Effects of Magnetically Treated Sedum Alfredii Seeds On The Dissolved Organic Matter Characteristics of Cd-Contaminated Soil During Phytoextraction

Effects of Magnetically Treated Sedum Alfredii Seeds On The Dissolved Organic Matter Characteristics of Cd-Contaminated Soil During Phytoextraction | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effects of Magnetically Treated Sedum Alfredii Seeds On The Dissolved Organic Matter Characteristics of Cd-Contaminated Soil During Phytoextraction Youjun Tang, Shuaizhi Ji, Dan Chen, Jiawei Wang, Min Cao, Jie Luo This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-560215/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 06 Nov, 2021 Read the published version in Environmental Science and Pollution Research → Version 1 posted 5 You are reading this latest preprint version Abstract The effects of magnetic field treatments on the two determining factors of phytoremediation, growth status and element uptake capacity, of Sedum alfredii have been thoroughly studied; however, minimal studies have been performed to determine the influence of the Cd hyperaccumulator Sedum alfredii , grown from magnetically treated seeds, on the dissolved organic matter (DOM) characteristics in its rhizosphere. A series of pot experiments were conducted to evaluate the variations in the DOM concentration and fractionations in the rhizosphere of S. alfredii treated with external magnetic fields. Compared with the untreated seeds, S. alfredii grown from magnetically treated seeds excreted more DOM in its rhizosphere. Additionally, the hydrophilic DOM fractionation proportion, which presented a greater capacity to mobilize Cd in the soil, increased from 42.7 % in the control sample to 47.2 % in the 150 mT magnetically treated S. alfredii sample. The water-soluble and exchangeable forms of Cd (extracted using deionized water and NH 4 NO 3 , respectively) in the rhizosphere of the magnetically treated S. alfredii were significantly lower than those of the control sample. Furthermore, the Cd extraction capacity of DOM from the rhizosphere of the magnetically treated S. alfredii was greater than that of the control sample, thereby increasing the Cd uptake ability of the magnetically treated species. These results suggest that the secretion of large amounts of DOM, especially acid and hydrophilic fractionations, is an essential mechanism of magnetically treated S. alfredii to mobilize Cd in the soil. Environmental Engineering Environmental Policy Phytoextraction Sedum alfredii Magnetic field Dissolved organic matter Rhizosphere Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction Cd, a well-known trace inorganic contaminant with high bioavailability and a long biological half-life, has detrimental effects on human health (Xiao et al., 2017 ). Cd-polluted soil decontamination using conventional technologies such as soil washing, excavation, in-situ solidification, and electrokinetic remediation is expensive and could damage the physical structure and ecological system of the soil (Komínková et al., 2018 ). Phytoextraction, the application of hyperaccumulators to remove pollutants from contaminated sites via plant uptake and migration to harvestable tissues, is an economical, ecologically friendly alternative to the traditional physical and chemical decontamination methods of contaminated sites. Various hyperaccumulator types, including Noccaea caerulescens for Cd and Zn (Escarre et al., 2000 ), Pteris vittata for As (Gonzaga et al., 2009 ), Arabis paniculate for Pb (Tang et al., 2009 ), and Phytolacca acinosa for Mn (Xue et al., 2004 ) have been identified and studied to evaluate the mobilization, extraction, transportation, hyperaccumulation, and detoxification mechanisms of contaminants in these plants. The major constraint restricting the widespread utilization of hyperaccumulators for soil remediation is the low biomass generation abilities of these plants (Li et al., 2018 ). Improving the dry weight and metal uptake efficiency of hyperaccumulators is critical for phytoextraction in metal-polluted fields; therefore, a thorough understanding of hyperaccumulator mechanisms that activate and accumulate metals is necessary. Previous studies have suggested that the application of an appropriate external magnetic field can enhance the phytoremediation efficiencies of Eucalyptus globulus (Luo et al., 2019a ), N. caerulescens (Luo et al., 2019b ), and Festuca arundinacea (Luo et al., 2020 ). For instance, an appropriate magnetic field can improve biomass yield and metal uptake efficiency by activating membrane transporters in plant tissues, providing energy for Mg 2+ to biosynthesize chlorophyll, and enhancing the antioxidant enzyme activities in plants to scavenge excessive free radicals as a result of balancing the endogenous biological magnetic field. Root-soil interactions can also modulate the metal activity in the rhizosphere and consequent alterations in the metal uptake abilities of plants. Many factors, including pH, cation exchange capacity, alkalinity, organic matter, microbial activity, and moisture content, can influence metal activity in the rhizosphere. The variation in pH in hyperaccumulator-growing soils has been thoroughly studied; however, inconsistent results have been reported. For example, Loosemore et al. ( 2004 ) found that the pH in the Nicotiana tabacum rhizosphere significantly decreased after growth occurred; however, McGrath et al. ( 1997 ) found that the pH in the N. caerulescens rhizosphere did not change, and Kim et al. ( 2010 ) reported that rhizosphere pH increased by 1.3 after B. juncea cultivation. These inconsistent results suggest that pH reduction in the rhizosphere is not the only metal activating mechanism in the rhizosphere by accumulators. Previous studies suggest that dissolved organic matter (DOM) controls metal fractionation and bioavailability rather than pH when the latter is higher than 6.5; therefore, DOM features in the rhizosphere should be considered during the phytoremediation process (Christensen and Christensen, 2000 ). DOM that can pass through a 0.45-µm filter has an affinity for bonding with metals, thus increasing metal bioavailability in the rhizosphere (Beiyuan et al., 2018 ; Bradney et al., 2019 ; El-Naggar et al., 2018 ). Kim et al. ( 2010 ) reported that plants could spontaneously adjust their rhizosphere to improve the uptake of beneficial elements by various mechanisms, such as acidification, lowering of oxidation-reduction potential, and organic matter excretion. Furthermore, hyperaccumulators secrete more DOM into their rhizosphere when subjected to high-metal environments, resulting in a decreased extraction rate of the corresponding metal hyperaccumulation by these plants (Guilpain et al., 2018 ; Tao et al., 2020 ). Based on previous studies on the major DOM characteristics and its impact on metal bioavailability in the rhizosphere, it has been determined that DOM can significantly reduce Cu, Pb, Ni, and Cd adsorption by soil particles, which increases the solubilities of these metals (Li et al., 2013c ). However, the effects of magnetically treated plants on DOM fractionation and metal bioavailability are not thoroughly understood; therefore, further evaluation is necessary to determine their effects on the rhizosphere of these plants during the phytoextraction process. S. alfredii , a Crassulacean acid metabolism species found in South China, is a candidate for phytoextraction in Cd-polluted fields owing to its physiological features (Li et al., 2013b ). Previous studies reported that an appropriate external magnetic field stimulated the growth rates of hyperaccumulators and high-biomass producing plants and improved their metal uptake capacities (Luo et al., 2019a , 2019b , 2020 ). Therefore, magnetic field application could alter the DOM characteristics in the rhizosphere of S. alfredii . The primary goals of this study were to evaluate the effects of magnetic field treatment on DOM concentrations and fractionations in the rhizosphere of S. alfredii , investigate the Cd extraction capacity of DOM from the rhizosphere of magnetically treated S. alfredii , and verify the feasibility of using magnetic fields to improve the phytoremediation efficiency of S. alfredii . 