate the suitability of elutriate bioassays and microbial toxicity tests for the assessment of sewage sludge-treated soil. Experiments were carried out on grey forest soil (Haplic Greyzem) treated with existent sewage sludge obtained from the Municipal Wastewater Treatment Plant of the city of Kazan, Tatarstan, Russia. Portions of sludge were preliminary spiked by adding water-soluble chlorides or nitrates containing the following metals/metalloids: As, Cd, Cr, Cu, Hg, Mn, Ni, Pb, Zn. The test battery included four organisms —bacteriumPseudomonas putida, protozoanParamecium caudatum, water fleaDaphnia magna and higher plantRaphanus sativus. In addition, the toxicity of the soils treated with sludge was measured using microbial bioassays based on the estimation of microbial respiration, microbial biomass, N2-fixation activity and calculation of the metabolic quotient. The comparison of the toxicity values (expressed as the lowest dilution factor which exhibits 10% inhibition of an estimated parameter) of three model samples of sewage sludge prepared by us showed different sensitivities for all test organisms. The most sensitive was observed in theDaphnia test, while a similar sensitivity was noted for theParamecium test. The other two tests proved to be less sensitive to the components of the sludge. The analysis of the soils treated with the sewage sludge samples, using the previously mentioned tests, showed that all metal-contaminated sludge treatments as well as the control soil were not significantly inhibitory to all elutriate tests except for that withDaphnia. The microbial toxicity studies including estimation of the microbial biomass C, respiration and N2-fixating activity appear to be more sensitive than the elutriate tests for toxicity in the case of soil treated with metal-contaminated sewage sludge. An integrated ecotoxicological approach to classify the sewage sludge into hazard classes is proposed based on toxicological analyses. The test battery including four bioassays is elaborated to establish the toxicity status of the sewage sludge. The proposed procedure is easy to apply, cost effective and relatively fast. The application of microbial toxicity tests can be very useful for risk assessment in the case of soil treated with metal-contaminated sewage sludge. The application of the bioassays appears to be very promising mainly for the risk identification of highly contaminated matrices with hardly identified composition such as sewage sludge or soil-associated contaminants. However, the future success in elaborating reliable toxicity tests can be achieved only by the progress in standardizing, national legislators and efforts of researchers.

Speciation, Accumulation of Heavy Metals in Vegetation Grown on Sludge Amended Soils and their Transfer to Human Food Chain - A Case Study

The continuous use of heavy metal contaminated sewage sludge as a fertilizer can result in transfer and accumulation of toxic metals from sludge to soil. The subsequent uptake and accumulation of metals in the edible parts of vegetative tissue results in a direct pathway into the human food chain. Speciation study has great importance for monitoring the entry of heavy metals into the human food chain as these are associated in a variable manner in different phases (or species). To assess the extent of heavy metal contamination of soils and also of the vegetables grown on soil amended with sludge, speciation studies were carried out using BCR sequential extraction scheme proposed by European Commission. The vegetables that were grown on sludge amended agricultural fields were analyzed for toxic heavy metals to assess the extent of uptake of heavy metals by different plants. This study reflects the extent of contamination of soil and vegetation due to application of sludge as manure and element transfer to the human food chain.

Mineralization of soil and legume nitrogen in soils treated with metal-contaminated sewage sludge

