Soil dissipation of juvenile hormone analog insecticide pyriproxyfen and its effect on the bacterial community

This investigation was undertaken to examine the dissipation rate of pyriproxyfen as well as the change in the soil bacterial community. Residues of pyriproxyfen were measured using high performance liquid chromatography (HPLC) and the changes in bacterial community were determined by comparing the 16S rDNA bands on patterns by denaturing gradient gel electrophoresis (DGGE). The dissipation of pyriproxyfen was affected by both the concentration applied and incubation temperature. Lower concentrations (1 mg Kg(-1)) and higher incubation temperatures (30 and 40°C) showed more rapid dissipation rates. The population of microbial community decreased rapidly after incubation with 10 mg Kg(-1) of pyriproxyfen for 91 days, indicating the toxicity of pyriproxyfen toward bacterial communities in a closed soil ecosystem. Lower concentrations of pyriproxyfen showed less toxicity toward the microbial community. From cluster analysis, the structure of the bacterial community showed roughly a 60 % similarity throughout the experiment period in the control experiment, indicating the stability within soil microbiota without chemical agitation. However, the similarity was lower than 50 % both in the one and 10 mg Kg(-1) of insecticide pyriproxyfen spiked experiment, indicating the soil bacterial community changed after the insecticide pyriproxyfen was applied.     

Degradation and sorption of imidacloprid in dissimilar surface and subsurface soils

Degradation and sorption/desorption are important processes affecting the leaching of pesticides through soil. This research characterized the degradation and sorption of imidacloprid (1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine) in Drummer (silty clay loam) and Exeter (sandy loam) surface soils and their corresponding subsurface soils using sequential extraction methods over 400 days. By the end of the incubation, approximately 55% of imidacloprid applied at a rate of 1.0 mg kg(-1) degraded in the Exeter sandy loam surface and subsurface soils, compared to 40% of applied imidacloprid within 300 days in Drummer surface and subsurface soils. At the 0.1 mg kg(-1) application rate, dissipation was slower for all four soils. Water-extractable imidacloprid in Exeter surface soil decreased from 98% of applied at day 1 to >70% of the imidacloprid remaining after 400 d, as compared to 55% in the Drummer surface soil at day 1 and 12% at day 400. These data suggest that imidacloprid was bioavailable to degrading soil microorganisms and sorption/desorption was not the limiting factor for biodegradation. In subsurface soils > 40% of (14)C-benzoic acid was mineralized over 21 days, demonstrating an active microbial community. In contrast, cumulative (14)CO(2) was less than 1.5% of applied (14)C-imidacloprid in all soils over 400 d. Qualitative differences in the microbial communities appear to limit the degradation of imidacloprid in the subsurface soils.     

Fungicide impacts on microbial communities in soils with contrasting management histories

The impacts of the fungicides azoxystrobin, tebuconazole and chlorothalonil on microbial properties were investigated in soils with identical mineralogical composition, but possessing contrasting microbial populations and organic matter contents arising from different management histories. Degradation of all pesticides was fastest in the high OM/biomass soil, with tebuconazole the most persistent compound, and chlorothalonil the most readily degraded. Pesticide sorption distribution coefficient (K(d)) did not differ significantly between the soils. Chlorothalonil had the highest K(d) (97.3) but K(d) for azoxystrobin and tebuconazole were similar (13.9 and 12.4, respectively). None of the fungicides affected microbial biomass in either soil. However, all fungicides significantly reduced dehydrogenase activity to varying extents in the low OM/biomass soil, but not in the high OM/biomass soil. The mineralization of subsequent applications of herbicides, which represents a narrow niche soil process was generally reduced in both soils by azoxystrobin and chlorothalonil. 16S rRNA-PCR denaturing gradient gel electrophoresis (DGGE) indicated that none of the fungicides affected bacterial community structure. 18S rRNA PCR-DGGE analysis revealed that a small number of eukaryote bands were absent in certain fungicide treatments, with each band being specific to a single fungicide-soil combination. Sequencing indicated these represented protozoa and fungi. Impacts on the specific eukaryote DGGE bands showed no relationship to the extent to which pesticides impacted dehydrogenase or catabolism of herbicides.     

