Crop suitability assessment in remediation of Zn contaminated soil

Enhancing zinc biofortification and mitigating cadmium toxicity in soil-earthworm-spinach systems using different zinc sources

Cadmium (Cd) pollution poses significant threats to soil organisms and human health by contaminating the food chain. This study aimed to assess the impact of various concentrations (50, 250, and 500 mg·kg-1) of zinc oxide nanoparticles (ZnO NPs), bulk ZnO, and ZnSO4 on morphological changes and toxic effects of Cd in the presence of earthworms and spinach. The results showed that Zn application markedly improved spinach growth parameters (such as fresh weight, plant height, root length, and root-specific surface area) and root morphology while significantly reducing Cd concentration and Cd bioconcentration factors (BCF-Cd) in spinach and earthworms, with ZnO NPs exhibiting the most pronounced effects. Earthworm, spinach root, and shoot Cd concentration decreased by 82.3 %, 77.0 %, and 75.6 %, respectively, compared to CK. Sequential-step extraction (BCR) analysis revealed a shift in soil Cd from stable to available forms, consistent with the available Cd (DTPA-Cd) results. All Zn treatments significantly reduced Cd accumulation, alleviated Cd-induced stress, and promoted spinach growth, with ZnO NPs demonstrating the highest Cd reduction and Zn bioaugmentation efficiencies compared to bulk ZnO and ZnSO4 at equivalent concentrations. Therefore, ZnO NPs offer a safer and more effective option for agricultural production and soil heavy metal pollution management than other Zn fertilizers.

Chemical speciation and environmental risk assessment of heavy metals in ash from smouldering combustion of oily sludge

Smouldering combustion of oily sludge (OS) was carried out to learn the characteristics of heavy metals (HMs) in ash products. Ash collected from four different height layers of the column reactor was analysed for the chemical speciation and environmental risk of six HMs, including Cr, Ni, Cu, Zn, As, and Pb. The results showed that after smouldering combustion, only 21.3-32.2 % of the total HMs was remained in the ash products. The retention of HMs in ash was closely relevant to the carbonaceous destruction efficiency of OS. Smouldering combustion led to the decrease of HMs in acid-soluble/exchangeable fraction from 21.5-49.3 to 0.8-19.8% and oxidizable fraction from 22.6-49.6 to 5.3-21.3, and the increase of reducible fraction from 13.6-38.0 to 30.5-89.1% and residue fraction from 7.8-27.3 to 24.1-63.6%. Upward migration of HMs during smouldering was evidenced by their occurrence in the top clean sand layer, which was dominated in acid-soluble/exchangeable and reducible fractions, accounting for 89.7-99.1% in total. Toxicity extraction and environmental risk studies indicated that smouldering combustion would effectively reduce the toxicity and pollution risk of HMs; however, attention should be paid to the disposal of the top sand layer after smouldering operation due to its high pollution risk of HMs according to the evaluation of Risk assessment code.

Physiological response and phytoremediation potential of dioecious Hippophae rhamnoides inoculated with arbuscular mycorrhizal fungi to Pb and Zn pollution

Plant-microorganism combined remediation of heavy metal pollution has been reported, but little attention has been paid to the effect of arbuscular mycorrhizal (AM) fungi on phytoremediation of dioecious plants under heavy metal pollution. In this study, the growth, physiological responses and phytoremediation traits of Hippophae rhamnoides were determined to evaluate whether sex-specific ecophysiological responses and phytoremediation capacities of females and males are affected by additional AM fungi (Glomus mosseae) under heavy metal treatments. The results showed that excess Pb and Zn stresses inhibited photosynthetic capacities of both sexes. However, inoculated AM fungi treatment increased the activity of photosynthesis, content of photosynthetic pigment, activity of superoxide dismutase, the content of proline and root Pb content and enrichment coefficient of males while decreased root Pb content of females under Pb stress. On the other hand, inoculated AM fungi treatment increased the photosynthetic activities and Pro accumulation of females, and activity of superoxide dismutase and transport coefficient of males under Zn stress. These results demonstrate that H. rhamnoides inoculated AM fungi showed significant sex-specific responses on the growth, physiological traits and phytoremediation potential to Pb and Zn stress. AM fungi significantly improved the tolerance of males to Pb stress and both sexes to Zn stress, which indicates H. rhamnoides and AM fungi can be used as a plant-microbial combined remediation method for Pb and Zn contaminated soil. More attention should be paid on sexual-specific responses and phytoremediation of dioecious plants to heavy metals in the future.

