The combined effects of atrazine and lead (Pb): relative microbial activities and herbicide dissipation

Characteristics of bacterial community and extracellular enzymes in response to atrazine application in black soil

The ecological functioning of black soil largely depends on the activities of various groups of microorganisms. However, little is known about how atrazine, a widely used herbicide with known harmful effects on the environment, influences the microbial ecology of black soil, and the extracellular enzymes related to the carbon, nitrogen and phosphorus cycles. Here, we evaluated the change in extracellular enzymes and bacterial community characteristics in black soil after exposure to various concentrations of atrazine. Low concentrations of applied atrazine (10 - 20 mg kg-1) were almost completely degraded after 120 days. At high concentrations (80 - 100 mg kg-1), about 95% of the applied atrazine was degraded over the same period. Additionally, linear fitting of data indicated that the total enzymatic activity index (TEI) and bacterial α-diversity index were negatively correlated with atrazine applied concentration. The atrazine had a greater effect on bacterial beta diversity after 120 days, which differentiated species clusters treated with low and high atrazine concentrations. Soil bacterial community structure and function were affected by atrazine, especially at high atrazine concentrations (80 - 100 mg kg-1). Key microorganisms such as Sphingomonas and Nocardioides were identified as biomarkers for atrazine dissipation. Functional prediction indicated that most metabolic pathways might be involved in atrazine dissipation. Overall, the findings enhance our understanding of the factors driving atrazine degradation in black soil and supports the use of biomarkers as indicators of atrazine dissipation.

Changes of atrazine dissipation and microbial community under coexistence of graphene oxide in river water

The coexistence of herbicide atrazine (ATZ) and the nanomaterial graphene oxide (GO) in natural water bodies will be an inevitable scenario due to their widespread application and consequent release into aquatic ecosystems. But the dissipation of ATZ with GO and the response of the microbial community to their combination are still not clear. Here, we investigated the dissipation dynamics and transformation of ATZ with and without GO in river water after 21-d incubation. In the presence of GO, ATZ residue significantly decreased by 11%-43%; the transformation of ATZ markedly increased by 11%-17% when ATZ concentrations were not above 1.0 mg∙L-1. The direct adsorption of ATZ on GO, mainly via π-π interactions, proton transfer and hydrogen bonding, contributed 54%-68% of the total increased ATZ dissipation by GO. ATZ and ATZ+GO exerted effects of similar magnitude on microbial OTU numbers with an increase of bacterial diversity. The coexisting GO increased the relative abundance of ATZ-degradation bacteria and Chitinophagales, thus improving ATZ transformation. This work indicated that the coexistence of GO at environmentally relevant concentrations can effectively reduce ATZ residues and promote the transformation of ATZ to degradation products in river water; nevertheless, the potential risk of GO acting as an ATZ carrier should be given more prominence.

Phytoremediation of cadmium-trichlorfon co-contaminated water by Indian mustard (Brassica juncea): growth and physiological responses

In this study, the morphological and physiological responses of Brassica juncea to the stresses of Cadmium (Cd) and trichlorfon (TCF), and the phytoremediation potential of B. juncea to Cd and TCF were investigated under hydroponics. Results showed that Cd exhibited strong inhibition on biomass and root morphology of B. juncea as Cd concentration increased. The chlorophyll a fluorescence intensity and chlorophyll content of B. juncea decreased with the increased Cd concentration, whereas the malondialdehyde and soluble protein contents and superoxide dismutase activity increased. TCF with different concentrations showed no significant influence on these morphological and physiological features of B. juncea. The biomass and physiological status of B. juncea were predominantly regulated by Cd level under the co-exposure of Cd and TCF. B. juncea could accumulate Cd in different plant parts, as well as showed efficient TCF degradation performance. A mutual inhibitory removal of Cd and TCF was observed under their co-system. The present study clearly signified the physiological responses and phytoremediation potential of B. juncea toward Cd and TCF, and these results suggest that B. juncea can be used as an effective phytoremediation agent for the Cd-TCF co-contamination in water.

Ecotoxicity of soil Pb pollution reflected by soil β-glucosidase: Comparison of extracellular and intracellular enzyme pool

Lead (Pb) is a major environmental pollutant that threatens the soil environment and human health. Monitoring and assessing Pb toxicity on soil health are of paramount importance to the public. To use soil enzymes as biological indicators of Pb contamination, herein, the responses of soil β-glucosidase (BG) in different pools of soil (total, intracellular and extracellular enzyme) to Pb contamination were investigated. The results indicated that the intra-BG (intracellular BG) and extra-BG (extracellular BG) responded differently to Pb contamination. While the addition of Pb caused a significant inhibition of the intra-BG activities, the extra-BG activities were only slightly inhibited. Pb showed a non-competitive inhibition to extra-BG, while both non-competitive and uncompetitive inhibition were observed for intra-BG in the tested soils. The dose-response modeling was used to calculate ecological dose ED₁₀, which represents the concentration of Pb pollutant that causes a 10 % reduction in V_max, to express the ecological consequences of Pb contamination. A positive correlation was found between ecological dose ED₁₀ values of intra-BG and soil total nitrogen (p < 0.05), which suggests soil properties may influence Pb toxicity to soil BG. Based on the differences in ED₁₀ and inhibition rate among different enzyme pools, this study suggests that the intra-BG is more sensitive for Pb contamination assessment. From this, we propose that intra-BG should be considered when evaluating Pb contamination using soil enzymes as indicators.

