Bioavailability of Heavy Metals in Biosolids‐Amended Soil

Abstract

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

Chemical structure of humic acids in biosolids-amended soils as revealed by NMR spectroscopy

We used NMR spectroscopy to characterize humid acids extracted from soils that had received long-term application of 2 levels of biosolids to evaluate the soil organic matter (SOM) stability in biosolids-amended soils. The study also quantified fulvic acids (FAs), humic acids (HAs) and Fe/Al oxides. The soils were collected in 2004 from 7 fields, in Fulton County, southwestern Illinois, which received biosolids at a cumulative rate of 0 (control), 554 (low biosolids) and 1,066 (high biosolids) Mg ha⁻¹. The application of biosolids increased both FA and HA contents, but biosolids-amended soil and control soil did not differ in FA/HA ratio. Biosolids application had no effect on water-soluble organic carbon content. Biosolids application increased the presence of Fe/Al in the SOM complex and lowered its C/Fe and C/Al ratios. ¹³C NMR spectra showed increased alkyl C and decreased aromatic C content in soil HAs with the application of biosolids, and the extent of such changes was higher with high than low biosolids treatment. Under biosolids application, the soil HAs’ C structure shifts from O-alkyl-dominant to alkyl-dominant. Biosolids application does not decrease SOM stability but rather increases the stability of soil humic substances.     

On-Site Assessment of Extractable Soil Nutrients after Long-Term Biosolid Applications to Perennial Forage

The objective of this study was to evaluate soil nutrient loading and depth distributions of extractable nitrogen (N), phosphorus (P), and potassium (K) after long-term, continuous annual surface applications of anaerobically digested class B biosolids at a municipal recycling facility in central Texas. Commercial forage production fields of coastal bermudagrass (Cynodon dactylon L.) were surface applied at 0, 20, 40, or 60 Mg dry biosolids ha^?1 y^?1 for 8 years. Application duration was evaluated in fields treated with 20 Mg dry biosolids ha^?1 y^?1 for 0, 8, or 20 years. Total soil loads of extractable inorganic N and P increased linearly with application rate, but only extractable P increased with duration. Neither total load nor soil distribution of extractable K was affected by biosolid applications. Mineralization of biosolid-derived organic N and P likely contributed to elevated concentrations of nitrate throughout the soil profile (0–110 cm) and orthophosphate in surface soils (0–40 cm).     

Photocatalytic Degradation of Phytotoxic Substances in Waste Nutrient Solution by Various Immobilized Levels of Nano-TiO2

Biosolids are nutrient-rich waste products often used as soil amendments. To evaluate the impact of repeated application of biosolids on heavy metal accumulation and lability, composite soils (at 0–15- and 15–30-cm depths) were collected from 0-, 2-, 5-, and 25-year biosolid-applied Genesee silt loam (fine loamy, mixed, nonacid, mesic Typic Udifluvent). While the biosolid application did not influence the pH, the electrical conductivity and heavy metal concentration varied significantly. Among the heavy metals, the concentration of total and residual cadmium (Cd) was the highest (3 and 2.8 times), and copper (Cu) was the lowest (1.3 to 1.2 times) in the 25-year biosolid-applied field than in the control. The exchangeable chromium (Cr) concentration was the highest (6 times), and Cu was the lowest (1.9 times) in the 25-year biosolid-applied field as compared with the control. The lability of Cr, lead (Pb), cobalt (Co), zinc (Zn), Cu, nickel (Ni), and arsenic (As) significantly increased by 20, 11, 9, 9, 8, 6, and 4 %, respectively, in the 25-year biosolid-applied field compared to the control field. The extractable Pb, As, Zn, and Cu concentrations were significantly higher at 0–15 cm than at 15–30 cm depth. The labilities of Pb, Cu, and Ni were significantly varied between depths. Biosolids × depth significantly influenced the total As and residual As, Cu, and Pb concentrations. All the heavy metals except total and residual As significantly correlated with the total organic carbon. Results suggest that the accumulation and lability of heavy metals are related to complexation of heavy metals with organic carbon in response to years of biosolid application.     