2. Materials And Methods 2.1 Plant and soil materials S. alfredii seeds were gathered from a lead-zinc mine in Shangfang Town, Quzhou City, China, and separated into four groups. Three groups were placed in non-magnetic containers and treated with a 50, 100, and 150 mT intensity static magnetic field for 20 min each day for one week, and the fourth group was exposed to a geomagnetic field and used as the control. Each treatment had five replicates. A field generator was used to produce the static magnetic field, and intensity stability was monitored using a Gauss magnetometer. After pre-sowing treatment, all plants were cultivated for 14 days under greenhouse conditions in a nutrient solution (Lu et al., 2008 ). The heavily contaminated soils used in this experiment were obtained from the top layer (20 cm) of a well-known electronic waste recycling center, Guiyu, located in south China, in which the soils have been heavily polluted owing to electronic waste dismantling activities using primitive recycling methods (Jiang et al., 2019 ). After air-drying, the soils were ground and sieved using a 2-mm nylon mesh. Four rounds of the wetting and drying processes were performed to equilibrate the soil, and then the mixed soil was divided into 6 kg aliquots and stored in PVC pots for S. alfredii transplantation. The Cd concentrations in the homogenized soil were 3.83 ± 0.46 mg kg − 1 . Four pre-cultured seedlings were transplanted into each pot. During the growth process, soil moisture was maintained at 70 % field capacity using deionized water and measuring the weights every day. Ninety days after the start of the experiment, all plants were carefully harvested, and rhizosphere soils from each pot were obtained by shaking the roots (Luster et al., 2009 ). The soil pH was measured before and after transplantation using a pH meter in a 1:2.5 soil-water suspension. The harvested plant tissues were washed with tap water to remove foreign materials and then soaked in a 10 mM EDTA-Na 2 solution to eliminate adsorbed ions. The cleaned tissues were dried in a 70°C oven until a constant weight was achieved. 2.2 DOM extraction and fractionation The DOM in the rhizosphere of S. alfredii was extracted following the procedures suggested by Jones and Willett ( 2006 ) with minor modifications, wherein 2.5 g of the rhizosphere soil was mixed with 25 mL of distilled water and shaken at 200 rpm for 120 min. The extracts were centrifuged at 10,000 × g for 20 min, and the recovered supernatant was sieved using 0.45-µm filters. A total organic carbon analyzer (TOC-5000, Shimadzu, Japan) was used to measure the dissolved organic carbon in the filtrate. Leenheer and Croue ( 2003 ) developed a dissolved organic carbon fractionation method that classified dissolved organics into six fractionations, including hydrophilic acid, hydrophilic base, hydrophilic neutral, hydrophobic acid, hydrophobic base, and hydrophobic neutral based on their polarity, alkaline-acidic properties, and specific compound characteristics. The DOM fractionation was performed according to the method suggested by Leenheer and Croue ( 2003 ), and the six DOM fractions were measured using the total organic carbon analyzer (TOC-5000, Shimadzu, Japan). 2.3 Cd extraction capacity of DOM Cations in the extracted DOM were removed using an Amberlite cation exchange H resin (Sigma-Aldrich, USA). After dilution to 100 mg L − 1 with deionized water, 20 mL of the solution was mixed with 2 g of soil in a centrifugal tube and centrifuged at 8000 × g for 30 min. After being sieved through a 0.45-µm membrane, the Cd concentration in the recovered supernatant was measured using an inductively coupled plasma mass spectrometer (ICP-MS) (Agilent 7700, USA). The soil samples (2 g) were mixed with deionized water and prepared using the same procedure as the control. 2.4 Cd analysis The dried plant and rhizosphere soil samples were ground and sieved using a 74-µm nylon mesh. The plant samples were digested through the aqua regia dissolution method (Ok et al., 2011 ), wherein the prepared plant samples were digested in a solution of HNO 3 and HCl (1:3 ratio) and heated for 100 min at 130°C. After cooling, the digestion was filtered through a 0.45-µm membrane, and the Cd concentration in the recovered filtrate was analyzed via ICP-MS (Agilent 7700, USA). The water-soluble and exchangeable fractions of Cd in the rhizosphere were determined according to the methodology described in a previous study (He et al., 2020 ), wherein 1.2 g of dried soil was shaken with 25 mL CaCl 2 (10 mmol L − 1 ) in a centrifugal tube at 200 rpm for 30 min and then centrifuged at 4000 × g for 15 min. After filtration through a 0.45-µm filter, the supernatant was collected and the water-soluble Cd was measured using the ICP-MS (Agilent 7700, USA). The residue recovered from the water-soluble extraction was shaken with 25 mL of NH 4 NO 3 (1 mol L − 1 ) in a centrifugal tube at 200 rpm for 30 min. During shaking, the pH of the mixture was maintained at 7.0 (He et al., 2020 ). The centrifugation and analysis procedures in the exchangeable Cd fraction were identical to those used for the water-soluble fraction. Carbonate, Fe-Mn oxide, organic matter, and the residual fractions of Cd in the remaining sedimentation reclaimed from the exchangeable extraction were extracted using 1 M NaOAc, a mixture of 0.04 M NH 2 .OH.HCl and 25 % HOAc, a mixture of 0.02 M HNO 3 and 30 % H 2 O 2 , and aqua regia in sequence, according to Li and Thornton ( 2001 ). The sum of these six chemical fractions represented the total Cd concentration in the rhizosphere. Quality control was performed by analyzing two reference materials, GBW07410 (soil) and GBW10012 (plant), which were obtained from the China Standard Materials Research Center (Beijing, China). 2.5 Statistical analysis The data are represented as the average of the five replicates used in this experiment. The magnetic field treatment effects on the dry weight of S. alfredii , DOM concentration and fractionations in the rhizosphere, mobile Cd in the soil, and Cd extraction ability of DOM were tested via one-way analysis of variance. Means of significant difference ( p < 0.05) were evaluated by Fisher’s protected least significant difference test, and SPSS 15.0 software was used to conduct the statistical analyses in this study. 3. Results And Discussion 3.1 Soil pH A significant pH variation among the treatments was not observed during this study. Compared with the initial soil pH (6.1 ± 0.2) at transplantation, the magnetically treated and untreated S. alfredii reduced the pH by 0.2 and 0.1, respectively, in its rhizosphere during the 90-day experiment. These results indicate that the soil used can buffer the plant-induced pH variation, and the pH changes in the S. alfredii rhizosphere are not responsible for Cd mobilization in this experiment. Soil pH significantly influences the metal activity in the substrate, which is enhanced by reducing the pH value (Hu et al., 2018 ; Römkens et al., 2002 ; Wang et al., 2020 ). The pH variation in the rhizosphere soil has been reported for other plants. For instance, N. tabacum reduces the pH in the rhizosphere (Loosemore et al., 2004 ), B. juncea increases the soil pH significantly (Kim et al., 2010 ), and N. caerulescens has no effect on the pH in the rhizosphere (McGrath et al., 1997 ). The pH variation in the soil is a complex process that is dependent on soil properties, plant types, and weather conditions. Generally, Cd is present as an oxyanion in soil, and its uptake by S. alfredii could enhance the pH in the rhizosphere because the plant releases OH − in the soil to maintain the charge balance after extracting Cd. Decreased soil pH was caused by increased root secretions, including rhamnolipids, surfactin, and humic and fulvic acids (Davin et al., 2018 ). The combination of these two processes could determine the final soil pH during phytoremediation. 