Assessment of Arsenic and Selenium Concentration with Chlorophyll Contents of Sugar Beet (Beta vulgaris var.saccharifera) and Wheat (Triticum aestivum) Exposed to Municipal Sewage Sludge Doses

The present study was conducted to assess the suitability of sewage sludge amendment in soil for Beta vulgaris var. saccharifera (sugar beet) and Triticum aestivum (wheat) by evaluating the arsenic and selenium accumulation and physiological responses of plants grown at 10%, 25%, and 50% sewage sludge amendment rate. Sewage sludge amendment was modified by the physicochemical properties of soil, thus increasing the availability of heavy metals in the soil and consequently with higher accumulation in plant parts. The chlorophyll contents increased after the sewage sludge treatments except for 50%. The sewage sludge amendment led to a significant increase in arsenic and selenium concentrations of the soil. The heavy metal accumulation in the soil after the treatments did not exceed the limits for the land application of sewage sludge recommended by the US Environmental Protection Agency. The increased concentration of heavy metals in the soil due to the sewage sludge amendment led to increases in heavy metal uptake in the leaves and root concentrations of arsenic and selenium in plants as compared to those grown on unamended soil. Accumulation was more in roots than shoots and leaves for most of the heavy metals. Concentrations of arsenic and selenium were more than the permissible limits of national standards in the edible portion of sugar beet and wheat grown on different sewage sludge amendments ratios. The study concludes that the sewage sludge amendment in the soil for growing sugar beet and wheat may not be a good option due to risk of contamination of arsenic and selenium.     

污泥与施污土壤重金属生物活性及生态风险评价

将城市污泥以不同质量比施于土壤中构成污泥混合土壤,研究各污泥配比土壤中重金属的生物活性,并采用三种重金属评价方法(地累积指数法、潜在生态风险指数法、综合毒性指数模型)和黑麦草对重金属的吸收富集效果来对施污土壤中重金属具有的生态风险性进行评价。结果表明:污泥的添加使土壤中生物活性态Cd、Cu和Zn含量显著增加,对三种重金属具有活化作用,但对Pb却起到钝化作用。生态风险评价结果表明:污泥的添加使土壤中Pb呈现无污染和低生态风险;Cu和Zn呈现中度污染和低生态风险;Cd达到强度污染和重度生态风险,重金属潜在生态风险(RI)总体处于强度生态风险水平。当污泥添加比例大于6:10(污泥S3处理)时,施污土壤中重金属的综合毒性指数高于土壤对照。黑麦草对Cd、Pb、Cu和Zn的富集浓度与施污土壤中对应重金属的生物活性态含量存在显著正幂指数关系,同时黑麦草对施污土壤中Cd、Cu和Pb的富集能力大小与地累积指数法和潜在生态风险指数法对三种重金属具有的生态风险性的评价结果具有一致性。     

Soil microbial community structure affected by 53 years of nitrogen fertilisation and different organic amendments

The Ultuna long-term soil organic matter experiment in Sweden (59′82° N, 17′65° E) was started in 1956 to study the effects of different N fertilisers and organic amendments on soil properties. In this study, samples were taken from 11 of the treatments, including unfertilised bare fallow and cropped fallow, straw with and without N addition, green manure, peat, farmyard manure, sawdust, sewage sludge, calcium nitrate and ammonium sulphate, with n = 4 for each treatment. Samples were taken from topsoil (0–20 cm) and subsoil (27–40 cm depth) and analysed for concentrations of phospholipid fatty acids (PLFAs), organic C, total N and pH. The results showed that the subsoil samples reflected the total PLFA content of the topsoil, but not the microbial community structure. Total PLFA content was well correlated with total organic C and total N in both topsoil and subsoil. Total PLFA content in topsoil samples was highest in the sewage sludge treatment (89 ± 22 nmol PLFA g dw⁻¹). This contradicts earlier findings on microbial biomass in this sewage sludge-treated soil, which indicated inhibition of microorganisms, probably by heavy metals added with sludge. A switch towards microbial growth and faster decomposition of organic matter occurred around 2000, coinciding with lowered heavy metal content in the sludge. According to the PLFA data, the microbial community in the sewage sludge treatment is now dominated by Gram-positive bacteria. A lack of Gram-negative bacteria was also observed for the ammonium sulphate treatment, obviously caused by a drop in pH to 4.2.     

