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.     

Stabilized municipal sewage sludge addition to improve properties of an acid mine soil for plant growth

Purpose

Degraded soils, such as those encountered in areas of mine activities, need to be ameliorated by liming to correct soil acidity and by addition of organic inputs to improve soil properties and fertility.

Materials and methods

Non-amended mine soil and soil amended with stabilized sewage sludge were incubated for 45 days. Soil physicochemical and biological indicators were periodically measured along incubation and other enzyme activities at the end of incubation. In improved soils, a study of plant development in 250-g pots was carried out with three vegetal species: tomato, rye grass and ahipa. Germination and mortality rates, biomass production and photosynthetic pigments were measured.

Results and discussion

Soil incubation with sewage sludge slightly increased soil pH and led to an enhancement of soil electrical conductivity, organic carbon and dehydrogenase activity, especially for the higher doses (5 and 10%). However soil respiration was more promoted with the 2% dose, pointing to a possible toxic effect of the sludge. At the end of incubation, physicochemical and biological properties were in general enhanced. Biomass production was improved in tomato and rye grass by sewage sludge addition (more at the 2% dose), whilst ahipa growth was not affected by sewage sludge treatments. Tomato mortality reached 73% with high sludge doses (10%).

Conclusions

According to this set of parameters, amendment with sewage sludge of a limed acid mine soil would be considered as a good strategy for soil amelioration in view of plant establishment and development.     

Changes in the microbial activity of an arid soil amended with urban organic wastes

Changes produced in the biological characteristics of an arid soil by the addition of various urban wastes (municipal solid waste, sewage sludge and compost) at different doses, were evaluated during a 360-day incubation experiment. The addition of organic materials to the soil increased the values of biomass carbon, basal respiration, biomass C/total organic C ratio and metabolic quotient (qCO₂), indicating the activation of soil microorganisms. These biological parameters showed a decreasing tendency with time. Nevertheless, their values in amended soils were higher than in control soil, which clearly indicates the improvement of soil biological quality brought about by the organic amendment. This favorable effect on soil biological activity was more noticeable with the addition of fresh wastes (municipal solid waste or sewage sludge) than with compost. In turn, this effect was more permanent when the soil was amended with municipal solid waste than when it was amended with sewage sludge. Received: 28 May 1996     

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.     

Carbon and nitrogen mineralization of sewage sludge compost in soils with a different initial pH

Soil properties may affect the decomposition of added organic materials and inorganic nitrogen (N) production in agricultural soils. Three soils, Potu (Pu), Sankengtzu (Sk) and Erhlin (Eh) soils, mixed with sewage sludge compost (SSC) at application rates of 0 (control), 25, 75 and 150 Mg ha⁻¹ were selected from Taiwan for incubation for 112 days. The aim of the present study was to examine the effects of SSC application rates on the carbon decomposition rate, N transformation and pH changes in three soils with different initial soil pH values (4.8–7.7). The results indicated that the highest peaks of the CO₂ evolution rate occurred after 3 days of incubation, for all treatments. The Pu soil (pH 4.8) had a relatively low rate of CO₂ evolution, total amounts of CO₂ evolution and percentage of added organic C loss, all of which resulted from inhibition of microbial activity under low pH. For the Pu and Sk soils, the concentration of NH₄⁺-N reached its peak after 7–14 days of incubation, which indicated that ammonification might have occurred in the two soils with low initial pH values. NO₃⁻-N rapidly accumulated in the first 7 days of incubation in the Eh soil (pH 7.7). The direction and extent of the soil pH changes were influenced by the N in the SSC and the initial soil pH. Ammonification of organic N in the SSC caused the soil pH to increase, whereas nitrification of mineralized N caused the soil pH to decline. Consequently, the initial soil pH greatly affected the rate of carbon decomposition, ammonification and nitrification of SSC.     