3.2 DOM concentration and fractionation The final DOM concentrations in the rhizospheres of the 50, 100, and 150 mT treated S. alfredii were 156.9, 192.5, and 198.3 mg kg − 1 , respectively, which are higher than the initial concentration of the experiment (129.3 mg kg − 1 ). Compared with the control that was exposed to a geomagnetic field, the 100 and 150 mT treatments significantly increased the DOM concentrations in the rhizosphere of S. alfredii (Fig. 1 ), while the 50 mT treatment had a slightly lower value ( p > 0.05). The 100 and 150 mT treated samples showed 17.6 % and 21.1 % increases in the DOM concentrations in the soil, respectively, relative to the control; however, no significant DOM concentration differences were observed between the 100 and 150 mT treatments (Fig. 1 ). The different fraction distributions in the soil of the various treatments are presented in Fig. 2 . The DOM fractions in the S. alfredii soil exhibited different distribution trends among various treatments, and the hydrophobic fraction concentrations (hydrophobic acid, base, and neutral) did not significantly vary among the treatments during phytoremediation. In contrast, the hydrophilic fraction concentrations (hydrophilic acid, base, and neutral) were significantly enhanced when treated by 100 and 150 mT than those treated with the geomagnetic field and 50 mT. Generally, the control had higher hydrophobic fraction concentrations than the other treatments, while the 150 mT treatment showed higher hydrophilic fraction concentrations than the other treatments. The acid fraction concentrations (hydrophilic and hydrophobic acid) were significantly higher in the 100 and 150 mT treatments than in the control and 50 mT treatments. Furthermore, the acid fraction was the predominant DOM component in all the soil treatments, accounting for 77.8 %, 76.5 %, 77.1 %, and 74.9 % of the DOM in the soil of the control, 50, 100, and 150 mT treatments, respectively. Meanwhile, the hydrophobic fraction proportion decreased from 57.3 % to 43.0 %, and the hydrophilic fraction proportion increased from 42.7 % to 57.2 % in the control and 150 mT treatment, respectively. During the phytoremediation process, root secretion is one of the most significant DOM sources (Martin et al., 2017 ); therefore, the elevated DOM concentrations in the soils at the end of the experiment might be induced by the presence of S. alfredii . Although the effects of external magnetic fields on root secretion have not been determined, many studies suggest that magnetic fields regulate plant metabolism and growth rate on the cellular, subcellular, and molecular levels (Ćirković et al., 2017 ; Shokrollahi et al., 2018 ; Teixeira da Silva and Dobránszki, 2016 ). Therefore, the increased DOM concentration in the rhizosphere of magnetically treated S. alfredii could be partially induced by the stimulation of the applied magnetic fields. DOM contains many functional components, including sulfhydryl, amino, and phosphoryl groups, which have a strong affinity to metals and increase the metal bioavailability in soil (Borggaard et al., 2019 ; Zhou et al., 2019 ). The chemical activity of hydrophilic acids is greater than that of hydrophobic acids because the former have more functional groups that can activate the metals in soil (Huang et al., 2019 ). Therefore, the chemical components of DOM in the rhizospheres of the 100 and 150 mT treated S. alfredii are more conducive to increasing Cd mobility. The mechanisms in which magnetic fields regulate root exudation regarding the physiological characteristics of plants require further investigation. 3.3 Chemical fractions of Cd in the rhizosphere S. alfredii growth did not change the total Cd concentration in the soil compared to the initial state; however, it increased the variable coefficient of the metal (Fig. 3 ), which concurs with the findings of a previous study, suggesting that plant growth can heterogenize the soil over time (He et al., 2020 ). The chemical fractions of Cd in the soil were analyzed at the beginning and end of the experiment. The initial water-soluble and exchangeable Cd concentrations in the soil decreased significantly after the cultivation of S. alfredii under all treatments, and the reduction in the rhizosphere increased with increasing magnetic intensity. Compared with the control, the 50 mT treatment did not significantly change the water-soluble Cd concentration, but it significantly decreased the exchangeable fraction concentration (Fig. 3 ). However, the 100 and 150 mT treatments decreased the water-soluble and exchangeable Cd concentrations, and their lowest concentrations were observed in the S. alfredii rhizosphere exposed to 150 mT. Li et al. ( 2013a ) compared the water-soluble Cd concentration variations in soils after the growth of hyperaccumulating and non-hyperaccumulating ecotypes of S. alfredii . They found that the hyperaccumulating ecotype of S. alfredii decreased the water-soluble Cd in slightly contaminated soil, which agrees with the results of this study. In contrast, the non-hyperaccumulating ecotype of S. alfredii increased the water-soluble Cd in the rhizosphere because S. alfredii activates Cd in the soil through increased root excretions, and Cd solubilization is faster than the Cd uptake by the plant. The water-soluble and exchangeable Cd concentrations in the soils decreased significantly after the S. alfredii growth with or without magnetic field treatment. However, the chemical fraction reductions in the rhizosphere were less than 1 % of the extraction amount in the S. alfredii , indicating that more than 99 % of the Cd uptake migrated from other fractions. Soils can spontaneously maintain a dynamic balance of a specific element during plant growth via a pair of opposite processes, i.e., mobilizing the element in soil and its extraction by plants. This study shows that the effective Cd extraction root system of S. alfredii can eliminate water-soluble and exchangeable Cd faster than the rate Cd is supplemented from other chemical fractions, and S. alfredii , particularly the magnetically treated species, can mobilize Cd that is not initially bioavailable. The mobilizing ability of S. alfredii increased with increasing magnetic field intensity. Increased DOM in the rhizosphere of the species can regulate the Cd mobilization process. As shown in Fig. 3 , the proportion of organic matter Cd increased significantly from 11.0 % in the control to 25.7 % and 25.9 % in the 100 and 150 mT treatments, respectively, while no significant changes in the carbonate, Fe-Mn oxide, and residual fraction proportions were observed. This finding is consistent with the results suggested by Krishnamurti et al. ( 2004 ) who suggested that Cd-DOM complexes can be taken up intact by Chlorococcum sp. and result in increased Cd toxicity. Generally, soil pH and DOM concentration are the two dominant factors controlling bioavailability and chemical activity in soil (Christensen and Christensen, 2000 ). In this study, soil pH did not vary after plant growth; therefore, the variation in the chemical fraction of Cd in the soil is mainly driven by the increased DOM in the rhizosphere. 3.4 Cd extraction capacity of DOM from the rhizosphere The DOM in the magnetically treated soils of S. alfredii significantly enhanced the Cd solubility in the soils relative to deionized water (Fig. 4 ). For example, the Cd concentration extracted using 100 mg L − 1 DOM from the control and the 50, 100, and 150 mT treatments were 48.3 %, 58.6 %, 137.9 %, and 151.7 % higher, respectively, than that extracted using deionized water, indicating that the Cd extraction capacity of DOM improved with increasing magnetic field intensity. Borggaard et al. ( 2019 ) reported that increased DOM concentration improved the metal solubility in the soil and attributed the metal extractability to hydrophilic fractions. Chen et al. ( 2018 ) studied the influence of DOM on Zn extractability in calcareous soil and suggested that the addition of maize straw resulted in increased amounts of applied Zn recovered by the DTPA-extractable fraction, which is attributed to the increased DOM concentration and consequently, increased formation of Zn-fulvic acid complexes. Additionally, the DOM from the soils of the 100 and 150 mT treated S. alfredii had a greater ability to mobilize Cd in the rhizosphere compared with the control and 50 mT treatment. For instance, the Cd concentration extracted by the DOM in the 150 mT treatment was 0.73 mg L − 1 , which was 0.70 and 0.59 times greater than that of the control and 50 mT treatments, respectively. This indicates that the DOM fractions under the different magnetic field treatments varied and potentially influenced Cd mobilization. Furthermore, the magnetically treated S. alfredii had a greater capacity to generate chelates with Cd compared with the control. 