Assessing risk to human health from tropical leafy vegetables grown on contaminated urban soils

Fifteen tropical leafy vegetable types were sampled from farmers' gardens situated on nine contaminated sites used to grow vegetables for commercial or subsistence consumption in and around Kampala City, Uganda. Trace metal concentrations in soils were highly variable and originated from irrigation with wastewater, effluent discharge from industry and dumping of solid waste. Metal concentrations in the edible shoots of vegetables also differed greatly between, and within, sites. Gynandropsis gynandra consistently accumulated the highest Cd, Pb and Cu concentrations, while Amaranthus dubius accumulated the highest Zn concentration. Cadmium uptake from soils with contrasting sources and severity of contamination was consistently lowest in Cucurbita maxima and Vigna unguiculata, suggesting these species were most able to restrict Cd uptake from contaminated soil. Concentrations of Pb and Cr were consistently greater in unwashed, than in washed, vegetables, in marked contrast to Cd, Ni and Zn. The risk to human health, expressed as a ‘hazard quotient’ (HQ_M), was generally greatest for Cd, followed successively by Pb, Zn, Ni and Cu. Nevertheless, it was apparent that urban cultivation of leafy vegetables could be safely pursued on most sites, subject to site-specific assessment of soil metal burden, judicious choice of vegetable types and adoption of washing in clean water prior to cooking.     

Effects of willow stands on heavy metal concentrations and top soil properties of infrastructure spoil landfills and dredged sediment-derived sites

The effects of willow stand development on top soil properties of uncontaminated infrastructure spoil landfills (ISL) and contaminated dredged sediment landfills (DSL) were assessed. For the ISL, significant increases in Cd, Zn and organic C levels in the top soil (0-10 cm) were detected more than 20 years after disposal. The increases in Cd and Zn concentrations in the top soil were attributed to leaf-associated metal transfer and leaf fall: the relatively high Cd and Zn concentrations in willow leaves resulted in top soil enrichment for these elements. Higher absolute amounts of Cd, Zn and Mn were taken up and recycled during leaf fall on DSL than on ISL, but did not result in significant differences between top soil and deeper soil (10-30 cm) for the DSL. Direct comparison of top soil development between both types of sites is not possible due to differences in stand age and time since disposal. The DSL were characterised by a higher short-range variance for the Cd, Cr, Cu, Pb and Zn concentrations in the top soil than the ISL. During the first years of ripening and dewatering, significant sulphate leaching occurred in the top soil of the DSL.     

Heavy metal and micronutrient uptake in soybeans as influenced by sewage sludge amendment

Soybean (Glycine max (L.) Merr.) uptake of the elements, Cd, Ni, Pb, Cu, Zn and Mn, from a sewage sludge-amended Mecklenburg soil was conducted in the greenhouse. “Bragg” soybeans were grown in pots for five weeks at which time the tops and roots were sampled separately for elemental analysis. Soil samples from each pot were extracted with DTPA (diethylenetriaminepentaacetic acid) and the concentration of extractable elements correlated with the elemental content in the soybean plant. There was a significant increase in dry matter production with sludge treatment. Concentrations of Cd, Ni and Pb in the soybean shoots and roots increased from sludge-amended soil as compared to the control. The metal concentration in the soybean tissue increased with increasing levels of sludge amendment. Uptake of the heavy metals was greater by the roots than by the shoots indicating some barrier to movement of the metals from roots to shoots. The DTPA extractable Cd in sludge-amended soil increased significantly, and showed correlation to the soybean tissue metal concentrations. As for the micronutrients, Cu increased in the soybean shoot as the extractable Cu increased. There was no significant relationship between soybean tissue Zn and Mn and extractable Zn and Mn.     

