Growth response and phytoextraction of copper at different levels in soils by Elsholtzia splendens

Phytoremediation is a promising approach for cleaning up soils contaminated with heavy metals. Information is needed to understand growth response and uptake mechanisms of heavy metals by some plant species with exceptional capability in absorbing and superaccumulating metals from soils. Greenhouse study, field trial, and old mined area survey were conducted to evaluate growth response and Cu phytoextraction of Elsholtzia splendens in contaminated soils, which has been recently identified to be tolerant to high Cu concentration and have great potential in remediating contaminated soils. The results from this study indicate that the plant exhibited high tolerance to Cu toxicity in the soils, and normal growth was attained up to 80 mg kg(-1) available soil Cu (the NH4OAc extractable Cu) or 1000 mg kg(-1) total Cu. Under the field conditions, a biomass yield of 9 ton ha(-1) was recorded at the soil available Cu level of 77 mg kg(-1), as estimated by the NH4OAc extraction method. Concentration-dependent uptake of Cu by the plant occurred mainly at the early growth stage, and at the late stage, there is no difference in shoot Cu concentrations grown at different extractable soil Cu levels. The extractability of Cu from the highly polluted soil is much greater by the roots than that by the shoots. The NH4OAc extractable Cu level in the polluted soil was reduced from 78 to 55 mg kg(-1) in the soil after phytoextraction and removal of Cu by the plant species for one growth season. The depletion of extractable Cu level in the rhizosphere was noted grown in the mined area, even at high Cu levels, the NH4OAc extractable Cu in the rhizosphere was 30% lower than that in the bulk soil. These results indicate that phytoextraction of E. splendens can effectively reduce the plant-available Cu level in the polluted soils.     

A model for evaluation of the phytoavailability of trace elements to vegetables under the field conditions

Currently, several single extraction methods are used for the evaluation of the phytoavailability of metals using pot experiments. A systematic comparison, however, is lacking. It is especially true for the field studies. This study was to investigate the phytoavailability of trace elements to vegetables grown on metal-contaminated soils under the field conditions. All soils collected were typical calcareous soils in northern China. Four frequently used methods using CaCl2, diethylenetriaminepentaacetic acid (DTPA), CH3COOH, and water as extractants were compared for phyto-availability. The concentrations of metals extracted by these four extraction methods ranged from 3.42 to 815, 1.51- 6965, 0.732-24473, 0.688-7863, 0.246-685, 1.99-5337 0.203-4649 ng/g for Cr, Ni, Zn, Cu, Cd, Pb and REEs, respectively. Simple correlation analysis indicated that a significant correlation (Cr: r = 0.5411**; Zn: r = 0.6352**; Cd: r = 0.6979**; Pb: r = 0.5537** and REEs: r = 0.5185** -0.6684**) was observed between the CaCl2-extractable metals in soil solution and that in Chinese cabbage. In addition, soil pH, organic matter (OM), and cation exchange capacity (CEC) also affect the phytoavailability. An empirical model was developed to express the combined effect of soil properties on the phytoavailability. The stepwise multiple regression analysis demonstrated that the phytoavailability of trace elements strongly correlated with the extractable fraction by CaCl2, total metal concentration in soils, and soil pH, OM, CEC. This model can describe approximately 75-95% of the variability of metal uptake and the r2 values ranged from 0.741** to 0.954**, which were much better than the single correlation analysis. For celery and cole, a strong correlation was obtained for Cr, Ni, Zn, Cu, Cd, La, Ce, Pr and Nd. For spinach and Chinese cabbage, however, a positive correlation was only observed for 1 and 3 metals, respectively. Generally, the developed empirical model can integrate the combined effects of soil properties, extractable metal fractions in soil solutions and plant species on the phytoavailability of metals to vegetables in the field conditions.     

