Environmental consequences from the use of sewage sludge in soil restoration related to microbiological pollution

Purpose

This article analyzed the survival of Escherichia coli, total coliforms, and Salmonella spp. in a soil amended with urban sewage sludge due to its potential use in soil rehabilitation and to the risk of microbial pollution.

Materials and methods

The survival of E. coli, total coliforms, and Salmonella spp. was determined in a soil amended with different doses of four different urban sewage sludge based on equivalent nitrogen fertilization of 0, 85, 170, and 340 kg N/ha. After the topsoil/sludge mixtures were made, they were wet to 18% moisture and analyzed for 2 months to determine the presence of bacteria, and then again after 1 year.

Results and discussion

The results indicate that the presence of microorganisms was strongly conditioned by the type of biosolid and the dose applied. Soil moisture diminished as the experiment progressed and seemed to play a role in controlling the presence of the bacteria.

Conclusions

The initial concentrations of bacteria depend on the sewage sludge treatment. The evolution of E. coli had a similar trend as total coliforms, and Salmonella spp. was absent after 8 weeks although a positive presence was detected in some soils after a year. As a conclusion, long periods of time reduce the risk from the presence of pathogens in soils, and the persistence may be closely related to the treatment of sewage sludge and the initial amount of microorganisms in the sewage sludge.

     

Long-term impacts of zinc and copper enriched sewage sludge additions on bacterial, archaeal and fungal communities in arable and grassland soils

Long-term impacts of metal contamination derived from sewage sludge on soil microbial communities have been widely evaluated, but confounding effects have made it difficult to draw firm conclusions and thus to advise on safe metal limits. Here we used Multiplex-terminal restriction length fragment polymorphism (M-TRFLP) to assess the long-term impact of sludge-borne Zn and Cu contamination on the structure of bacterial, fungal and archaeal communities across seven different soils at metal levels relevant to current guideline limits. Despite strong effects of site on microbial community structure, analysis of similarity (ANOSIM) demonstrated a small but significant effect of Zn on bacteria (P < 0.001), archaea (P < 0.001), and fungi (P < 0.001). Significant effects of Cu on bacteria (P < 0.001), archaea (P < 0.001) and fungi (P < 0.001) were also observed. Several bacterial and fungal T-RFs were identified as responding to Zn or Cu. For example the bacterial T-RF 72 was negatively correlated with Zn and Cu, and T-RF 259 was positively correlated with Zn. Attempts to identify these bacterial markers of Zn and Cu contamination suggest a negative impact of Cu on Acidobacteria in arable soils. These results demonstrate for the first time, that despite a strong influence of site on microbial community structure, effects of Zn and Cu derived from sewage sludge can be detected as shifts in bacterial, fungal and archaeal communities indicating a common response more than 11 years after sludge addition.     

Fungal community composition in sodic soils subjected to long-term rice cultivation

ABSTRACT

Rice cultivation is widely used to improve saline-sodic soils in Northeast China. However, the chronological effect of rice cultivation on soil fungal communities has not been studied. Therefore, this study investigated the variation of soil fungal communities in different rice cultivation years. Compared with the blank area, the rice cultivation for 20 years significantly decreased the pH and electrical conductivity by 16.6% and 70.1%, while significantly increased the soil organic carbon and available phosphorus by 90.6% and 17.4 times, respectively. Meanwhile, the relative abundance of Ascomycota and Basidiomycota significantly decreased and increased after rice cultivation, respectively. Moreover, some genera of arbuscular mycorrhizal fungi (AMF), e.g. Corymbiglomus, Glomus, and Rhizophagus, and pathogenic fungi (Thanatephorus cucumeris) disappeared, while Dentiscutata (one genus of AMF) appeared after rice cultivation. In addition, the fungal richness significantly increased when the rice cultivation less than 15 years and the fungal communities were similar between 15 and 20 years. Our findings showed that rice cultivation improved the nutrient availability and reduced salinity-alkalinity stress of sodic soils. After the rice cultivation for 15 years, the soil fungal community tended to be stable.     