Mammalian hair as an accumulative bioindicator of metal bioavailability in Australian terrestrial environments

The current study represents the first investigation of the suitability of marsupial and eutherian mammalian hair as indicator tissue for metal exposure and accumulation within contaminated Australian terrestrial ecosystems. A soil metal contamination gradient was established across 22 sites at increasing distances from a decommissioned Lead/Zinc smelter in NSW, Australia. Within each site, soil and small mammal populations were sampled. An Australian native marsupial, the insectivorous Brown Antechinus, Antechinus stuartii: Dasyuridae, and introduced rodents, the omnivorous Brown or Norway Rat, Rattus norvegicus: Muridae and the Black Rat, Rattus rattus: Muridae were assessed for hair concentrations of Cadmium (Cd), Copper (Cu), Lead (Pb) and Zinc (Zn). Metals in soil were most elevated at sites within close proximity to the smelter, with soil metal concentrations decreasing with distance from the smelter. The non-essential metals Pb and Cd were accumulated in hair, both metals exhibiting positive linear relationships with environmental exposure (soil metal concentrations). When the variables of weight and snout-vent length were considered, no further contribution in terms of explaining the variability in hair Cd or Pb was observed for all species examined. The essential metals Cu and Zn were regulated in hair, remaining similar across the metal contamination gradient. A significant negative correlation between snout-vent length and hair Cu concentration was found for the Brown Rat; greater hair Cu concentrations were found in smaller individuals of this species. Accumulation of Pb to hair was similar among species while concentrations of Cd in Brown Rat hair were higher than both Black Rat and Brown Antechinus hair. As each of the three aforementioned species exhibit similar bioaccumulation relationships for Pb, we suggest that sampling hair from introduced rodents (pest species) may provide a suitable proxy for the assessment of Pb bioavailability for a range of small mammals within Australian urban remnants.     

The effects of Aphis fabae infestation on the antioxidant response and heavy metal content in field grown Philadelphus coronarius plants

The purpose of this study was to explore a possible relationship between the soil availability of metals and their concentrations in various parts of Philadelphuscoronarius plants. Moreover, the possible impact of an aphid infestation on the contamination and antioxidant response of plants from the urban environment of Kraków and the reference rural area of Zagaje Stradowskie (southern Poland) was analyzed. The contents of the glutathione, proline, non-protein − SH groups, antioxidants, and phosphorous and the levels of guaiacol peroxidase and catalase activity in leaves and shoots either infested or not by the aphid Aphis fabae Scop., were measured. The potential bioavailability of metals (Cd; Cu; Ni; Pb; Zn) in the soil and their concentrations in P. coronarius plants originating from both sites were compared.The antioxidant responses were generally elevated in the plants in the polluted area. Such reactions were additionally changed by aphid infestation. Generally, the concentrations of metals in the HNO₃ and CaCl₂ extractants of the soils from two layers at the 0-20 and 20-40 cm depths from the polluted area were higher than in those from the reference area. Such differences were found for nickel and lead (in all examined extractants), zinc (in soil extractants from the layer at 20-40 cm) and cadmium (in HNO₃ extractants). Significant positive relationships between the lead concentrations in the soil and in the plants were found. In the parts of plants from the polluted area, higher concentrations of Pb and Zn (leaves and shoots) and Cd (shoots) were recorded. The shoots and leaves of plants infested with aphids had higher concentrations of Zn but lower Pb. Moreover, their leaves had higher contaminations of Cu and Ni. In conclusion, aphids affected not only the antioxidant response of the plants but also their contamination with metals, especially contamination of the leaves.     

Health risk from heavy metals via consumption of food crops in the vicinity of Dabaoshan mine, South China