Eighty percent of urban sewage sludge in southeastern Australia is destined to be reused on agricultural land to improve soil fertility. However, this sludge is usually contaminated with industrial pollutants, in particular with heavy metals. As heavy metals are known to be toxic to microorganisms, concern has been raised that treating soils with these sludges may adversely affect the mineralization of the organic N in the soil, sludge or plant material incorporated into the amended soils. In the absence of historically contaminated soils, dewatered sewage sludges with total heavy metal contents to 4658 mg kg −1 were ground and mixed with different soils at rates up to 240 t ha −1. The soil–sludge mixtures were then ‘aged’ through seven cycles of wetting and drying, in the presence of plants, over a period of twelve months. Total heavy metal concentrations to 1026 mg kg −1 soil, with individual metal concentrations (mg kg −1 soil) to Zn (481), Cu (249), Cr (187), Ni (86), Pb (80) and Cd (2.5), were achieved with the treatments. Samples of the processed soils were incubated at the field capacity water contents, with or without incorporated lucerne, before determining the concentration of available N (nitrate+ammonium) in the soil. More available N occurred in soils treated with sludge than in unamended soils, and available N increased with the amount of sewage sludge added. The N in lucerne was mineralized in all treatments, and there were few cases in which the increase in available N due to lucerne was reduced in sludge-amended soils. These cases involved nitrate loss and could not be ascribed to an effect of high concentrations of heavy metals in soil. Soil amendment with sludge increased the concentrations of total N, C and exchangeable cations, as well as pH, in the soil. Of these factors, only total N and exchangeable cations were positively correlated with available N. Higher concentrations of C (or heavy metals) in soil and higher pH were associated with less available N, but these effects were quantitatively inferior to the positive effects of total N and exchangeable cations. Based on the results of these studies the current limits on the allowable concentrations of heavy metals in soils, as defined by the New South Wales Environmental Protection Agency, can be substantially increased without affecting the benefits in N obtained from the incorporation of legume N with soil.

Measurement of symbiotic nitrogen-fixation in leguminous host-plants grown in heavy metal-contaminated soils amended with sewage sludge

Rates of nitrogen fixation by Rhizobium in symbiosis with leguminous host-plants including white clover, broad bean and peas have been established in soils that have been amended experimentally with heavy metal-contaminated sewage sludges. Results from 15N-dilution experiments for the measurement of N2 fixation have shown that adverse heavy metal effects are apparent on symbiotic N2 fixation rates for white clover grown in inter-specific competition with ryegrass under mixed sward conditions, compared to white clover grown in pure sward. Further experiments on broad bean and pea indicated a significant, but minor-inhibitory metal-related effect on the rate of N2 fixation compared to untreated soils and soils amended with a relatively uncontaminated sludge. The implications of the results with respect to sludge utilisation in agriculture are discussed.

Assessing the tolerance to heavy metals of arbuscular mycorrhizal fungi isolated from sewage sludge-contaminated soils

Different fungal ecotypes were isolated from soils which had received long-term applications of metal-contaminated sewage sludge with the aim of studying the degree of tolerance and adaptation to heavy metals of arbuscular mycorrhizal (AM) fungi. The development and structural aspects of AM colonization produced by the different fungal isolates were studied using two host plants, Allium porrum and Sorghum bicolor, which were grown in either contaminated or non-contaminated soils. Four different AM fungi were successfully isolated from the experimental field plots: (i) Glomus claroideum, isolated from plots receiving only inorganic fertilizer; (ii) another apparently similar ecotype of Glomus claroideum, but isolated from plots with 300 m 3 ha −1 year −1 of contaminated sludge added, (iii) an unidentified Glomus sp., present only in the less contaminated plots (100 m 3 ha −1 year −1 of unamended sludge) and (iv) Glomus mosseae, isolated from plots receiving 100 or 300 m 3 ha −1 year −1 of amended or unamended sludge (intermediate rates of contamination). There were consistent differences in behaviour among the four AM fungi tested with regard to the colonization levels they produced in non-contaminated and contaminated soils. Both total and arbuscular colonization were affected by heavy metal contamination. The main conclusions of this study are that Glomus sp. and G. mosseae isolates are strongly inhibited by heavy metals, which acted mainly by interfering with the growth of the external mycelium, and also by limiting the production of arbuscules. Our results suggest that G. claroideum isolates, particularly the ecotype which was isolated from the plots receiving the highest dose of metal-contaminated sludge, shows a potential adaptation to increased metal concentration in soil.