3.5 Phytoextraction efficiency The average soil Cd concentration difference before and after transplantation could not be used to estimate the phytoextraction efficiency of S. alfredii because the duration of the experiment was not long enough for the S. alfredii to change the total Cd concentration in its rhizosphere. Therefore, the Cd uptake in the harvested tissues was calculated to evaluate the phytoremediation effect of S. alfredii . The Cd uptake of S. alfredii was defined as the product of the plant’s dry weight (belowground and aerial plant parts) and Cd concentration. As shown in Table 1 , the 150 mT treated S. alfredii had the largest dry weight, and that of the 100 and 50 mT treatments and the control were successively lower. Except for the 50 mT treatment, magnetic field treatments significantly improved the dry weight. Similarly, Cd concentrations in the belowground and aerial plant parts increased with increasing magnetic field intensity (Table 1 ), consistent with the DOM concentration and fraction variation trends in the S. alfredii rhizosphere. Table 1 Dry weight, Cd concentrations, and Cd decontamination capacity of S. alfredii in different treatments Dry weight (g) Cd content Cd accumulation (mg pot − 1 ) Roots Shoots Roots Shoots control 0.73 ± 0.12 8.21 ± 1.25 22.3 ± 2.9 71.2 ± 5.6 2.4 50 mT 0.69 ± 0.08 9.22 ± 1.03 25.6 ± 2.2 66.5 ± 6.1 2.5 100 mT 1.02 ± 0.09 10.63 ± 1.52 29.7 ± 3.1 90.5 ± 9.1 4 150 mT 1.12 ± 0.15 12.39 ± 2.56 38.1 ± 2.6 95.7 ± 7.1 4.9 Based on the dry weight and Cd content, 2.4, 2.5, 4.0, and 4.9 mg of Cd were extracted from the control, 50, 100, and 150 mT treatment pots. The excess Cd in each pot that required removal was calculated as the product of the difference between the initial soil Cd concentration (3.83 mg kg − 1 ) and its safe threshold (0.30 mg kg − 1 ) and the soil mass (6 kg). The required number of planting cycles to clean the soil was determined by the excess Cd divided by the Cd uptake capacity of S. alfredii ; therefore, 9, 9, 6, and 5 planting cycles are required for S. alfredii to decontaminate the soil in the control, 50, 100, and 150 mT treatments, respectively. A previous study concluded that an appropriate magnetic field improves the plant dry weight by increasing antioxidant enzyme activity that scavenges oxyradicals resulting from stresses, balancing the internal magnetic field in plants, and increasing cytomembrane permeability to enhance nutrient pass through (Luo et al., 2020 ). The study focused on the plant improvement effects of magnetic fields rather than effects on the rhizosphere. Previous studies have shown that many plants can modulate the chemical properties of their rhizosphere to improve pollutant extraction via various pathways, including acidification, stimulating microbial proliferation, and DOM excretion (Cao et al., 2018 ; Gattullo et al., 2015 ). This study found that the increased Cd uptake capacity of magnetically treated S. alfredii was related to increased DOM concentrations and its acid and hydrophilic fractions. Furthermore, higher Cd extractability was observed for DOM under magnetic field treatments, particularly when exposed to 150 mT, indicating that DOM from magnetically treated soils exhibits a greater capacity to mobilize Cd in the soil by generating a complex. The Cd extraction capacity of magnetically treated S. alfredii , particularly at 150 mT, was much greater than that of the control and decreased the required remediation time by half. This study demonstrated that applying an appropriate magnetic field is an effective method to enhance the phytoextraction efficiency of S. alfredii based on the variation in DOM concentrations and its fractions in the soil. Conclusions This study determined that magnetic field treatment, particularly a 150 mT treatment, improves the phytoextraction of S. alfredii by increasing its dry weight and Cd uptake capacity. Compared to the control, the DOM concentrations and acid fractions were significantly higher in the 100 and 150 mT treatments. Additionally, the hydrophilic fraction proportions significantly influence metal mobilization in the soil, which increases with increasing magnetic intensity. Compared with the initial state, water-soluble and exchangeable Cd in the rhizosphere decreased significantly after the S. alfredii growth, and the decreases were significantly higher in the magnetically treated plants than in the control. The Cd extraction capacity of DOM from the rhizosphere of magnetically treated S. alfredii was significantly higher than that of the control, thus increasing the Cd concentrations in the plant tissues. This study reveals that the primary mechanism to increase Cd extraction in S. alfredii is the DOM in magnetically treated S. alfredii , which increases the Cd mobilization capacity in the rhizosphere by generating DOM-Cd (organic matter Cd fraction) and consequently increases the phytoextraction efficiency of S. alfredii in Cd-polluted soil. Declarations Corresponding author's email address : [email protected] Ethical Approval : Not applicable. This work did not describe experiments with animals, human subjects, or human tissue samples. Consent to Participate : Not applicable. This work did not describe experiments with animals, human subjects, or human tissue samples. Consent to Publish : The manuscript entitled, “Effects of magnetically treated Sedum alfredii seeds on the dissolved organic matter characteristics of Cd-contaminated soil during phytoextraction” is prepared in accordance with the Guide for authors available on the journal’s website and it has not been published elsewhere in part or in its entirety. All authors attest to the validity of its contents, and agree to its submission in Environmental Science and Pollution Research. Authors Contributions : Conceptualization, Youjun Tang, Jie Luo; Data curation, Shuaizhi Ji, Jiawei Wang; Methodology, Jie Luo, Dan Chen; Funding acquisition, Jie Luo; Writing – original draft, Youjun Tang, Shuaizhi Ji, Jie Luo; Writing – review & editing, Min Cao, Dan Chen. All authors read and approved the final manuscript. Funding : The authors thank the National Natural Science Foundation of China (Project No. 21876014) for financial support to carry out this study. 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Processes 7:1–16 Jiang S, Luo J, Ye Y, Yang G, Pi W, He W (2019) Using Pb Isotope to Quantify the Effect of Various Sources on Multi-Metal Polluted soil in Guiyu. Bull Environ Contam Toxicol 102:413–418 Jones DL, Willett VB (2006) Experimental evaluation of methods to quantify dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) in soil. Soil Biol Biochem 38:991–999 Kim KR, Owens G, Naidu R, Kwon S, lk (2010) Influence of plant roots on rhizosphere soil solution composition of long-term contaminated soils. Geoderma 155:86–92 Komínková D, Fabbricino M, Gurung B, Race M, Tritto C, Ponzo A (2018) Sequential application of soil washing and phytoremediation in the land of fires. J Environ Manage 206:1081–1089 Krishnamurti GSR, Megharaj M, Naidu R (2004) Bioavailability of cadmium-organic complexes to soil alga - An exception to the free ion model. J Agric Food Chem 52:3894–3899 Leenheer JA, Croue JP (2003) Characterizing dissolved aquatic organic matter. Environ Sci Technol 1:19–26 Li JT, Gurajala HK, Wu LH, Van Der Ent A, Qiu RL, Baker AJM, Tang YT, Yang XE, Shu WS (2018) Hyperaccumulator Plants from China: A Synthesis of the Current State of Knowledge. Environ Sci Technol 52:11980–11994 Li T, Liang C, Han X, Yang X (2013a) Mobilization of cadmium by dissolved organic matter in the rhizosphere of hyperaccumulator Sedum alfredii. Chemosphere 91:970–976 Li T, Tao Q, Han X, Yang X (2013b) Effects of elevated CO2 on rhizosphere characteristics of Cd/Zn hyperaccumulator Sedum alfredii. Sci Total Environ 454–455:510–516 Li T, Tao Q, Liang C, Shohag MJI, Yang X, Sparks DL (2013c) Complexation with dissolved organic matter and mobility control of heavy metals in the rhizosphere of hyperaccumulator Sedum alfredii. Environ Pollut 182:248–255 Li X, Thornton I (2001) Chemical partitioning of trace and major elements in soils contaminated bymining and smelting activities. Appl Geochem 16:1693–1706 Loosemore N, Straczek A, Hinsinger P, Jaillard B (2004) Zinc mobilisation from a contaminated soil by three genotypes of tobacco as affected by soil and rhizosphere pH. Plant Soil 260:19–32 Lu LL, Tian SK, Yang XE, Wang XC, Brown P, Li TQ, He ZL (2008) Enhanced root-to-shoot translocation of cadmium in the hyperaccumulating ecotype of Sedum alfredii. J Exp Bot 59:3203–3213 Luo J, He W, Qi S, Wu