Effects of atrazine on microbial metabolic limitations in black soils: Evidence from enzyme stoichiometry

Long-term input of agricultural chemicals such as pesticides into the soil can increase soil pollution, thereby affecting the productivity and quality of black soil. Triazine herbicide atrazine has been shown to have long-lasting residual effects in black soil. The atrazine residues affected soil biochemical properties, further leading to microbial metabolism restriction. It is necessary to explore the strategies to mitigate the limitations on microbial metabolism in atrazine-contaminated soils. Here, we evaluated the effect of the atrazine on microbial nutrient acquisition strategies as indicated by extracellular enzyme stoichiometry (EES) in four black soils. Atrazine degradation in soil followed the first-order kinetics model across various concentrations ranging from 10 to 100 mg kg^-1. We found that the atrazine was negatively correlated with the EES for C-, N-, and P-acquisition. Vector lengths and angles decreased and increased significantly with an increase of atrazine concentration in tested black soils except for Lishu soils. Moreover, the vector angles were >45° for tested four black soils, indicating that atrazine residue had the greatest P-limitation on soil microorganisms. Interestingly, microbial C- and P-limitations with different atrazine concentrations showed a strong linear relationship, especially in Qiqihar and Nongan soils. Atrazine treatment significantly negatively affected microbial metabolic limitation. Soil properties and EES interaction explained up to 88.2% for microbial C-/P-limitation. In conclusion, this study confirms the EES as a useful method in evaluating the effects of pesticides on microbial metabolic limitations.

Impact of Paenarthrobacter ureafaciens ZF1 on the soil enzyme activity and microbial community during the bioremediation of atrazine-contaminated soils

Bioremediation of atrazine-contaminated soil is considered a safe and effective approach in removing contaminates from the soil. However, the effects of adding foreign organisms to assist bioremediation on soil environmental quality and ecosystem are unclear. Here, the ecological remediation potential of strain Paenarthrobacter ureafaciens ZF1 on atrazine-contaminated soil was investigated through miniature experiments using variations in soil enzymes and bacterial communities as indicators. The results showed that strain ZF1 accelerated atrazine degradation, which could completely degrade atrazine at concentrations of 100 mg·L^− 1 atrazine within 2 h in liquid medium and could remove up to 99.3% of atrazine (100 mg·kg^− 1 in soil) within 6 days. During soil bioremediation, atrazine promoted the activities of urease and cellulase, and inhibited the activities of sucrase and catalase, while the strain ZF1 significantly promoted the activities of these four enzymes. High-throughput sequencing of 16S rRNA genes showed that ZF1 affected the relative abundance and bacterial community structure, and promoted bacterial diversity and evenness. Furthermore, redundancy analysis revealed a certain correlation among the strain ZF1, atrazine residue, soil enzyme activity, and soil bacterial community. The strain ZF1 in this work demonstrated remarkable potential for ecological restoration, and can be an effective and environmentally friendly alternative in remediating atrazine-contaminated soil.

Ameliorative effects of biochar on persistency, dissipation, and toxicity of atrazine in three contrasting soils

The presence of atrazine a persistent herbicide in soil poses a serious threat to the ecosystem. The biochar amendment in soil altered the fate of this herbicide by modifying the soil properties. The present study examines the dissipation and toxicity of atrazine in three contrasting soils (silty clay, sandy loam, and sandy clay) without and with biochar amendment (4%). The experiment was performed for 150 days with three application rates of atrazine (4, 8, and 10 mg kg^-1). The speciation and degradation of atrazine, metabolite content, microbial biomass, and enzymatic activities were evaluated in all treatments. Three kinetic models and soil enzyme index were calculated to scrutinize the degradation of atrazine and its toxicity on soil biota, respectively. The goodness of fit statistical indices suggested that the first-order double exponential decay (FODE) model best described the degradation of atrazine in silty clay soil. However, a single first order with plateau (SFOP) was best fitted for atrazine degradation in sandy loam and sandy clay soils. The half-life of atrazine was higher in sandy clay soil (27-106 day^-1) than silty clay (28-77 day^-1) and sandy loam soil (27-83 day^-1). The variations in the dissipation kinetics and half-life of the atrazine in three soil were associated with atrazine partitioning, availability of mineral content (silica, aluminum, and iron), and soil microbial biomass carbon. Biochar amendment significantly reduced the plateau in the kinetic curve and also reduced the atrazine toxicity on soil microbiota. Overall, biochar was more effective in sandy clay soil for the restoration of soil microbial activities under atrazine stress due to modulation in the pH and more improved soil quality.