Naturally-assisted metal phytoextraction by Brassica carinata: Role ofroot exudates

Due to relatively high chelant dosages and potential environmental risks it is necessary to explore different approaches in the remediation of metal-contaminated soils. The present study focussed on the removal of metals (As, Cd, Cu, Pb and Zn) from a multiple metal-contaminated soil by growing Brassica carinata plants in succession to spontaneous metallicolous populations of Pinus pinaster, Plantago lanceolata and Silene paradoxa. The results showed that the growth of the metallicolous populations increased the extractable metal levels in the soil, which resulted in a higher accumulation of metals in the above-ground parts of B. carinata. Root exudates of the three metallicolous species were analysed to elucidate their possible role in the enhanced metal availability. The presence of metals stimulated the exudation of organic and phenolic acids as well as flavonoids. It was suggested that root exudates played an important role in solubilising metals in soil and in favouring their uptake by roots.     

Evaluation of plant availability of soil trace metals by chemical fractionation and multiple regression analysis

Soil samples with a range of chemical and physical properties were collected from 10 different rural regions of China. Trace metals (Ni, Co, Cu, and Pb) in the soils were partitioned by a sequential extraction procedure into Mg(NO(3))(2) extractable (F1), CH(3)COONa extractable (F2), NH(2)OH.HCl extractable (F3), HNO(3)?H(2)O(2) extractable (F4), and residual (F5) fractions. Chemical fractionation showed that F1 fraction of the metals was less than 1% and residue was the dominant form for Cu and Ni in all samples, and for Co in most of the samples. Significant interrelationships of the fractions varied considerably with the different metals. Winter wheat (Triticum aestivum L.) and alfalfa (Medicago sativa L.) had been grown on the soils in a pot-culture experiment under greenhouse conditions for 40 days. Metal availability to the plants was evaluated by simple and multiple regression analysis. The Mg(NO(3))(2) extractable Co (F1) was significantly correlated with Co concentrations in different parts of wheat and in the whole of alfalfa. For the other metals, the independent variables of the multiple regression models, chosen by stepwise selection, were given as: F1 and F2 + F3 + F4 for Ni; F1, F2 + F3 and F4 for Cu; and F3 + F4 for Pb. The results of this study demonstrate that the sequential extraction procedure, in conjunction with multiple regression models using a combination of correlated fractions as an independent variable, may be useful for the prediction of plant absorption of trace metals in soils.     

Trace metal speciation and bioavailability in urban soils

Urban soils often contain concentrations of trace metals that exceed regulatory levels. However, the threat posed by trace metals to human health and the environment is thought to be dependent on their speciation in the soil solution rather than the total concentration. Three inactive railway yards in Montréal, Québec, were sampled to investigate the speciation and bioavailability of Cd, Cu, Ni, Pb and Zn. Soil solutions were obtained by centrifuging saturated soil pastes. In the soil solutions, up to 59% of the dissolved Cd was in its free ionic form. For Cu, Pb and Zn, organic complexes were the predominant species. Over 40% of Ni was present as inorganic complexes if the solution pH exceeded 8.1. Multiple regression analyses showed that pH and total metals in soil were significantly correlated with the activities of free metal ions, except for Cd(2+), which only had a weak correlation with soil pH. Free, dissolved and total soil metals were tested for their ability to predict metal uptake by plants in the field. However, none of these metal pools were satisfactory predictors. The results indicated that in these urban soils, trace metals were mainly in stable forms and bioavailability was extremely low.     

Effect of chemical amendments on the concentration of cadmium and lead in long-term contaminated soils

The availability of metal in contaminated soil can be reduced by the addition of soil amendments. The objectives of this study are to study the effects of applying different soil amendments on the concentration of Cd and Pb in soil solution, DTPA or EDTA extractable Cd and Pb, and the uptake of Cd and Pb by wheat (Triticum vulgare) when growing in long-term Cd and Pb-contaminated soils, more than 20 years. The soil amendments, including check, compost, zinc oxide, calcium carbonate, calcium carbonate mixed with zinc oxide, and calcium carbonate mixed with compost, were conducted in a four replicates pot cultural study. The amended soils were incubated for six months under 60% of water holding capacity. Following incubation, wheat was grown for four months in greenhouse. Analyses of Cd concentration demonstrated a significant decrease in soil solution concentration and DTPA or EDTA extractable in soils amended with calcium carbonate or calcium carbonate mixed with ZnO (or compost) (p<0.01). These amendments can significantly reduce the Cd concentration in the grain, leaf and stem, or reduce the total Cd uptake in all parts of wheat species grown in highly contaminated soil amended with calcium carbonate or calcium carbonate mixed with ZnO (or compost) (p<0.01). The concentration of Cd in soil solution and extracted with DTPA or EDTA can predict the Cd concentration in wheat, especially for soil solution.     