J, Gu XS (2020) A novel phytoremediation method assisted by magnetized water to decontaminate soil Cd based on harvesting senescent and dead leaves of Festuca arundinacea. J Hazard Mater 383:1–6 Luo J, He W, Xing X, Wu J, Gu XWS (2019a) The phytoremediation efficiency of Eucalyptus globulus treated by static magnetic fields before sowing. Chemosphere 226:891–897 Luo J, He W, Yang D, Wu J, Sophie. Gu XW (2019b) Magnetic field enhance decontamination efficiency of Noccaea caerulescens and reduce leaching of non-hyperaccumulated metals. J Hazard Mater 368:141–148 Luster J, Göttlein A, Nowack B, Sarret G (2009) Sampling, defining, characterising and modeling the rhizosphere-the soil science tool box. Plant Soil 321:457–482 Martin BC, Statton J, Siebers AR, Grierson PF, Ryan MH, Kendrick GA (2017) Colonizing tropical seagrasses increase root exudation under fluctuating and continuous low light. Limnol Oceanogr 63:381–391 McGrath SP, Shen ZG, Zhao FJ (1997) Heavy metal uptake and chemical changes in the rhizosphere. Plant Soil 188:153–159 Ok YS, Kim SC, Kim DK, Skousen JG, Lee JS, Cheong YW, Kim SJ, Yang JE (2011) Ameliorants to immobilize Cd in rice paddy soils contaminated by abandoned metal mines in Korea. Environ Geochem Health 33:23–30 Römkens P, Bouwman L, Japenga J, Draaisma C (2002) Potentials and drawbacks of chelate-enhanced phytoremediation of soils. Environ Pollut 116:109–121 Shokrollahi S, Ghanati F, Sajedi RH, Shari M (2018) Possible role of iron containing proteins in physiological responses of soybean to static magnetic fi eld 226, 163–171 Tang YT, Qiu RL, Zeng XW, Ying RR, Yu FM, Zhou XY (2009) Lead, zinc, cadmium hyperaccumulation and growth stimulation in Arabis paniculata Franch. Environ Exp Bot 66:126–134 Tao Q, Zhao J, Li J, Liu Y, Luo J, Yuan S, Li B, Li Q, Xu Q, Yu X, Huang H, Li T, Wang C (2020) Unique root exudate tartaric acid enhanced cadmium mobilization and uptake in Cd-hyperaccumulator Sedum alfredii. J Hazard Mater 383:1–10 Teixeira da Silva JA, Dobránszki J (2016) Magnetic fields: how is plant growth and development impacted? Protoplasma 253:231–248 Wang G, Wang L, Ma F, You Y, Wang Y, Yang D (2020) Integration of earthworms and arbuscular mycorrhizal fungi into phytoremediation of cadmium-contaminated soil by Solanum nigrum L. J Hazard Mater 389:1–11 Xiao W, Li D, Ye X, Xu H, Yao G, Wang J, Zhang Q, Hu J, Gao N (2017) Enhancement of Cd phytoextraction by hyperaccumulator Sedum alfredii using electrical field and organic amendments. Environ Sci Pollut Res 24:5060–5067 Xue SG, Chen YX, Reeves RD, Baker AJM, Lin Q, Fernando DR (2004) Manganese uptake and accumulation by the hyperaccumulator plant Phytolacca acinosa Roxb. (Phytolaccaceae) Environmental Pollution 131:393–399 Zhou Q, Li X, Lin Y, Yang C, Tang W (2019) Effects of copper ions on removal of nutrients from swine wastewater and on release of dissolved organic matter in duckweed systems. Water Res 158:171–181 Cite Share Download PDF Status: Published Journal Publication published 06 Nov, 2021 Read the published version in Environmental Science and Pollution Research → Version 1 posted Reviews received at journal 21 Jun, 2021 Reviewers invited by journal 18 Jun, 2021 Editor invited by journal 03 Jun, 2021 Editor assigned by journal 25 May, 2021 First submitted to journal 24 May, 2021 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-560215","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":34342731,"identity":"34382304-d696-4f52-91d2-9e0ff8dda0c3","order_by":0,"name":"Youjun Tang","email":"","orcid":"","institution":"Yangtze University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Youjun","middleName":"","lastName":"Tang","suffix":""},{"id":34342732,"identity":"c2111012-ddaf-4af5-b8ab-680e50a8c3f1","order_by":1,"name":"Shuaizhi Ji","email":"","orcid":"","institution":"sinopec Zhongyuan oilfield","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Shuaizhi","middleName":"","lastName":"Ji","suffix":""},{"id":34342733,"identity":"4fe7212a-05bb-4ad3-b1bf-5c6bd666be95","order_by":2,"name":"Dan Chen","email":"","orcid":"","institution":"Yangtze University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Dan","middleName":"","lastName":"Chen","suffix":""},{"id":34342734,"identity":"3e95a346-3868-46ef-a8cd-0cec6575a518","order_by":3,"name":"Jiawei Wang","email":"","orcid":"","institution":"Yangtze University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Jiawei","middleName":"","lastName":"Wang","suffix":""},{"id":34342735,"identity":"8c1ca8a4-3899-4b87-9058-4ba4fb9336d5","order_by":4,"name":"Min Cao","email":"","orcid":"","institution":"University of Leicester","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Min","middleName":"","lastName":"Cao","suffix":""},{"id":34342736,"identity":"913b827a-62ca-4855-ab67-d2e7227553c3","order_by":5,"name":"Jie Luo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA8UlEQVRIiWNgGAWjYFACHhAhIcfG3nwMzGdjJ06LhTEfz7E0BoYEoBZm4rRUJM6T8DEDa2EgpEXe/ezBxxUVEoxtEjzfHnz8sU2ej5mB8cPHHNxaDM/kJRueOSPBzCbdu91wRsJtwzZmBmbJmdvwaJnBYybZ2CbBxiZzdps0T8JtRqAWNmZe/FrMfwK18LBJ5DwDabEnqEVegseMEahFAqiFDaQlkaAWA54cY8mGMxIGbDzHzCRnpN1ObmNmbMbrF/n2M4YfGyrq6ue3Nz+T+GBz2xbIOPjhIz5bDmCKMTbgVg+yBb/0KBgFo2AUjAIgAAAJB0eDSt9WEgAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0001-6543-9346","institution":"Yangtze University","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Jie","middleName":"","lastName":"Luo","suffix":""}],"badges":[],"createdAt":"2021-05-26 02:12:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-560215/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-560215/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11356-021-17312-w","type":"published","date":"2021-11-06T06:59:02+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":10644529,"identity":"d16e59b8-cabc-4b80-8758-0ed733eac17b","added_by":"auto","created_at":"2021-06-22 14:58:44","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1133950,"visible":true,"origin":"","legend":"DOM concentrations in the rhizosphere of S. alfredii\nDifferent letters represent significant differences in DOM content (p \u003c 0.05) evaluated by Fisher’s LSD post-hoc tests in different treatments.","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-560215/v1/ce6026b6ec558577b2795f7e.jpeg"},{"id":10644533,"identity":"03e15793-c6aa-4f1b-b1ca-75cd48fea330","added_by":"auto","created_at":"2021-06-22 14:58:44","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":158625,"visible":true,"origin":"","legend":"DOM fractions in the rhizosphere of S. alfredii\nDifferent letters represent significant differences in DOM fractions (p \u003c 0.05) evaluated by Fisher’s LSD post-hoc tests in different treatments.","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-560215/v1/861f95104bef4f1e7b59b4a0.jpeg"},{"id":10644535,"identity":"731fb4bd-9488-47d0-8af3-1526996c9076","added_by":"auto","created_at":"2021-06-22 14:58:44","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1766667,"visible":true,"origin":"","legend":"Chemical fractions of Cd in the rhizosphere of S. alfredii\nDifferent letters represent significant differences in Cd fractions (p \u003c 0.05) evaluated by Fisher’s LSD post-hoc tests in different treatments.","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-560215/v1/2a727f90628d0c93abe1aae0.jpeg"},{"id":10645356,"identity":"83f790b0-1796-4014-8722-16e64a1e480a","added_by":"auto","created_at":"2021-06-22 15:01:44","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1107775,"visible":true,"origin":"","legend":"Cd extraction abilities of DOM in different treatments\nDifferent letters represent significant differences in Cd extraction (p \u003c 0.05) evaluated by Fisher’s LSD post-hoc tests in different treatments.","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-560215/v1/5c508d4a69392061b3c8ed4b.jpeg"},{"id":18877203,"identity":"583baabf-e8a9-4694-903a-49ccab21bc1c","added_by":"auto","created_at":"2022-03-05 06:59:09","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":593964,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-560215/v1/1cb0b290-bc78-4e16-b6c4-8734ee1bf90f.pdf"}],"financialInterests":"","formattedTitle":"\u003cp\u003eEffects of Magnetically Treated \u003cem\u003eSedum Alfredii\u003c/em\u003e Seeds On The Dissolved Organic Matter Characteristics of Cd-Contaminated Soil During Phytoextraction\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":" \u003cp\u003eCd, a well-known