Soil microbial biomass and microbial activity in soils treated with heavy metal contaminated sewage sludge

Soil microbial biomass carbon (C mic) and respiration were measured in soils from two long-term field experiments with recent sewage sludge applications. A moderately contaminated sludge was applied as received from the sewage treatment plant and after additional metal contamination. Generally, the low metal sludge had beneficial effects on C mic and on the soil microbial activity. Higher heavy metal contamination of soils resulted in a substantial decrease in C mic. The ratio C mic C org even decreased when low metal sludge was applied. Soil respiration and especially the respiration per unit biomass ( qCO 2) increased with increasing amounts of heavy metals. Concomitantly the contribution of fungi to soil respiration increased. Judging from these results the qCO 2 appears to be a sensitive indicator of effects that are resulting from the effects of heavy metals on the soil microflora.

The possibility of in situ heavy metal decontamination of polluted soils using crops of metal-accumulating plants

The decontamination of soils and wastes polluted with heavy metals presents one of the most intractable problems for soil clean-up. Present technology relies upon metal extraction or immobilization processes, both of which are expensive and which remove all biological activity in the soil during decontamination. They may only be appropriate for small areas of valuable redevelopment land. In this paper the use of metal-accumulating plants is explored for the removal of metals from superficially-contaminated soils such as those resulting from the long-term application to land of metal-contaminated sewage sludges. Green remediation employs plants native to metalliferous soils with a capacity to bioaccumulate metals such as zinc and nickel to concentrations greater than 2% in the aerial plant dry matter (hyperaccumulators). Growing such plants under intensive crop conditions and harvesting the dry matter is proposed as a possible method of metal removal and for ‘polishing’ contaminated agricultural soils down to metal concentrations below statutory limits. Not only are the biological activity and physical structure of soils maintained but the technique is potentially cheap, visually unobtrusive and offers the possibility of biorecovery of metals. The limitations of the process are reviewed and the future requirements for the development of efficient phytoremediators are outlined.

Dehydrogenase activity of the microbial biomass in soils from a field experiment amended with heavy metal contaminated sewage sludges

Dehydrogenase activity (DHA) of the microbial biomass was measured in sewage sludge amended soil samples collected from the Braunschweig experimental site, Germany. The site had received additions of sludge with or without heavy metals at two application rates (100 m 3/ha per year and 300 m 3/ha per year) on soils of ‘low’ (4.8–5.8) and ‘high’ (5.4–7.0) pH since 1980. DHA was found to be a sensitive and precise assay for determining the effect of heavy metals on substrate-induced (glucose) microbial biomass in sewage sludge amended soils. Effects on DHA were determined in relation to heavy metal concentrations and other soil factors. Addition of relatively uncontaminated sludge enhanced DHA, but this was dependent on the level and type of sludge addition. Adverse metal effects were only significant in the most contaminated soils where sludge had been added to the ‘high’ andd ‘low’ pH treatments at Braunschweig. However, these effects were small compared to the effects of high rates of sludge addition alone, despite exceeding statutory limits for Zn and Cu, where concentrations reached 341 and 99 μg/g, respectively.

Enumeration of indigenous Rhizobium leguminosarum biovar Trifolii in soils previously treated with metal-contaminated sewage sludge