The impact of sludge amendment on methanogen community structure in an upland soil

In the EU, municipal sewage sludge application to agricultural land has increased dramatically since the ban on dumping at sea came into effect in 1998. There are many concerns related to potential contamination and reduction in plant productivity. In this study, the aim was to assess the impact of repeated long-term soil amendment with anaerobically digested sewage sludge on methanogen diversity in an upland soil ecosystem. Sludge-treated and untreated upland soil samples as well as samples of the sludge used, were analysed for the diversity of methanogens using TGGE, PCR-RFLP and DNA sequence analysis of approximately 490 bp of the mcrA operon. PCR analysis using mcrA specific oligonucleotide primers confirmed the presence of methanogen DNA in treated and untreated soil samples and in sewage sludge. TGGE was used to describe the diversity of methanogen mcrA sequences and the differences in community structure between samples. Ninety-six mcrA gene PCR products were screened using RFLP analysis representing methanogen DNA amplified from anaerobically digested sewage sludge, control soils and sludge treated soils. Fourteen RFP's were detected in all treatments, five of which were common to all three treatments. Thirty-eight cloned amplimers were selected for sequencing and phylogenetic analysis. These included representatives of each RFP. From control soils, sludge and sludged soil samples 15, 16 and 7 clones were sequenced, respectively. Phylogenetic analysis suggested that they represented hitherto uncharacterised mcr genes; 35 of the clones fell into 7 clusters supported by moderate to high bootstrap values. The diversity of methanogens in an upland soil (treated and untreated) and sludge was evaluated and marked differences in the diversity of the methanogen communities was observed between the treatments. Our results indicate that sludge application may reduce soil methanogen community diversity.     

Short-term effect of tillage and crop rotation on microbial community structure and enzyme activities of a clay loam soil

A field study was carried out to analyze the short-term (2 years) effect of tillage and crop rotation on microbial community structure and enzyme activities of a clay loam soil. The experimental design was a split-plot arrangement of treatments, consisting of two tillage treatments—ridge tillage (RT) and no-tillage (NT)—in combination with two crop rotation treatments—corn (Zea mays L.) monoculture and a 2-year corn-soybean (Glycine max L.) rotation. Phospholipid fatty acid (PLFA) profiles were used to assess soil microbial community structure. No-tillage resulted in significantly higher total PLFAs compared to the RT treatment, which was accompanied by higher activities of protease, β-glucosaminidase, and β-glucosidase. This suggests a close link between soil microbial communities and enzyme activities in response to tillage. The increase of total microbial lipid biomass in the NT soils was due to the increase in both fungal and bacterial PLFAs. Crop rotation had little effect on soil bacterial communities and enzyme activities, but it significantly influenced soil fungal communities, particularly arbuscular mycorrhizal fungi. Soils under monoculture corn had higher fungal biomass than soils under corn-soybean rotation regardless of tillage treatment.     

Soil fungal communities and enzyme activities in a sandy, highly weathered tropical soil treated with biochemically contrasting organic inputs

The regulative effect of long-term application of biochemically contrasting organic inputs such as rice straw (4.7 g?N; 6.5 g polyphenols), groundnut stover (22.8 g?N; 12.9 g polyphenols) and leaf litter of tamarind (13.6 g?N; 31.5 g polyphenols) and dipterocarp (5.7 g?N; 64.9 g polyphenols) on fungal decomposers was studied in a tropical sandy soil. Fungal decomposers were assayed by 18S rRNA gene-based community profiling and were combined with measurements of selected enzyme activities. Dipterocarp residue application depressed fungal abundance, but promoted specialized decomposers (e.g., Aspergillus fumigatus and Anguillospora longissima) with increases in polyphenol oxidase activity. The degree of functional redundancy for invertase and B-glucosidase activities was induced after the addition of easily decomposable rice straw and groundnut stover. Higher N availability in the tamarind treatment increased, in contrast to low N rice straw, fungal abundance (i.e., Fusarium oxysporum, Myceliopthora thermophila, and Aspergillus versicolor) and promoted invertase and B-glucosidase activities, while peroxidase activity was depressed. In addition, N availability seemed to regulate not only decomposing soil fungi, but also the abundance of protozoan decomposers whose actual contribution to N turnover in soils is still poorly understood. Prospective research should thus consider apart from studying decomposing fungi also protozoa and bacteria to better understand the microbially mediated degradation of complex organic materials in soils.     