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

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

Clay addition to lime-amended biosolids overcomes water repellence and provides nitrogen supply in an acid sandy soil

The addition of clay to lime-amended biosolids has been proposed as a soil amendment (LaBC^®) for remediating acidic-sandy soil. We investigated whether the presence of clay in LaBC^® altered soil microbial processes over a 30-week period. Aerobic-incubation and CO₂ respiration assays were used to monitor water repellence and chemical and microbiological properties of amended soil. Dry LaBC^® was applied at equivalent wet weight of 50, 100 and 150 t ha^?1. In addition, dry components of LaBC^® (lime, clay, lime + clay (LAC) and lime + biosolids (LAB)) were applied separately at rates equivalent to their fractions within LaBC^®. Inclusion of clay in LaBC^® was effective in eliminating water repellence at all application rates. Inclusion of clay in LaBC^® decreased nitrogen (N) release from the biosolids (by 58 %), even at 50 t ha^?1, but only during the first 2 weeks of incubation. LaBC^® consistently decreased soil microbial respiration compared with LAB alone when applied at 150 t ha^?1 thereby protecting organic matter decomposition. There was no significant N release with lime and clay amendment alone or in combination (LAC) in the absence of the biosolids. There may be a complex interaction between clay, lime and organic matter, but each may have had a role in altering N release from biosolids at different times during the 30-week incubation. Addition of clay to LAB increased its effectiveness in remediating this water-repellent, acidic-sandy soil and prolonged the N release from the biosolids following soil amendment.     

Composted biosolids enhance fertility of a sandy loam soil under dairy pasture

Field and pot trials were established to assess potential benefits and adverse effects of amending a sandy loam soil, under grazed ryegrass-clover pasture, with compost manufactured from wastewater biosolids, wood waste and green waste. Compost was applied to the field trial site annually for 4 years and the pot trials used soil from the field trial site each year after compost application. The pot trials demonstrated that yield of silver beet (Beta vulgaris L.) increased with increasing compost application rate and that plant metal uptake was (except for Zn) unrelated or inversely related to soil metal concentrations. In samples from the field trial, soil total C, N, P and Olsen P increased markedly with increasing compost application rate. Cation exchange capacity, exchangeable cations and total-extractable and EDTA-extractable metals (Cd, Cr, Cu, Ni, Pb and Zn) were also elevated, total Cu to the limit allowable in biosolids-amended soil. Soil basal respiration, microbial biomass C and anaerobically mineralisable N were significantly increased in the amended plots. Anaerobically mineralisable N was highly correlated with respiration (r =0.98, n =24) and only weakly related to microbial biomass C, probably indicating that a high proportion of the N mineralised was from the compost organic matter. Sulphatase and phosphatase activities increased, but not significantly, and there were no measurable effects on rhizobial numbers or on sensitive microbial biosensors (Rhizotox C and lux-marked Escherichia coli). Biosolids compost application enhanced soil fertility, productivity and microbial biomass and activity, with no apparent adverse effects attributable to heavy metals.     

Long‐Term Application of Biosolids on Apricot Production

Abstract

The use and disposal of biosolids, or wastewater treatment sludge, as a fertilizer and soil amendment is becoming increasingly widespread. We evaluated the multiyear use of biosolids in apricot (Prunus armeniaca L.) production, grown on productive agricultural soils. Class A biosolids were initially applied annually at rates of 0, 1.9, 5.8, and 11.7 Mg · ha^?1 (dry basis) to a 2‐year‐old apricot orchard on the USDA‐ARS research site on the eastern side of the San Joaquin Valley, CA. These application rates provided estimated rates of 0 (control), 57, 170, and 340 kg total N · ha^?1 yr^?1, respectively. Compared to the control treatment, the applications of biosolids significantly increased soil salinity (electrical conductivity from 1:1 soil–water extract) and total concentrations of nutrients [e.g., calcium (Ca), magnesium (Mg), sulfur (S), phosphorus (P), zinc (Zn), and copper (Cu)] after 7 years but did not increase the concentrations of selected metals [cadmium (Cd), chromium (Cr), cobalt (Co), nickel (Ni), and lead (Pb)] between 0‐ and 60‐cm soil depths. Mean concentrations of total nitrogen (N) and carbon (C) in soils (0‐ to 15‐cm depth) ranged from a low of 1.3 g kg^?1 to a high of 5.2 g · kg^?1 and from 14.1 g · kg^?1 to 45.7 g · kg^?1 for the control and high biosolids treated soils, respectively.