Effects of lead and cadmium nitrate on biomass and substrate utilization pattern of soil microbial communities

A study was conducted to evaluate the effects of different concentrations of lead (Pb) and cadmium (Cd) applied as their nitrates on soil microbial biomass carbon (C(mic)) and nitrogen (N(mic)), and substrate utilization pattern of soil microbial communities. The C(mic) and N(mic) contents were determined at 0, 14, 28, 42 and 56 days after heavy metal application (DAA). The results showed a significant decline in the C(mic) for all Pb and Cd amended soils from the start to 28 DAA. From 28 to 56 DAA, C(mic) contents changed non-significantly for all other treatments except for 600 mgkg(-1) Pb and 100 mgkg(-1) Cd in which it declined significantly from 42 to 56 DAA. The N(mic) contents also decreased significantly from start to 28 DAA for all other Pb and Cd treatments except for 200 mgkg(-1) Pb which did not show significant difference from the control. Control and 200 mgkg(-1) Pb had significantly lower soil microbial biomass C:N ratio as compared with other Pb treatments from 14 to 42 DAA, however at 56 DAA, only 1000 mgkg(-1) Pb showed significantly higher C:N ratio compared with other treatments. No significant difference in C:N ratio for all Cd treated soils was seen from start to 28 DAA, however from 42 to 56 DAA, 100 mgkg(-1) Pb showed significantly higher C:N ratio compared with other treatments. On 56 DAA, substrate utilization pattern of soil microbial communities was determined by inoculating Biolog ECO plates. The results indicated that Pb and Cd addition inhibited the functional activity of soil microbial communities as indicated by the intensity of average well color development (AWCD) during 168 h of incubation. Multivariate analysis of sole carbon source utilization pattern demonstrated that higher levels of heavy metal application had significantly affected soil microbial community structure.     

Influence of lead acetate on soil microbial biomass and community structure in two different soils with the growth of Chinese cabbage (Brassica chinensis)

A greenhouse pot experiment was conducted to evaluate the impact of different concentrations of lead acetate on soil microbial biomass and community structure during growth of Chinese cabbage (Brassica chinensis) in two different soils. The field soils were used for a small pot, short-term 60-day growth chamber study. The soils were amended with different Pb concentrations, ranging from 0 to 900mgkg(-1) soil. The experimental design was a 2 soilx2 vegetation/non-vegetationx6 treatments (Pb)x3 replicate factorial experiment. At 60 days the study was terminated and soils were analyzed for microbial parameters, namely, microbial biomass, basal respiration and PLFAs. The results indicated that the application of Pb at lower concentrations (100 and 300mgkg(-1)) as lead acetate resulted in a slight increase in soil microbial biomass, whereas Pb concentrations >500mgkg(-1) caused an immediate gradual significant decline in biomass. However, the degree of impact on soil microbial biomass and basal respiration by Pb was related to management (plant vegetation) or the contents of clay and organic matter in soils. The profiles of 21 phospholipid fatty acids (PLFAs) were used to assess whether observed changes in functional microbial parameters were accompanied by changes in the composition of the microbial communities after Pb application at 0, 300 and 900mg Pbkg(-1) soil. The results of principal component analyses (PCA) indicated that there were significant increases in fungi biomarkers of 18:3omega6c, 18:1omega9c and a decrease in cy17:0, which is an indicator of gram-negative bacteria for the high levels of Pb treatments In a word, soil microbial biomass and community structure, therefore, may be sensitive indicators reflecting environmental stress in soil-Pb-plant system. However, further studies will be needed to better understand how these changes in microbial community structure might actually impact soil microbial community function.     