The Phytotoxicity Changes of Sewage Sludge-Amended Soils

The aim of the present study was the estimation of changes in the phytotoxicity of soils amended with sewage sludge with relation to Lepidium sativum, Sinapis alba and Sorghum saccharatum. The study was realised in the system of a plot experiment for a period of 29?months. Samples for analyses were taken at the beginning of the experiment, and then after 5, 17 and 29?months. Two kinds of sewage sludge, with varying properties, were added to a sandy soil (soil S) or a loamy soil (soil L) at the dose of 90?t/ha. The addition of sewage sludge to the soils at the start of the experiment caused a significant reduction of both seed germination capacity and root length of the test plants, the toxic effect being distinctly related to the test plant species. With the passage of time the negative effect of sewage sludge weakened, the extent of its reduction depending both of the kind of sewage sludge applied and on the type of soil. Phytotoxicity of the soils amended with the sewage sludges was significantly lower at the end of the experiment than at the beginning. The species of the plants grown on the soils also had a significant effect on their phytotoxicity. The greatest reduction of toxicity was observed in the soil on which no plants were grown (sandy soil) and in the soil under a culture of willow (loamy soil). Solid phase of sewage sludge-amended soils was characterised by higher toxicity than their extracts.     

Effect of sewage sludge on suppressiveness to soil-borne plant pathogens

The objective of this study was to evaluate the effect of sewage sludge on soil suppressiveness to the pathogens Fusarium oxysporum f. sp. lycopersici on tomato, Sclerotium rolfsii on bean, Sclerotinia sclerotiorum on tomato, Rhizoctonia solani on radish, Pythium spp. on cucumber, and Ralstonia solanacearum on tomato. Soil samples were collected from an experimental corn field in which sewage sludge had been incorporated once a year, since 1999. Sludge from two sewage treatment stations in Brazil (Franca and Barueri, SP) were applied at the rates of one (1N), two (2N), four (4N) and eight (8N) times the N recommended doses for the corn crop. Soil suppressiveness was evaluated by methods using indicator host plants, baits and mycelial growth. There was no effect of sewage sludge on soil suppressiveness to Fusarium oxysporum f. sp. lycopersici in tomato plants. For S. rolfsii, reduction of the disease in bean was inversely proportional to the dose of Franca sludge. The incidence of dead plants, caused by S. sclerotiorum, was directly proportional to sludge doses applied. For R. solani and R. solanacearum, there was a linear trend with reduction in plant death in soils treated with increasing amounts of sludge from Franca. There was an increase in the pathogen community of Pythium spp., proportional to the amounts of sewage applied. The effects of sewage sludge varied depending on the pathogen, methodology applied and on the time interval between the sewage sludge incorporation and soil sampling.     

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 m³ ha⁻¹ year⁻¹ of contaminated sludge added, (iii) an unidentified Glomus sp., present only in the less contaminated plots (100 m³ ha⁻¹ year⁻¹ of unamended sludge) and (iv) Glomus mosseae, isolated from plots receiving 100 or 300 m³ 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.     

Major nutrients,heavy metals and PBDEs in soils after long-term sewage sludge application

Purpose

Two contrasting soils receiving long-term application of commercial sewage sludge fertilizers in China were investigated to determine the concentrations of selected nutrients, heavy metals (HMs) and polybrominated diphenyl ethers (PBDEs) present to evaluate the impact of sewage sludge fertilizer on soil fertility and environmental risk.