Study on the spatial distribution of ureolytic microorganisms in farmland soil around tailings with different heavy metal pollution

Ureolytic microorganisms, a kind of microorganism which can secrete urease and decompose urea, have great potential in remediation of soil heavy metals based on microbial induced carbonate precipitation. However, the horizontal and vertical distribution of ureolytic microbial community in heavy metals contaminated soils is poorly understood. In this study, urease genes in agricultural soils surrounding tailings were first investigated using metagenomic in two dimensions: heavy metal pollution (Low-L, Middle-M, High-H) and soil depth (0-20 cm, 20-40 cm, 40-60 cm, 60-80 cm, 80-100 cm). Results showed that the effect of heavy metal concentration on ureolytic microorganisms was indeed significant, while the changes of ureolytic microorganisms with increasing soil depth varied in the vertical direction at the same level of heavy metal contamination. H site had the highest diversity of ureolytic microorganisms except for the topsoil. And at the same heavy metal contamination level, the ureolytic microbial diversity was lower in deeper soils. Proteobacteria, Actinobacteria and Thaumarchaeota (Archaea) were the dominant phyla of ureolytic microorganisms in all three sites, accounting for more than 80% of the total. However, the respond to the heavy metal concentrations of three phyla were different, which were increasing, decreasing and essentially unchanged, respectively. Besides, other environmental factors such as SOM and pH had different effects on ureolytic microorganisms, with Proteobacteria being positively correlated and Actinobacteria being the opposite. Another phenomenon was that Actinobacteria and Verrucomicrobia were biomarkers of group L, which could significantly explain the difference with the other two sites. These results provided valuable information for further research on the response mechanism and remediation of heavy metal pollution by ureolytic microbial system.

Influence of Cd on atrazine degradation and the formation of three primary metabolites in water under the combined pollution

To understand the influence of Cd on atrazine (ATZ) degradation in aqueous solution, the degradation of different initial levels of ATZ (0.1, 0.5, 1.0, and 2.0 mg·L^-1) was investigated in the presence and absence of Cd²⁺ in a 20-day laboratory experiment. It was found that Cd²⁺ caused a significant decrease in ATZ degradation and increased its half-life from 17-34 days to 30-57 days (p < 0.0001). Regarding the three most common metabolites of ATZ, deethylatrazine (DEA) and deisopropylatrazine (DIA) were detected in water earlier than hydroxyatrazine (HYA). The DEA content was several times higher than the DIA and HYA contents, regardless of the presence or absence of Cd²⁺. In the presence of Cd²⁺, the DIA content was significantly lower and the HYA content was significantly higher. Furthermore, Cd²⁺ had a dose-dependent effect on HYA formation. Our results indicated that the coexistence of Cd²⁺ and ATZ resulted in greater herbicide persistence, thereby possibly increasing the risk of environmental contamination. DEA was still the predominant ATZ degradation product detected in water under the combined pollution, which was similar to the ATZ tendency.

Effects of the long-term application of atrazine on soil enzyme activity and bacterial community structure in farmlands in China

Atrazine has been used on Chinese farmlands for a long time and over a wide range. The concentration of atrazine (1.86-1100 mg kg^-1) has exceeded the allowable limit in the soil (1.0 mg kg^-1), and concern is increasing about the potential harm to farmland soil. Four treatments (AT₀, AT₆, AT₁₀, AT₁₆) were established to reveal the effects of the long-term application of atrazine on soil health. The results showed a nonlinear regulation of the atrazine residue concentrations in the four treatments. The highest concentration of atrazine residue was in AT₆, at 167 mg kg^-1, and the lowest concentration of atrazine residue was in AT₁₆, at 102 mg kg^-1, but there was no significant difference between AT₁₀ and AT₁₆. The soil urease activity decreased significantly with the increase in the years of atrazine application, the saccharase and cellulase activities in the AT₆ were significantly higher than those observed in the other three treatments, the catalase activity gradually decreased with the increase in atrazine application years, and the activity in AT₆ was significantly higher than that in AT₁₆. A total of 238 genera were identified by Illumina MiSeq sequencing, and 28 dominant genera were screened. Atrazine significantly increased the relative abundance of Actinobacteria and contributed to the relative abundance of Rubrobacter, Blastococcus, Promicromonospora, Jiangella, Psychroglaciecola and Acetobacteraceae_uncultured, which exhibited significantly higher abundance in AT₁₆ than in AT₀. Although there were atrazine-degrading bacteria in the soil, and the atrazine residue decreased with the increase in application years, the concentration of the atrazine residue was still nearly 100 times higher than the allowable limit in the soil, which is a great threat to the soil health.