Biosolids applications did not lead to fruit yield reductions, although fruit maturation was generally delayed and more fruits appeared at picking times at the high rate of application. Yellow fruits collected from all biosolids applications were significantly firmer than were fruit collected from control trees, and they had higher concentrations of Ca, potassium (K), S, iron (Fe), and Zn in the fruit. Among the fruit quality parameters tested, the juice pH, total acidity, and fruit skin color were not significantly affected by biosolids applications. Malic acid concentrations decreased most of the time, while citric acid concentrations increased with increasing rates of biosolids applications. Overall, our results suggest that nonindustrial biosolids applied at an annual rate at or less than 11.7 Mg N · ha^?1 (340 kg N · ha^?1) can be safely used for apricot production on sandy loam soils.     

Heavy-Metal Uptake and Growth of Bouteloua Species in Semi-arid Soils Amended with Biosolids

An experiment was conducted in three wild grass species of Bouteloua genus to compare the effect of application of biosolids on the accumulation of heavy metals, biomass production, and nutrient uptake. Biosolids were applied at rates of 0, 3.5, 7, and 10.5 Mg ha^?1 to two different soil types collected from the southern Chihuahuan Desert. The shoot biomass of B. gracilis (BOGR) and B. scorpioides (BOSC) was two to three times greater than the control while in B. curtipendula (BOCU) in was 75% in both soils. In BOGR, nitrogen and phosphorus uptake were 4–4.5 times greater than the control in calcareous soil. With few exceptions, there was no significant difference of heavy-metal concentrations (copper, chromium, arsenic, nickel, and zinc) in the plant tissues compared to the control. The application of biosolids showed interspecies variability in growth response and nutrient uptake with a minimal risk of heavy-metal uptake for Bouteloua wild species.     

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

Purpose

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

Materials and methods

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

Results and discussion

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

Conclusions

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

Influence of Biosolids Rate on Chemical Properties of an Oxisol in São Paulo,Brazil

Abstract

Organic residues may cause major health and environmental problems. This is the case in our study area, where more than 10 billion L per year of residential and industrial waste are produced. Land application of biosolids can be an economical solution by recycling waste and can provide valuable fertilizer if used correctly. The aim of this work was to study the effect of biosolids on the chemical properties of an Oxisol. The experiment was located at Ilha Solteira northwest of São Paulo State, Brazil. The soil was cropped to Sorghum bicolor. The field experimental design consisted of random blocks with six treatments and four replications of each treatment. Biosolids were surface applied to four treatments at rates of 5, 10, 20, and 40 Mg ha^?1 on a dry matter basis; in addition, a treatment with mineral fertilizer and a control were included. One year after biosolids application, soil samples were taken at 0–10, 10–20, and 20–40 cm. Organic matter content (Walkley–Black) and pH (CaCl₂) were routinely determined. Cation exchange capacity, exchangeable bases (Ca, Mg, K), and P were determined by exchange resin extraction. No significant differences in any of the analyzed properties were found below the 20‐cm depth. Extractable phosphorus (P) and potassium (K) increased with increasing biosolids rate in the top 20 cm, whereas calcium (Ca) and (Ma) magnesium content were not significantly influenced by biosolids. Soil pH decreased with increasing biosolids application. The sewage sludge application did not influence the sorghum production in the first year of culture, under unfavorable soil moisture conditions, but it influenced the dry matter.     

3种有机酸对伴矿景天修复效率及土壤微生物数量的影响

伴矿景天(Sedumplumbizincicola)是一种Cd和Zn的超积累植物,常用于Cd污染土壤的植物修复。有机酸能够提高土壤重金属的有效性,促进植物对重金属的积累,对重金属污染土壤的植物修复效率具有强化作用,并对土壤微生物数量有重要影响。以河潮土和红黄泥为供试土壤,探讨了乙二胺四乙酸(EDTA)、柠檬酸、草酸对伴矿景天修复效率和土壤微生物数量的影响。结果表明,有机酸能显著提高土壤有效态Cd含量,柠檬酸处理的效果最好,河潮土和红黄泥中有效态Cd含量较单种伴矿景天分别增加72.73%,12.99%(P<0.05);伴矿景天地上部Cd含量在河潮土和红黄泥中以EDTA处理最高,在河潮土和红黄泥中分别比单种伴矿景天增加99.24%和33.32%;与单种伴矿景天相比,添加有机酸处理河潮土和红黄泥中伴矿景天修复效率显著提高。添加有机酸比单种伴矿景天显著增加土壤中微生物数量,其中柠檬酸处理河潮土中细菌和真菌数量分别增加34.38%和68.42%(P<0.05),草酸处理红黄泥中放线菌数量增加150.00%。研究结果可为重金属污染土壤的植物强化修复提供理论支撑。     