Materials and methods

Soil samples were collected from Tangshan City, Hebei province and Ningbo City, Zhejiang province and divided into two portions, one of which was air-dried and sieved through 2-, 0.25- and 0.149-mm nylon mesh for determination of nutrients and heavy metals. The other portion was frozen at ?20°C, freeze-dried and sieved through 2-mm nylon mesh for PBDE analysis. The concentrations of nutrients, heavy metals and PBDEs were determined in all samples.

Results and discussion

Concentrations of nutrients and heavy metals in soils amended with low rates of sewage sludge fertilizer (SSF) and conventional fertilizer were compared. After long-term excessive amendment with SSF from Ningbo City (SSF-N), the concentrations of soil total N, P, aqua regia-extractable HMs and DTPA extractable HMs were higher than the control, especially in the arable layer. Moreover, the concentration of aqua regia-extractable Zn (457 mg kg^?1) exceeded the recommended China Environmental Quality Standard for soils (GB15618-1995). All 8 target PBDE congeners were found in fertilizer SSF-N and soil with excessive amendment with SSF-N for 12 years, but the concentrations of 8 different PBDEs in SSF-N-amended soil were not significantly different from control soil.

Conclusions

Both economic and environmental benefits can be obtained by careful application of sewage sludge fertilizer to recycle plant nutrients. Repeated and excessive application rates of sewage sludge fertilizer may pose environmental risk, especially in respect of soil heavy metal and PBDE contamination, and high concentrations of phosphorus may also be environmentally detrimental.

     

Assessment of Heavy Metals in Spinach (Spinacia oleracea L.) Grown in Sewage Sludge–Amended Soil

The assessment of heavy metals in spinach (Spinacia oleracea) grown in sewage sludge–amended soil was investigated. The results revealed that sewage sludge significantly (P < 0.01) increased the nutrients and heavy metals such as cadmium (Cd), chromium (Cr), copper (Cu), manganese (Mn), and zinc (Zn) in the soil. The contents of metals were found to be below the maximum levels permitted for soils in India. The most agronomic performance and biochemical components of S. oleracea were found at 50% concentrations of sewage sludge in both seasons. The contents of Cd, Cr, Cu, Mn, and Zn in S. oleracea were increased from 5% to 100% concentrations of sewage sludge in both seasons. The order of contamination factor (Cf) of different heavy metals was Mn > Cd > Cr > Zn > Cu for soil and Cr > Cd > Mn > Zn > Cu for S. oleracea plants after application of sewage sludge. Therefore, use of sewage sludge increased concentrations of heavy metals in soil and S. oleracea.     

Remediation of Metal Contaminated Soil by Organic Metabolites from Fungi I—Production of Organic Acids

Investigations were made on living strains of fungi in a bioremediation process of three metal (lead) contaminated soils. Three saprotrophic fungi (Aspergillus niger, Penicillium bilaiae, and a Penicillium sp.) were exposed to poor and rich nutrient conditions (no carbon availability or 0.11 M d-glucose, respectively) and metal stress (25 µM lead or contaminated soils) for 5 days. Exudation of low molecular weight organic acids was investigated as a response to the metal and nutrient conditions. Main organic acids identified were oxalic acid (A. niger) and citric acid (P. bilaiae). Exudation rates of oxalate decreased in response to lead exposure, while exudation rates of citrate were less affected. Total production under poor nutrient conditions was low, except for A. niger, for which no significant difference was found between the poor and rich control. Maximum exudation rates were 20 µmol oxalic acid g^?1 biomass h^?1 (A. niger) and 20 µmol citric acid g^?1 biomass h^?1 (P. bilaiae), in the presence of the contaminated soil, but only 5 µmol organic acids g^?1 biomass h^?1, in total, for the Penicillium sp. There was a significant mobilization of metals from the soils in the carbon rich treatments and maximum release of Pb was 12% from the soils after 5 days. This was not sufficient to bring down the remaining concentration to the target level 300 mg kg^?1 from initial levels of 3,800, 1,600, and 370 mg kg^?1in the three soils. Target levels for Ni, Zn, and Cu, were 120, 500, and 200 mg kg^?1, respectively, and were prior to the bioremediation already below these concentrations (except for Cu Soil 1). However, maximum release of Ni, Zn, and Cu was 28%, 35%, and 90%, respectively. The release of metals was related to the production of chelating acids, but also to the pH-decrease. This illustrates the potential to use fungi exudates in bioremediation of contaminated soil. Nonetheless, the extent of the generation of organic acids is depending on several processes and mechanisms that need to be further investigated.     