Application of composted sewage sludges contaminated with heavy metals to an agricultural soil

To elucidate the mechanism of transfer of heavy metals into the food chain, an experiment was carried out with a calcareous soil, to which two different doses of a sewage sludge compost contaminated with either Cd or Zn, Cd, Cu, and Ni were applied. A crop of lettuce was then grown in the amended soils. The application of sewage sludge composts to a calcareous soil lowered the soil's pH, although the value was always around 8 at the end of the experiment. Electric conductivity rose with organic amendment. As anticipated, such an amendment improved the nutritional level of the soils, particularly Nand P, both total and available. Plant yields were negatively affected by organic amendments contaminated with heavy metals, the most dangerous in our experiment being Cd and Zn since this metals easily taken up by plants. As Ni and Cu form insoluble complexes with the organic matter of the sewage sludge composts they are not readily absorbed. Of the metals studied, Cd and Zn showed the highest bioavailability index.     

Determination of Heavy-Metal Concentration with Chlorophyll Contents of Wheat (Triticum aestivum) Exposed to Municipal Sewage Sludge Doses

The present study was conducted to assess the suitability of sewage-sludge amendment in soil for Triticum aestivum (wheat) by evaluating the heavy-metal accumulation and physiological responses of plants grown at 10, 25, and 50% sewage sludge amendment rate. Sewage sludge amendment modified the physicochemical properties of soil, thus increasing the availability of heavy metals in soil and consequently greater accumulation in plant parts. The chlorophyll contents generally increased after the sewage sludge treatments. Heavy-metal accumulation in the soil after the treatments did not exceed the limits for land application of sewage sludge recommended by the U.S. Environmental Protection Agency. Recycling sewage sludge as fertilizer will generate economical profits. However, the use of sewage sludge amendment in the soil for growing wheat may not be a good option due to risk of contamination of some heavy metals.     

Potential of Gamma Irradiated Sewage Sludge as Fertilizer in Radish: Evaluating Heavy-Metal Accumulation in Sandy Loam Soil

Soil application of sewage sludge as an amendment in crop plants has became a popular method of municipal sewage-sludge disposal in many countries. However, the presence of heavy metals in untreated sewage sludge has raised concerns of adverse effects on crop growth, quality of product, and environmental health. Gamma irradiation is one of the treatments for hygienization of sewage sludge before use as fertilizer. To evaluate the potential of gamma-irradiated sewage sludge as fertilizer in vegetable crops, the field investigation was conducted in a root crop, radish (Raphanus sativus L.), during the 2005–06 and 2006–07 growing seasons in a sandy loam soil. Treatments consisted of three source of fertilizers [farmyard manure (FYM), gamma-irradiated sewage sludge (GISS), and nonirradiated sewage sludge (NISS)]; each were compared at six application levels (1, 3, 6, 7, 9, and 11 t ha^?1). The physicochemical properties of all the three fertilizers used in this study were compared. Growth parameters and yields of radish were not significantly influenced by source of fertilizers or their application levels, except plant stand, which was influenced by type of fertilizers used. There was no significant difference observed between source of fertilizer treatments with respect to any of the measured soil properties, including major nutrients [nitrogen (N), phosphorus (P), and potassium (K)], metallic micronutrients [copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn)], and heavy metals [nickel (Ni), lead (Pb), cadmium (Cd), and cobalt (Co)]. Soil P and Zn were influenced by the various level of fertilizers. However, the interaction effect of source and level of fertilizer was absent for all the measured parameters. The maximum pollutant limits in sewage sludge and soil for agricultural use in different countries were compared. The concentration of metallic micronutrients and heavy metals in soil were less than the prescribed limit of the United States Environmental Protection Agency (USEPA), and no significant accumulation was noted after 2 years of application of GISS and NISS even at higher application rates.     