Effects of Biosolids Based Soil Products on Soil Physical and Chemical Properties in Urban Gardens

A study was conducted to evaluate the effect of two biosolids based soil products on soil physical properties in urban community gardens in Tacoma, Washington. The Tagro soil product is made from Class A biosolids cake mixed with sand and sawdust. The GroCo biosolids compost is produced from biosolids cake and sawdust. Both products meet regulatory requirements for unrestricted use and are locally available to gardeners. Plots were established in 3 community gardens and maintained for 2 growing seasons. Amendments were applied at 200 Mg ha^?1 dry weight in the first year of the study. Plots were split for the 2^nd year with half of the amendment plots receiving an additional 200 Mg ha^?1 of amendments. Although lower in most metals than the soils they were added to, biosolids did not have a significant effect on total soil metal concentrations. Biosolids addition increased the water infiltration rate from 10.1 ± 0.95 ml min^?1 in control soils to between 51 ± 6.1 and 212 ± 34 ml min^?1, depending on the product used. Bulk density was decreased from 1.07 ± 0.01 g cm^?3 to between 0.51 ± 0.08 and 0.77 ± 0.03 g cm^?3. Total carbon, nitrogen and available phosphorus were increased in the biosolids amended plots in comparison to the control plots. Biosolids addition resulted in decreased pH (5.35-5.75) in comparison to the control soils (6.43). The results from this study indicate that biosolids-based soil amendments can be a suitable amendment for urban community gardens.     

Nitrogen Availability for Maize from a Rolling Pampa Soil After Addition of Biosolids

Abstract

The objective of this paper was to evaluate the influence of different rates of biosolids on the soil nitrogen (N) availability for maize and its residuality. A field experiment was developed in a typic Argiudol located in the NE of the Buenos Aires Province. Maize was sown for two consecutive years 1997–1999. Biosolids from a sewage treatment plant of Buenos Aires outskirts were superficially applied to the soil and incorporated by plowing. There were eight treatments: Check; 8, 16, and 24 Mg of dry biosolid ha^?1; 8 and 16 Mg of dry biosolid ha^?1 applied one year before, 100 and 150 kg N ha^?1 of calcium ammonium nitrate (CAN). The sampling and determinations were done during the second maize cycle. At presowing (PS), sowing (S), 6 expanded leaves (V6), 12 expanded leaves (V12), and Flowering (Fl) composite soil samples from 0–40 cm depth were obtained to determine ammonium and nitrate contents. At Fl maize plants were sampled in order to determine total biomass and N content. The N‐nitrate content in the soil was significantly increased by the biosolids application (p < 0.05), and varied for each increment depending on the biosolids rates and the phenological stage. After 30 days from the incorporation the increases of 1.19, 1.34, and 2.05% were observed for N‐nitrates for 8, 16, and 24 Mg ha^?1, respectively. The contribution of mineral N from the biosolids was 2.48, 6.46, and 5.01 kg N Mg^?1 when the rates were incremented from 0–8, 8–16, and 16–24 Mg ha^?1, respectively. The nitrogen mineralization followed a release pattern with a maximum value of 296 kg N‐nitrate ha^?1 at sowing for 24 Mg ha^?1. Since then, the release of mineral nitrogen decreased significantly till Fl. The N‐nitrates values variation with the temperature adjusted to polinomic functions. The mineral N released from the biosolids increased as a response to the increment of soil temperature and then decreased due to the maize nitrogen absorption and the potentially mineralized nitrogen exhaustion. The application of 150 kg N ha^?1 as CAN incremented significantly the soil N‐nitrate content and equalized 16 and 24 Mg of dry biosolids ha^?1 at V6. But, no synchronism between the high nitrate releasing from biosolids and the increment in the nitrogen absorption by maize was observed. This fact generates a surplus of nitrate that incremented the potential of nitrogen loss by lixiviation. We observed a residual effect from the biosolids that were applied the previous year. This contribution represented the 35% of the maize requirements and was similar to the nitrate content observed in Bio 16. The biosolids might be a valuable source of nitrogen for maize crop if the synchronism between the soil supply and maize demand is observed in order to avoid nitrates surplus.     