Survival of faecal indicators and enteroviruses in soil after land-spreading of municipal sewage sludge

One concern in regard to the spreading of sewage sludge is the potential for contamination of soil and water by pathogens present in sludge. We studied the survival of enteric micro-organisms in sewage sludge following direct land-spreading. The sludge produced by a wastewater treatment plant (capacity equivalent to 2000 inhabitants; sludge storage tank of 700 m³) was spread on a soil, at a rate of 80 m³/ha. The tested micro-organisms included three of specific sanitary interest: faecal indicators, spores of Clostridium perfringens and enteroviruses. The results highlighted three types of behaviour associated with these three groups of micro-organisms. The enteroviruses were not detected 2 weeks after spreading on the soil whereas the concentration of faecal indicators fell slowly with an observed decrease of between 1.2 and 1.8 logarithmic units over 2 months (but without the initial level of the soil being reached). Lastly, the concentrations of C. perfringens remained stable. The different survival times of the studied micro-organisms in soil, confirm the necessity to include several indicators (bacteria and viruses) to estimate reliably the sanitary risk related to sludge spreading.     

Sewage Sludge Application and Soil Properties Effects on Short-Term Zinc Leaching in Soil Columns

Eight soils from the Gulf of Kalloni in Lesvos Island, Greece, most of which were of low clay and low organic matter content, were used in a series of leaching experiments. The aim was to investigate the role of sewage sludge on Zn leaching and determine the soil properties that affect it. It was found that sludge addition at a low application rate (10 t ha^?1) decreased Zn leaching significantly by 30%. From a regression analysis it was found that eluted Zn was reduced with increasing Fe oxides content, probably due to Zn adsorption onto them, and that Zn transport increased with clay content. This indicates that Zn leaching was facilitated by the downward movement of clay particles, which was also suggested by the erratic Zn breakthrough observed in many soils. This was further confirmed by the fact that colloid concentrations increased with clay content in the soils (R?=?0.85, P?<?0.05). The results show that the addition of sewage sludge to low organic matter and clay content soils at moderate application rates enhances soil organic carbon and increases metal retention capacity.     

Evolution of mercury content in agricultural soils due to the application of organic and mineral fertilizers

Purpose

Mercury pollution in agricultural soils associated to the use of fertilizers and its influence on crops is a cause of major concern. The purpose of this work was to investigate the impact of the application of different organic and mineral fertilizers on the Hg concentration in the agricultural soils and its uptake by barley.

Materials and methods

Hg concentration was studied through a field test in an agricultural land located in the province of Palencia (Spain) over a 5-year period. The impact of irrigation and of four different fertilizers (a mineral one and three different organic waste materials, namely municipal solid waste compost, sewage sludge, and dehydrated sewage sludge) was assessed. The amounts of the mineral and organic fertilizers added to the soil were determined according to agricultural fertilization needs. The experimental crop was barley (Hordeum vulgare L.), planted as an annual crop. Mercury analyses were conducted using a direct mercury analyzer and validated according to EPA Method 7473. BCR-141R was used as a certified reference material.

Results and discussion

After 5 years, whereas the application of the mineral fertilizer did not increase the mercury content in the agricultural soils, the application of the organic residues led to Hg contents 1.7–7.6 times higher than that of the control soil. The treatment with solid municipal waste compost (MSWC) led to the largest increase in Hg content in the soil, followed by composted sewage sludge (CSS) and by dehydrated sewage sludge (DSS). No significant differences were observed in the Hg content in the barley grains, although the highest values were associated to the sludge-treated plots.