The root-associated Fusarium isolated based on fungal community analysis improves phytoremediation efficiency of Ricinus communis L. in multi metal-contaminated soils

Phytoremediation is a widely accepted bioremediation method of treating heavy metal contaminated soils. Nevertheless, the remediation efficiency in multi-metal contaminated soils is still unsatisfactory attributable to susceptibility to different metals. To isolate root-associated fungi for improving phytoremediation efficiency in multi-metal contaminated soils, the fungal flora in root endosphere, rhizoplane, rhizosphere of Ricinus communis L. in heavy metal contaminated soils and non-heavy metal contaminated soils were compared by ITS amplicon sequencing, and then the critical fungal strains were isolated and inoculated into host plants to improve phytoremediation efficiency in Cd, Pb, and Zn-contaminated soils. The fungal ITS amplicon sequencing analysis indicated that the fungal community in root endosphere was more susceptible to heavy metals than those in rhizoplane and rhizosphere soils and Fusarium dominated the endophytic fungal community of R. communis L. roots under heavy metal stress. Three endophytic strains (Fusarium sp. F2, Fusarium sp. F8, and Fusarium sp. F14) isolated from Ricinus communis L. roots showed high resistances to multi-metals and possessed growth-promoting characteristics. Biomass and metal extraction amount of R. communis L. with Fusarium sp. F2, Fusarium sp. F8, and Fusarium sp. F14 inoculation in Cd-, Pb- and Zn-contaminated soils were significantly higher than those without the inoculation. The results suggested that fungal community analysis-guided isolation could be employed to obtain desired root-associated fungi for enhancing phytoremediation of multi-metal contaminated soils.

Joint effects of gamma radiation and zinc on duckweed Lemna minor L

When assessing the consequences of combined chemical and radiation pollution on bodies of water, it is important to take into account the interaction of different factors, especially the possible synergistic increase in the toxic effect on growth, biochemical and physiological processes of living organisms. In this work, we studied the combined effect of γ-radiation and zinc on freshwater duckweed Lemna minor L. Irradiated plants (doses were 18, 42, and 63 Gy) were placed on a medium with an excess of zinc (3.15, 6.3, 12.6 μmol/L) for 7 days. Our results showed that the accumulation of zinc in tissues increased in irradiated plants when compared to non-irradiated plants. The interaction of factors in assessing their effect on the growth rate of plants was most often additive, but there was also a synergistic increase in the toxic effect at a zinc concentration of 12.6 μmol/L and irradiation at doses of 42 and 63 Gy. When comparing the combined and separate effects of gamma radiation and zinc, it was found that a reduction in the area of fronds (leaf-like plates) was caused exclusively due to the effects of radiation. Zinc and γ-radiation contributed to the enhancement of membrane lipid peroxidation. Irradiation stimulated the production of chlorophylls a and b, as well as carotenoids.

Binding characteristics of Pb and Zn to low-temperature feces-based biochar-derived DOM revealed by EEM-PARAFAC combined with general and moving-window two-dimensional correlation analyses