trace inorganic contaminant with high bioavailability and a long biological half-life, has detrimental effects on human health (Xiao et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Cd-polluted soil decontamination using conventional technologies such as soil washing, excavation, in-situ solidification, and electrokinetic remediation is expensive and could damage the physical structure and ecological system of the soil (Kom\u0026iacute;nkov\u0026aacute; et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePhytoextraction, the application of hyperaccumulators to remove pollutants from contaminated sites via plant uptake and migration to harvestable tissues, is an economical, ecologically friendly alternative to the traditional physical and chemical decontamination methods of contaminated sites. Various hyperaccumulator types, including \u003cem\u003eNoccaea caerulescens\u003c/em\u003e for Cd and Zn (Escarre et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2000\u003c/span\u003e), \u003cem\u003ePteris vittata\u003c/em\u003e for As (Gonzaga et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2009\u003c/span\u003e), \u003cem\u003eArabis paniculate\u003c/em\u003e for Pb (Tang et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2009\u003c/span\u003e), and \u003cem\u003ePhytolacca acinosa\u003c/em\u003e for Mn (Xue et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) have been identified and studied to evaluate the mobilization, extraction, transportation, hyperaccumulation, and detoxification mechanisms of contaminants in these plants. The major constraint restricting the widespread utilization of hyperaccumulators for soil remediation is the low biomass generation abilities of these plants (Li et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eImproving the dry weight and metal uptake efficiency of hyperaccumulators is critical for phytoextraction in metal-polluted fields; therefore, a thorough understanding of hyperaccumulator mechanisms that activate and accumulate metals is necessary. Previous studies have suggested that the application of an appropriate external magnetic field can enhance the phytoremediation efficiencies of \u003cem\u003eEucalyptus globulus\u003c/em\u003e (Luo et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2019a\u003c/span\u003e), \u003cem\u003eN. caerulescens\u003c/em\u003e (Luo et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2019b\u003c/span\u003e), and \u003cem\u003eFestuca arundinacea\u003c/em\u003e (Luo et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). For instance, an appropriate magnetic field can improve biomass yield and metal uptake efficiency by activating membrane transporters in plant tissues, providing energy for Mg\u003csup\u003e2+\u003c/sup\u003e to biosynthesize chlorophyll, and enhancing the antioxidant enzyme activities in plants to scavenge excessive free radicals as a result of balancing the endogenous biological magnetic field.\u003c/p\u003e \u003cp\u003eRoot-soil interactions can also modulate the metal activity in the rhizosphere and consequent alterations in the metal uptake abilities of plants. Many factors, including pH, cation exchange capacity, alkalinity, organic matter, microbial activity, and moisture content, can influence metal activity in the rhizosphere. The variation in pH in hyperaccumulator-growing soils has been thoroughly studied; however, inconsistent results have been reported. For example, Loosemore et al. (\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) found that the pH in the \u003cem\u003eNicotiana tabacum\u003c/em\u003e rhizosphere significantly decreased after growth occurred; however, McGrath et al. (\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e1997\u003c/span\u003e) found that the pH in the \u003cem\u003eN. caerulescens\u003c/em\u003e rhizosphere did not change, and Kim et al. (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) reported that rhizosphere pH increased by 1.3 after \u003cem\u003eB. juncea\u003c/em\u003e cultivation. These inconsistent results suggest that pH reduction in the rhizosphere is not the only metal activating mechanism in the rhizosphere by accumulators.\u003c/p\u003e \u003cp\u003ePrevious studies suggest that dissolved organic matter (DOM) controls metal fractionation and bioavailability rather than pH when the latter is higher than 6.5; therefore, DOM features in the rhizosphere should be considered during the phytoremediation process (Christensen and Christensen, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). DOM that can pass through a 0.45-\u0026micro;m filter has an affinity for bonding with metals, thus increasing metal bioavailability in the rhizosphere (Beiyuan et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Bradney et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; El-Naggar et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Kim et al. (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) reported that plants could spontaneously adjust their rhizosphere to improve the uptake of beneficial elements by various mechanisms, such as acidification, lowering of oxidation-reduction potential, and organic matter excretion. Furthermore, hyperaccumulators secrete more DOM into their rhizosphere when subjected to high-metal environments, resulting in a decreased extraction rate of the corresponding metal hyperaccumulation by these plants (Guilpain et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Tao et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Based on previous studies on the major DOM characteristics and its impact on metal bioavailability in the rhizosphere, it has been determined that DOM can significantly reduce Cu, Pb, Ni, and Cd adsorption by soil particles, which increases the solubilities of these metals (Li et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2013c\u003c/span\u003e). However, the effects of magnetically treated plants on DOM fractionation and metal bioavailability are not thoroughly understood; therefore, further evaluation is necessary to determine their effects on the rhizosphere of these plants during the phytoextraction process.\u003c/p\u003e \u003cp\u003e \u003cem\u003eS. alfredii\u003c/em\u003e, a Crassulacean acid metabolism species found in South China, is a candidate for phytoextraction in Cd-polluted fields owing to its physiological features (Li et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2013b\u003c/span\u003e). Previous studies reported that an appropriate external magnetic field stimulated the growth rates of hyperaccumulators and high-biomass producing plants and improved their metal uptake capacities (Luo et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2019a\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2019b\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Therefore, magnetic field application could alter the DOM characteristics in the rhizosphere of \u003cem\u003eS. alfredii\u003c/em\u003e. The primary goals of this study were to evaluate the effects of magnetic field treatment on DOM concentrations and fractionations in the rhizosphere of \u003cem\u003eS. alfredii\u003c/em\u003e, investigate the Cd extraction capacity of DOM from the rhizosphere of magnetically treated \u003cem\u003eS. alfredii\u003c/em\u003e, and verify the feasibility of using magnetic fields to improve the phytoremediation efficiency of \u003cem\u003eS. alfredii\u003c/em\u003e.