Heavy metal contamination of soils resulting from mining and smelting is causing major concern due to the potential risk involved. This study was designed to investigate the heavy metal (Cu, Zn, Pb and Cd) concentrations in soils and food crops and estimate the potential health risks of metals to humans via consumption of polluted food crops grown at four villages around the Dabaoshan mine, South China. The heavy metal concentrations in paddy and garden soils exceeded the maximum allowable concentrations for Chinese agricultural soil. The paddy soil at Fandong village was heavily contaminated with Cu (703 mg kg^− 1), Zn (1100 mg kg^− 1), Pb (386 mg kg^− 1) and Cd (5.5 mg kg^− 1). Rice tended to accumulated higher Cd and Pb concentration in grain parts. The concentrations of Cd, Pb and Zn in vegetables exceeded the maximum permissible concentration in China. Taro grown at the four sampled villages accumulated high concentrations of Zn, Pb and Cd. Bio-accumulation factors for heavy metals in different vegetables showed a trend in the order: Cd > Zn > Cu > Pb. Bio-accumulation factors of heavy metals were significantly higher for leafy than for non-leafy vegetable. The target hazard quotient (THQ) of rice at four sites varied from 0.66-0.89 for Cu, 0.48-0.60 for Zn, 1.43-1.99 for Pb, and 2.61-6.25 for Cd. Estimated daily intake (EDI) and THQs for Cd and Pb of rice and vegetables exceeded the FAO/WHO permissible limit. Heavy metal contamination of food crops grown around the mine posed a great health risk to the local population through consumption of rice and vegetables.     

Seasonal and time variability of heavy metal content and of its chemical forms in sewage sludges from different wastewater treatment plants

Sewage sludges obtained from seven wastewater treatment plants from the province of Salamanca, Spain, were periodically sampled to determine seasonal and time variation of their elemental composition over 2000 to 2002. The aim of this paper was to provide additional insight to evaluate the potential environmental impact following soil incorporation of these materials as amendments. Aqua regia extractable metals (pseudo total content) of Cd, Cr, Cu, Ni, Pb and Zn were determined and furthermore, the main chemical forms of metals within the sludge were evaluated using a five-step fractionation procedure. All the studied sludges displayed high fertility properties due to their richness of OC, P and K. Total mean concentrations of Cd, Cr, Cu, Ni, Pb and Zn in the sludges were within the regulation of the Spanish legislation. Using an multifactor analysis of variance, significant differences between Cr, Cu, Ni, Pb and Zn pseudo total contents (p<0.01) of sludges at different sites were found while the Cd content was statistically similar. Also significant differences were found between these pseudo total contents of heavy metals in samples collected along the time after three years (0.001相似文献   

Heavy metal pollution associated with mining activity in the Kouh-e Zar region,NE Iran

The Kouh-e Zar mining area is located in the central part of the “Khaf–Bardaskan” volcanic-plutonic zone, NE Iran. Mining activity has resulted in pollution of soil and water resources by potentially toxic elements including arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), antimony (Sb), nickel (Ni) and zinc (Zn). In this study, the major source of heavy metal pollution and elucidating the probable environmental risks associated with this area were determined by quantifying pollution in soils and water resources. Concentrations of Cd, Cr, Cu, Pb and Zn in the Kouh-e Zar mining area varied in the range of 5–470, 33–442, 25–5125, 81.15–12,096.27 and 55–4210 mg/kg, respectively. The geo-accumulation index for Cd in all samples was extremely high (I_geo > 5) and the enrichment factor also shows an extremely high amount (EF > 40), both representing evidence for highly polluted soil in the area. However, the coefficients of aqueous migration (K_x) of Cd, Cr, Cu, Pb and Zn were K_x < 0.1, so they are classified as “least mobile and inert” grade. Also, the heavy metals tend to remain in soil (solid environment). Cluster analysis (CA) determined the lithogenic origin for Zn, Cu, Cr and Cd, and the anthropogenic origin (mining activity) for Pb in the soils of the mining area. The concentrations of Cd, Cu, Pb and Zn in water are controlled by free Fe and Mn oxy-hydroxide content in the soils. Both water–rock interaction and mining activity have contributed to pollution in the area.