Characterization of humic acids extracted from sewage sludge-amended oxisols by electron paramagnetic resonance

In tropical soils, the high turnover rate and mineralization of organic matter (OM) associated with intensive agricultural use, generally leads to faster soil degradation than that observed in temperate climatic zones. The application of sewage sludge to the soils is one proposed method of maintaining soil organic matter, and is also an alternative method of disposing of this waste product. As well as containing large quantities of OM, sludge is also a significant source of supplementary nitrogen, phosphorus and other essential nutrients for plant growth. However, it is necessary to understand the qualitative and quantitative changes that take place in the OM in soil treated with sewage sludge. The approach of the present study was intended to identify possible structural changes caused by sewage sludge applications on soil humic acids (HAs). The HAs extracted from a Typic Achrortox under sewage sludge applications were characterized by electron paramagnetic resonance (EPR) spectroscopy. The soil samples were collected from a field experiment designed to evaluate the effects of different doses of sewage sludge on corn growth and development in Brazil. The sewage sludge originated from urban waste treated at the sewage sludge treatment station in the city of Franca, state of Sao Paulo, Brazil. The following soil treatments were studied: control (non-cultivated soil under natural vegetation (NC)), control soil amended with NPK (conventional corn fertilization) and four treatments N1, N2, N4 and N8 with applications of 3.5, 7, 14 and 28 Mg ha⁻¹ of sewage sludge (dry matter), respectively. HAs were extracted from the surface layer using the methodology of the International Humic Substance Society (IHSS). Fe³⁺ and VO²⁺ ions complexed with HAs, and also semiquinone-type free radical (SFR) at concentrations of approximately 2.0 × 10¹⁸ spins g⁻¹ HA were identified in EPR spectra. The levels of SFR were lower for treatments where the applied sewage sludge doses were equivalent to four and eight times the normal doses of N mineral fertilization, reaching values of 1.7 × 10¹⁸ and 1.24 × 10¹⁸ spins g⁻¹ HA, respectively. The observed decrease in SFR content as sewage sludge dose was increased, was probably associated with the incorporation of less aromatic components into HAs originating from the sewage sludge.     

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.     

Sorption of s-Triazine Herbicides in Organic Matter Amended Soils: Fresh and Incubated Systems

Fresh amendment of soil with sewage sludge and composted sewage sludge resulted in increased sorption of three s-triazine herbicides: atrazine, ametryn and terbuthylazine. The extent of increased sorption (as evaluated by sorption coefficients K_d or K_f) was a function of soil type, such that sorption in amended organic carbon-poor soil (0.4% OC) was more enhanced than in amended organic carbon-rich soil (1.55% OC). Despite significant differences between the organic amendments in terms of humic and fulvic acid content, humin content, soluble organic matter content, total organic matter content, and H/C and O/C atomic ratios, organic matter composition had no discernible effect on either sorption distribution coefficients or on isotherm linearity in amended soils. Soils amended with composted sludge had the same sorption potential as did soils amended with the analogous uncomposted sludge. After incubating soil-sludge mixtures for a year at room temperature, organic matter content decreased to original pre-amendment levels. Sorption coefficients for the three compounds similarly decreased to initial pre-amendment values. Organic carbon normalized sorption coefficients (K_oc) were essentially identical in the soils, amended soils, and incubated amended soils, indicating that sludge and compost derived organic matter does not have a significantly different sorption capacity as compared with the original soils, despite compositional differences.     

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

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

Mineralization of Organic-Amendment-Derived Nitrogen in Two Mediterranean Soils with Different Organic-Matter Contents

The use of organic fertilizers in lands with low organic-matter content, such as those found in the Mediterranean region, is an attractive option for enhancing soil quality and fertility status. However, it is difficult to assess the nitrogen (N) mineralization rate and the quantity of the organic amendment to be added. Thus we conducted a 300-day incubation trial, where four commonly found organic amendments (three different animal manures and one sewage sludge) were mixed with two soils. Our aim was to assess the potentially mineralizable N with the use of the first-order exponential model. Our findings indicated that the N-mineralization data fitted well to the model we used and that the active N fraction (eluted available N per total N added) ranged from 28.4% to 50.3%, depending on indigenous organic carbon (C) content, as well as on the organic amendment C/N ratio and total N content.