Persistence of repeated triadimefon application and its impact on soil microbial functional diversity

The effects of repeated applications of the fungicide triadimefon in agricultural soil on the microbial functional diversity of the soil and on the persistence of the fungicide in the soil were investigated under laboratory conditions. The degradation half-lives of triadimefon at the recommended dosage, simulated by a first-order kinetic model, were 23.90, 22.95, and 21.52 days for the first, second, and third applications, respectively. Throughout this study, no significant inhibition of the Shannon-Wiener index (H') was observed. However, the Simpson index (1/D) and the McIntosh index (U) were obviously reduced (p ≤ 0.05) during the initial 3 days after the first triadimefon application and thereafter, gradually recovered to or exceeded the level of the control soil. A similar trend in variation but with a faster recovery in the 1/D and U was observed after the second and third triadimefon applications, respectively. Taken together, the above results indicate that the repeated application of triadimefon enhanced the degradation rate of the fungicide and the recovery rate of the soil microbial functional diversity. It is concluded that repeated triadimefon applications in soil have a transient or temporary inhibitory effect on soil microbial communities.     

Mercury baseline levels in Flemish soils (Belgium)

It is important to establish contaminant levels that are normally present in soils to provide baseline data for pollution studies. Mercury is a toxic element of concern. This study was aimed at assessing baseline mercury levels in soils in Flanders. In a previous study, mercury contents in soils in Oost-Vlaanderen were found to be significantly above levels reported elsewhere. For the current study, observations were extended over two more provinces, West-Vlaanderen and Antwerpen. Ranges of soil Hg contents were distinctly higher in the province Oost-Vlaanderen (interquartile range from 0.09 to 0.43 mg/kg) than in the other provinces (interquartile ranges from 0.7 to 0.13 and 0.7 to 0.15 mg/kg for West-Vlaanderen and Antwerpen, respectively). The standard threshold method was applied to separate soils containing baseline levels of Hg from the data. Baseline concentrations for Hg were characterised by a median of 0.10mg Hg/kg dry soil, an interquartile range from 0.07 to 0.14 mg/kg and a 90% percentile value of 0.30 mg/kg. The influence of soil properties such as clay and organic carbon contents, and pH on baseline Hg concentrations was not important. Maps of the spatial distribution of Hg levels showed that the province Oost-Vlaanderen exhibited zones with systematically higher Hg soil contents. This may be related to the former presence of many small-scale industries employing mercury in that region.     

Persistence of repeated triadimefon application and its impact on soil microbial functional diversity

The effects of repeated applications of the fungicide triadimefon in agricultural soil on the microbial functional diversity of the soil and on the persistence of the fungicide in the soil were investigated under laboratory conditions. The degradation half-lives of triadimefon at the recommended dosage, simulated by a first-order kinetic model, were 23.90, 22.95, and 21.52 days for the first, second, and third applications, respectively. Throughout this study, no significant inhibition of the Shannon-Wiener index (H′) was observed. However, the Simpson index (1/D) and the McIntosh index (U) were obviously reduced (p ≤ 0.05) during the initial 3 days after the first triadimefon application and thereafter, gradually recovered to or exceeded the level of the control soil. A similar trend in variation but with a faster recovery in the 1/D and U was observed after the second and third triadimefon applications, respectively. Taken together, the above results indicate that the repeated application of triadimefon enhanced the degradation rate of the fungicide and the recovery rate of the soil microbial functional diversity. It is concluded that repeated triadimefon applications in soil have a transient or temporary inhibitory effect on soil microbial communities.     

Transfer of hexazinone and glyphosate through undisturbed soil columns in soils under Christmas tree cultivation