Microbial indicators of heavy metal contamination in urban and rural soils

Urban soils and especially their microbiology have been a neglected area of study. In this paper, we report on microbial properties of urban soils compared to rural soils of similar lithogenic origin in the vicinity of Aberdeen city. Significant differences in basal respiration rates, microbial biomass and ecophysiological parameters were found in urban soils compared to rural soils. Analysis of community level physiological profiles (CLPP) of micro-organisms showed they consumed C sources faster in urban soils to maintain the same level activity as those in rural soils. Cu, Pb, Zn and Ni were the principal elements that had accumulated in urban soils compared with their rural counterparts with Pb being the most significant metal to distinguish urban soils from rural soils. Sequential extraction showed the final residue after extraction was normally the highest proportion except for Pb, for which the hydroxylamine-hydrochloride extractable Pb was the largest part. Acetic acid extractable fraction of Cd, Cu, Ni, Pb and Zn were higher in urban soils and aqua regia extractable fraction were lower suggesting an elevated availability of heavy metals in urban soils. Correlation analyses between different microbial indicators (basal respiration, biomass-C, and sole C source tests) and heavy metal fractions indicated that basal respiration was negatively correlated with soil Cd, Cu, Ni and Zn inputs while soil microbial biomass was only significantly correlated with Pb. However, both exchangeable and iron- and manganese-bound Ni fractions were mostly responsible for shift of the soil microbial community level physiological profiles (sole C source tests). These data suggest soil microbial indicators can be useful indicators of pollutant heavy metal stress on the health of urban soils.     

Determination of chemical availability of cadmium and zinc in soils using inert soil moisture samplers

A rapid method for extracting soil solutions using porous plastic soil-moisture samplers was combined with a cation resin equilibration based speciation technique to look at the chemical availability of metals in soil. Industrially polluted, metal sulphate amended and sewage sludge treated soils were used in our study. Cadmium sulphate amended and industrially contaminated soils all had > 65% of the total soil solution Cd present as free Cd2+. However, increasing total soil Cd concentrations by adding CdSO4 resulted in smaller total soil solution Cd. Consequently, the free Cd2+ concentrations in soil solutions extracted from these soils were smaller than in the same soil contaminated by sewage sludge addition. Amendment with ZnSO4 gave much greater concentrations of free Zn2+ in soil solutions compared with the same soil after long-term Zn contamination via sewage sludge additions. Our results demonstrate the difficulty in comparing total soil solution and free metal ion concentrations for soils from different areas with different physiochemical properties and sources of contamination. However, when comparing the same Woburn soil, Cd was much less available as Cd2+ in soil solution from the CdSO4 amended soils compared with soil contaminated by about 36 years of sewage sludge additions. In contrast, much more Zn was available in soil solution as free Zn2+ in the ZnSO4 amended soils compared with the sewage sludge treated soils.     