The effects of heavy metals from metal-contaminated sewage sludge on the indigenous population of Rhizobium leguminosarum biovar trifolii in the soils of two well controlled field experiments at Braunschweig in northeast Germany are reported. Both experiments were in the same field, but one was on an old arable soil and the other on an ex-woodland soil. The following treatments were applied to both experiments: inorganic fertilizer at 180 kg N ha −1 yr −1 (control soils); uncontaminated sludge at 100m 3 ha −1 yr −1 or 300m 3 ha −1 yr −1; or metal-contaminated sludge at the same two rates. In the ex-woodland experimental soil, rhizobial numbers decreased by an order of magnitude in plots treated with 100 m 3 yr −1 contaminated sludge compared to the control plots and plots receiving the same amount of uncontaminated sludge. The total Zn and Cd concentrations in these plots were close to the German limits, but were well below the U.K. and E.C. upper limits. Even with 300m 3 yr −1 uncontaminated sludge rhizobial numbers decreased by several orders of magnitude compared to the control soils, in both field experiments: at both sites one of the four replicate plots had no rhizobia, whereas the other three plots had decreased numbers. Metal concentrations in these plots were well below the U.K. and E.C. upper limits, except Zn which was close to the German limit, and ranged from (mg kg −1): Zn, 200–250; Cu, 46–62; Ni, 16–23 and Cd, 0.9–1.6. In plots receiving 300m 3 yr −1 contaminated sludge, in both field experiments, rhizobial numbers were further decreased compared to plots receiving uncontaminated sludge at the same rate and were several orders of magnitude smaller than in the control plots. For example, in the old arable site, three out of four plots had no rhizobia, with one plot containing 20 cell g −1soil. In the ex-woodland site, two plots out of four had no rhizobia, whereas, the other two had <4 and 9 cells g −1 soil. Metal concentrations in these plots were above the U.K. and E.C. upper limits for Zn, but were still below the corresponding limits for Cd. Copper concentrations in one old arable plot and three ex-woodland plots were slightly above the U.K. limits for soils of pH 5–6. The German limits for Zn, Cu, and Cd, but not Ni, were considerably exceeded in these plots. The major factor influencing rhizobial numbers, in both field experiments, was not the soil pH, nor the organic carbon content, but rather the metal concentrations in the soil. The smaller N contents, chlorosis and stunting of clover plants grown in soils from plots containing no, or very few, rhizobia was not due to phytotoxicity, but was shown by the addition of nitrogen fertilizer to be due to lack of N 2-fixation. Our results indicate that although several metals were accumulated simultaneously in both field experiments there was a strong Zn effect on the numbers of rhizobia in these soils. Significant reductions in rhizobial numbers occurred even at metal concentrations well below the current U.K. and E.C. upper limits for all metals, but close to the much more strict German limits for Zn and Cd. These results are discussed in relation to the setting of safe limits for the long-term protection of soils.

Rhizobium leguminosarum bv. trifolii in soils amended with heavy metal contaminated sewage sludges

Soils from a well controlled field experiment were screened for the presence and number of cells of Rhizobium leguminosarum bv. trifolii capable of effectively nodulating the host plant, white clover ( Trifolium repens). Soils had been amended with anaerobically-digested or undigested sewage sludge at rates of 0, 100 and 300 m 3 ha −1 yr −1on plots of differing pH since 1980 and up to the present. Applications of anaerobically-digested sludge included additions with or without heavy metal salts. Rhizobium were present in all of the treatments, apart from the most metal-contaminated treatment in the soil of lower pH, despite the absence of the host plant from the field sward. Lack of nodulation and nitrogen fixation (acetylene reduction activity) for T. repens growing in soils was, in some cases, probably caused by the high concentrations of extractable nitrate present as plants subsequently grown in N-free media were effectively nodulated. Important effects on the size of the effective rhizobial population were apparent in relation to the soil pH, sludge type and addition rates, and the concentration of heavy metals present.

The use of the cotton-strip assay to assess cellulose decomposition in heavy metal-contaminated sewage sludge-amended soils

Cellulose decomposition in soils amended over twenty year ago with heavy metal-contaminated sewage sludges was assessed by using the cotton-strip assay. The soils of the Luddington Experiment now contain concentrations of Cu, Ni and Zn in selected plots that approximate to or exceed the statutory limits for these elements in sewage sludge-amended soils. The rates of cellulose decomposition were generally lower in the plots with elevated metal concentrations, relative to uncontaminated sludge-amended and unsludged controls. Generally, the metal-rich plots showed reductions in the time taken to reach 50% cotton-tensile-strength loss (CTSL). However, the reductions could not be consistently related to any one metal. The difference in decomposition rates between treatments was systematically reduced over the duration of a time-course experiment. A lower intial population of the appropriate decomposer community of micro-organisms may account for the observed short-term lag in decomposition rates.