Toxic effect of cadmium and nickel on soil enzymes and the influence of adding sewage sludge

Sewage sludge is increasingly used as an organic amendment to soil, especially to soil containing little organic matter. However, little is known about the utility of this organic amendment in the reclamation of soil polluted with heavy metals. We studied the effects of adding sewage sludge on enzymatic activities of a semi-arid soil contaminated with Cd or Ni in the laboratory. The activities of urease, phosphatase, β-glucosidase and protease-BAA were measured in soil containing concentrations of Cd or Ni in the range 0–8000 mg kg⁻¹ soil, and their inhibition was compared with those of the enzymatic activities in the same soil amended with sewage sludge and containing similar concentrations of the heavy metals. The inhibition was tested for three different incubation times to determine changes in the effect of the heavy metals on hydrolase activity with the time elapsed after contamination. Ecological dose (ED) values of Cd and Ni were calculated from three mathematical models which described the inhibition of the enzymatic activities with increasing concentrations of heavy metal in the soil. For urease and phosphatase activities, the ED values for Cd and Ni increased after application of sewage sludge to soil, indicating a decrease in Cd and Ni toxicity. The other two enzymes (β-glucosidase and protease-BAA) were less sensitive to Cd or Ni contamination, and it was more difficult to determine whether addition of sewage sludge had affected the inhibition of these enzymes by the heavy metals.     

Effect of differently post-treated dewatered sewage sludge on β-glucosidase activity,microbial biomass carbon,basal respiration and carbohydrates contents of soils from limestone quarries

This work has evaluated the effects of thermally dried (TDS) or composted (CDS) dewatered sewage sludge on β-glucosidase activity, total (TCH) and extractable (ECH) carbohydrate content, microbial biomass carbon and basal respiration of soils from limestone quarries under laboratory conditions. Two doses (low and high) of the dewatered sludge (DS) or of the respective TDS or CDS were applied to a clayey and a sandy soil, both coming from working quarries. The soil mixtures and the controls (soils with no added sludge) were incubated for 9 months at 25°C and 30% of field capacity. The addition of sludge increased all the studied soil parameters, and the increase depended on the amount of sludge. Except in the case of TCH and ECH, the enhancing effect decreased with time, but at the end of incubation, parameters of the treated soils were higher than those of the control. The rank order of the initial stimulating effect was soil–TDS ≥ soil–DS ≥ soil–CDS, and probably, this order depended on the proportion of stable organic matter, which was the lowest in the TDS. Values of metabolic quotient (qCO₂) were higher at the lower dose, and they did not change during incubation in the CDS-treated soils. Both TCH and ECH were the parameters with the greatest significant sludge and dose effects. Basal respiration, microbial biomass carbon and β-glucosidase activity were the best measured parameters in distinguishing the long-term effects of the three sludge types over the soils.     

污泥农用对土壤和作物重金属累积及作物产量的影响

以3 a定位试验为基础,比较3种不同处理的污泥肥料（消化污泥、污泥堆肥及污泥复混肥）农田施用下土壤养分、土壤和作物籽粒中Mn、Cu、Zn、Pb、Cd 5种重金属的积累以及作物产量的变化情况,以阐明污泥农用对土壤及作物的影响。研究表明,3种污泥肥料提高了土壤中氮素和有机质的含量;与空白和普通化肥处理相比,3种污泥肥料增加了土壤中Mn和Cu的含量,而对土壤交换态重金属含量没有显著影响;3种污泥处理均增加了小麦籽粒中Zn的含量;相对普通化肥处理,3种污泥肥料处理对小麦和玉米产量均无显著影响。合理施用污泥肥料可以有效地提高作物产量;污泥肥料施用对土壤重金属有一定累积效应,但短期施用对土壤比较安全。     