Is an adjusted rhizosphere-based method valid for field assessment of metal phytoavailability? Application to non-contaminated soils

Previously recommended rhizosphere-based method (RHIZO) applied to moist rhizosphere soils was integrated with moist bulk soils, and termed adjusted-RHIZO method (A-RHIZO). The A-RHIZO and RHIZO methods were systematically compared with EDTA, DTPA, CaCl2 and the first step of the Community Bureau of Reference (BCR1) methods for assessing metal phytoavailability under field conditions. Results suggested that moist bulk soils are equally suited or even better than rhizosphere soils to estimate metal phytoavailability. The A-RHIZO method was preferred to other methods for predicting the phytoavailability of Ni, Cu, Zn, Cd, Pb and Mn to wheat roots with correlation coefficients of 0.730 (P<0.001), 0.854 (P<0.001), 0.887 (P<0.001), 0.739 (P<0.001), 0.725 (P<0.001) and 0.469 (P<0.05), respectively. When including soil properties, other extraction methods were also able to predict phytoavailability reasonably well for some metals. Soil pH, organic matter and Fe-Mn oxide contents, and cation-exchange capacity mostly influenced the extraction and phytoavailability of metals.     

Antimony accumulation in Achillea ageratum, Plantago lanceolata and Silene vulgaris growing in an old Sb-mining area

Preliminary data of a biogeochemical survey concerning antimony transfer from soil to plants in an abandoned Sb-mining area are presented. Achillea ageratum, Plantago lanceolata and Silene vulgaris can strongly accumulate antimony when its extractable fraction in the soil is high (139-793 mg/kg). A. ageratum accumulates in basal leaves (1367 mg/kg) and inflorescences (1105 mg/kg), P. lanceolata in roots (1150 mg/kg) and S. vulgaris in shoots (1164 mg/kg). In these plant species, the efficiency of antimony accumulation decreases when the antimony availability in the soil is high. In A. ageratum and S. vulgaris, the death of the epigeal target part at the end of the growing season contributes to a reduction of the antimony load in the plant. A study to test the use of these species as bioindicators of antimony availability in soil is suggested by our results.     

The influence of EDDS on the uptake of heavy metals in hydroponically grown sunflowers

Phytoextraction is an environmentally friendly in situ technique for cleaning up metal contaminated land. Unfortunately, efficient metal uptake by remediation plants is often limited by low phytoavailability of the targeted metals. Chelant assisted phytoextraction has been proposed to improve the efficiency of phytoextraction. Phytoremediation involves several subsequent steps: transfer of metals from the bulk soil to the root surfaces, uptake into the roots and translocation to the shoots. Nutrient solution experiments address the latter two steps. In this context we investigated the influence of the biodegradable chelating agent SS-EDDS on uptake of essential (Cu and Zn) and non-essential (Pb) metals by sunflowers from nutrient solution. EDDS was detected in shoots and xylem sap for the first time, proving that it is taken up into the above ground biomass of plants. The essential metals Cu and Zn were decreased in shoots in the presence of EDDS whereas uptake of the non-essential Pb was enhanced. We suggest that in the presence of EDDS all three metals were taken up by the non-selective apoplastic pathway as the EDDS complexes, whereas in the absence of EDDS essential metal uptake was primarily selective along the symplastic pathway. This shows that synthetic chelating agents do not necessarily increase uptake of heavy metals, when soluble concentrations are equal in the presence and absence of chelates.     

Difference of lead,copper and zinc concentrations between interiors and exteriors of peds in some contaminated soils

Solute transport of elements in soils depends on the soil structural and hydraulic properties, and it is controlled by sorption and diffusion, which both limit the mobility and distribution of elements in soils. This study was conducted to compare lead (Pb), copper (Cu) and zinc (Zn) concentrations between ped exteriors and interiors of some contaminated soils. The results show that the differences of the heavy metals between exteriors and interiors decreased in the order clayey soil, clayey loam soil, loam soil. For same soils, the differences decreased from Pb to Cu to Zn. The differences in readily extractable concentrations of the three metals between ped exteriors and interiors were much larger than the differences in their total metals, this may indicate that extractable metals were more recently deposited. The higher Pb and Cu concentrations in the ped exteriors than interiors may additionally be explained by anthropogenic input, movement and downward through preferential flow.     