Pyrolysis carbonization of human feces has shown potential for converting feces biomass into a soil amendment. However, little is known about the interactions of DOM derived from feces-based biochar produced at low-temperature with heavy metals (HMs). In this study, the binding properties of Pb(II) and Zn(II) with DOM derived from feces-based biochar produced at low pyrolysis temperatures were investigated using EEM-PARAFAC combined with general, and moving-window two-dimensional correlation analyses (2D-COS). The results revealed that DOM from biochar produced at 280 °C exhibited a higher Pb(II) and Zn(II) affinity and more binding sites than DOM produced at 380 °C. The fulvic-like and humic-like components exhibited obvious fluorescence quenching after the heavy metal addition, and the complexes formed with Pb(II) and Zn(II) were more stable. C-H groups exhibited the fastest response to Pb(II) and Zn(II) binding in the FB280 DOM, while the COO- groups of carboxylic acids in the FB380 DOM exhibited the fastest response to Pb(II) and Zn(II). Moreover, the mutation concentration range of components and functional groups in DOM, as analyzed by MW2D-COS, was greater for Zn(II) than for Pb(II). These results provide a more detailed molecular-level understanding of the interaction mechanisms between heavy metals and feces-based biochar-derived DOM and the effect of HM concentration on DOM binding. Further, these results will help to provide a reasonable reference for feces management and feces-based biochar in controlling soil HMs.

Phytoaccumulation of zinc and its associated impact on the growth performance and tolerance index of six non-food crop plants grown in Zn-contaminated soil

Crop plant remediation and detoxification of Zn-contaminated soils may pose a significant threat to food safety and, thus, human health. Therefore, the current study was carried out to assess the ability of six non-food crop plants (NFCP); Zea mays L. cultivar 360 (T360), Z. mays cultivar 123 (T123), Helianthus annuus L., Brassica juncea (L.) Czern., Ricinus communis L., and Simmondsia chinensis (Link) C.K. Schneid to remediate and restore Zn-contaminated soils. The investigated plants tolerate 150 mg/kg of Zn content of the soil, where they had tolerance index (TI) > 1 for all growth criteria, except the root dry weight (DW) of S. chinensis. Z. mays T123 and R. communis were the most susceptible plants, while B. juncea and S. chinensis were moderately tolerant, while H. annuus was the most tolerant to high Zn concentrations in a growing medium. Increasing the soil Zn content led to a significant increase (p < 0.05) in Zn concentration in the various tissues of the six NFCPs. The studied NFCP did not translocate Zn to their grains/seeds; consequently, they can be used safely for Zn-contaminated soils. The Zn content in root and shoot was negatively correlated with the TI of their length and weight, while the translocation factor (TF) of Zn from root to shoot was positively correlated to the TI of the root length and weight. The six studied NFCPs were arranged based on their phytoremediation efficiency as follows: B. juncea (31.86%) > Z. mays T123 (31.14%) > Z. mays T360 (27.59%) > H. annuus (20.85%) > S. chinensis (20.29%) > R. communis (15.3%). All tested NFCPs accumulated significant concentrations of Zn in their roots and shoots, a high Zn uptake potential, and biomass at 150-450 mg/kg of Zn treatments, indicating that these plants are good candidates for the implementation of a new strategy of cultivating NFCP for phytoremediation of Zn-contaminated soils.

Comparative zinc tolerance and phytoremediation potential of four biofuel plant species

Soil pollution has become a serious environmental problem worldwide due to rapid industrialization and urbanization. Zinc (Zn) contamination has raised concerns about potential effects on plants and human health. This study was conducted to assess the capability of four biofuel plants: Abelmoschus esculentus, Avena sativa, Guizotia abyssinica, and Glycine max to remediate and restore Zn contaminated soil. Selected plants were grown in soil exposed to different Zn treatments (50, 100, 200, 300, 400, 600, 800 and 1000 mg Zn kg^-1) for 12 weeks. Soil without spike taken as control. Zn induced toxicity significantly (p < 0.05) reduced seed germination and inhibited plant growth and leaf chlorophyll content. The investigated plants can tolerate a soil content of 800 mg Zn kg^-1 with the exception of A. sativa, which was most tolerant to high Zn concentrations (1000 mg Zn kg^-1) for all growth criteria. Moreover, increasing Zn content in soil resulted in a significant (p < 0.05) increase in Zn accumulation in various tissues of the four biofuel plants. According to phytoremediation efficiency, the four biofuel plants studied were arranged as follows: A. sativa (5.05%) > A. esculentus (4.15%) > G. max (2.31%) > G. abyssinica (1.17%). This study concluded that all tested biofuel plants species, especially A. sativa exhibited high Zn concentrations in roots and shoots, high Zn uptake capability, high tolerance, and high biomass at 50-800 mg Zn kg^-1 treatments. Consequently, these biofuel plants are excellent candidates for phytoremediation in Zn contaminated soils.