Dissipation of chlorothalonil in the presence of chlortetracycline and ciprofloxacin and their combined effects on soil enzyme activity

The long-term application of substantial amounts of fungicides and antibiotic-polluted organic manure (OM) in greenhouse has caused the co-existence of fungicides and antibiotics in soils. However, little is known about the effects of antibiotics on the persistence of fungicides in soils or their combined effects on soil enzyme activity. In this study, fungicide chlorothalonil (CTL) alone and in combination with antibiotic chlortetracycline (CTC) or ciprofloxacin (CIP) were repeatedly added to OM-amended soil to investigate the changes in the residual characteristics of CTL and in soil dehydrogenase and urease activity. The results showed that CTL rapidly dissipated in soils with the corresponding half-lives of 0.9-3.2, which initially increased, then decreased and finally stabilized with an increased treatment frequency. The dissipation of CTL was inhibited by CTC and CIP during the first several treatments. The soil dehydrogenase and urease activity in CTL-treated soils was inhibited during the first six treatments and then recovered afterwards. Compared with the OM-amended soil+CTL treatment, the OM-amended soil+CTL+CTC and OM-amended soil+CTL+CIP treatments had stronger inhibitory effects on soil enzyme activity during the first six repeated treatments but exhibited slight stimulating effects afterwards. Therefore, the results obtained in this study suggested that the long-term co-existence of CTL, CTC, and CIP altered the dissipation characteristics of CTL in soil and affected the soil enzyme activity levels. The prudent application of large and frequent of fungicides and OM-containing antibiotic residues in greenhouses should therefore be carefully considered in order to reduce the long-term combined pollution in soils.

Bioremediation of cadmium-trichlorfon co-contaminated soil by Indian mustard (Brassica juncea) associated with the trichlorfon-degrading microbe Aspergillus sydowii: Related physiological responses and soil enzyme activities

Soil co-contaminated with heavy metals and organics is often difficult to remediate. In this study, pot experiments were conducted to investigate the concurrent removal of cadmium (Cd, two levels: Cd_L [10 mg kg^-1] and Cd_H [50 mg kg^-1]) and trichlorfon (TCF, 100 mg kg^-1) from co-contaminated soil by comparing the following remediation methods: natural remediation (NR), soil inoculated with Aspergillus sydowii (AS), soil planted with Brassica juncea (BJ), and soil planted with B. juncea and inoculated with A. sydowii (BJ-AS). The physiological responses of B. juncea and soil enzyme activities after remediation were also studied. B. juncea grew well in co-contaminated soil at both Cd levels. The biomass and chlorophyll content of B. juncea in Cd_H soil were lower than those in Cd_L soil, whereas the malondialdehyde content and activities of catalase, peroxidase and superoxide dismutase of B. juncea in Cd_H soil were higher than those in Cd_L soil. Cd accumulation in B. juncea was high in Cd_H soil, whereas high Cd removal efficiency was observed in Cd_L soil. TCF could be thoroughly degraded within 35 days in NR at both Cd-level soils. AS, BJ and BJ-AS promoted TCF degradation and enhanced the activities of catalase, urease, sucrase and alkaline phosphatase in soil compared with the NR. BJ-AS showed the highest phytoextraction ratio (3.32% in Cd_L and 1.34% in Cd_H soil) and TCF degradation rate (half-life of 2.18 and 2.37 days in Cd_L and Cd_H soil, respectively). These results demonstrate that BJ-AS could effectively remove Cd and TCF from soil and is thus a feasible technology for the bioremediation of these co-contaminated soil.

Contamination of pyrethroids and atrazine in greenhouse and open-field agricultural soils in China

A national-scale survey was conducted to assess the levels and distribution of two extensively used pesticides (pyrethroids and atrazine) in greenhouse and open-field soils in 20 provinces across China. Concentrations between 1.30 and 113 ng/g and 0.51-85.4 ng/g for the total pyrethroids (PYs) and of LOD-137 ng/g and LOD-134 ng/g for atrazine were found in greenhouse and open-field soils, respectively. Higher contaminations were found in the greenhouse than in the open fields. The levels of total pyrethroids in 80% of the greenhouses and of atrazine in 60% of the greenhouses were significantly higher than those in the nearby open-field soils (p < 0.05), respectively. The contamination of PYs and atrazine was generally more serious in the northern provinces of China, such as Heilongjiang, Jilin, Liaoning, Beijing, and Hebei. Pearson correlation analysis revealed that the contamination of PYs was significantly correlated with the soil total organic carbon (TOC) value in both greenhouse and open-field soils. Canonical correspondence analysis (CCA) showed that PYs might have an impact on the microbial alpha diversity, while cyhalothrin and cypermethrin may be the key factors affecting the microbial community in the greenhouse and open-field soils. The soil samples containing pesticide residues showed distinct taxonomic and functional communities, where an increased diversity and abundance of microorganisms able to degrade pesticides was observed with high-level PYs contamination. These findings provide useful information for evaluating PYs and atrazine pollution and for contamination management in greenhouse agriculture.