Evaluation of extractants for estimation of the phytoavailable trace metals in soils

Despite its environmental (and financial) importance, there is no agreement in the literature as to which extractant most accurately estimates the phytoavailability of trace metals in soils. A large data set was taken from the literature, and the effectiveness of various extractants to predict the phytoavailability of Cd, Zn, Ni, Cu, and Pb examined across a range of soil types and contamination levels. The data suggest that generally, the total soil trace metal content, and trace metal concentrations determined by complexing agents (such as the widely used DTPA and EDTA extractants) or acid extractants (such as 0.1M HCl and the Mehlich 1 extractant) are only poorly correlated to plant phytoavailability. Whilst there is no consensus, it would appear that neutral salt extractants (such as 0.01 M CaCl(2) and 0.1 M NaNO(3)) provide the most useful indication of metal phytoavailability across a range of metals of interest, although further research is required.     

Ascorbic acid, β-carotene,sugars, phenols,and heavy metals in sweet potatoes grown in soil fertilized with municipal sewage sludge

Municipal sewage sludge (MSS) used for land farming typically contains heavy metals that might impact crop quality and human health. A completely randomized experimental design with three treatments (six replicates each) was used to monitor the impact of mixing native soil with MSS or yard waste (YW) mixed with MSS (YW +MSS) on: i) sweet potato yield and quality; ii) concentration of seven heavy metals (Cd, Cr, Mo, Cu, Zn, Pb, and Ni) in sweet potato plant parts (edible roots, leaves, stem, and feeder roots); and iii) concentrations of ascorbic acid, total phenols, free sugars, and β-carotene in sweet potato edible roots at harvest. Soil samples were collected and analyzed for total and extractable metals using two extraction procedures, concentrated nitric acid (to extract total metals from soil) as well as CaCl₂ solution (to extract soluble metals in soil that are available to plants), respectively. Elemental analyses were performed using inductively coupled plasma mass spectrometry (ICP-MS). Overall, plant available metals were greater in soils amended with MSS compared to control plots. Concentration of Pb was greater in YW than MSS amendments. Total concentrations of Pb, Ni, and Cr were greater in plants grown in MSS+YW treatments compared to control plants. MSS+YW treatments increased sweet potato yield, ascorbic acid, soluble sugars, and phenols in edible roots by 53, 28, 27, and 48%, respectively compared to plants grown in native soil. B-carotene concentration (157.5 μg g^?1 fresh weight) was greater in the roots of plants grown in MSS compared to roots of plants grown in MSS+YW treatments (99.9 μg g^?1 fresh weight). Concentration of heavy metals in MSS-amended soil and in sweet potato roots were below their respective permissible limits.     

Rhizosphere characteristics of indigenously growing nickel hyperaccumulator and excluder plants on serpentine soil

The role of rhizosphere processes in metal hyperaccumulation is largely unexplored and a matter of debate, related field data are virtually not available. We conducted a field survey of rhizosphere characteristics beneath the Ni hyperaccumulator Thlaspi goesingense Hálácsy and the metal-excluder species Silene vulgaris L. and Rumex acetosella L. growing natively on the same serpentine site. Relative to bulk soil and to the rhizosphere of the excluder species, we found significantly increased DOC and Ni concentrations in water extracts of T. goesingense rhizosphere, whereas exchangeable Ni was depleted due to excessive uptake of Ni. Chemical speciation analysis using the MINTEQA2 software package revealed that enhanced Ni solubility in Thlaspi rhizosphere is driven by the formation of Ni-organic complexes. Moreover, ligand-induced dissolution of Ni-bearing minerals is likely to contribute to enhanced Ni solubility. Increased Mg and Ca concentrations and pH in Thlaspi rhizosphere are consistent with ligand-induced dissolution of orthosilicates such as forsterite (Mg(2)SiO(4). Our field data reinforce the hypothesis that exudation of organic ligands may contribute to enhanced solubility and replenishment of metals in the rhizosphere of hyperaccumulating species.     