Tolerance and Heavy Metal Accumulation Characteristics of Sasa argenteostriata (Regel) E.G. Camus under Zinc Single Stress and Combined Lead–Zinc Stress

Sasa argenteostriata (Regel) E.G. Camus is a gramineous plant with the potential for phytoremediation. In this study, we aimed to determine its tolerance to zinc stress and combined lead–zinc stress and the effect of zinc on its absorption and accumulation characteristics of lead. The results showed that S. argenteostriata had good tolerance to zinc stress, and S. argenteostriata was not significantly damaged when the zinc stress concentration was 600 mg/L. Under both zinc stress and combined lead–zinc stress, the root was the main organ that accumulated heavy metals in S. argenteostriata. The presence of zinc promoted the absorption of lead by the root of S. argenteostriata, and the lead content in the root under PZ1, PZ2, PZ3 and PZ4 treatments was 2.15, 4.31, 4.47 and 6.01 times that of PZ0 on the 20 days. In the combined lead–zinc stress treatments, the toxicity of heavy metals to S. argenteostriata was mainly caused by lead. Under high concentrations of combined lead–zinc stress (PZ4), the proportion of zinc in the leaf of S. argenteostriata on the 20 days increased, which was used as a tolerance strategy to alleviate the toxicity of lead.

Wood vinegar facilitated growth and Cd/Zn phytoextraction of Sedum alfredii Hance by improving rhizosphere chemical properties and regulating bacterial community

Soil Cd and Zn contamination has become a serious environmental problem. This work explored the performance of wood vinegar (WV) in enhancing the phytoextraction of Cd/Zn by hyperaccumulator Sedum alfredii Hance. Rhizosphere chemical properties, enzyme activities and bacterial community were analyzed to determine the mechanisms of metal accumulation in this process. Results demonstrated that, after 120 days growth, different times dilution of WV increased the shoot biomass of S. alfredii by 85.2%-148%. In addition, WV application significantly increased soil available Cd and Zn by lowing soil pH, which facilitated plant uptake. The optimal Cd and Zn phytoextraction occurred from the 100 times diluted WV (D100), which increased the Cd and Zn extraction by 188% and 164%, compared to CK. The 100 and 50 times diluted WV significantly increased soil total and available carbon, nitrogen and phosphorus, and enhancing enzyme activities of urease, acid phosphatase, invertase and protease by 10.1-21.4%, 29.1-42.7%,12.2-38.3% and 26.8-85.7%, respectively, compared to CK. High-throughput sequencing revealed that the D 100 significantly increased the bacterial diversity compared to CK. Soil bacterial compositions at phylum, family and genera level were changed by WV addition. Compared to CK, WV application increased the relative abundances of genus with plant growth promotion and metal mobilization function such as, Bacillus, Gemmatimonas, Streptomyces, Sphingomonas and Polycyclovorans, which was positively correlated to biomass, Cd/Zn concentrations and extractions by S. alfredii. Structural equation modeling analysis showed that, soil chemical properties, enzyme activities and bacterial abundance directly or indirectly contributed to the biomass promotion, Cd, and Zn extraction by S. alfredii. To sum up, WV improved phytoextraction efficiency by enhancing plant growth, Cd and Zn extraction and increasing soil nutrients, enzyme activities, and modifying bacterial community.