Growth and metal uptake of edamame [Glycine max (L.) Merr.] on soil amended with biosolids and gypsum

ABSTRACT

Although biosolids are a rich source of plant nutrients, there is concern about the potential heavy metal uptake by crops grown on biosolid-amended soils. This study was conducted to determine the effects of limed or composted biosolids and flue gas desulfurization gypsum (FGDG) on edamame growth, nodule development, and metal uptake. Two consecutive crops of edamame were grown on 40 and 80 T ha^?1 biosolid-amended soil with and without 10 T ha^?1 FGDG. Biosolids with or without FGDG did not reduce biomass, nodules, or grain yields in the first harvest and increased yields of all three tissues in the second harvest. Lead and cadmium concentrations in grain and biomass were below the instrument detection limits. Copper, manganese, and zinc were within the ranges normally found in soybean grain. In this pot study, biosolids and FGDG did not reduce edamame growth or increase grain metal concentrations to levels of concern.     

Nursery Crop Growth Response to Municipal Biosolids: Species Salt and Xeric Adaptation a Key Factor?

ABSTRACT

Growth responses of potted ornamental crops to municipal biosolids in the semiarid southwestern USA are not adequately known. In 10- to 11-wk greenhouse pot studies, we evaluated the effects of dried biosolids-amended growing media on four ornamental crop species: Garden chrysanthemum (Dendranthema Xgrandiflorum ‘Megan’), butterfly bush (Buddleia davidii ‘Nanho Blue’), Japanese honeysuckle (Lonicera japonica ‘Purpurea’), and blanket flower (Gaillardia Xgrandiflora ‘Goblin’). The biosolids were composted without bulking agents (100% sewage sludge) and incorporated into growing media at rates ranging from 0 to 593 kg m^?3, or 0 to 72% by volume. Biosolids increased substrate pH from 5.8 to 7.2 and electrical conductivity (EC) from 2.6 to 47.3 dS m^?1. Any addition of biosolids (≥30 kg m^?3) reduced total plant dry matter (DM) of chrysanthemum. Conversely, shoot DM of blanket flower and butterfly bush increased by four- to five-fold at biosolids rates of 59 to 148 kg m^?3 (7 to 18% by volume) with corresponding increases in shoot N and P concentrations. Biosolids rates higher than 148 kg m^?3 reduced top growth of the latter two species and of Japanese honeysuckle. For all species, growth reductions with excessive biosolids rates likely resulted from osmotic stress and specific NH₄ toxicity. However, based on the substantial growth stimulations at moderate biosolids rates, xeric and salt-adapted species, such as blanket flower and butterfly bush, may be ideally suited for expanding the use of highly saline biosolids at semiarid nursery production sites.     

Water Treatment Residuals and Biosolids Co‐applications Affect Phosphatases in a Semi‐arid Rangeland Soil

Co‐application of biosolids and water treatment residuals (WTR) land has not been extensively studied but may be beneficial by sorbing excess biosolid‐borne or soil phosphorus (P) onto WTR, reducing the likelihood of off‐site movement. Reduction of excess soil P may affect the role of specific P‐cleaving enzymes. The research objective was to understand the long‐term effects of single co‐applications and the short‐term impacts of repeated co‐applications on soil acid phosphomonoesterase, phosphodiesterase, pyrophosphatase, and phytase enzyme activities. Test plots were 7.5 × 15 m with treatments consisting of three different WTR rates with a single biosolids rate (5, 10, and 21 Mg WTR ha^?1; 10 Mg biosolids ha^?1) surface co‐applied once in 1991 or reapplied in 2002. Control plots consisted of those that received no WTR–biosolids co‐applications and plots that received only 10 Mg biosolids ha^?1. Plots were sampled to a 5‐cm depth in 2003 and 2004, and soil phosphatases and phytase enzyme activities were measured. Soil phosphodiesterase activity decreased in WTR‐amended plots, and pyrophosphatase activity decreased with increasing WTR application rates. In contrast, acid phosphatase and phytase activity increased with WTR addition, with WTR application possibly triggering a deficiency response causing microorganisms or plants to secrete these enzymes. Biosolids and WTR co‐applications may affect enzymatic strategies for P mineralization in this study site. Reductions in phosphodiesterase activity suggest less P mineralization from biomass sources, including nucleic acids and phospholipids. Increased acid phosphatase and phytase activities indicate that ester‐P and inositol‐P may be important plant‐available P sources in soils amended with WTR.     