Field studies monitoring pesticide pollution in the Morvan region (France) have revealed surface water contamination by some herbicides. The purpose of this study was to investigate in greater detail the transport of two herbicides, used in Christmas tree production in the Morvan, under controlled laboratory conditions. Thus, the leaching of hexazinone (3-cyclohexyl-6-dimethyl-amino-1-methyl-1,3,5-triazine-2,4 (1H,3H) dione) and glyphosate (N-(phosphono-methyl-glycine)) through structured soil columns was studied using one loamy sand and two sandy loams from sites currently under Christmas tree cultivation in the Morvan. The three soils were cultivated sandy brunisol [Sound reference base for soils, D. Baize, M.C. Girard (Coord.), INRA, Versailles, 1998, 322 p] or, according to the FAO [FAO, World reference base for soil resources, ISSS-ISRIC-FAO, FAO, Rome, Italy, 1998], the La Garenne was an arenosol and the two other soils were cambisols. The clay contents of the soils ranged from 86 to 156 g kg(-1) and the organic carbon ranged from 98 to 347 g kg(-1). After 160 mm of simulated rainfall applied over 12 days, 2-11% of the applied hexazinone was recovered in the leachate. The recovery was much higher than that of glyphosate, which was less than 0.01%. The greater mobility of hexazinone might be related to its much lower adsorption coefficient, K(oc), 19-300 l kg(-1), compared with 8.5-10231 l kg(-1) for glyphosate (literature values). Another factor that may explain the higher amounts of hexazinone recovered in the leachates of the three soil columns is its greater persistence (19.7-91 days) relative to that of glyphosate (7.9-14.4 days). The mobility of both herbicides was greater in the soils with higher gravel contents, coarser textures, and lower organic carbon contents. Moreover, glyphosate migration seems negatively correlated not only to soil organic carbon, but also to aluminium and iron contents of soils. This soil column study suggests that at the watershed scale, surface water contamination by hexazinone could occur via the horizontal subsurface flow in upper centimeters of soil. In contrast, the surface water contamination with glyphosate by this mechanism appears unlikely.     

Behavior of triadimefon in two Lebanese soils.

The retention and fate of triadimefon fungicide were studied under two environmental conditions. Field studies were conducted on two soils, a sandy loam soil (Fanar) and a clay soil (Raouda). Fanar is a wet coastal area while Raouda is a dry agricultural area of the Bekaa plain located at an elevation of 870 m above sea level. Triadimefon was applied with a jet sprayer at 267 g a.i.ha(-1) and 200 g a.i.ha(-1) at Fanar and Raouda, respectively. Reconstituted soil columns (600 x 30 mm) glasses, were used to study the fungicide movement and metabolism in the two soils. Analyses of triadimefon and its metabolites were carried out using gas chromatography (GC) and high performance liquid chromatography (HPLC). The results indicated a weak reversibility of the adsorbed fraction in the clay soil. Clay is considered an important factor in triadimefon adsorption. Triadimefon mobility in the sandy-loam soil was relatively high in comparison with behavior in the clay soil where about half of the applied fungicide was detected in the upper 25 cm of soil, six days after treatment. Rapid degradation of triadimefon to triadimenol was observed in the two soils. The observed half-life was 8 days in sandy-loam and 13 days in clay soils.     

Biological degradation and microbial function effect of norfloxacin in a soil under different conditions

This paper investigated the degradation kinetics of norfloxacin in a soil, and its effects on soil respiration and nitrogen transformation under different conditions. Compared to the sterile control, the degradation rates of norfloxacin in the non-sterile soil were greatly enhanced, suggesting that microorganisms played a major role in the degradation. Accelerated degradation for norfloxacin in the soil was observed with decreasing concentrations (30 mg/kg to 5 mg/kg) with its half-life decreasing from 62 days to 31 days. Amending swine manure into the soil and increasing the soil moisture level enhanced the biological degradation of norfloxacin. No obvious inhibition of norfloxacin on soil respiration was observed in the soil, while only slight effect on nitrogen transformation was found. The results suggested that norfloxacin at the reported environmental concentrations (<100 mg/kg) would have little effect on microbial activity and functions in the soils.     

Variations of Bacterial Community Structure in Flooded Paddy Soil Contaminated with Herbicide Quinclorac

The denaturing gradient gel electrophoresis (DGGE) method was applied to determine the relative genetic complexity of microbial communities in flooded paddy soil treated with herbicide quinclorac (3,7-dichloro-8-quinoline-carboylic acid). The results obtained showed a significant effect of quinclorac on the development of bacterial populations in soils contaminated with different concentrations of the herbicide at the early time after application. In general, however, the number of populations of the same soil sample treated with the same concentration of the quinclorac differed obviously with increasing incubation time within the early 8 weeks. The scale of differences in banding patterns-showed that the microbial community structures of the quinclorac-treated and non-quinclorac-treated soils were not significantly different after 21 weeks of incubation. Quantification, as demonstrated in this paper, was studied by establishing dose-response relationships. Significant pattern variations were quantified. Prominent DGGE bands were excised, cloned and sequenced to gain insight into the identities of predominant bacterial populations. The majority of DGGE band sequences were related to bacterial genera Clostridium, Sphingobacterium, Xanthomonas and Rhodococcus.     