Composition and function of soil fungal community during the establishment of Quercus acutissima (Carruth.) seedlings in a Cd-contaminated soil

This study was designed to explore the functions of soil fungal communities in the Cd tolerance of Q. acutissima seedling. Three Cd levels of 15, 30, and 40 mg kg^-1 were set up using the soils collected from Q. acutissima forests. The benomyl was applied to inhibit the fungal communities in the soil. Following a 100-day pot cultivation of Q. acutissima seedlings, the plant growth, Cd content, N uptake, and fungal communities were evaluated. The results showed that the root dry weights were significantly reduced after the benomyl addition at the Cd concentrations of 30 and 40 mg kg^-1. Root fungi colonization was enhanced under higher Cd concentrations when soil fungi are present (without the benomyl treatment). The fungi associated with root increased the Cd accumulation in the roots while decreased the Cd transfer to the shoot at 40 mg Cd kg^-1. The ¹⁵N enrichment in root tip was positively correlated with enzyme activities of soil catalase and urease. And the activities of acid phosphatase, catalase, and urease were inhibited at each Cd level. The abundance of the dominant fungal genus differed in their response to Cd contamination. The ectomycorrhizal fungi of Tomentella and Cortinarius were identified under the higher Cd levels (40 mg kg^-1). Our results implied Tomentella and Cortinarius could be applied to enhance the capacity of Quercus acutissima in the bioremediation of Cd polluted soil.

Simultaneous degradation of trichlorfon and removal of Cd(II) by Aspergillus sydowii strain PA F-2

Co-contamination with heavy metals and pesticides is a severe environmental problem, but little information is available regarding the simultaneous removal of these pollutants. In this study, we showed that Aspergillus sydowii strain PA F-2 isolated from soil contaminated with heavy metal and pesticides can simultaneously degrade trichlorfon (TCF) and adsorb Cd(II) from mineral salt medium. The maximum removal rates for TCF and Cd(II) were 55.52% and 57.90%, respectively, in the treatment containing 100 mg L^-1 TCF and 2 mg L^-1 Cd(II). As the initial Cd(II) concentration increased (2, 5, and 10 mg L^-1), the PA F-2 biomass, TCF degradation rate, and Cd(II) adsorption efficiency decreased, whereas the Cd(II) adsorption capacity by PA F-2 increased. The addition of exogenous glucose and sucrose significantly increased the PA F-2 biomass as well as the removal of TCF and Cd(II). Moreover, the TCF degradation pathway and Cd(II) adsorption mechanism were investigated by gas chromatography-mass spectrometry, scanning electron microscopy, and Fourier transform infrared spectroscopy. These results suggest that PA F-2 has potential applications in the bioremediation of TCF and Cd(II) co-contamination.

Zn2+ and Cd2+ assisted photo-catalytic degradation of chlorpyrifos in soil

The Cd²⁺ and Zn²⁺ assisted photo-catalytic degradation of soil incorporated chlorpyrifos (CLP) was reported in current study. The soil samples fortified with CLP and metals were irradiated in photo-reactor for different time intervals to check maximum degradation. Soil samples extracted with acetonitrile were analyzed by HPLC. The results of the study revealed a complete mineralization of insecticide from soil that followed first–order Langmuir-Hinshelwood (L-H) kinetic model. The CLP degradation rate in soil was higher in photoreactor than control with variation in half-life from 41 days to 20 days. The degradation of CLP in photoreactor was 5 fold augmented after Zn²⁺ fortification of soil while Cd²⁺ had negligible effect on CLP photodegradation. Thus Zn²⁺ fortification of soil will not only replenish the important nutrient for plant growth but will also help in alleviating the harmful effects of CLP on soil flora and fauna by enhancing its rate of photodegradation.

Combined pollution of copper nanoparticles and atrazine in soil: Effects on dissipation of the pesticide and on microbiological community profiles

Copper nanoparticles (NCu) have been proposed as an antimicrobial agent in agriculture. Therefore, NCu may interact with numerous pollutants including pesticides. Little is known about the combined effects of NCu and pesticides in soil. This study aimed at assessing the impact of NCu combined with the herbicide atrazine (ATZ) on soil. We focused on assessing the adsorption and dissipation of ATZ in the presence of NCu and the changes in microbial community profiles. First, ATZ adsorption isotherms (described using the Freundlich equation) were evaluated. After that, soil samples were spiked with NCu (40-60 nm) at 0.05 and 0.15% w/w and ATZ (3 mg a.i kg^-1) and incubated for 30 days. The results showed that ATZ adsorption is favored by the presence of NCu. On the other hand, NCu at 0.15% w/w caused a significant decrease in ATZ dissipation, increasing its half-life from 6 to 37 days. Microbial community profiles (bacteria, fungi and nitrifying bacteria) remained relatively stable throughout the evaluated period. Therefore, our findings suggest that NCu can increase the persistence of ATZ in soil, which may be mostly associated to physical-chemical interaction with soil particles more than a microbial impact.

Combined effects of antimony and sodium diethyldithiocarbamate on soil microbial activity and speciation change of heavy metals. Implications for contaminated lands hazardous material pollution in nonferrous metal mining areas

The combined effects of antimony (Sb) and sodium diethyldithiocarbamate (DDTC), a common organic flotation reagent, on soil microbial activity and speciation changes of heavy metals were investigated for the first time. The results showed that the exchangeable fraction of Sb was transformed to a stable residual fraction during the incubation period, and the addition of DDTC promoted the transformation compared with single Sb pollution, probably because DDTC can react with heavy metals to form a complex. In addition, the presence of DDTC and Sb inhibited the soil microbial activity to varying degrees. The growth rate constant k of different interaction systems was in the following order on the 28th day: control group ≥ single DDTC pollution > combined pollution > single Sb pollution. A correlation analysis showed that the concentration of exchangeable Sb was the primary factor that affected the toxic reaction under combined pollution conditions, and it significantly affected the characteristics of the soil microorganisms. All the observations provide useful information for a better understanding of the toxic effects and potential risks of combined Sb and DDTC pollution in antimony mining areas.