Enzyme Activities in a Sandy Soil Amended with Sewage Sludge and Coal Fly Ash

Previous studies showed that coal fly ash could stabilize sewage sludge by reducing metal availability, but fly ash may cause an adverse effect on soil microbial activities. Therefore, an experiment was performed to evaluate the effects of amendment of soil with anaerobically digested dewatered sewage sludge, stabilised with alkaline coal fly ash, on soil enzyme activity and the implications for soil nutrient cycling. Sewage sludge was amended with 0, 5, 10, 35 and 50% w/w of fly ash, and then the ash-sludge mixtures were incubated with a sandy soil at 1:1 (v/v). Dehydrogenase activity decreased with an increase in fly ash amendment level and the time of incubation. Soil receiving 5% ash-sludge amendment had a higher dehydrogenase activity than other treatments. Soil receiving 10% ash-sludge mixture had the highest urease activity and in general, urease activity decreased with increasing incubation time. Phosphatase activity was the highest at 5% ash-sludge mixture amended soil and no general trend was observed with time. Water-soluble Zn, Mn and Cu contents were suppressed by the addition of fly ash. The present experiment indicated that addition of 10% ash-sludge mixture should have a positive benefit on the activity of soil microorganisms, N and P nutrient cycling, and reduce the availability of heavy metals.     

Effects of N-enriched sewage sludge on soil enzyme activities

Sewage sludge is increasingly used as an organic amendment to soil, especially to soil containing little organic matter. However, little is known about utility of this organic amendment with N-enriched or adjusted C:N ratios in soil. We studied the effects of adding of different doses (0, 100, 200 and 300 t ha⁻¹) and C:N ratios (3:1, 6:1 and 9:1) of sewage sludge on enzyme activities (β-glucosidase, alkaline phosphatase, arylsulphatase and urease) in a clay loam soil at 25 °C and 60% soil water holding capacity. Nitrogen was added in the form of (NH₄)₂ SO₄ solution to the sludge to reduce the C:N ratio from 9:1 to 6:1 and 3:1. The addition of different doses and C:N ratios of the sludge caused a rapid and significant in the enzymatic activities in soils, this increase was specially noticeable in soil treated with high doses of the sludge. In general, enzymatic activities in sludge-amended soils tended to decrease with the incubation time. All activities reached peak values at 30 days incubation and then gradually decreased up to 90 days of incubation. Sewage sludges also the increased available metal (Cu, Ni, Pb and Zn) contents in the soils. However, the presence of available soil metals due to the addition of the sludge at all doses and C:N ratios did negatively affect all enzymatic activities in the soils. This experiment indicated that all doses and C:N ratios of sewage sludge applied to soil would have harmful effects on enzymatic activity. Some heavy metals found in sewage sludge may negatively influence soil enzyme activities during the decomposition of the sludge.     

蚯蚓活动对城市生活污泥重金属的影响

采用塑料温棚内垄式堆积污泥培养蚯蚓方式,研究了蚯蚓处理对污泥重金属的影响。结果表明,污泥经蚯蚓处理后,理化性质发生了显著的变化,污泥的pH值、有机质、总氮和总磷都有不同程度的降低;蚯蚓能吸收富集污泥中的重金属,其中对重金属Cd有较强的富集能力;蚯蚓处理使污泥中重金属含量均出现不同程度的下降,重金属Cr、Zn、Pb、Cd、Cu、Ni分别减少27.98%、31.46%、32.81%、13.85%、23.86%和22.92%。利用盆栽试验,研究了污泥施用于土壤后生菜体内重金属积累的情况,结果表明,生菜体内重金属Zn、Cu、Pb和Ni的含量为污泥处理高于蚓粪处理;Cr和Cd则分别为差异不显著和略有降低。     

Recycling gamma irradiated sewage sludge as fertilizer: A case study using onion (Alium cepa)

Recycling sewage sludge into fertilizer for agricultural purposes may improve soil fertility by influencing the physical, chemical, and biological properties of the land. However, there is concern regarding elevated levels of heavy metals and pathogenic microorganisms, which may result from the use of untreated sewage sludge. Gamma radiation is found to be an efficient tool in the hygienization of municipal sewage sludge. In order to evaluate the agricultural potential of gamma irradiated sewage sludge and to assess the safety of this fertilizer, field experiments were performed in a root crop, onion (Alium cepa), during the 2003–2004 and 2004–2005 winter months. The influence over major nutrients, metallic micronutrients, and heavy metals in soil and crop plant were key factors to be analyzed. Treatments consisted of three source of fertilizers {S₁: farmyard manure (FYM); S₂: gamma irradiated sewage sludge (GISS); and S₃: non-irradiated sewage sludge (NISS)}, each at three separate levels (L₁: 5 t ha^?1; L₂: 10 t ha^?1; and L₃: 15 t ha^?1), which were evaluated and compared. The growth parameters and onion yield were not significantly influenced by the different sources of fertilizer, or the different application levels. Values of pH, organic carbon, organic N, available P and K, metallic micronutrients (Zn, Mn, Fe, Cu) and heavy metals (Ni, Cd, Pb, Co) indicate no negative effects on either soil or plant properties. Concentrations of heavy metals in soil and plant were slightly higher in NISS treatment in compare to GISS; however, the concentrations did remain within the prescribed limit, and no significant increase was consistently noted. The results prove that the gamma irradiated sludge material was of equal quality compared to the conventional FYM.     

