Effects of intercropping on safe agricultural production and phytoremediation of heavy metal-contaminated soils

Applying cross-scale regulations to Sedum plumbizincicola for strengthening the bioremediation of the agricultural soil that contaminated by electronic waste dismantling and revealing the underlying mechanisms by multi-omics

Electronic waste dismantling has induced the surrounding agricultural soils suffered from combined contamination of heavy metals and organic pollutants. Lower efficiency and complex mechanisms of bioremediation remain to be resolved. Here, we adopted regulations to Sedum plumbizincicola cross aboveground and belowground scales to strengthen the bioremediation efficiency. Results showed that the S. plumbizincicola intercropping with the Astragalus sinicusL. that inoculated with Rhizobium had the highest performance in reductions of Cd, PBDEs and PCBs from soils by 0.11 mg/kg, 67.93 μg/kg and 38.91 μg/kg, respectively. Rhizosphere soil metabolomics analysis demonstrated that reductions in Cd and PBDEs significantly associated with 2-Methylhippuric acid and L-Saccharopine, which were involved in phenylalanine metabolism, biosynthesis of amino acids and lysine. Metagenomics analysis revealed that these functional pathways were mediated by Frankia, Mycobacterium, Blastococcus, etc. microbial taxa, which were also significantly altered by regulations. Moreover, regulation regimes significantly affected transcription genes of S. plumbizincicola. Functional annotation revealed that cross-scale regulations significantly improved bioremediation efficiency through microorganisms and metabolites in the rhizosphere and transcription genes of S. plumbizincicola, which were illustrated to promote plant growth and tolerance to environmental stress. Our integration of multi-omics provides comprehensive and deep insights into molecular mechanisms in the cross-scale regulations of S. plumbizincicola, which would favor remediation techniques advances for the soil contaminated by electronic waste dismantling.

Enhancing soil quality in soybean cultivation: Mycorrhizal technology combined with intercropping under high cadmium stress

Cadmium (Cd) contamination presents a serious challenges for sustainable agriculture. This study evaluated the combined impact of arbuscular mycorrhizal fungi (AMF) inoculation and intercropping with Solanum nigrum on soil microbial diversity, enzyme activity, and environmental factors in soybean cultivation under high Cd stress. The combined treatment effectively reduced bioavailable Cd in soil, with the acid-soluble Cd fraction at 19.57 mg/kg and the reducible Cd fraction at 61.35 %, resulting in safe soybean grain Cd levels (2.63 mg/kg, below the 3 mg/kg organic standard). Illumina NovaSeq sequencing analysis revealed that key bacterial taxa, including Bradyrhizobium and PMMR1, were correlated with reduced Cd uptake in grains. Although bacterial α diversity increased, microbial network stability decreased in response to Cd, AMF inoculation, and intercropping. The combined treatment also enhanced soil enzyme activity by regulating the relative abundance of dominant or key genera such as Subgroup_6, Rokubacteriales and Pseudarthrobacter. Notably, catalase activity was 97.25 % higher in the combined treatment compared to monoculture without AMF colonization under high Cd conditions. These findings demonstrate the synergistic potential of AMF inoculation and S. nigrum intercropping as a sustainable approach to mitigate Cd contamination in crops while improving soil health in Cd-contaminated environments.

Intercropping can accelerate the phytoremediation of Cd-contaminated soil

Remediation of heavy metal (HM)-contaminated farmland has attracted much attention. Intercropping shows great potential in the remediation of HM-contaminated farmland and has been extensively studied. However, it remains uncertain how intercropping influences phytoremediation. In this study, a meta-analysis was performed to evaluate the effects of intercropping on plant growth, plant Cd uptake, residual soil Cd concentration, and the related impact factors. The results showed that compared with monocropping, intercropping decreased plant biomass (-13.0 %) and Cd uptake (-26.9 %), whereas it increased Cd translocation factor (5.23 %) and soil pH (2.74 %). However, intercropping decreased residual soil Cd concentration by -2.08 % under the conditions of low plant biomass and less Cd uptake. These indicate that phytoextraction may contribute little to soil Cd removal in intercropping, and the indirect effects of intercropping are more important than its direct effects. Intercropping effects were affected by experimental and environmental factors. In terms of decreasing residual soil Cd concentration, intercropping produced large effect under at least one of the following conditions: monocots-dicots combination, intercropping with hyperaccumulator, in the soil contaminated by industrial pollution-derived Cd, at soil Cd level of ≤10.0 mg/kg, in acid soil (pH ≤ 6.50), and at the medium experimental duration of 61-120 d. This study confirms that intercropping can accelerate the phytoremediation of Cd-contaminated soil, and indirect effects of intercropping may play important roles.