Effects of acidic solutions on element dynamics in the monsoon evergreen broad-leaved forest at Dinghushan,China

BACKGROUND: Soil metal dynamics are affected by acid deposition. Little knowledge is available about the process in the lateritic soils under the monsoon forest in south China. METHODS: Samplings of Acmera acuminatissima, Cryptocarya concinna and Schima superba were grown from October, 2000 to July, 2002 in pots with a natural acid lateritic forest soil from Dinghushan. Pots were watered weekly with an acid solution (pH 3.05, 3.52, 4.00 or 4.40) or with tap water. Fe, Mn, Cu and Al were measured in soils, leachates and sapling leaves. RESULTS: Soil extractable Fe and leachate Al and Mn concentrations increased with a decreasing treatment pH. Soil reactive Al exhibited the opposite trend and decreased over time. The Ca/Al and Mg/ (Al+Mn) ratios did not decrease in the leaves of Schima superba, but decreased with a decreasing treatment pH for Cryptocaria concinna. Both ratios only decreased in the pH 3.05 treatment for Acmena CONCLUSIONS: Cu will not be toxic for plants since soil extractable Cu was not high and Fe will not be toxic either given that its root uptake was inhibited by Mn. Acid rains will lead to increased Mn and Al mobility in soil. Cryptocaria concinna will be the most sensible species to these changes (nutrient deficiency and direct Mn toxicity), while Schima superba should retain a good growth.     

Comparative tolerance of Pinus radiata and microbial activity to copper and zinc in a soil treated with metal-amended biosolids

A study was conducted to evaluate the effects of elevated concentrations of copper (Cu) and zinc (Zn) in a soil treated with biosolids previously spiked with these metals on Pinus radiata during a 312-day glasshouse pot trial. The total soil metal concentrations in the treatments were 16, 48, 146 and 232 mg Cu/kg or 36, 141, 430 and 668 mg Zn/kg. Increased total soil Cu concentration increased the soil solution Cu concentration (0.03–0.54 mg/L) but had no effect on leaf and root dry matter production. Increased total soil Zn concentration also increased the soil solution Zn concentration (0.9–362 mg/L). Decreased leaf and root dry matter were recorded above the total soil Zn concentration of 141 mg/kg (soil solution Zn concentration, >4.4 mg/L). A lower percentage of Cu in the soil soluble?+?exchangeable fraction (5–12 %) and lower Cu²⁺ concentration in soil solution (0.001–0.06 μM) relative to Zn (soil soluble?+?exchangeable fraction, 12–66 %; soil solution Zn²⁺ concentration, 4.5–4,419 μM) indicated lower bioavailability of Cu. Soil dehydrogenase activity decreased with every successive level of Cu and Zn applied, but the reduction was higher for Zn than for Cu addition. Dehydrogenase activity was reduced by 40 % (EC₄₀) at the total solution-phase and solid-phase soluble?+?exchangeable Cu concentrations of 0.5 mg/L and 14.5 mg/kg, respectively. For Zn the corresponding EC₅₀ were 9 mg/L and 55 mg/kg, respectively. Based on our findings, we propose that current New Zealand soil guidelines values for Cu and Zn (100 mg/kg for Cu; 300 mg/kg for Zn) should be revised downwards based on apparent toxicity to soil biological activity (Cu and Zn) and radiata pine (Zn only) at the threshold concentration.     

A comparison of physiologically based extraction test (PBET) and single-extraction methods for release of Cu, Zn, and Pb from mildly acidic and alkali soils