Preliminary ecotoxicological characterization of a new energetic substance, CL-20

A new energetic substance hexanitrohexaazaisowurtzitane (or CL-20) was tested for its toxicities to various ecological receptors. CL-20 (epsilon-polymorph) was amended to soil or deionized water to construct concentration gradients. Results of Microtox (15-min contact) and 96-h algae growth inhibition tests indicate that CL-20 showed no adverse effects on the bioluminescence of marine bacteria Vibrio fischeri and the cell density of freshwater green algae Selenastrum capricornutum respectively, up to its water solubility (ca. 3.6 mg l(-1)). CL-20 and its possible biotransformation products did not inhibit seed germination and early seedling (16-19 d) growth of alfalfa (Medicago sativa) and perennial ryegrass (Lolium perenne) up to 10,000 mg kg(-1) in a Sassafras sandy loam soil (SSL). Indigenous soil microorganisms in SSL and a garden soil were exposed to CL-20 for one or two weeks before dehydrogenase activity (DHA) or potential nitrification activity (PNA) were assayed. Results indicate that up to 10,000 mg kg(-1) soil of CL-20 had no statistically significant effects on microbial communities measured as DHA or on the ammonium oxidizing bacteria determined as PNA in both soils. Data indicates that CL-20 was not acutely toxic to the species or microbial communities tested and that further studies are required to address the potential long-term environmental impact of CL-20 and its possible degradation products.     

Effects of copper fungicide residues on the microbial function of vineyard soils

The use of copper-based fungicides leads to an accumulation of copper (Cu) in vineyard soils, potentially causing adverse effects to the microbial function and fertility of the soil. This study used a soil microcosm approach to assess the effects of Cu accumulation on microbial function in vineyard soils. Surface soil samples were collected from 10 vineyards and a number of un-impacted reference sites in each of three different viticultural regions of Australia. The field-collected soils were transferred to microcosms and maintained for up to 93 days in the laboratory at 20–22 °C and 60 % of their maximum water-holding capacity. The microbial function of the soils was indicated by measuring phosphomonoesterase, arylsulfatase, urease, and phenol oxidase activities. In general, the vineyard soils had greater concentrations of Cu and lower enzyme activities than in the reference soils, although a weak negative relationship between Cu and enzyme activity could only be found for phosphomonoesterase activity. The results show that soil physical–chemical properties (i.e., organic carbon, pH) are greater determinants of soil enzyme activity than increased soil Cu concentration at the Cu concentrations present in vineyard soils.     

The EDTA effect on phytoextraction of single and combined metals-contaminated soils using rainbow pink (Dianthus chinensis)