Elevated CO2 benefits the soil microenvironment in the rhizosphere of Robinia pseudoacacia L. seedlings in Cd- and Pb-contaminated soils

Soil contamination by heavy metals in combination with elevated atmospheric CO₂ has important effects on the rhizosphere microenvironment by influencing plant growth. Here, we investigated the response of the R. pseudoacacia rhizosphere microenvironment to elevated CO₂ in combination with cadmium (Cd)- and lead (Pb)-contamination. Organic compounds (total soluble sugars, soluble phenolic acids, free amino acids, and organic acids), microbial abundance and activity, and enzyme activity (urease, dehydrogenase, invertase, and β-glucosidase) in rhizosphere soils increased significantly (p < 0.05) under elevated CO₂ relative to ambient CO₂; however, l-asparaginase activity decreased. Addionally, elevated CO₂ alone affected soil microbial community in the rhizosphere. Heavy metals alone resulted in an increase in total soluble sugars, free amino acids, and organic acids, a decrease in phenolic acids, microbial populations and biomass, and enzyme activity, and a change in microbial community in rhizosphere soils. Elevated CO₂ led to an increase in organic compounds, microbial populations, biomass, and activity, and enzyme activity (except for l-asparaginase), and changes in microbial community under Cd, Pb, or Cd + Pb treatments relative to ambient CO₂. In addition, elevated CO₂ significantly (p < 0.05) enhanced the removal ratio of Cd and Pb in rhizosphere soils. Overall, elevated CO₂ benefited the rhizosphere microenvironment of R. pseudoacacia seedlings under heavy metal stress, which suggests that increased atmospheric CO₂ concentrations could have positive effects on soil fertility and rhizosphere microenvironment under heavy metals.

Long-term field application of sewage sludge increases the abundance of antibiotic resistance genes in soil

Sewage sludge and manure are common soil amendments in crop production; however, their impact on the abundance and diversity of the antibiotic resistome in soil remains elusive. In this study, by using high-throughput sequencing and high-throughput quantitative PCR, the patterns of bacterial community and antibiotic resistance genes (ARGs) in a long-term field experiment were investigated to gain insights into these impacts. A total of 130 unique ARGs and 5 mobile genetic elements (MGEs) were detected and the long-term application of sewage sludge and chicken manure significantly increased the abundance and diversity of ARGs in the soil. Genes conferring resistance to beta-lactams, tetracyclines, and multiple drugs were dominant in the samples. Sewage sludge or chicken manure applications caused significant enrichment of 108 unique ARGs and MGEs with a maximum enrichment of up to 3845 folds for mexF. The enrichment of MGEs suggested that the application of sewage sludge or manure may accelerate the dissemination of ARGs in soil through horizontal gene transfer (HGT). Based on the co-occurrence pattern of ARGs subtypes revealed by network analysis, aacC, oprD and mphA-02, were proposed to be potential indicators for quantitative estimation of the co-occurring ARGs subtypes abundance by power functions. The application of sewage sludge and manure resulted in significant increase of bacterial diversity in soil, Proteobacteria, Acidobacteria, Actinobacteria and Chloroflexi were the dominant phyla (>10% in each sample). Five bacterial phyla (Chloroflexi, Planctomycetes, Firmicutes, Gemmatimonadetes and Bacteroidetes) were found to be significantly correlated with the ARGs in soil. Mantel test and variation partitioning analysis (VPA) suggested that bacterial community shifts, rather than MGEs, is the major driver shaping the antibiotic resistome. Additionally, the co-occurrence pattern between ARGs and microbial taxa revealed by network analysis indicated that four bacterial families might be potential hosts of ARGs. These results may shed light on the mechanism underlining the effects of amendments of sewage sludge or manure on the occurrence and dissemination of ARGs in soil.

A proposal of "core enzyme" bioindicator in long-term Pb-Zn ore pollution areas based on topsoil property analysis

To study the effects of long-term mining activities on the agricultural soil quality of Mengnuo town in Yunnan province, China, the heavy metal and soil enzyme activities of soil samples from 47 sites were examined. The results showed that long-term mining processes led to point source heavy metal pollution and Pb, Cd, Zn and As were the primary metal pollutants. Polyphenoloxidase was found the most sensitive soil enzyme activity and significantly correlated with almost all the metals (P < 0.05). Amylase (for C cycling), acid phosphatase (for P cycling) and catalase (for redox reaction) activities showed significantly positive correlations (P < 0.05) with Pb, Cd, Zn and As contents. The correlations between soil enzymes activities and Cd, Pb and Zn contents were verified in microcosm experiments, it was found that catalase activity had significant correlations (P < 0.05) with these three metals in short-term experiments using different soils under different conditions. Based on both field investigation and microcosm simulation analysis, oxidoreductases activities (rather than a specific enzyme activity) were suggested to be used as "core enzyme", which could simply and universally indicate the heavy metal pollution degrees of different environments. And hydrolases (for C, N, P and S recycling) could be used as a supplement to improve correlation accuracy for heavy metal indication in various polluted environments.

