Effects of soil amendments on cadmium transfer along the lettuce-snail food chain: Influence of chemical speciation

Co-exposure of polystyrene microplastics influence cadmium trophic transfer along the "lettuce-snail" food chain: Focus on leaf age and the chemical fractionations of Cd in lettuce

Cadmium (Cd) and polystyrene microplastics (PS) co-contamination always occurs in environment; however, the trophic transfer of Cd and PS is still poorly understood. A hydroponic experiment was conducted to investigate the behavior of Cd in lettuce, together with the root or foliar exposure of different sized PS. Accumulation and chemical form distributions of Cd in leaves were distinguished into young and mature leaves. Subsequently, a 14-day snail feeding experiment was performed. Data showed that Cd accumulation in roots, rather than in leaves, are significantly affected by PS coexistence. However, mature leaves had a higher Cd content than young leaves under the root exposure of PS, while a reverse effect was observed in the foliar exposure. There existed a positive correlation between the food-chain transfer associated Cd (CdFi+Fii+Fiii) in mature leaves and Cd content in snail soft tissue (r = 0.705, p < 0.001), but not in young leaves. Though no bio-amplification of Cd in food chain was observed, an increase of Cd transfer factor (TF) from lettuce to snail was noted in the root exposure of 5 μm PS and the foliar exposure of 0.2 μm PS. Moreover, we observed a highest increase rate of 36.8 % in TF values from lettuce to snail viscera, and a chronic inflammatory response in snail stomach tissue. Therefore, more attentions should be paid to study the ecological risks of heavy metals and microplastics co-contamination in environment.

Response mechanism of lettuce (Lactuca sativa L.) under combined stress of Cd and DBDPE: An integrated physiological and metabolomics analysis

DBDPE and Cd are representative contaminants commonly found in electronic waste (e-waste), which tend to be gradually discharged and accumulated in the environment during e-waste dismantling, resulting in frequent outbreaks and detection of these pollutants. The toxicity of both chemicals to vegetables after combined exposure has not been determined. The accumulation and mechanisms of phytotoxicity of the two compounds, alone and in combination, were studied using lettuce. The results showed that the enrichment ability of Cd and DBDPE in root was significantly higher than that in aerial part. Exposure to 1 mg/L Cd + DBDPE reduced the toxicity of Cd to lettuce, while exposure to 5 mg/L Cd + DBDPE increased the toxicity of Cd to lettuce. The absorption of Cd in the underground part of lettuce of 5 mg/L Cd + DBDPE was significantly increased by 108.75 % compared to 5 mg/L Cd. The significant enhancement of antioxidant system activity in lettuce under 5 mg/L Cd + DBDPE exposure, and the root activity and total chlorophyll content were decreased by 19.62 % and 33.13 %, respectively, compared to the control. At the same time, the organelles and cell membranes of lettuce root and leaf were significantly damaged, which was significantly worse than that of single Cd and DBDPE treatment. Combined exposure significantly affected the pathways related to amino acid metabolism, carbon metabolism and ABC transport in lettuce. This study filled the safety gap of DBDPE and Cd combined exposure on vegetables and would provide a theoretical basis for the environmental behavior and toxicological study of DBDPE and Cd.

Can mollusks or insects serve as bioindicators of the risk element polluted area? Gastropods (Gastropoda) versus leaf beetles (Coleoptera: Chrysomelidae)