Biodegradation and effects of EDDS and NTA on Zn in soil solutions during phytoextraction by alfalfa in soils with three Zn levels

In chelator-enhanced Zn phytoremediation studies, it is crucial to understand how the degradable chelators and the competition from other ions influence the concentration of Zn in soil solutions. This study investigated the biodegradability of two chelators (EDDS: Ethylenediamine-N,N'-disuccinic acid, and NTA: Nitrilotriacetic acid) and their effects on the Zn concentration in the soil solution during the growth of alfalfa (Medicago Sativa L.). The chelators were added at four doses (0, 0.5, 2 and 5 mmol kg^-1) in soils with varying Zn levels (189, 265 and 1496 mg kg^-1). The results showed that the lag phase before EDDS and NTA biodegradation varied from 0 to 7 days in the three soils. EDDS and NTA were completely decomposed within the assessed 57 days regardless of the applied dosage, with a half-life of 1.3-3.0 days in highly Zn-contaminated soil and 4.2-10.8 days in the two other soils. In soil solutions, the change in solubilized Zn was in line with EDDS and NTA degradation kinetics. Cu, Al, Fe and Mn were the main metal ions that competed against Zn for chelation. Besides, Ni competed with Zn in the whole process. Ca did not compete effectively in the three soils, while Mg was a competitor only at the initial stage. Our results show the importance of considering both the biodegradation rate and the competition between the target cation and other elements present in the soil when using chelators to enhance phytoremediation. A 30-day explorative incubation experiment is recommended to evaluate the appropriate application time of chelators and the target Zn exposure time for plants during phytoremediation.

Zinc in soil-plant-human system: A data-analysis review

Zinc (Zn) plays an important role in the physiology and biochemistry of plants due to its established essentiality and toxicity for living beings at certain Zn concentration i.e., deficient or toxic over the optimum range. Being a vital cofactor of important enzymes, Zn participates in plant metabolic processes therefore, alters the biophysicochemical processes mediated by Zn-related enzymes/proteins. Excess Zn can provoke oxidative damage by enhancing the levels of reactive radicals. Hence, it is imperative to monitor Zn levels and associated biophysicochemical roles, essential or toxic, in the soil-plant interactions. This data-analysis review has critically summarized the recent literature of (i) Zn mobility/phytoavailability in soil (ii) molecular understanding of Zn phytouptake, (iii) uptake and distribution in the plants, (iv) essential roles in plants, (v) phyto-deficiency and phytotoxicity, (vi) detoxification processes to scavenge Zn phytotoxicity inside plants, and (vii) associated health hazards. The review especially compares the essential, deficient and toxic roles of Zn in biophysicochemical and detoxification processes inside the plants. To conclude, this review recommends some Zn-related research perspectives. Overall, this review reveals a thorough representation of Zn bio-geo-physicochemical interactions in soil-plant system using recent data.

Cu phytoextraction and biomass utilization as essential trace element feed supplements for livestock

Copper (Cu), as an essential element, is added to animal feed to stimulate growth and prevent disease. The forage crop alfalfa (Medicago sativa L.) produced during Cu phytoextraction may be considered a biofortified crop to substitute the Cu feed additives for livestock production, beneficially alleviating Cu contamination in soils and reducing its input into agriculture systems. To assess this, alfalfa was grown in three similar soils with different Cu levels, i.e., 11, 439 and 779 mg kg^-1 for uncontaminated soil (A), moderately Cu-contaminated soil (B) and highly Cu-contaminated soil (C), respectively. EDDS (Ethylenediamine-N,N'-disuccinic acid) was applied to the soils seven days before the first cutting at four rates (0, 0.5, 2 and 5 mmol kg^-1) to enhance bioavailable Cu uptake. Alfalfa grew well in soils A and B but not in the highly Cu-contaminated soil. After applying EDDS, a significant biomass reduction of the first cutting shoot was only observed with 5 mmol kg^-1 EDDS in the highly Cu-contaminated soil, with a 45% (P < 0.05) decrease when compared to the control. Alfalfa grown in the three soils gradually wilted after the first cutting with 5 mmol kg^-1 EDDS, and Cu concentrations in the first cutting shoot were augmented strongly, by 250% (P < 0.05), 3500% (P < 0.05) and 6700% (P < 0.05) compared to the controls, respectively. Cu concentrations in alfalfa shoots were found to be higher in this study than in some fodder plants and further augmented in soils with higher Cu levels and with EDDS application. These findings suggest that alfalfa grown on clean soils or soils with up to 450 mg Cu kg^-1 (with appropriate EDDS dosages) has the potential to be considered as a partial Cu supplementation for livestock. This research laid the foundation for the integration between Cu-phytoextraction and Cu-biofortification for livestock.