Pb(II), Cr(VI) and atrazine sorption behavior on sludge-derived biochar: role of humic acids

Pyrolyzing municipal wastewater treatment sludge into biochar can be a promising sludge disposal approach, especially as the produced sludge-derived biochar (SDBC) is found to be an excellent sorbent for heavy metals and atrazine. The aim of this study was to investigate how and why the coexisting humic acids influence the sorption capacity, kinetic, and binding of these contaminants on SDBC surface. Results showed humic acids enhanced Pb(II)/Cr(VI) sorption binding, and increased the corresponding Pb(II) Langmuir sorption capacity at pH 5.0 from 197 to 233 μmol g(-1), and from 688 to 738 μmol g(-1) for Cr(VI) at pH 2.0. It can be mainly attributed to the sorbed humic acids, whose active functional groups can offer the additional sites to form stronger inner-sphere complexes with Pb(2+), and supply more reducing agent to facilitate the transformation of Cr(VI) to Cr(III). However, humic acids reduced the atrazine adsorption Freundlich constant from 1.085 to 0.616 μmol g(-1). The pore blockage, confirmed by the decreased BET-specific surface area, as well as the more hydrophilic surface with more sorbed water molecules may be the main reasons for that suppression. Therefore, the coexisting humic acids may affect heavy metal stabilization or pesticide immobilization during SDBC application to contaminated water or soils, and its role thus should be considered especially when organic residues are also added significantly to increase the humic acid content there.

Atrazine immobilization on sludge derived biochar and the interactive influence of coexisting Pb(II) or Cr(VI) ions

Sludge derived biochars (SDBCs) may have the potential to simultaneously remove heavy metals and organic contaminants in relation to their various active sorption sites for both metal ions and organic compounds. SDBCs have been proven to provide a considerable capacity for immobilizing Pb(II) and Cr(VI) ions in solution, and in this study their ability to sorb atrazine, in addition to their corresponding interactive influences with coexisting metal ions, is extensively investigated. The results indicate that all atrazine adsorption isotherms fit well with the Freundlich equation, and the greatest value of 16.8 mg g(-1) sorption capacity occurred with SDBCs pyrolyzed at 400°C for 2h. The slow sorption kinetics fit well with the Lagergren's 2nd order reaction, and depend upon the initial atrazine concentration, indicating the significance of a site-specific process. The ionic strength-dependence of the atrazine adsorption behavior further consolidates the involvement of the mechanism of the H-bond with hydroxyl groups on SDBC. However, when Pb(II)/Cr(VI) metal ions coexist in solution, they substantially suppress atrazine adsorption, probably because the inner complex between the hydroxyl groups on SDBCs and Pb(II)/Cr(III) ions intrude the weak H-bond with atrazine. As a result, metal adsorption was found to be unaffected by the coexisting atrazine. Therefore, although SDBC is applicable for atrazine removal/immobilization in most of environmentally relevant conditions, a two-step process may be required if heavy metal ions coexist.

The bioavailability and adverse impacts of lead and decabromodiphenyl ether on soil microbial activities

Lead (Pb) and decabromodiphenyl ether (BDE209) are the main pollutants at electronic waste (e-waste) recycling sites (EWRSs), and their potential toxic effects on soil organisms have received extensive attention. However, the impact on soil microorganisms of joint exposure to the two chemicals remains almost unknown. Therefore, indoor incubation tests were performed to explore the adverse impacts of Pb and BDE209 on soil microbial activities and chemical transformation for the first time. The results have demonstrated that BDE209 was barely degraded in all treated groups, indicating that the presence of Pb hardly affected BDE209 dissipation. The fractions analysis according to Tessier suggested that Pb gradually transformed towards more stable fractions in the slightly alkaline soil, thus reducing the bioavailability of Pb. Additionally, increased Pb doses caused significantly higher bioavailability (p < 0.05), and the same trend was clearly observed after simultaneous exposure to BDE209. Generally, single Pb or BDE209 exposure markedly inhibited (p < 0.05 or 0.01) soil microbial biomass C (C mic), while soil basal respiration (SBR) indicated the opposite response trend (inhibition or stimulation for BDE209 or Pb alone, respectively). Compared to the controls, Pb dramatically (p < 0.01) facilitated soil metabolic quotient (qCO2) during the incubation period. After joint exposure to Pb and BDE209, C mic generally declined with increasing exposure concentration, following certain dose-response relationships. However, SBR and qCO2 were highly significantly stimulated (p < 0.01), and more doses of Pb and BDE209 resulted in higher values. The results of these observations have provided a basic understanding of the potential ecological risk of Pb and BDE209 in soil at EWRSs.