Rainbow pink (Dianthus chinensis), a potential phytoextraction plant, can accumulate high concentrations of Cd from metal-contaminated soils. The soils used in this study were artificially added with different metals including (1) CK: original soil, (2) Cd-treated soil: 10 mg Cd kg(-1), (3) Zn-treated soil: 100 mg Zn kg(-1), (4) Pb-treated soil: 1000 mg Pb kg(-1), (5) Cd-Zn-treated soil: 10 mg Cd kg(-1) and 100 mg Zn kg(-1), (6) Cd-Pb-treated soil: 10 mg Cd kg(-1) and 1000 mg Pb kg(-1), (7) Zn-Pb-treated soil: 100 mg Zn kg(-1) and 1000 mg Pb kg(-1), and (8) Cd-Zn-Pb-treated soil: 10 mg Cd kg(-1), 100 mg Zn kg(-1), and 1000 mg Pb kg(-1). Three concentrations of 2Na-EDTA solutions (0 (control), 2, and 5 mmol kg(-1) soil) were added to the different metals-treated soils to study the influence of applied EDTA on single and combined metals-contaminated soils phytoextraction using rainbow pink. The results showed that the Cd, Zn, Pb, Fe, or Mn concentrations in different metals-treated soil solutions significantly increased after applying 5 mmol EDTA kg(-1) (p<0.05). The metal concentrations in different metals-treated soils extracted by deionized water also significantly increased after applying 5 mmol EDTA kg(-1) (p<0.05). Because of the high extraction capacity of both 0.005 M DTPA (pH 5.3) and 0.05 M EDTA (pH 7.0), applying EDTA did not significantly increase the Cd, Zn, or Pb concentration in both extracts for most of the treatments. Applying EDTA solutions can significantly increase the Cd and Pb concentrations in the shoots of rainbow pink (p<0.05). However, this was not statistically significant for Zn because of the low Zn concentration added into the contaminated soils. The results from this study indicate that applying 5 mmol EDTA kg(-1) can significantly increase the Cd, Zn, or Pb concentrations both in the soil solution or extracted using deionized water in single or combined metals-contaminated soils, thus increasing the accumulated metals concentrations in rainbow pink shoots. The proposed method worked especially well for Pb (p<0.05). The application of 2 mmol EDTA kg(-1) might too low to enhance the phytoextraction effect when used in silty clay soils.     

Degradation and sorption of imidacloprid in dissimilar surface and subsurface soils

Degradation and sorption/desorption are important processes affecting the leaching of pesticides through soil. This research characterized the degradation and sorption of imidacloprid (1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine) in Drummer (silty clay loam) and Exeter (sandy loam) surface soils and their corresponding subsurface soils using sequential extraction methods over 400 days. By the end of the incubation, approximately 55% of imidacloprid applied at a rate of 1.0 mg kg^?1 degraded in the Exeter sandy loam surface and subsurface soils, compared to 40% of applied imidacloprid within 300 days in Drummer surface and subsurface soils. At the 0.1 mg kg^?1 application rate, dissipation was slower for all four soils. Water-extractable imidacloprid in Exeter surface soil decreased from 98% of applied at day 1 to > 70% of the imidacloprid remaining after 400 d, as compared to 55% in the Drummer surface soil at day 1 and 12% at day 400. These data suggest that imidacloprid was bioavailable to degrading soil microorganisms and sorption/desorption was not the limiting factor for biodegradation. In subsurface soils > 40% of ¹⁴C-benzoic acid was mineralized over 21 days, demonstrating an active microbial community. In contrast, cumulative ¹⁴CO₂ was less than 1.5% of applied ¹⁴C-imidacloprid in all soils over 400 d. Qualitative differences in the microbial communities appear to limit the degradation of imidacloprid in the subsurface soils.     

Interactions between a polycyclic aromatic hydrocarbon mixture and the microbial communities in a natural freshwater sediment

The toxicity of a polycyclic aromatic hydrocarbon (PAH) mixture was assessed on the indigenous microbial communities of a natural freshwater sediment. The fate and effects of the PAH mixture (phenanthrene, fluoranthene and benzo(k)fluoranthene) were studied over 28 days. Bacterial communities were described by bacterial counts (total bacteria and viable bacteria), and by some hydrolytic enzyme activities (beta-glucosidase and leucine-aminopeptidase), PAH concentrations were measured in the overlying waters and in the sediments. No effect of PAH was detected at 30 mg/kg for all bacterial parameters. At 300 mg/kg, the quantity of total bacteria and the proportion of viable bacteria markedly decreased, compared to the control (0 mg PAH/kg). At 300 mg/kg, an increase of the beta-glucosidase activity and a decrease of the leucine-aminopeptidase activity were observed. For all treatments, the benzo(k)fluoranthene concentration in the sediment was stable over 28 days whereas, in the same time, only 3-6% of the initial concentrations of phenanthrene and fluoranthene remained. This study shows that (1) PAH induce perturbations of sediment microbial communities in terms of density and metabolism (but not always as an inhibition), (2) indigenous bacteria of sediments might be used for toxicity assessment of specific organic pollutants, (3) native microorganisms of sediment seem to have a high capacity for PAH degradation, depending on the physico-chemical properties and the bioavailability of the substance encountered.     