Predicting bioavailable barium transfer in soil-bok choy systems: A study induced by shale gas extraction in Chongqing, China

Barium (Ba) is a significant contaminant from shale gas extraction and is also used in various other industries. However, there has been very limited attention paid to Ba. Elucidating the Ba in soil-crop system are of great significance for both human health risk assessment and pollution control. In this study, the bioavailability of Ba in soils was studied by using various characterization methods. Then the major factors dominating the transfer of Ba in soil-bok choy system and a suitable predicted model was derived. The results showed that Ba was mainly accumulated in the roots (transfer factor < 0.3). The relationships between Ba in shoots and the bioavailability of Ba characterizing with different methods increased in the order of CH3COOH (R2 = 0.81) < ethylenediamine tetraacetic acid (R2 = 0.87) < pore water (R2 = 0.89) < diffusive gradients in thin film (R2 = 0.90) < CaCl2 (R2 = 0.91). The major soil properties affecting Ba in shoots were pH (r = -0.32, P > 0.05), cation exchange capacity (r = -0.43, P < 0.01) and labile Al (r = 0.38, P < 0.05). Bioavailability of Ba can preferably model the Ba transfer in soil-bok choy system. The best reliable model was LogBa[shoot] = 0.591LogBa[soil-Pore water] + 1.749 (R2 = 0.963, P < 0.001). This model without measuring soil physicochemical properties, making it easier and more convenient to use in practice. Overall, these results highlight the role of metal bioavailability in predicting their transfer in soil-plant systems.

Application of rhizobium inoculation in regulating heavy metals in legumes: A meta-analysis

Rhizobium inoculation has been widely applied to alleviate heavy metal (HM) stress in legumes grown in contaminated soils, but it has generated inconsistent results with regard to HM accumulation in plant tissues. Here, we conducted a meta-analysis to assess the performance of Rhizobium inoculation for regulating HM in legumes and reveal the general influencing factors and processes. The meta-analysis showed that Rhizobium inoculation in legumes primarily increased the total HM uptake by stimulating plant biomass growth rather than HM phytoavailability. Inoculation had no significant effect on the average shoot HM concentration (p > 0.05); however, it significantly increased root HM uptake by 61 % and root HM concentration by 7 % (p < 0.05), indicating safe agricultural production while facilitating HM phytostabilisation. Inoculation decreased shoot HM concentrations and increased root HM uptake in Vicia, Medicago and Glycine, whereas it increased shoot HM concentrations in Sulla, Cicer and Vigna. The effects of inoculation on shoot biomass were suppressed by nitrogen fertiliser and native microorganisms, and the effect on shoot HM concentration was enhanced by high soil pH, organic matter content, and phosphorous content. Inoculation-boosted shoot nutrient concentration was positively correlated with increased shoot biomass, whereas the changes in pH and organic matter content were insufficient to significantly affect accumulation outcomes. Nitrogen content changes in the soil were positively correlated with changes in root HM concentration and uptake, whereas nitrogen translocation changes in the tissues were positively correlated with changes in HM translocation. Phosphorus solubilisation could improve HM phytoavailability at the expense of slight biomass promotion. These results suggest that the diverse growth-promoting characteristics of Rhizobia influence the trade-off between biomass-HM phytoavailability and HM translocation, impacting HM accumulation outcomes. Our findings can assist in optimising the utilisation of legume-Rhizobium systems in HM-contaminated soils.

Intraspecific variation in tomato: Impact on production quality and cadmium phytoremediation efficiency in intercropping systems with hyperaccumulating plant