     

Effect of biodegradable amendments on uranium solubility in contaminated soils

Chelate-assisted phytoextraction has been proposed as a potential tool for phytoremediation of U contaminated sites. In this context, the effects of five biodegradable amendments on U release in contaminated soils were evaluated. Three soils were involved in this study, one with a relatively high background level of U, and two which were contaminated with U from industrial effluents. Soils were treated with 5 mmol kg(-1) dry weight of either citric acid, NH(4)-citrate/citric acid, oxalic acid, S,S-ethylenediamine disuccinic acid or nitrilotriacetic acid. Soil solution concentration of U was monitored during 2 weeks. All amendments increased U concentration in soil solution, but citric acid and NH(4)-citrate/citric acid mixture were most effective, with up to 479-fold increase. For oxalic acid, S,S-ethylenediamine disuccinic acid and nitrilotriacetic acid, the increase ranged from 10-to 100-fold. The highest concentrations were observed 1 to 7 days after treatment, after which U levels in soil solution gradually decreased. All amendments induced a temporary increase of soil solution pH and TOC that could not be correlated with the release of U in the soil solution. Thermodynamic stability constants (log K) of complexes did not predict the relative efficiency of the selected biodegradable amendments on U release in soil solution. Amendments efficiency was better predicted by the relative affinity of the chelate for Fe compared to U.     

The distribution and fixation of trace elements by the Vertisols of Malaga, southern Spain

The response of Vertisols to contamination by heavy metals (As, Cd, Pb, Hg) and micronutrients (Co, Cu, Cr, Ni, V, Zn) in toxic concentrations was studied in a series of experiments undertaken with soil samples from the province of Malaga, in Southern Spain. The soil samples were placed in methacrylate columns, passing through a contaminant solution of selected elements in order to study their distribution and fixation the soil. Systematic sampling and sequential extraction analysis show that the Vertisols behave as a buffer to all elements, except cadmium.     

Heavy metal pollution downstream the abandoned Coval da Mó mine (Portugal) and associated effects on epilithic diatom communities

This study examined trace-element concentrations in 39 sediment samples collected in the vicinity of the abandoned Coval da Mó mine, and evaluated the anthropogenic contaminant effects and other environmental variables in the taxonomic composition, structure and morphological changes of benthic diatom communities.The results show the existence of extremely high contamination in Pb, Zn and Cd (the mean values exceed the background values 376, 96 and 19 times, respectively) on the first 2.5 km in the water flow direction. Also Co, Cu, Mn and Ni are present in high concentrations. Dilution by relatively uncontaminated sediment reduces metal concentrations downstream, but Zn concentrations increase downstream Fílvida stream, as a result of several factors such as sewage and agriculture.To evaluate the biological effects caused by Pb, Cd and Zn, three sites were selected. In the stressed environment, near the mining area (C232), diatoms were extremely rare, however there was a slight recovery at site C79 located 2 km downstream. Fragilaria capucina var. rumpens, Fragilaria cf. crotonensis and Achnanthidium minutissimum showed abnormal valves which may be related to high levels of metals.Six km downstream, in Fílvida stream (C85), an increase in species richness and diversity was registered while the relative percentage of valve teratologies was lower. In the absence of OM, nutrients and low pH the diatom community patterns must be attributed to the metal concentration at some sites. Considering that community diversity can be affected by abiotic and biotic variables and valve deformations are caused by a small number of variables, basically metals, and acid conditions, we consider the presence of teratologies as an indication of the presence of metals.     

磷石膏堆场对周围农田土壤重金属含量的影响

对什邡市两家磷化工厂的磷石膏和磷石膏堆周围土壤中Zn、Cd、As、Cu、Pb、Hg和U的含量及空间分布特征进行了检测和分析,根据重金属含量对土壤环境质量进行评价,并结合研究区地形地貌、气候特征研究重金属分布和来源。结果表明：磷石膏中重金属的含量较高,磷石膏的堆放使得磷石膏中的重金属在周边耕作层土壤中形成了较大的累积;9个土样中,各元素的检出率均为100%,Cd、Cu、Zn和Pb的含量最高,Cd的最高含量超土壤三级标准近3倍,Cu和Zn平均含量超过了土壤二级标准;磷石膏堆周围土壤中重金属含量在平面上与磷石膏堆距离成负相关,在纵向剖面上,重金属含量也基本上随着深度的增加而降低,各元素含量下降的程度不一样,Cd和Pb的下降程度最大。     