利用粉煤灰污泥混料改良石灰岩质土壤后的重金属累积状况

通过盆栽试验研究以粉煤灰和污泥混合物为主料,改良石灰岩质土壤后对玉米生长发育的影响,根据重金属富集系数,探讨重金属在土壤和玉米中的累积状况。试验结果表明:石灰岩质土壤中添加粉煤灰污泥混合物后能显著促进玉米的生长和提高玉米的干物重,其中以1:1重量比处理玉米的平均株高、株径及根部、地上部、总干物重分别是对照样的1.56、1.71、1.36、3.81和2.37倍。以不同配比添加混合物后的土壤相对于国家土壤背景值和国家土壤环境质量标准,Hg和Cd表现强烈富集,整体符合国家二级土壤质量标准,且玉米根部几乎所有有害元素的含量均高于对照组,而地上部分除Cu、Zn外均显著低于对照组,但均未达到玉米毒害浓度。另外,在土壤中添加混合物后玉米中几乎所有的有害元素主要累积在根部,有利于其地上部分的生长,减少了通过食物链危害人类的机率。     

Chemical Changes and Heavy Metal Partitioning in an Oxisol Cultivated with Maize (Zea mays, L.) after 5 Years Disposal of a Domestic and an Industrial Sewage Sludge

The need for solutions to minimize the negative environmental impacts of anthropogenic activities Fhas increased. Sewage sludge is composed of predominantly organic matter and can be used to improve soil characteristics, such as fertility. Therefore, its application in agriculture is an adequate alternative for its final disposal. However, there is a lack of information on its long-term effects on soil changes in tropical areas. Thus, the objectives of this study were to determine (i) the effect of sewage sludge application on heavy metal build-up in soil and maize grains and leaves, and (ii) the effects of soil amendment with sewage sludge on the chemical properties of a Brazilian oxisol. Besides the increasing levels of Zn, Cu, Ni, and Cr, amending soil with sewage sludge also alters the distribution of these metals by increasing the mobile Phases, which correlated significantly with the increase in metal extraction with two single extractants, Mehlich 1 and DTPA (Diethylene triamine pentaacetic acid). The levels of Fe, Mn, Zn, and Cu in maize grains and leaves increased with the type and rate of sewage sludge application. Nevertheless, metal build-up in soil and plants was within the allowed limits. Significant differences were also found in soil characteristics like humic fractionation with the applied sewage doses. The data obtained does not indicate any expressive drawbacks in the use of sewage sludge as a soil amendment, as the heavy metal concentrations observed are unlikely to cause any environmental or health problems, even overestimated loadings, and are in accordance with the Brazilian regulations on farming land biosolid disposal.     

EFFECT OF SOIL AMENDMENT WITH THIN STILLAGE AND GLYCEROL ON PLANT GROWTH AND SOIL PROPERTIES

Controlled environment experiments were set up in 2007 and 2008 to evaluate the potential of using by-products of the biofuel industry as soil amendments to improve fertility and plant growth in Saskatchewan soils. Trials were run with thin stillage (a by-product of ethanol production) and glycerol (by-product of biodiesel production). Canola (B. napus L.) and wheat (T. aestivum) were grown as the test crop in amended pots. Plant yield, composition, and soil properties were measured after five weeks. The stillage was found to be an effective soil amendment for increasing plant biomass yield. Per unit of nitrogen (N) added, canola yields were less than that of urea when nitrogen was the only limitation, due to only a portion of the nitrogen in the thin stillage becoming available over the five week period. However, when nutrients other than nitrogen were limiting, canola dry matter yields with thin stillage amendment approached or exceeded that of urea, due to the ability of the amendments to supply other nutrients such as phosphorus in addition to nitrogen. Glycerol, an amendment that only contains carbon, hydrogen and oxygen, was effective in increasing soil organic carbon content, but required supplemental fertilizer to account for nutrient tie-up by microorganisms during decomposition in the soil. The amendments did not have any biologically significant effects on other soil chemical parameters measured, including soluble metals, pH or salinity.     