Biosolids and Biosolids-ash as Sources of Heavy Metals in a Plant-Soil System

Generally, the potential for biosolids (digested or composted)to contribute heavy metals to the soil-plant system has beencompared with commercial fertilizers and other organic wastesbut not with biosolids-ash. An column study was conducted in agreenhouse to determine the availability, extractability andleachability of metals in a degraded, non-calcareous soilamended with different biosolids (200 Mg ha^-1). Thebiosolids investigated were dewatered, anaerobically digestedbiosolids, composted biosolids and biosolids-ash. The columns(26 cm) were planted with wheat (Triticum aestivum L. cvMexa). The addition of digested biosolids decreased the drymatter yield of wheat. Treatments including organic biosolidsincreased Cu and Zn concentrations in wheat roots, straw andgrain, whereas the addition of biosolids-ash did not affect theconcentrations of these metals in wheat. Concentrations of Ni,Co, Pb, Cr and Cd in wheat were below reliable detection limits(0.06, 0.05, 0.1, 0.06 and 0.02 mg kg^-1, respectively).After harvesting, total and AB-DTPA extractable Cu, Zn and Pbincreased in the upper layer of the soil amended with thedifferent biosolids studied, whereas levels of AB-DTPAextractable Ni and Co were affected only when the soil wasamended with digested or composted biosolids. Total chromiumincreased only in treatments including organic biosolids. TheAB-DTPA extractable Cu, Zn and Pb in the lower layer of thesoil in treatments including biosolids evidenced downwardmovement of these metals. However, absence of these metals incolumn leachates indicates that this movement was gradual.     

Heavy Metal Content in Lettuce Plants Grown in Biosolids Compost

The objective of this work was to evaluate the affects of the application of composted biosolids on the accumulation of heavy metals (Cd, Cu, Ni, Pb and Zn) in lettuce leaves. Pots containing different proportions (0 to 100%) of composted biosolids were used to grow lettuce plants under greenhouse conditions. Dry and fresh weight, leaf area and Cd, Cu, Ni, Pb and Zn uptake were determined after harvest. It was found that the dry and fresh matter productions of the plants were significantly lower in the control treatment. The addition of composted biosolids caused a 20 and 40% increase in biomass accumulation. Cd and Pb concentrations in leaves were below detection limits (0.05 mg kg^?1) in all treatments. Zn concentration in leaves increases as compost proportion decreases, ranging from 57.2 to 80.4 mg kg^?1. Composted biosolids application increased the Cu and Ni plant concentrations, ranging from 5.1 to 9.8 mg Cu kg^?1 and 2.3 to 3.7 mg Ni kg^?1. In all treatments the proportions of heavy metals in plants were below the international standards of toxicity. The results allow us to suggest that, in short-term applications, composted biosolids could be used as soil amendment for lettuce production, without toxic effects in the chemical composition of the plant.     

Use of Drinking Water Treatment Residuals in Reducing Bioavailability of Metals in Biosolid-Amended Alkaline Soils

This greenhouse study evaluated the use of drinking-water-treatment residuals (WTRs) to reduce the bioavailability of metals in the biosolid-amended agricultural alkaline soils. Results showed that increasing the application rate of biosolids increased the accumulation of lead (Pb), nickel (Ni), copper (Cu), and cadmium (Cd) in corn (Zea mays cv. single hybride 10), with greater metal concentrations in roots than in shoots. However, the addition of WTRs (1–4%, w/w) to the soil amended with 3% biosolids significantly (P < 0.05) decreased the concentrations of soil diethylenetriaminepentaacetic acid (DTPA)–extractable metals. The accumulation of Pb, Ni, Cu, and Cd in corn significantly correlated with the DTPA-extractable metal concentrations in the soils. Plant metal concentrations were significantly affected by the soil type, application rates of biosolids and WTRs, and the ratio of WTRs to biosolids in the soils. The 1:1 application ratio of WTRs to biosolids at the 3% application rate effectively reduced the accumulation of metals in corn tissues.