Patterns of metal bioaccumulation in two filter-feeding macroinvertebrates: Exposure distribution, inter-species differences and variability across developmental stages

This study focused on the metal bioaccumulation of two aquatic insects (Ephoron virgo and Hydropsyche spp.) in order to evaluate the spatial distribution of metals, the interspecific differences between both filter-feeders and the bioaccumulation dynamics during E. virgo development stages. Hg, Cd, Ni, Cr, As, Pb, Cu, Ti, Zn and Mn were quantified in insects and in suspended particulate matter (SPM) sampled downstream and upstream of a chemical plant, where more than 300,000 t of polluted sediments are deposited. Hg concentrations were one order of magnitude higher downstream of the sediment dump, which showed that the Hg pollution originated in the chemical plant. Cd, Ni, Cr, Pb, Ti, Zn and Mn in invertebrates revealed that metal pollution was present upstream in other parts of the river. Interspecific differences were observed for all metals but Mn; significantly higher concentrations were observed in E. virgo over Hydropsyche exocellata, except for Cd, which showed 10-fold higher values. Hg and Cd increased until E. virgo nymphs reached 11 mm and decreased afterwards in late instars when nymphs were about to emerge. Cr, Pb, Ti and Mn decreased along early instars followed by a steady state in late instars. Similar values were obtained for Cu, As and Zn along all instars. Sexual differences between males and females of E. virgo were observed for Cd, Cu and Mn. Hg and Cd persistence was strong across developmental stages since high concentrations were found in eggs and emerging adults. Because the behavior of different metals varied for the two species and during the developmental stages of E. virgo, care should be taken in the interpretation of insect metal concentrations when analyzing the food chain transfer of metals in river ecosystems.     

Intra- and inter-annual variation of Cd, Zn, Mn and Cu in foliage of poplars on contaminated soil

The uptake of trace metals in the leaves of fast-growing woody species is a crucial factor in ecological risk assessment and in the evaluation of phytoextraction potentials. In this study, we present a long-term data series of foliar Cd, Zn, Mn and Cu concentrations in poplar (Populus trichocarpa x P. deltoides). Leaves were collected every three weeks from 2001 until 2007 on three sites, (i) a new plantation on an alluvial soil polluted by river sediments, (ii) a new plantation on an unpolluted soil and (iii) a 10-year old plantation on a polluted dredged sediment soil. In addition, tree rings were measured on the alluvial soil in order to better assess growth over the past seven years. Foliar concentrations of Cd, Zn and Mn decreased considerably with time in the new plantation on polluted soil. Concentrations of Zn and Mn decreased in the new plantation on unpolluted soil as well. The older plantation on polluted soil did not show changes in foliar concentrations for Cd, Zn or Mn. Foliar Cu concentrations slightly increased for all sites. Within one growing season, foliar concentrations of Cd, Zn, Cu and Mn increased towards the end of the season. The tree ring data of the poplars on the alluvial soil indicated a strong decrease in growth due to declining tree condition from 2005 onwards, the same year that foliar Cd and Zn concentrations markedly decreased. Lower transpiration rates probably induced a lower uptake of dissolved trace metals. It is concluded that stand health and growth rate have a strong impact on the variation of foliar trace metal concentrations over time.     

Trace element availability and plant growth in a mine-spill contaminated soil under assisted natural remediation I. Soils

We evaluated the effects of different amendments and/or a plant cover on reclamation of a trace element contaminated soil. Seven treatments were established: four organic (leonardite (LEO), litter (LIT), municipal waste compost (MWC), biosolid compost (BC)), one inorganic (sugar beet lime (SL)) and two controls (control without amendment but with Agrostis (CTRP) and control without amendment and without Agrostis (CTR)). Results showed that total organic C was significantly higher in organic treatments in all samplings. Water-soluble C was lower in CTR compared to other treatments, but no significant differences were observed between organic treatments and SL and CTR. SL, BC and MWC treatments increased soil pH and reduced 0.01 M CaCl2-extractable Cd, Cu and Zn concentrations more efficiently, especially in the first 2 years. At the end of the experiment 0.01 M CaCl2-extractable trace element concentrations were similar in all treatments. 0.01 M CaCl2-extractable As and Pb were below the detection limit. Addition of amendments showed no clear reduction in 0.05 M EDTA-extractable trace element concentrations and some amendments even increased 0.05 M EDTA-extractable As and Cu with time. Pseudo-total trace element concentrations were higher for As in controls. On the other hand, mean values of Cu and Zn were higher in MWC treatment. BC and SL treatments also showed higher Zn mean concentration than controls. No amendment effect was observed for Cd and Pb.     