Application of 16S rDNA-PCR amplification and DGGE fingerprinting for detection of shift in microbial community diversity in Cu-, Zn-, and Cd-contaminated paddy soils

Seven soils were sampled from farmland at different distances (0.01-5 km) from a copper and zinc smelter. The total contents of heavy metals in these soils ranged from 46 to 4895 mg Cu kg-1, 96 to 1133 mg Zn kg-1, and 6.9 to 28.8 mg Cd kg-1, respectively. The available fractions were highly correlated with total contents of the metals. In order to assess the impact of combined contamination of heavy metals on soil bacterial communities, denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR) amplicons of 16S rDNA sequence of bacteria in soil was used. Bacterial community structure was affected to some extent by heavy metals. The number of DGGE bands in soils increased with increasing distance from the copper and zinc smelter. Clustering analysis of the DGGE profiles showed that bacteria in the seven soils belonged to three clusters. Bacterial communities in three soils sampled at 0.01-0.60 km from the smelter belonged to one cluster, and those in three soils sampled at 0.8-1.2 km from the smelter belong to another cluster. Bacterial community in soil farthest from the smelter belonged to a single cluster. This study demonstrated that heavy metal contamination decreased both biomass and diversity of bacterial community in the soil.     

Effects of incubation on solubility and mobility of trace metals in two contaminated soils

Much research has focused on changes in solubility and mobility of trace metals in soils under incubation. In this experiment, changes in solubility and mobility of trace metals (Pb, Cu and As) and Fe in two contaminated soils from Tampa, Florida and Montreal, Canada were examined. Soils of 30 g were packed in columns and were incubated for 3-80 days under water-flooding incubation. Following incubation, metal concentrations in pore water (water soluble) and in 0.01 M CaCl2 leachates (exchangeable+water soluble) were determined. While both soils were contaminated with Pb (1600-2500 mg kg(-1)), Tampa soil was also contaminated with As (230 mg kg(-1)). Contrast to the low pH (3.8) of Tampa soil, Montreal soil had an alkaline pH of 7.7 and high Ca of 1.6%. Concentrations of Fe(II) increased with incubation time in the Tampa soil mainly due to reductive Fe dissolution, but decreased in the Montreal soil possibly due to formation of FeCO3. The inverse relationship between concentrations of Pb and Fe(II) in pore water coupled with the fact that Fe(II) concentrations were much greater than those of Pb in pore water may suggest the importance of Fe(II) in controlling Pb solubility in soils. However, changes in concentrations of Fe(II), Pb, Cu and As in pore water with incubation time were similar to those in leachate, i.e. water soluble metals were positively related to exchangeable metals in the two contaminated soils. This research suggests the importance of Fe in controlling metal solubility and mobility in soils under water-flooded incubation.     

Effects of lead contamination on soil microbial activity and rice physiological indices in soil-Pb-rice (Oryza sativa L.) system

The effect of lead (Pb) treatment on the soil microbial activities (soil microbial biomass and soil basal respiration) and rice physiological indices were studied by greenhouse pot experiment. Pb was applied as lead acetate at six different levels in two different paddy soils, namely 0 (control), 100, 300, 500, 700, 900 mg kg-1 soil. The results showed that the application of Pb at lower level (<300 mg kg-1) as lead acetate resulted in a slight increase in soil microbial activities compared with the control, and had an inhibitory influence at high concentration (>500 mg Pb kg-1 soil), which might be the critical concentration of Pb causing a significant decline in the soil microbial activities. However, the degree of influence on soil microbial activities by Pb was related to the clay and organic matter contents of the soils. On the other hand, when the level of Pb treatments increased to 500 mg kg-1, there was ecological risk for both soil microbial activities and plants. The results also revealed that there was a consistent trend that the chlorophyll contents increased initially, and then decreased gradually with increase in Pb concentration. Pb was effective in inducing proline accumulation and its toxicity causes oxidative stress in rice plants. In a word, soil microbial activities and rice physiological indices, therefore, may be sensitive indicators reflecting environmental stress in soil-Pb-rice system.