Intercropping with hyperaccumulators can facilitate the safe utilization of cadmium-contaminated soil. However, the effectiveness of this approach is influenced by plant species and varieties, which necessitates research on optimal plant consortia. In this study, 8 tomato varieties (3 cherry tomatoes and 5 common large-fruit tomatoes) were intercropped with Sedum alfredii in a moderately Cd-contaminated vegetable field. The results showed that the Cd concentration in the fruits of common large-fruit tomato varieties under monoculture was 1.03-1.50 mg/kg, while that in the fruits of cherry tomato varieties was 0.67-0.71 mg/kg. After intercropping with S. alfredii, the fruit Cd concentrations of Hangza 501, Hangza 503, and Hangza 108 decreased by 16.42 %, 19.72 %, and 6.76 %, respectively, while those of the other varieties significantly increased, except for those of Hangza 8. In contrast, the shoot Cd concentration of cherry tomatoes was greater than that of large-fruit tomatoes under monoculture. Furthermore, a significant increase in the shoot Cd concentration was noted in the Hangza 501, Hangza 503 and Hangza 603 plants following intercropping. Additionally, intercropping with S. alfredii increased the concentration of soluble sugars in the fruits of Hangza 8, Hangza 501, Hangza 503 and Hangza 603 by 4.66 %, 17.91 %, 10.60 % and 17.88 %, respectively. Intercropping with tomatoes resulted in a decrease in both the biomass and Cd uptake of S. alfredii. Interestingly, the inhibitory effect on S. alfredii was less pronounced when intercropped with cherry tomatoes than when intercropped with large-fruit tomatoes. Among the intercropping treatments, S. alfredii exhibited the greatest total Cd accumulation (0.06 mg/plant) when intercropped with Hangza 503. In conclusion, the cherry tomato variety Hangza 503 was the most suitable for intercropping with S. alfredii and can be used safely for vegetable production and simultaneous phytoremediation of polluted soil. Our findings suggest that strategic selection of tomato varieties can optimize the effectiveness of "phytoextraction coupled with agro-safe production" technology for managing soil Cd concentrations.

Simultaneous immobilization of V and Cr availability, speciation in contaminated soil and accumulation in ryegrass by using Fe-modified pyrolysis char

In this study, municipal waste pyrolytic char (PEWC) was prepared by pyrolysis from municipal solid waste extracted in landfills, and Fe-based modified pyrolytic char (Fe-PEWC) was prepared by modification. Focusing on the evaluation of the stabilization capacity of Fe-PEWC for vanadium (V) and chromium (Cr) in soils, the effects of PEWC addition on soil properties, bioavailability and morphological distribution of V and Cr, ryegrass growth, and V and Cr accumulation were thoroughly investigated. The results of pot experiment showed that the application of PEWC and Fe-PEWC significantly (P < 0.05) improved soil properties (such as pH, EC, total nitrogen, available phosphorus, available potassium, and organic matter). After 42 days of cultivation, Fe-PEWC has a better fixation effect on heavy metals, and the bioavailable V and Cr of 3% Fe-PEWC decreased by 14.96% and 19.48%, respectively. The exchangeable state and reducible state decreased, while the oxidizable state and residual state increased to varying degrees. The Fe-PEWC can effectively reduce the accumulation of V and Cr in ryegrass by 71.25% and 76.43%, respectively, thereby reducing their toxicity to plants. In summary, modified pyrolytic char can effectively solidify heavy metals in soil, improve soil ecology and reduce the toxicity to plants. The use of excavated waste as a raw material for the preparation of soil heavy metal curing agent has the significance of resource recycling, low price, and practical application.

Effect of soil organic matter-mediated electron transfer on heavy metal remediation: Current status and perspectives

Soil contamination by heavy metals poses major risks to human health and the environment. Given the current status of heavy metal pollution, many remediation techniques have been tested at laboratory and contaminated sites. The effects of soil organic matter-mediated electron transfer on heavy metal remediation have not been adequately studied, and the key mechanisms underlying this process have not yet been elucidated. In this review, microbial extracellular electron transfer pathways, organic matter electron transfer for heavy metal reduction, and the factors affecting these processes were discussed to enhance our understanding of heavy metal pollution. It was found that microbial extracellular electrons delivered by electron shuttles have the longest distance among the three electron transfer pathways, and the application of exogenous electron shuttles lays the foundation for efficient and persistent remediation of heavy metals. The organic matter-mediated electron transfer process, wherein organic matter acts as an electron shuttle, promotes the conversion of high valence state metal ions, such as Cr(VI), Hg(II), and U(VI), into less toxic and morphologically stable forms, which inhibits their mobility and bioavailability. Soil type, organic matter structural and content, heavy metal concentrations, and environmental factors (e.g., pH, redox potential, oxygen conditions, and temperature) all influence organic matter-mediated electron transfer processes and bioremediation of heavy metals. Organic matter can more effectively mediate electron transfer for heavy metal remediation under anaerobic conditions, as well as when the heavy metal content is low and the redox potential is suitable under fluvo-aquic/paddy soil conditions. Organic matter with high aromaticity, quinone groups, and phenol groups has a stronger electron transfer ability. This review provides new insights into the control and management of soil contamination and heavy metal remediation technologies.