Trace elements in soils and plants in temperate forest plantations subjected to single and multiple applications of mixed wood ash

Wood ash, a by-product generated in power plants, can be used to fertilize forest plantations to replenish nutrients lost during harvesting. Although wood ash generally contains low levels of trace metals, release of some of these may occur soon after ash application in acid soils. The risk of heavy metal contamination associated with application of mixed wood ash was assessed in six Pinus radiata D. Don plantations, on two types of mineral soil differing in texture, drainage and CECe. Four of the stands received a single application of 4500 kg ha(-1) (March 2003), and in the other two stands the same treatment was applied over three consecutive years (2003-2005). Trace metal (Cd, Cr, Cu, Mn, Ni, Pb, Zn) concentrations were monitored throughout the 3 years in different components of the forest ecosystem--soil solid fraction, soil solution, tree needles, ground vegetation and different mushroom species. Repeated applications of wood ash led to moderate increases in soil extractable Mn and Zn, and Mn in all mushrooms species. However, the maximum concentrations did not reach levels potentially harmful to organisms. Concentrations of Zn, Cu and Cd decreased in some mushroom species, probably because of increased soil pH caused by the treatment. Heavy metal concentrations in tree needles and ground vegetation were not altered. Although the risk of heavy metal contamination appears to be low, the long-term effects of wood ash application must be assessed.     

Application of phytotoxicity data to a new Australian soil quality guideline framework for biosolids

To protect terrestrial ecosystems and humans from contaminants many countries and jurisdictions have developed soil quality guidelines (SQGs). This study proposes a new framework to derive SQGs and guidelines for amended soils and uses a case study based on phytotoxicity data of copper (Cu) and zinc (Zn) from field studies to illustrate how the framework could be applied. The proposed framework uses normalisation relationships to account for the effects of soil properties on toxicity data followed by a species sensitivity distribution (SSD) method to calculate a soil added contaminant limit (soil ACL) for a standard soil. The normalisation equations are then used to calculate soil ACLs for other soils. A soil amendment availability factor (SAAF) is then calculated as the toxicity and bioavailability of pure contaminants and contaminants in amendments can be different. The SAAF is used to modify soil ACLs to ACLs for amended soils.The framework was then used to calculate soil ACLs for copper (Cu) and zinc (Zn). For soils with pH of 4-8 and OC content of 1-6%, the ACLs range from 8 mg/kg to 970 mg/kg added Cu. The SAAF for Cu was pH dependant and varied from 1.44 at pH 4 to 2.15 at pH 8. For soils with pH of 4-8 and OC content of 1-6%, the ACLs for amended soils range from 11 mg/kg to 2080 mg/kg added Cu. For soils with pH of 4-8 and a CEC from 5-60, the ACLs for Zn ranged from 21 to 1470 mg/kg added Zn. A SAAF of one was used for Zn as it concentrations in plant tissue and soil to water partitioning showed no difference between biosolids and soluble Zn salt treatments, indicating that Zn from biosolids and Zn salts are equally bioavailable to plants.     

Bromine,copper, manganese and lead content of the leaves of Taraxacum officinale (dandelion)

Neutron activation analysis and atomic absorption spectrometry have been used to determine the Br, Cu, Mn and Pb content of leaves of Taraxacum officinale (dandelion). The elemental content of the leaves correlated with the extent of anthropogenic pollution of the region where the plant developed. The dependence of the element concentrations on the distance from the source has been evaluated. A positive linear correlation was observed between the Cu and Pb, Cu and Sb, Pb and Cd, and Pb and Zn content of the leaves.     