Two groups of invertebrates, terrestrial mollusks (Gastropoda) and Chrysomela populi leaf beetles (Coleoptera: Chrysomelidae), were sampled to estimate the suitability of these organisms as bioindicators of risk element pollution (predominantly Cd, Pb, and Zn) alongside the risk element concentration gradient in the contaminated area (former mining/smelting areas in the vicinity of Příbram city, Central Bohemia, Czech Republic). The individuals representing ten species of terrestrial snails and imagoes of C. populi were collected manually at five sampling sites, differing in the level of soil contamination with risk elements. The findings showed high variability of the results regardless of the element determined, animal species, and sampling location. Among the elements, higher accumulation ability was observed for Cd and Zn, given the higher bioaccessibility of these elements in soils compared to Pb, Cr, and Cu. Higher Cd and Zn accumulation in the soft tissues of gastropods (without any statistically significant differences among the species) compared to C. populi was also recorded. Medians of the bioaccumulation factors (BAFs) reached up to 33.2 for Cd and 5.8 for Zn, in gastropods while reaching up to 3.4 for Cd, and 2.3 for Zn, for C. populi. For both groups of organisms, paradoxically, a higher rate of accumulation of risk elements was observed in all analyzed organisms in sites with lower soil contamination compared to heavily contaminated sites. This indicated the ability of the organisms living in extreme conditions to avoid the uptake of these elements or to move among areas of different contamination levels. Thus, terrestrial gastropods and C. populi proved to be unsuitable bioindicators for assessing soil pollution.

Microbiological mechanism for "production while remediating" in Cd-contaminated paddy fields: A field experiment

Security utilization measures (SUMs) for "production while remediating" in moderate and mild Cd-polluted paddy fields had been widely used. To investigate how SUMs drove rhizosphere soil microbial communities and reduced soil Cd bioavailability, a field experiment was conducted using soil biochemical analysis and 16S rRNA high-throughput sequencing. Results showed that SUMs improved rice yield by increasing the number of effective panicles and filled grains, while also inhibiting soil acidification and enhancing disease resistance by improving soil enzyme activities. SUMs also reduced the accumulation of harmful Cd in rice grains and transformed it into FeMn oxidized Cd, organic-bound Cd, and residual Cd in rhizosphere soil. This was partly due to the higher degree of soil DOM aromatization, which helped complex the Cd with DOM. Additionally, the study also found that microbial activity was the primary source of soil DOM, and that SUMs increased the diversity of soil microbes and recruited many beneficial microbes (Arthrobacter, Candidatus_Solibacter, Bryobacter, Bradyrhizobium, and Flavisolibacter) associated with organic matter decomposition, plant growth promotion, and pathogen inhibition. Besides, special taxa (Bradyyrhizobium and Thermodesulfovibrio) involved in sulfate/sulfur ion generation and nitrate/nitrite reduction pathway were observably enriched, which effectively reduced the soil Cd bioavailability through adsorption and co-precipitation. Therefore, SUMs not only changed the soil physicochemical properties (e.g., pH), but also drove rhizosphere microbes to participate in the chemical species transformation of soil Cd, thus reducing Cd accumulation in rice grains.

Effect of phosphorus sources on growth and cadmium accumulation in wheat under different soil moisture levels

Both cadmium (Cd) toxicity and water limited stress in crop plants are serious concerns worldwide while little is known about the impact of various phosphorus (P) sources on Cd accumulation in cereals especially under water limited stress. A study was conducted to explore the efficiency of three frequently available P fertilizers on Cd accumulation in wheat under different soil moisture levels. Three different P sources including diammonium phosphate (DAP), single super phosphate (SSP), and nitrophos (NP) were applied in the soil with three levels (0, 50 and 100 mg/kg). The drought stress was applied to half treatments during the latter growth stages of wheat and plants were harvested at maturity. The results demonstrated that water-limited stress decreased the growth and yield of plants than respective treatments without water stress. P supply increased the growth of wheat irrespective of water-limited stress. The effect on growth and yield varied with the sources and levels of P and maximum effects was observed in DAP treatment (100 mg/kg). The P amendments enhanced the leaf photosynthesis and activities of SOD, POD, CAT and decreased the leaf oxidative burst. Water limited stress enhanced the Cd concentrations in shoots, roots, and grains whereas P amendments minimized the Cd concentrations and enhanced the P concentrations in these parts of plants. The results obtained demonstrated that P supply in the form of DAP might be effective in minimization of Cd in grains and can be used for safe cultivation of metal-contaminated soils.