Phytoremediation and environmental effects of three Amaranthaceae plants in contaminated soil under intercropping systems

Intercropping is a widely used agricultural system; however, the effect of intercropping between accumulator plants on phytoextraction in heavy metal-contaminated soils remains unknown. Here, a field experiment was conducted to investigate the phytoextraction efficiency and related environmental effects of three Amaranthaceae plants (Amaranthus hypochondriacus, Celosia argentea, and Pfaffia glomerata) using mono- and intercropping models. In monocropping, the total biomass of A. hypochondriacus was only 51.2 % of that of C. argentea. Compared with monocropping, intercropping reduced the fresh weight per plant of A. hypochondriacus by 53.0 % (intercropping with C. argentea) and 40.5 % (intercropping with P. glomerata) but increased the biomass per plant of C. argentea and P. glomerata by 128.2 and 14.2 %, respectively. The Cd uptake of the three plants in the monocropping models showed the following trend: C. argentea > P. glomerata > A. hypochondriacus. Interplanting A. hypochondriacus and C. argentea further increased the phytoextraction efficiency by 361.2 % (compared with A. hypochondriacus monocropping) and 52.0 % (compared with C. argentea monocropping). Soil exchangeable Cd, Pb, Cu, Zn, K, and P, soil N-NO3- and N-NH4+, soil common bacteria and arbuscular mycorrhiza (AM) fungi, and soil total organic carbon (TOC) play key roles in Cd and Pb uptake by the three accumulator plants (p < 0.05). The biomass of common bacteria, Gm+, Gm- bacteria, fungi, AM fungi, and actinomycetes increased with the three accumulators planted in the mono- and intercropping models. Compared with C. argentea monocropping, the biomass of soil microbes in the rhizosphere soil was obviously increased in the intercropping A. hypochondriacus and C. argentea models. These results suggest that interplanting A. hypochondriacus and C. argentea can increase Cd removal efficiency from Cd-contaminated soils, and this model could be recommended to remediate Cd-contaminated soils on a field scale.

Competitive adsorption behaviors and mechanisms of Cd, Ni, and Cu by biochar when coexisting with microplastics under single, binary, and ternary systems

In this study, the effects of coexistence with microplastics and co-ageing with the soil on adsorption behaviors and mechanisms of biochar for heavy metals were investigated. Adsorption experiments of Cd, Ni, and Cu by microplastics, biochar, and their combination were conducted in single, binary, and ternary systems. The results indicated that the heavy metal adsorption by microplastics was ranked as Ni > Cd > Cu, which increased with decreasing particle size, and the adsorption capacity of microplastics was enhanced after dry-wet and freeze-thaw ageing. Biochar preferentially adsorbed Cd in the single system, while the maximum adsorption of Cu was observed in the binary and ternary systems due to the minimizing impact of competition on the Cu adsorption by biochar. The heavy metal adsorption by the combination of microplastics and biochar was less than that by single biochar, and the smaller the particle size of microplastics, the greater the negative effects on heavy metal adsorption. Coexistence with microplastics reduced Cd adsorption of biochar by 0.72 %-50.35 %, Ni adsorption by 1.17 %-30.43 %, and Cu adsorption by 5.78 %-47.88 %, respectively. Moreover, coexistence with microplastics exacerbated the adverse impacts of competition on biochar adsorption for heavy metals. The contribution percentages of biochar mineral mechanisms for heavy metal adsorption were ranked as Cu > Cd > Ni. When coexisting with microplastics or after ageing, the mineral mechanisms of heavy metal adsorption by biochar significantly decreased. This study investigated the competitive adsorption behaviors and mechanisms of heavy metals by biochar when coexisting with microplastics, which highlighted that the application of biochar for the remediation of heavy metal pollution should be concerned with the impacts of microplastics.