Long-term effects of farmyard manure and sewage sludge on some soil biochemical characteristics

 Changes in some soil biochemical properties were investigated following repeated applications of aerobically digested sewage sludge (SS) under field conditions over 12 years, and compared with those of an adjacent soil cultivated and amended with 5 t ha^–1 year^–1 (dry weight) farmyard manure (FYM) for at least 40 years, as well as with those of an adjacent uncultivated soil, in order to ascertain changes in soil quality. A short-term aerobic incubation was used to determine the potential of the samples to mineralize the organic C supplied. Results indicated that cultivation caused a reduction in total, humified and potentially mineralizable organic C, total N, light-fraction (LF) C, total and water-soluble carbohydrates, phenolic compounds, cation-exchange capacity (CEC), microbial biomass C, specific respiration, hydrolytic and urease activities, and an increase in the heavy metal content. Total and water-soluble carbohydrates and phenolic compounds expressed as a percentage of total organic C (TOC) were similar in the differently managed plots. Of the two amendments, FYM treatments showed higher amounts of TOC and N, LF-C, total and water-soluble carbohydrates, phenolic substances, CEC, specific respiration of biomass, hydrolytic and urease activities, similar amounts and characteristics of humified organic matter and lower concentrations of Cu, Zn and Cr. Both FYM and SS were inadequate treatments for the restoration of soil organic matter lost as a consequence of cultivation. Received: 20 October 1998     

Temporal response of soil biochemical properties in a pastoral soil after cultivation following high application rates of undigested sewage sludge

Soil biochemical properties were measured annually between 1995 and 1999 in soil from an 8-ha site that had received over 1,000 wet tonnes ha^–1 undigested sewage sludge, 1–4 years earlier. Basal respiration generally declined with time and was usually greatest in the untreated control area. This trend was attributed to a similar trend in soil moisture content. In contrast, microbial biomass C increased with time and also generally increased with sludge treatment age. Microbial biomass C, and to a lesser extent sulphatase activity, accurately predicted the order of sludge application to the site. This was perceived as a function of time since tillage and pasture establishment, with activities increasing in parallel to the build up of C residues in the soil, and not an effect of sludge or its composition. Except immediately after sludge application, there was no effect on N mineralisation and nitrification. None of the biochemical properties was strongly correlated with heavy metal concentrations. Our results suggest that there was little effect on soil biochemical properties, either adverse or beneficial, of adding raw sewage sludge to this site. Although a companion study showed considerable mobility and plant uptake of heavy metals, this difference could mainly be attributed to a different sampling strategy and the effects of intensive liming of the site.     

疏浚底泥的养分特征及污染化学性质研究

本文以运河 (杭州段 )待疏浚底泥为材料 ,分析了疏浚底泥的养分特征及其污染化学性质。结果发现 ,疏浚底泥中含有较高的养分 ,其中有机质、P、K含量较高而N偏低 ;施入土壤后 ,有机质矿化很快 ,P的供应较为持久。疏浚底泥重金属含量较一般城市污泥低 ,基本低于污泥农用中污染物控制标准 ,可考虑直接使用以节约处置费用 ;重金属的有效态含量不高 ,相对活性顺序为Cd Cu Pb Zn,直接大量施用会对种子发芽产生一定的毒害。供试作物种子对疏浚底泥毒害作用的反应不同 ,抗性顺序为青菜 羊茅草 三叶草。直接利用疏浚底泥比城市污泥具有更大的安全性 ,只要物种选取合适、用量恰当 ,是一种很好的肥源和复垦用土。     

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.