Novel insights into probabilistic health risk and source apportionment based on bioaccessible potentially toxic elements around an abandoned e-waste dismantling site

The study of potentially toxic element (PTE) hazards around e-waste recycling areas has attracted increasing attention but does not consider elemental bioaccessibility. Here, the respiratory and oral bioaccessibilities were incorporated into probabilistic health risk evaluation and source contribution apportionment. The results showed that soil Cd yielded the highest respiratory and oral bioaccessibility, whereas Cr in soils and vegetables attained the lowest oral bioaccessibility. When incorporating metal bioaccessibility into health risk assessment, a 48.3%-55.7% overestimation of non-cancer and cancer risks can be avoided relative to the risk assessment based on the total concentrations of PTEs. More importantly, priority control metals were misidentified without consideration of bioaccessibility. Cadmium, As, and Cr were screened as the priority metal(loid)s for targeted risk control based on the total PTEs, whereas Cd, Zn, and Cu were the priority metal(loid)s based on the bioaccessible PTEs. Furthermore, source apportionment revealed that >50% of oral bioaccessible Cd, Cu, Ni, Pb, and Zn in farmland were contributed by e-waste dismantling activities, whereas bioaccessible As and Cr mainly originated from agrochemical applications and natural sources, respectively. This study emphasizes the refinement of risk estimation and source apportionment through metal bioaccessibility adjustment, which facilitates the realistic assessment of adverse health effects in humans and the precise identification of high-risk sources.

Distribution characteristics, bioaccumulation and trophic transfer of heavy metals in the food web of grassland ecosystems

In this study, according to the classification of biological "classes" and the different trophic levels of the food web, the distribution characteristics, bioaccumulation of heavy metals (HMs) and their trophic transfer in the food web of typical grassland ecosystems were studied and predicted. The results indicated that the accumulation of toxic As was the highest in small mammals and reptiles, Cu was the highest in insects, and the micronutrient Zn in large mammals was higher than that in plants. The metal transfer factor (MTF) by plants at the first trophic level showed that Leymus chinensis had the best ability to absorb HMs from soil. The trophic transfer factor (TTF) of HMs in the second-trophic level insects, birds and some mammals were Zn > As > Cu > Ni > Pb > Co = Cr > Mn > V, in which, biomagnified on Zn, As, and Cu. Organisms at the third trophic level including birds, reptiles and some mammals had the strongest accumulation ability for Pb, V and As, and all were biomagnified. The biomagnification on As and Co of the fourth trophic level Siberian weasel was obviously higher than that of Dione's rat-snake, which had significant biomagnification effect on As by preying on Steppe toad-headed agama. The study showed that the bioaccumulation levels of HMs in organisms at different trophic levels varied significantly with species, prey, and organ type, but they all showed strong bioaccumulation capacity to toxic As, which indicated that As could produce certain toxic effects on animals in the food web through trophic transfer. In addition, organisms at low-trophic levels were more likely to biomagnify Zn, while organisms at high-trophic levels were more likely to biodilute Pb.

Contrasting impacts of mobilisation and immobilisation amendments on soil health and heavy metal transfer to food chain

Heavy metal mobilisation or immobilisation have been widely applied in situ for soil remediation. However, the consequences of the mobilisation or immobilisation amendments on soil health and heavy metal transfer are rarely compared. In this study, four mobilisation additives (EDTA, humic acid, oxalic acid and citric acid) and four immobilisation additives (calcium silicate, lime, biochar and pig manure) were applied in soils contaminated with Cd, Zn, and Pb to investigate their effects on soil microbial and nematode communities, chemical speciation of metals in Amaranthus tricolour L., and metal food chain transfer in soil-plant-insect system. We found that mobilisation amendments inhibited plant growth and EDTA reduced microbial biomass indicated by phospholipid fatty acids. In contrast, immobilisation amendments promoted plant growth. However, abundances of microbe and nematode were reduced by calcium silicate and lime, while they were substantially increased by biochar and pig manure. We also realised that the immobilisation amendments shifted the water-soluble and pectate-/protein-associated fractions to phosphate-/oxalate-associated fractions of metals in plant leaves, enhanced detoxification ability of Prodenia litura larvae, and reduced metal transfer along food chain. However, opposite changes were observed in mobilisation treatments. According to redundancy analysis, we found that the addition of biochar or pig manure improved soil health and function by reducing metal availability and increasing soil available N and P concentrations. Our results indicate that organic immobilisation amendments most effectively improve soil health and reduce metal transfer, and should be recommended for remediation of heavy metal-contaminated soils.