Residual effect of pre-emergence herbicides on microbial activities in relation to mineralization of C, N and P in the Gangetic alluvial soil of West Bengal, India

An experiment has been conducted under laboratory conditions to investigate the residual effect of three pre-emergence herbicides (thiobencarb, pendimethalin and pretilachlor) at fivefold field application rates (7.5, 10.0 and 2.5 kg a.i. ha(-1), respectively), on the changes of microbial activities and some biochemical processes in the Gangetic alluvial soil of West Bengal. Application of herbicides in general significantly increased microbial biomass resulting in greater mineralization of C, N and P in soil. The highest stimulation of microbial biomass C was recorded with thiobencarb (24.4%) followed by pendimethalin (23.4%). Microbial biomass N was highly induced under pretilachlor (54.5%) and thiobencarb (52.7%), while the stimulation of microbial biomass P was at par in the herbicide-treated soils. Compared to untreated control, the highest amount of organic C was retained with thiobencarb followed by pendimethalin. A similar trend was recorded with thiobencarb for total N, while pendimethalin induced exchangeable NH4 (+) and soluble NO3 (-) to the highest extent (42.2 and 34.5%, respectively). Regarding the availability of P in soil, pretilachlor manifested greater stimulation (33.1%) than thiobencarb (21.6%) and pendimethalin (11.4%). As compared to untreated control, thiobencarb harboured maximum number of bacteria (107.9%), while pretilachlor exerted the highest stimulations towards the proliferations of actinomycetes (132.6%) and fungi (149.5%) in soil.

Effects of decabromodiphenyl ether on lead mobility and microbial toxicity in soil

Lead (Pb) and decabromodiphenyl ether (BDE209) are the main pollutants at e-waste recycling sites (EWRSs). Focus on joint toxicological effects of the two chemicals has increasingly gained a great amount of interest. Therefore, the lab study was performed to determine the Pb mobility and microbial toxicity in a Pb-polluted soil in the presence of BDE209 for the first time. The results showed that BDE209 was barely degraded and could elicit the combined effects with Pb exposure during the entire incubation period. The exchangeable (EXCH) and carbonates fractions of Pb were transformed to organic, Fe/Mn oxides and residual fractions, and the addition of an appropriate amount (100mgkg(-1)) of BDE209 facilitated the transformation compared with Pb alone. In addition, soil microbial biomass C (Cmic), soil basal respiration (SBR) and metabolic quotient (qCO2) increased in the beginning of the experiment and then declined with the incubation period extension, and BDE209 addition might cause notable different response relative to the control. Significant correlations between EXCH or mobility factor (MF) of Pb and SBR, Cmic, or qCO2 in soil treated with BDE209 can be clearly observed. Results of the observations provide a better understanding of ecotoxicological effects of Pb and BDE209 joint exposure on indigenous microorganisms in soil at EWRSs.

Responses of soil ammonia-oxidizing microorganisms to repeated exposure of single-walled and multi-walled carbon nanotubes

The impacts of carbon nanotubes (CNTs) including single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) on soil microbial biomass and microbial community composition (especially on ammonium oxidizing microorganisms) have been evaluated. The first exposure of CNTs lowered the microbial biomass immediately, but the values recovered to the level of the control at the end of the experiment despite the repeated addition of CNTs. The abundance and diversity of ammonium-oxidizing archaea (AOA) were higher than that of ammonium-oxidizing bacteria (AOB) under the exposure of CNTs. The addition of CNTs decreased Shannon-Wiener diversity index of AOB and AOA. Two-way ANOVA analysis showed that CNTs had significant effects on the abundance and diversity of AOB and AOA. Dominant terminal restriction fragments (TRFs) of AOB exhibited a positive relationship with NH4(+), while AOA was on the contrary. It implied that AOB prefer for high-NH4(+) soils whereas AOA is favored in low NH4(+) soils in the CNT-contaminated soil.

Soil microbial community toxic response to atrazine and its residues under atrazine and lead contamination

Intensive use of atrazine and extensive dispersal of lead (Pb) have occurred in farmland with chemical agriculture development. However, the toxicological effect of their presence on soil microorganism remains unknown. The objective of this study was to investigate the impacts of atrazine or Pb on the soil microbiota, soil net nitrogen mineralization, and atrazine residues over a 28-day microcosm incubation. The Shannon-Wiener diversity index, typical microbe species, and a Neighbor-joining tree of typical species from sequencing denaturing gradient gel electrophoresis (DGGE) bands were determined across periodical sampling times. The results showed that the existence of atrazine or Pb (especially high concentration) in soils reduced microbial diversity (the lowest H value is 2.23) compared to the control (H = 2.59) after a 28-day incubation. The species richness reduced little (from 17~19 species to 16~17 species) over the research time. But soil microbial community was significantly affected by the incubation time after the exposure to atrazine or Pb. The combination of atrazine and Pb had a significant inhibition effect on soil net nitrogen nitrification. Atrazine and Pb significantly stimulated soil cumulative net nitrogen mineralization and nitrification. Pb (300 and 600 mg kg(-1)) accelerated the level of atrazine dissipation. The exposure might stimulate the significant growth of the autochthonous soil degraders which may use atrazine as C source and accelerate the dissipation of atrazine in soils.