\u003c/p\u003e "},{"header":"2. Materials And Methods","content":" \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Plant and soil materials\u003c/h2\u003e \u003cp\u003e \u003cem\u003eS. alfredii\u003c/em\u003e seeds were gathered from a lead-zinc mine in Shangfang Town, Quzhou City, China, and separated into four groups. Three groups were placed in non-magnetic containers and treated with a 50, 100, and 150 mT intensity static magnetic field for 20 min each day for one week, and the fourth group was exposed to a geomagnetic field and used as the control. Each treatment had five replicates. A field generator was used to produce the static magnetic field, and intensity stability was monitored using a Gauss magnetometer. After pre-sowing treatment, all plants were cultivated for 14 days under greenhouse conditions in a nutrient solution (Lu et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2008\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe heavily contaminated soils used in this experiment were obtained from the top layer (20 cm) of a well-known electronic waste recycling center, Guiyu, located in south China, in which the soils have been heavily polluted owing to electronic waste dismantling activities using primitive recycling methods (Jiang et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). After air-drying, the soils were ground and sieved using a 2-mm nylon mesh. Four rounds of the wetting and drying processes were performed to equilibrate the soil, and then the mixed soil was divided into 6 kg aliquots and stored in PVC pots for \u003cem\u003eS. alfredii\u003c/em\u003e transplantation. The Cd concentrations in the homogenized soil were 3.83\u0026thinsp;\u0026plusmn;\u0026thinsp;0.46 mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFour pre-cultured seedlings were transplanted into each pot. During the growth process, soil moisture was maintained at 70 % field capacity using deionized water and measuring the weights every day. Ninety days after the start of the experiment, all plants were carefully harvested, and rhizosphere soils from each pot were obtained by shaking the roots (Luster et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). The soil pH was measured before and after transplantation using a pH meter in a 1:2.5 soil-water suspension.\u003c/p\u003e \u003cp\u003eThe harvested plant tissues were washed with tap water to remove foreign materials and then soaked in a 10 mM EDTA-Na\u003csub\u003e2\u003c/sub\u003e solution to eliminate adsorbed ions. The cleaned tissues were dried in a 70\u0026deg;C oven until a constant weight was achieved.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 DOM extraction and fractionation\u003c/h2\u003e \u003cp\u003eThe DOM in the rhizosphere of \u003cem\u003eS. alfredii\u003c/em\u003e was extracted following the procedures suggested by Jones and Willett (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) with minor modifications, wherein 2.5 g of the rhizosphere soil was mixed with 25 mL of distilled water and shaken at 200 rpm for 120 min. The extracts were centrifuged at 10,000 \u0026times; g for 20 min, and the recovered supernatant was sieved using 0.45-\u0026micro;m filters. A total organic carbon analyzer (TOC-5000, Shimadzu, Japan) was used to measure the dissolved organic carbon in the filtrate. Leenheer and Croue (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) developed a dissolved organic carbon fractionation method that classified dissolved organics into six fractionations, including hydrophilic acid, hydrophilic base, hydrophilic neutral, hydrophobic acid, hydrophobic base, and hydrophobic neutral based on their polarity, alkaline-acidic properties, and specific compound characteristics. The DOM fractionation was performed according to the method suggested by Leenheer and Croue (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2003\u003c/span\u003e), and the six DOM fractions were measured using the total organic carbon analyzer (TOC-5000, Shimadzu, Japan).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Cd extraction capacity of DOM\u003c/h2\u003e \u003cp\u003eCations in the extracted DOM were removed using an Amberlite cation exchange H resin (Sigma-Aldrich, USA). After dilution to 100 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with deionized water, 20 mL of the solution was mixed with 2 g of soil in a centrifugal tube and centrifuged at 8000 \u0026times; g for 30 min. After being sieved through a 0.45-\u0026micro;m membrane, the Cd concentration in the recovered supernatant was measured using an inductively coupled plasma mass spectrometer (ICP-MS) (Agilent 7700, USA). The soil samples (2 g) were mixed with deionized water and prepared using the same procedure as the control.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Cd analysis\u003c/h2\u003e \u003cp\u003eThe dried plant and rhizosphere soil samples were ground and sieved using a 74-\u0026micro;m nylon mesh. The plant samples were digested through the aqua regia dissolution method (Ok et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), wherein the prepared plant samples were digested in a solution of HNO\u003csub\u003e3\u003c/sub\u003e and HCl (1:3 ratio) and heated for 100 min at 130\u0026deg;C. After cooling, the digestion was filtered through a 0.45-\u0026micro;m membrane, and the Cd concentration in the recovered filtrate was analyzed via ICP-MS (Agilent 7700, USA).\u003c/p\u003e \u003cp\u003eThe water-soluble and exchangeable fractions of Cd in the rhizosphere were determined according to the methodology described in a previous study (He et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), wherein 1.2 g of dried soil was shaken with 25 mL CaCl\u003csub\u003e2\u003c/sub\u003e (10 mmol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) in a centrifugal tube at 200 rpm for 30 min and then centrifuged at 4000 \u0026times; g for 15 min. After filtration through a 0.45-\u0026micro;m filter, the supernatant was collected and the water-soluble Cd was measured using the ICP-MS (Agilent 7700, USA).\u003c/p\u003e \u003cp\u003eThe residue recovered from the water-soluble extraction was shaken with 25 mL of NH\u003csub\u003e4\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e (1 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) in a centrifugal tube at 200 rpm for 30 min. During shaking, the pH of the mixture was maintained at 7.0 (He et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The centrifugation and analysis procedures in the exchangeable Cd fraction were identical to those used for the water-soluble fraction.\u003c/p\u003e \u003cp\u003eCarbonate, Fe-Mn oxide, organic matter, and the residual fractions of Cd in the remaining sedimentation reclaimed from the exchangeable extraction were extracted using 1 M NaOAc, a mixture of 0.04 M NH\u003csub\u003e2\u003c/sub\u003e.OH.HCl and 25 % HOAc, a mixture of 0.02 M HNO\u003csub\u003e3\u003c/sub\u003e and 30 % H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, and aqua regia in sequence, according to Li and Thornton (\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). The sum of these six chemical fractions represented the total Cd concentration in the rhizosphere.\u003c/p\u003e \u003cp\u003eQuality control was performed by analyzing two reference materials, GBW07410 (soil) and GBW10012 (plant), which were obtained from the China Standard Materials Research Center (Beijing, China).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Statistical analysis\u003c/h2\u003e \u003cp\u003eThe data are represented as the average of the five replicates used in this experiment. The magnetic field treatment effects on the dry weight of \u003cem\u003eS. alfredii\u003c/em\u003e, DOM concentration and fractionations in the rhizosphere, mobile Cd in the soil, and Cd extraction ability of DOM were tested via one-way analysis of variance. Means of significant difference (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) were evaluated by Fisher\u0026rsquo;s protected least significant difference test, and SPSS 15.0 software was used to conduct the statistical analyses in this study.\u003c/p\u003e \u003c/div\u003e "},{"header":"3. Results And Discussion","content":" \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Soil pH\u003c/h2\u003e \u003cp\u003eA significant pH variation among the treatments was not observed during this study. Compared with the initial soil pH (6.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2) at transplantation, the magnetically treated and untreated \u003cem\u003eS. alfredii\u003c/em\u003e reduced the pH by 0.2 and 0.1, respectively, in its rhizosphere during the 90-day experiment. These results indicate that the soil used can buffer the plant-induced pH variation, and the pH changes in the \u003cem\u003eS. alfredii\u003c/em\u003e rhizosphere are not responsible for Cd mobilization in this experiment.