The Renaissance of Wild Food Plants: Insights from Tuscany (Italy)

This paper provides an overview of wild food plants traditionally used in the gastronomy of Tuscany, an Italian region with high biological diversity and whose cultural heritage is well known. Forty-nine bibliographic sources, including five unpublished studies, were reviewed. A list of species with ecological characteristics, plant parts used, use category (food, liquor, or seasoning), methods of preparation (raw or cooked), and recipes is presented. The use of 357 taxa (3711 use reports, URs), was recorded, belonging to 215 genera and 72 botanical families. Over the total taxa, 12 are new for Tuscany, 52 seem not to be present in other Italian regions, and 54 were not detected in the consulted European ethnobotanical literature. Of these taxa, 324 (3117 URs) were used as food, while 49 (178 URs) and 81 (416 URs) were used for liquor and seasoning, respectively. Of the 17 different food recipes, cooked vegetables constituted the largest group, followed by salads, omelets, snacks, and fillings. The chemical composition of the recorded food plants and the possible safety risks associated to their consumption, as well as their traditional medicinal use, are also shown. This review highlights the richness of ethnobotanical knowledge in Tuscany. Such biocultural heritage can be a “source of inspiration” for agriculture. As a reservoir of potential new crops, wild edible flora may contribute to the development of emerging horticultural sectors such as vertical farming and microgreens production. Moreover, the nutrient content and healthy properties of many wild food plants reported in this study has the ability to meet consumer demand for functional foods.

In-situ remediation of zinc contaminated soil using phosphorus recovery product: Hydroxyapatite/calcium silicate hydrate (HAP/C-S-H)

This work discussed the feasibility and stability of utilizing C-S-H phosphorus recovered products, HAP/C-S-H, to remove Zn(Ⅱ) from aqueous solution and in-situ immobilize Zn(Ⅱ) in contaminated soil. The removal mechanisms of Zn(Ⅱ) by HAP/C-S-H were relatively complex, combining multiple reactions including electrostatic attraction, ion exchange, surface complexation and (co-)precipitation. The removal rate of Zn(Ⅱ) by HAP/C-S-H raised with the increase of pH value, reaching 99.47% at pH of 8 in aqueous solution. The ion strength of background solution negatively affected the adsorption efficiency. The pseudo-second-order model and Langmuir model were more suitable to fit the Zn(Ⅱ) adsorption experimental data for the adsorbent. The adsorption process was endothermic and spontaneous naturally according to thermodynamic parameter. The maximum adsorption capacity of HAP/C-S-H can reach 114.0 mg/g at 308 K. After 28 days of immobilization, the release of Zn(Ⅱ) in soil with HAP/C-S-H remarkably decreased to 0.6 mg/L, compared with control group (2.9 mg/L). BCR sequential extraction results indicated that HAP/C-S-H could convert acid-soluble Zn(Ⅱ) into reducible and residual Zn(Ⅱ), reducing the bioavailability and ecotoxicity of Zn(Ⅱ) in contaminated soil. pH-dependent leaching tests revealed that the soil with HAP/C-S-H had stronger resistance to acid impact.

Effects of biochar on berseem clover (Trifolium alexandrinum, L.) growth and heavy metal (Cd, Cr, Cu, Ni, Pb, and Zn) accumulation