Conclusions

The application of organic fertilizers such as sewage sludges and municipal solid wastes led to an increase in the mercury concentration in the agricultural soils, noticeable for soils with low initial Hg concentrations (similar to background levels). This increase differed depending on the type of waste and on the intra-organic matter diffusion mechanisms, as well as on the type of irrigation of the agricultural land. Conversely, no significant differences in the Hg content in grains were found among the soils with the different fertilization treatments, although the highest values were observed for those treated with sewage sludge. The resulting Hg levels in both soils and grains were within legal limits, posing no danger to the environment or to human health.

     

Impact of Sewage Sludge Application on the Long-Term Nutrient Balance in Acid Soils of Scots Pine (Pinus Sylvestris,L.) Forests

The objective of the investigation was to determine the effectsof sewage sludge application on nutrient concentrations in soil and plant biomass fractions in Scots pine forests (Pinus sylvestris, L.), situated on sandy soils with low pH, in a south to north temperature gradient in Sweden. Twenty tons dw ha^-1 of sewage sludge was applied in 50 to 60 yr old pine forests at foursites from Brösarp in South Sweden to Jukkasjärvi in thenorthern parts of the country.Application of 20 ton dw ha^-1 of sewage sludge significantlyincreased the concentrations of extractable N, P, K, Ca, Mg and Na, in both the mor layer and in the upper 10 cm of the mineral soil. Three years after sludge application K concentrations were only significantly increased in the upper 10 cm of the mineral soil. After 11 yr the concentrations of P were still at the samelevel in the mor layer as after three years. The concentrations of Ca, Mg and Na had slightly decreased only in the mor layer. There was, in most cases, a statistically significant positive correlation between the amount of applied sludge and nutrientconcentrations in the soil, as well as in pine needles and in leaves of Vaccinium vitis-idaea.In all sites, Mg concentrations in the mor layer was positivelyand significantly correlated with Mg concentrations in current-year pine needles. Similarly, concentrations of Ca, Mg,and P in the mor layer were correlated with concentrations of these elements in current-year shorts of Vaccinium vitis-idaea.     

Effects of two grass species on the composition of soil fungal communities

Many studies have shown effects of plants species on fungal communities, but these are often confounded with soil effects. Thus, the specific role of plant species in structuring rhizospheric and soil fungal communities is poorly described. Our study used microcosms in which plants were grown under artificial conditions to bridge this gap. Two perennial grasses dominating subalpine grasslands, Festuca paniculata and Dactylis glomerata, were grown at two levels of fertilization on standard soil. Fungal communities were determined by 454 pyrosequencing of the internal transcribed spacer 1 region. Among the fungal communities characterized by the primers used, original communities were associated to each plant species and also diverged between rhizosphere and bulk soils within each plant species, though there were no significant fertilization effects. Differences regarded global composition of the fungal communities and abundant molecular operational taxonomic units (MOTUs). Both plant species and location effects were reflected more in the abundance than in the composition of MOTUs. The observed differences in fungal communities coincide with differing strategies of plant root growth, with D. glomerata having greater root mass, length, and area than F. paniculata. Our study, by dissociating soil effects from plant effects, demonstrated that plant species exert a key control on soil fungi. We suggest that such effects may be linked to inter-specific differences in root traits and their consequences on nitrogen uptake.     

Toxic potential of different types of sewage sludge as fertiliser in agriculture: ecotoxicological effects on aquatic,sediment and soil indicator species

Purpose

Treated and processed sewage sludges (biosolids) generated during the treatment of wastewater usually contain substantial concentrations of nutrients, especially phosphorus, which is essential for plant growth. Sewage sludge therefore can be used as an alternative fertiliser in agriculture. But since sewage sludge could also contain pollutants, analysis and ecotoxicological tests on affected soil and stream water organisms are necessary in order to guarantee its harmless use.