Rhizospheric pore-water content predicts the biochar-attenuated accumulation, translocation, and toxicity of cadmium to lettuce

Metal bioavailability controls its behaviors in soil-plant system, especially involved in biochar amendment. This study compared a rhizospheric pore-water extraction against a BCR sequential extraction method to understand cadmium (Cd) bioavailability in two typical Chinese soils. Soils were spiked with five levels of Cd (CdCl2) and remediated with 3% corn-straw derived biochar. After 60 days of lettuce growth, Cd accumulation and enzyme activities in tissues were analyzed. Results showed that biochar increased soil properties (pH, CEC and SOM) compared to un-amended soils, but decreased contents of bioavailable Cd in soil pore-water (Cdpore-water) and BCR extracted Cd (CdFi+Fii). Contents of Cdpore-water were lower in yellow-brown soils than that in red soils. Pearson analysis showed that bioavailable Cd is negatively correlated with soil pH and CEC (p < 0.05). Cd accumulation in lettuce roots and leaves both were decreased by biochar addition, and the established linear equations proved that soil Cdpore-water is the best predictor for Cd accumulation in lettuce roots (r2 = 0.964) and in leaves (r2 = 0.953), followed by CdFi+Fii. Transfer factor (TF) values of Cd from roots to leaves were lower than 1, and slightly better correlated with soil Cdpore-water (r = -0.674, p < 0.01) than CdFi+Fii (r = -0.615, p < 0.01). Aggregated boosted tree (ABT) analyses indicated that soil properties together with Cdpore-water contribute more than 50% to root enzyme activities. Collectively, soil Cdpore-water is a promising predictor of Cd bioavailability, accumulation and toxicity in soil-plant system with biochar addition.

Use of Brassica juncea and Dactylis glomerata for the phytostabilization of mine soils amended with compost or biochar

Phytostabilization of mine soils contaminated by potentially toxic elements (PTEs) requires plants tolerant to PTE toxicity and to the poor soil physico-chemical characteristics of these areas. A pot experiment was carried out to assess the phytostabilization potential of Brassica juncea and Dactylis glomerata in mine soils amended with compost and biochar. Furthermore, the Environmental Risk of the soils and the effects of the phytostabilization process on the microbiological population size and activity in the soils were also determined. According to the Ecological Risk Index (ERI) the soils studied presented "very high risk" and As, Cd and Pb were the target elements for phytostabilization. Both amendments improved soil conditions (e.g., increasing total-N and total organic-C concentrations) and contributed to PTE (Cd, Pb and Zn) immobilization in the soil. Compost showed a more marked effect on soil microbial biomass and nutrients release in soil, which led to higher B. juncea and D. glomerata biomass in compost treated soils. Biochar treatment showed a positive effect only on D. glomerata growth, despite it provoked strong PTE immobilization in both soils. The addition of both amendments resulted in an overall reduction of PTE concentration in the plants compared to the control treatment. In addition, both plant species showed higher accumulation of PTE in the roots than in the shoots (transfer factor<1) independently of the treatment received. Therefore, they can be considered as good candidates for the phytostabilization of PTE contaminated mine soils in combination with organic amendments like biochar and compost.

Effect of amendments on soil Cd sorption and trophic transfer of Cd and mineral nutrition along the food chain