Urban soil pollution by heavy metals (HMs) is a pressing problem in the development of urban agriculture (UA). In this context, the use of amendments, such as biochar, and phytoremediation are considered potentially cost-effective alternatives to conventional methods, and can be also combined to improve the remediation of soils from HMs. A pot experiment was performed to investigate the combined effect of berseem clover (Trifolium alexandrinum, L.) and biochar amendment in remediating a sandy soil collected near a shooting range area co-contaminated with Cd, Cr, Cu, Ni, Pb, and Zn. The biochar, obtained from a wood-chip gasifier fed with a mix of Douglas (Pseudotsuga menziesii, Mirb.) and Black Pine (Pinus nigra, J.F.Arnold) wood, was applied at two rates (0.8% and 1.6%, w/w). Eighteen weeks after sowing, all plants were harvested. The roots and aboveground tissues of the crops were separately collected and analyzed. The tested biochar effectively adsorbed the HMs (Cd, Cr, Cu, Ni, Pb, and Zn) from the soil. Biochar increased DW production of aboveground and root tissues. Moreover, biochar significantly reduced the concentration of Cr, Cu, Ni, and Pb in the aboveground tissues of berseem clover, although a significant reduction was not detected for Cd and Zn. Results indicated that berseem clover was a Cr, Ni and Pb excluder. However, this species can be considered suitable for Cu phytoextraction and Cd and Zn phytostabilization of slightly polluted urban soil. Only the Cu levels in the aerial biomass were below the acceptable limit for use as fodder.

Irrigating digestate to improve cadmium phytoremediation potential of Pennisetum hybridum

The bioavailability of heavy metal and growth of hyperaccumulator are key factors controlling the phytoextraction of heavy metal from soil. In this study, the efficacy and potential microbial mechanisms of digestate application in enhancing Cd extraction from soil by Pennisetum hybridum were investigated. The results showed that digestate application significantly promoted the height, tiller number, and biomass yield of Pennisetum hybridum. The application also increased the activities of urease, sucrase, dehydrogenase, available Cd contents of rhizosphere soils (from 2.21 to 2.46 mg kg^-1), and the transfer factors of Cd from root to shoot and leaf. Assuming three annual harvests, digestate application would substantially reduce time needed for Pennisetum hybridum to completely absorb Cd from soil-from 15-16 yr-10 yr. Furthermore, the results of microbial community diversity analysis showed that digestate irrigation was more facilitated for the growth of the predominant bacteria, which were Actinobacteria and Chloroflexi at phylum level, and Sphingomonas and Nitrospiraat genus level, which mainly have the functions of promoted plant growth and metal resistance. The results suggested that the enhanced phytoextraction of Cd by Pennisetum hybridum with digestate application might mainly attributed to the increased Cd bio-availability and the enhanced plant growth, indicating that an approach combining digestate and Pennisetum hybridum could be a promising strategy for remediating Cd-contaminated soils.

Zn phytoextraction and recycling of alfalfa biomass as potential Zn-biofortified feed crop

Zn is an essential micronutrient for living organisms and, in that capacity, it is added to animal feed in intensive livestock production to promote growth and eliminate diseases. Alfalfa (Medicago sativa L.) may have the potential to compensate and substitute the need for chemical Zn additives in feeds as a Zn-biofortified feed crop when grown on Zn-enriched soils. Thus, this possibility was investigated with a greenhouse experiment using three soils with Zn concentrations (mg kg^-1) of 189 (soil A), 265 (soil B) and 1496 (soil C). Ethylenediamine-N,N'-disuccinate acid (EDDS) and Nitrilotriacetic acid (NTA) at different rates (0 as control, 0.5, 2 and 5 mmol kg^-1) were applied as soil additives to enhance the phytoextraction efficiency of alfalfa. The results showed that Zn was highly transferable in alfalfa tissues in the three soils even without additives. EDDS was more effective than NTA in enhancing Zn phytoextraction by alfalfa. The maximum Zn accumulation in the third cutting shoots was obtained with the EDDS concentration of 5 mmol kg^-1 in soil A and of 2 mmol kg^-1 in soil B, with a 462% and 162% increase compared with controls, respectively. However, the higher EDDS concentration resulted in a significant reduction in biomass production. In soil C, all EDDS concentrations resulted in similar Zn accumulations in the third shoot. To improve the phytoextraction efficacy of Zn while minimizing its phytotoxicity on alfalfa, the rate of 2 mmol kg^-1 EDDS proved to be optimal for soil B, and 0.5 mmol kg^-1 EDDS for soils A and C. Findings suggest that phytoextraction of Zn-enriched soil can be combined with Zn biofortification, thus allowing to recycle Zn into biomass that can, to an extent, substitute Zn feed additives. This study provided a primary data set for the combination of Zn-biofortification and Zn-phytoextraction.