Mass balance and distribution of sludge-borne trace elements in a silt loam soil following long-term applications of sewage sludge

Soil samples were collected at 15-cm increments to a depth of 75 cm from plots on a silt loam soil where until several years earlier and for 14 years, anaerobically digested sewage sludge had been annually applied by furrow irrigation. The study protocol consisted of four replications of 6.1 x 12.2-m plots with 0 (T0), 1/4-maximum (T1), 1/2-maximum (T2) and maximum (T3) sludge application rates randomized within blocks. When sludge applications were terminated, maximum sludge-treated plots had received 765 Mg ha-1 (dry weight equivalent) of sludge solids. Total soil concentrations of Cd, Cr, Cu, Ni, Pb and Zn had been significantly enhanced by all sludge application rates to a soil depth of 30 cm. Below the 30-cm depth, total soil Cd was increased to 75 cm, total Zn to 45 cm (T2 and T3 only), total Cr to 60 cm (T2 and T3 only), but total Cu, Pb, and Ni were not increased at depth. Despite the lack of significant increases in subsoil concentrations for some metals, mass balance calculations showed a relatively high proportion of all the above sludge-borne heavy metals to be unaccounted for in the soil profile for each application rate. Mass balance calculations of losses ranged from a high of 60% for Ni to a low of 36% for Cu and Pb. Similar losses were calculated from metal concentrations measured in soil samples taken at the time the sludge was applied. In soil surface samples (0-15 cm) from maximum sludge-treated plots, percentages of total metal concentration extracted with 4.0 M HNO3 ranged from a low of 31 for Zn to a high of 75 for Cu. Efficiency of metal extraction by HNO3 was inconsistent, depending on the soil horizon and sludge treatment, so that evaluation of HNO3-extractable metals is not a reliable method of estimating total metal retention in the profiles. In soil surface samples from maximum sludge-treated plots, the percentage of total metal contents extracted with DTPA ranged from a low of 0.03 for Cr to a high of 59 for Cd. The DTPA extractable levels of Cu, Ni, and Pb were higher in the subsoils of the sludge-treated soils, indicating that these metals had been redistributed from the surface layer to deeper zones in the profile of sludge-amended soil, despite the absence of elevated total concentrations of these three metals in the deeper subsoil.     

Cadmium solubility in paddy soils: effects of soil oxidation, metal sulfides and competitive ions

Cadmium (Cd) is a non-essential element for human nutrition and is an agricultural soil contaminant. Cadmium solubility in paddy soils affects Cd accumulation in the grain of rice. This is a human health risk, exacerbated by the fact that rice grains are deficient in iron (Fe) and zinc (Zn) for human nutrition. To find ways of limiting this potential risk, we investigated factors influencing Cd solubility relative to Fe and Zn during pre-harvest drainage of paddy soils, in which soil oxidation is accompanied by the grain-filling stage of rice growth. This was simulated in temperature-controlled “reaction cell” experiments by first excluding oxygen to incubate soil suspensions anaerobically, then inducing aerobic conditions. In treatments without sulfur addition, the ratios of Cd:Fe and Cd:Zn in solution increased during the aerobic phase while Cd concentrations were unaffected and the Fe and Zn concentrations decreased. However, in treatments with added sulfur (as sulfate), up to 34 % of sulfur (S) was precipitated as sulfide minerals during the anaerobic phase and the Cd:Fe and Cd:Zn ratios in solution during the aerobic phase were lower than for treatments without S addition. When S was added, Cd solubility decreased whereas Fe and Zn were unaffected. When soil was spiked with Zn the Cd:Zn ratio was lower in solution during the aerobic phase, due to higher Zn concentrations. Decreased Cd:Fe and Cd:Zn ratios during the grain filling stage could potentially limit Cd enrichment in paddy rice grain due to competitive ion effects for root uptake.