Phytotoxicity of cadmium (Cd) and its trophic transfer along a terrestrial food chain have been extensively investigated. However, few studies focused on the role of amendments on the trophic transfer of Cd and related mineral nutrients. In a 60-day pot experiment, soil Cd availability, accumulation of Cd, mineral nutrients (Ca and Si) in lettuce, and subsequent trophic transfer along the lettuce-snail system were investigated with or without 3% (w/w) soil amendment (biochar or micro-hydroxyapatite, μHAP). Soil CaCl2 extractable Cd (CdCaCl2) contents decreased by both amendments. μHAP amended soil increased the Freundlich sorption capacity of Cd2+ to a greater extent (15.9 mmol/kg) than biochar (12.6 mmol/kg). Cd, Ca and Si accumulation in lettuce tissues (roots and shoots) varied with amendment species and soil Cd levels. Linear regression analysis showed that root Cd contents are negatively correlated with root Ca and Si contents (r2 = 0.96, p < 0.05). But no significant correlation between shoot Cd and lettuce Ca and Si contents was found (p > 0.05). After 15 days snail feeding, nearly 90% content of Cd was found in snail viscera, while nearly 95% content of Ca was found in snail shells. Contents of Si distributed equally in snail tissues. Biomagnification of Cd, Ca and Si (TF > 1) was found in lettuce shoot - snail viscera system. Opposite tendency of TF variation between Cd and nutrient elements (Ca and Si) from shoots to snail tissues indicated that μHAP, rather than biochar, amendment is applicable to remediate soil Cd contamination in our study.

Bioavailability of cadmium to celery (Apium graveolens L.) grown in acidic and Cd-contaminated greenhouse soil as affected by the application of hydroxyapatite with different particle sizes

Cadmium contamination in greenhouse vegetable fields greatly limited the sustainable production especially of leafy vegetables. Hydroxyapatite (HAP), as a common soil amendment, has been widely used in the remediation of Cd-contaminated soils, while its remediation efficiency greatly depends on its particle sizes. In this study, a rhizobag pot experiment was conducted to investigate the effects of HAP (<60 nm, <12 μm and <80 μm) on bioavailability of Cd to celery grown in acidic and slightly Cd-contaminated greenhouse soil. The results suggested that HAP with the largest particle size (<80 μm) had the best effectiveness in reducing Cd uptake especially by the edible part of celery. Specifically, the increase in HAP (<80 μm) addition from 0.5% to 3% prominently reduced Cd concentrations in celery shoot by 19.6%-76.8% as compared with the untreated group. Also, adding HAP (<80 μm) especially at 3% significantly decreased translocation factor (TF) of Cd from celery root to shoot by 30.6% and reduced bioconcentration factor (BCF) of Cd from rhizosphere soil to celery shoot by 76.4%. These were predominantly associated with the significantly increased soil pH and the subsequently decreased soil CaCl₂-Cd concentration after adding HAP (<80 μm). Overall, although rhizosphere soil pH was the key factor in controlling Cd uptake by edible celery and regulating BCF and TF of Cd, insignificant root-induced acidification had limited effect on the immobilization efficiency of Cd by HAP (<80 μm). In conclusion, HAP (<80 μm) has good potential for the remediation of Cd-contaminated greenhouse soils.

Simultaneous Immobilization of Soil Cd(II) and As(V) by Fe-Modified Biochar

Remediation of soil heavy metal by biochar has been extensively studied. However, few studies focused on the role of biochar on the co-immobilization of cadmium (Cd(II)) and arsenate (As(V)) and related soil nutrient availability. Remediation tests were conducted with three types of pristine and ferric trichloride (FeCl₃) modified biochar (rice, wheat, and corn straw biochar) in Cd-As co-contaminated soil, with application rates of 1, 5, and 10% (w/w) and the incubation of 1, 7, 10, and 15 days. Using TCLP (Toxicity Characteristic Leaching Procedure) method, 10% of FeCl₃ modified corn-straw derived biochar (FCB) had the highest immobilization efficiency of Cd(II) (63.21%) and As(V) (95.10%) after 10 days of the incubation. Iron-modified biochar immobilized higher fractions of water-soluble (F1) and surface-absorbed (F2) metal fractions than pristine biochar. For FCB amendment, Cd was mostly presented in the organic matter (OM) and sulfides associated (F4) and residual (F5) fractions (88.52%), as was found in the Fe-Al (oxides and hydroxides) (F3), F4, and F5 fractions (75.87%). FCB amendment increased soil pH values and available iron contents (p < 0.05), while no changes in soil available phosphorus content (p > 0.05). This study showed that FCB application reduces the environmental mobility of metals in Cd-As contaminated soil, while it also increases soil pH and available nutrient mobility, improving soil environmental quality and reducing remediation costs.