Effects of tomato-Sedum alfredii Hance intercropping on crop production and Cd remediation as affected by soil types

Intercropping crops with hyperaccumulators is a proven model for coupling crop safety production and soil heavy metal remediation. And both crop genotypes and soil properties might have great impacts on the effect of intercropping. Therefore, a greenhouse pot experiment was designed to investigate the effects of intercropping different tomato varieties with the cadmium (Cd) hyperaccumulator Sedum alfredii Hance (S. alfredii Hance) on different soils. The results showed that intercropping promoted Cd uptake by S. alfredii Hance and reduced soil total Cd concentration. There was no significant effect of intercropping on tomato yield and Cd concentration. Different tomato varieties had different effects on tomato yield and Cd concentration. The yield of cherry tomato was 1.04 times higher than that of common large fruit tomato, while the Cd concentration in all parts was lower than that of common large fruit tomato. Different typical zonal soils had different effects on tomato production and soil remediation. And among the four studied soils, tomatoes grown on ZJ soil had the highest yields and lowest fruit Cd concentration, making them more suitable for remediation coupled with safety production. This study provided a comprehensive analysis of tomato production benefits and soil remediation effects, which could be useful as a guide in vegetable safety production coupled with soil remediation practices in the Cd-contaminated greenhouse.

Interspecific root interaction enhances cadmium accumulation in Oryza sativa when intercropping with cadmium accumulator Artemisia argyi

The contamination of arable land with heavy metals, such as Cd, is a serious concern worldwide. Intercropping with Cd accumulators can be used for efficient safe crop production and phytoremediation of Cd-contaminated soil. However, the effect of intercropping on Cd uptake by main crops and accumulators varies among plant combinations. Rhizosphere interaction may mediate Cd uptake by intercropped plants, but the mechanism is unclear. Thus, in the present study, we aimed to examine the effect of rhizosphere interaction on Cd uptake by intercropping rice (Oryza sativa L.) with mugwort (Artemisia argyi Levl. et Vant.) in Cd-contaminated paddy soil. We grew O. sativa and A. argyi in pots designed to allow different levels of interaction: complete root interaction (no barrier), partial root interaction (nylon mesh barrier), and no root interaction (plastic film barrier). Our results indicated that both complete and partial root interaction increased the shoot and root mass of A. argyi, but did not decrease the shoot, root, and grain mass of O. sativa. Interspecific root interaction significantly increased the Cd content in the shoots, roots, and grains of O. sativa and the shoots of A. argyi. Increased content of total organic acids in the rhizosphere, which increased the content of available Cd, was a possible mechanism of increased Cd uptake in both plants under interspecific root interaction. Our findings demonstrate that an intercropping system can extract more Cd from contaminated soil than a monocropping system of either A. argyi or O. sativa. However, the intercropping system did not facilitate safe crop production because it substantially increased grain Cd content in O. sativa.

Source apportionment and migration characteristics of heavy metal(loid)s in soil and groundwater of contaminated site

The rapid industrial growth has generated heavy metal(loid)s contamination in the soil, which poses a serious threat to the ecology and human health. In this study, 580 samples were collected in Henan Province, China, for source apportionment, migration characterization and health risk evaluation using self-organizing map, positive matrix factorization and multivariate risk assessment methods. The results showed that samples were classified into four groups and pollution sources included chromium slag dump, soil parent rock and abandoned factory. The contents of Cr, Pb, As and Hg were low in Group 1. Group 2 was characterized by total Cr, Cr(Ⅵ) and pH. The enrichment of total Cr and Cr(Ⅵ) in soil was mainly attributed to chromium slag dump, accounting for more than 84.0%. Group 3 was dominated by Hg and Pb. Hg and Pb were primarily attributed to abandoned factory, accounting for 84.7% and 70.0%, respectively. Group 4 was characterized by As. The occurrence of As was not limited to one individual region. The contribution of soil parent rock reached 83.0%. Furthermore, the vertical migration of As, Hg, Pb and Cr(Ⅵ) in soil was mainly influenced by medium permeability, pH and organic matter content. The trends of As, Pb, and Hg with depth were basically consistent with the trends of organic matter with depth, and were negatively correlated with the change in pH with depth. The trends of Cr(Ⅵ) with depth were basically consistent with the changes in pH with the depth. The content of Cr(Ⅵ) in the deep soil did not exceed the detection limits and Cr(Ⅵ) contamination occurred in the deep aquifer, suggesting that Cr(Ⅵ) in the deep groundwater originated from the leakage of shallow groundwater. The assessment indicated that the non-carcinogenic and carcinogenic risks for children and adults could not be neglected. Moreover, children were more susceptible than adults.