Materials and methods

Three test species were chosen to cover the environmental compartments, water, sediment and soil. The following test species and parameters were applied to evaluate the acute effects of three sewage sludge samples: Lemna minor (growth inhibition, discolouration and colony breakup), Gammarus fossarum (mortality, behaviour) and Eisenia fetida (avoidance behaviour). Chemical assessment included nutrients, organic pollutants and heavy metals.

Results and discussion

The assessment of a non-dewatered sludge (S1) sample resulted in an inhibition of growth of L. minor starting from 0.6 g total solid (TS)?l^?1 after 7 days (EC₅₀ 1.2 g TS l^?1). G. fossarum displayed significantly decreased movement activity at 0.5 and 1.2 g TS l^?1 sludge concentration during an exposure time of 2 days, leading to decreased survival after 4 days of exposure in 0.5 g TS l^?1 (LC₅₀ 0.5 g TS l^?1). After 2 days, E. fetida exhibited an increased avoidance behaviour of contaminated soil from 0.2 g TS kg^?1 sewage sludge (EC₅₀ 0.4 g TS kg^?1). The dewatered sludge samples (S2 and S3) had a lower toxic effect on the test organisms. G. fossarum was the most sensitive test species in the applied test setups. The realistic application amounts of the tested sewage sludge samples of approximately 6.0 g TS kg^?1 (maximum allowed application amount of sewage sludge) and approximately 3 g TS kg^?1 (maximum agronomical relevant application amount) in worst case studies are higher than the analysed EC₅₀/LC₅₀ values of S1 and of the LC₅₀ (G. fossarum) of S2 and S3.

Conclusions

All three tested sewage sludge samples have to be classified as toxic at high concentration levels under laboratory conditions. Realistic output quantities of S1 will negatively influence soil invertebrates and freshwater organisms (plants and crustacean), whereas the dewatered sludge samples will most likely not have any acute toxic effect on the test organisms in the field. Test with environmental samples should be conducted in order to support this hypothesis.

     

Availability of Cu and Zn in an acidic sludge-amended soil as affected by zeolite application and liming

Purpose  

Acidic soils exhibit high trace element availability compared to neutral pH soils, and thus, when trace metals are added (e.g. due to sewage sludge application), measures should be taken to reduce their mobility. In this experiment, we tested two such methods, liming and zeolite addition. The aim was to measure the availability, in ryegrass (Lolium perenne L.), of heavy metals (Cu and Zn) added to soil with sewage sludge in both acidic and limed soil.     

Four Swedish long-term field experiments with sewage sludge reveal a limited effect on soil microbes and on metal uptake by crops

Purpose

This study aims to study the effect of sewage sludge amendment on crop yield and on microbial biomass and community structure in Swedish agricultural soils.

Materials and methods

Topsoil samples (0–0.20 m depth) from four sites where sewage sludge had been repeatedly applied during 14–53 years were analysed for total C, total N, pH and phospholipid fatty acids (PLFAs). Heavy metals were analysed in both soil and plant samples, and crop yields were recorded.

Results and discussion

At all four sites, sewage sludge application increased crop yield and soil organic carbon. Sludge addition also resulted in elevated concentrations of some heavy metals (mainly Cu and Zn) in soils, but high concentrations of metals (Ni and Zn) in plant materials were almost exclusively found in the oldest experiment, started in 1956. PLFA analysis showed that the microbial community structure was strongly affected by changes in soil pH. At those sites where sewage sludge had caused low pH, Gram-positive bacteria were more abundant. However, differences in community structure were larger between sites than between the treatments.

Conclusions

At all four sites, long-term sewage sludge application increased the soil organic carbon and nitrogen content, microbial biomass and crop yield. Long-term sewage sludge application led to a decrease in soil pH. Concentrations of some metals had increased significantly with sewage sludge application at all sites, but the amounts of metals added to soil with sewage sludge were found not to be toxic for microbes at any site.