In vitro digestion test can be applied to evaluate the bioaccessibility of soil metals by measuring the solubility of the metals in synthetic human digestive tract. Physiologically based extraction test (PBET), composed of sequential digestion of gastric and intestinal phase, is one of the frequently used in vitro digestion tests. In this study, the PBET was chosen to determine the bioaccessibility of Cu, Zn, and Pb in 14 mildly acidic and alkali (pH 5.87–8.30) soils. The phytoavailability of Cu, Zn, and Pb in the same soils was also measured using six single-extraction methods (0.1 M HNO₃, 0.4 M HOAc, 0.1 M NaNO₃, 0.01 M CaCl₂, 0.05 M EDTA, and 0.5 M DTPA). The extraction efficiencies of the methods were compared. The PBET had a strong ability to extract metals from soil, which was much greater than neutral salt extraction and close to dilute acid and complex extraction in spite of the last 2 h neutral intestinal digestion. The amounts of bioaccessible Cu, Zn, and Pb in the gastric phase and in the gastrointestinal phase were both largely determined by the total content of soil Cu, Zn, and Pb. But the results of gastrointestinal digestion reflected more differences resulting from element and soil types than those of gastric digestion did. It was noticed that most of variations in the amounts of soil Cu, Zn, and Pb extracted by EDTA were well explained by the total soil Cu, Zn, and Pb, as same as the PBET. Moreover, the solubility of Cu, Zn, and Pb in the gastric phase and gastrointestinal phase were all positively linearly correlated with the results of EDTA. It was suggested that EDTA extraction can be used to predict the bioaccessibility of Cu, Zn, and Pb in mildly acidic and alkali (pH?>?5.8) soils, and the PBET and EDTA could be applied to measure, in a certain extent, the bioaccessibility and phytoavailability of Cu, Zn, and Pb in mildly acidic and alkali (pH?>?5.8) soils at the same time.     

Effect of ageing on the availability of heavy metals in soils amended with compost and biochar: evaluation of changes in soil and amendment properties

Remediation strategies using soil amendments should consider the time dependence of metal availability to identify amendments that can sustainably reduce available pollutant concentrations over time. Drying-wetting cycles were applied on amendments, soils and soil + amendment mixtures, to mimic ageing at field level and investigate its effect on extractable Cd, Cu, Ni, Pb and Zn concentrations from three contaminated soils. The amendments investigated were municipal waste organic compost and biochars. The amendments, soils and mixtures were characterised by their physicochemical properties at different ageing times. The amendments were also characterised in terms of sorption capacity for Cd and Cu. The sorption capacity and the physicochemical properties of the amendments remained constant over the period examined. When mixed with the soils, amendments, especially the compost, immediately reduced the extractable metals in the soils with low pH and acid neutralisation capacity, due to the increase in pH and buffering capacity of the mixtures. The amendments had a relatively minor impact on the metal availability concentrations for the soil with substantially high acid neutralisation capacity. The most important changes in extractable metal concentrations were observed at the beginning of the experiments, ageing having a minor effect on metal concentrations when compared with the initial effect of amendments.

     

Ascorbic acid, β-carotene, sugars, phenols, and heavy metals in sweet potatoes grown in soil fertilized with municipal sewage sludge

Municipal sewage sludge (MSS) used for land farming typically contains heavy metals that might impact crop quality and human health. A completely randomized experimental design with three treatments (six replicates each) was used to monitor the impact of mixing native soil with MSS or yard waste (YW) mixed with MSS (YW +MSS) on: i) sweet potato yield and quality; ii) concentration of seven heavy metals (Cd, Cr, Mo, Cu, Zn, Pb, and Ni) in sweet potato plant parts (edible roots, leaves, stem, and feeder roots); and iii) concentrations of ascorbic acid, total phenols, free sugars, and β-carotene in sweet potato edible roots at harvest. Soil samples were collected and analyzed for total and extractable metals using two extraction procedures, concentrated nitric acid (to extract total metals from soil) as well as CaCl? solution (to extract soluble metals in soil that are available to plants), respectively. Elemental analyses were performed using inductively coupled plasma mass spectrometry (ICP-MS). Overall, plant available metals were greater in soils amended with MSS compared to control plots. Concentration of Pb was greater in YW than MSS amendments. Total concentrations of Pb, Ni, and Cr were greater in plants grown in MSS+YW treatments compared to control plants. MSS+YW treatments increased sweet potato yield, ascorbic acid, soluble sugars, and phenols in edible roots by 53, 28, 27, and 48%, respectively compared to plants grown in native soil. B-carotene concentration (157.5 μg g?1 fresh weight) was greater in the roots of plants grown in MSS compared to roots of plants grown in MSS+YW treatments (99.9 μg g?1 fresh weight). Concentration of heavy metals in MSS-amended soil and in sweet potato roots were below their respective permissible limits.