\u003c/p\u003e \u003cp\u003eSoil pH significantly influences the metal activity in the substrate, which is enhanced by reducing the pH value (Hu et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; R\u0026ouml;mkens et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The pH variation in the rhizosphere soil has been reported for other plants. For instance, \u003cem\u003eN. tabacum\u003c/em\u003e reduces the pH in the rhizosphere (Loosemore et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2004\u003c/span\u003e), \u003cem\u003eB. juncea\u003c/em\u003e increases the soil pH significantly (Kim et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), and \u003cem\u003eN. caerulescens\u003c/em\u003e has no effect on the pH in the rhizosphere (McGrath et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). The pH variation in the soil is a complex process that is dependent on soil properties, plant types, and weather conditions. Generally, Cd is present as an oxyanion in soil, and its uptake by \u003cem\u003eS. alfredii\u003c/em\u003e could enhance the pH in the rhizosphere because the plant releases OH\u003csup\u003e\u0026minus;\u003c/sup\u003e in the soil to maintain the charge balance after extracting Cd. Decreased soil pH was caused by increased root secretions, including rhamnolipids, surfactin, and humic and fulvic acids (Davin et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The combination of these two processes could determine the final soil pH during phytoremediation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2 DOM concentration and fractionation\u003c/h2\u003e \u003cp\u003eThe final DOM concentrations in the rhizospheres of the 50, 100, and 150 mT treated \u003cem\u003eS. alfredii\u003c/em\u003e were 156.9, 192.5, and 198.3 mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively, which are higher than the initial concentration of the experiment (129.3 mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Compared with the control that was exposed to a geomagnetic field, the 100 and 150 mT treatments significantly increased the DOM concentrations in the rhizosphere of \u003cem\u003eS. alfredii\u003c/em\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), while the 50 mT treatment had a slightly lower value (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.05). The 100 and 150 mT treated samples showed 17.6 % and 21.1 % increases in the DOM concentrations in the soil, respectively, relative to the control; however, no significant DOM concentration differences were observed between the 100 and 150 mT treatments (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe different fraction distributions in the soil of the various treatments are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The DOM fractions in the \u003cem\u003eS. alfredii\u003c/em\u003e soil exhibited different distribution trends among various treatments, and the hydrophobic fraction concentrations (hydrophobic acid, base, and neutral) did not significantly vary among the treatments during phytoremediation. In contrast, the hydrophilic fraction concentrations (hydrophilic acid, base, and neutral) were significantly enhanced when treated by 100 and 150 mT than those treated with the geomagnetic field and 50 mT. Generally, the control had higher hydrophobic fraction concentrations than the other treatments, while the 150 mT treatment showed higher hydrophilic fraction concentrations than the other treatments. The acid fraction concentrations (hydrophilic and hydrophobic acid) were significantly higher in the 100 and 150 mT treatments than in the control and 50 mT treatments.\u003c/p\u003e \u003cp\u003eFurthermore, the acid fraction was the predominant DOM component in all the soil treatments, accounting for 77.8 %, 76.5 %, 77.1 %, and 74.9 % of the DOM in the soil of the control, 50, 100, and 150 mT treatments, respectively. Meanwhile, the hydrophobic fraction proportion decreased from 57.3 % to 43.0 %, and the hydrophilic fraction proportion increased from 42.7 % to 57.2 % in the control and 150 mT treatment, respectively.\u003c/p\u003e \u003cp\u003eDuring the phytoremediation process, root secretion is one of the most significant DOM sources (Martin et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2017\u003c/span\u003e); therefore, the elevated DOM concentrations in the soils at the end of the experiment might be induced by the presence of \u003cem\u003eS. alfredii\u003c/em\u003e. Although the effects of external magnetic fields on root secretion have not been determined, many studies suggest that magnetic fields regulate plant metabolism and growth rate on the cellular, subcellular, and molecular levels (Ćirković et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Shokrollahi et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Teixeira da Silva and Dobr\u0026aacute;nszki, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Therefore, the increased DOM concentration in the rhizosphere of magnetically treated \u003cem\u003eS. alfredii\u003c/em\u003e could be partially induced by the stimulation of the applied magnetic fields.\u003c/p\u003e \u003cp\u003eDOM contains many functional components, including sulfhydryl, amino, and phosphoryl groups, which have a strong affinity to metals and increase the metal bioavailability in soil (Borggaard et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Zhou et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The chemical activity of hydrophilic acids is greater than that of hydrophobic acids because the former have more functional groups that can activate the metals in soil (Huang et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Therefore, the chemical components of DOM in the rhizospheres of the 100 and 150 mT treated \u003cem\u003eS. alfredii\u003c/em\u003e are more conducive to increasing Cd mobility. The mechanisms in which magnetic fields regulate root exudation regarding the physiological characteristics of plants require further investigation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Chemical fractions of Cd in the rhizosphere\u003c/h2\u003e \u003cp\u003e \u003cem\u003eS. alfredii\u003c/em\u003e growth did not change the total Cd concentration in the soil compared to the initial state; however, it increased the variable coefficient of the metal (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), which concurs with the findings of a previous study, suggesting that plant growth can heterogenize the soil over time (He et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe chemical fractions of Cd in the soil were analyzed at the beginning and end of the experiment. The initial water-soluble and exchangeable Cd concentrations in the soil decreased significantly after the cultivation of \u003cem\u003eS. alfredii\u003c/em\u003e under all treatments, and the reduction in the rhizosphere increased with increasing magnetic intensity. Compared with the control, the 50 mT treatment did not significantly change the water-soluble Cd concentration, but it significantly decreased the exchangeable fraction concentration (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). However, the 100 and 150 mT treatments decreased the water-soluble and exchangeable Cd concentrations, and their lowest concentrations were observed in the \u003cem\u003eS. alfredii\u003c/em\u003e rhizosphere exposed to 150 mT. Li et al. (\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2013a\u003c/span\u003e) compared the water-soluble Cd concentration variations in soils after the growth of hyperaccumulating and non-hyperaccumulating ecotypes of \u003cem\u003eS. alfredii\u003c/em\u003e. They found that the hyperaccumulating ecotype of \u003cem\u003eS. alfredii\u003c/em\u003e decreased the water-soluble Cd in slightly contaminated soil, which agrees with the results of this study. In contrast, the non-hyperaccumulating ecotype of \u003cem\u003eS. alfredii\u003c/em\u003e increased the water-soluble Cd in the rhizosphere because \u003cem\u003eS. alfredii\u003c/em\u003e activates Cd in the soil through increased root excretions, and Cd solubilization is faster than the Cd uptake by the plant.\u003c/p\u003e \u003cp\u003eThe water-soluble and exchangeable Cd concentrations in the soils decreased significantly after the \u003cem\u003eS. alfredii\u003c/em\u003e growth with or without magnetic field treatment. However, the chemical fraction reductions in the rhizosphere were less than 1 % of the extraction amount in the \u003cem\u003eS. alfredii\u003c/em\u003e, indicating that more than 99 % of the Cd uptake migrated from other fractions. Soils can spontaneously maintain a dynamic balance of a specific element during plant growth via a pair of opposite processes, i.e., mobilizing the element in soil and its extraction by plants. This study shows that the effective Cd extraction root system of \u003cem\u003eS. alfredii\u003c/em\u003e can eliminate water-soluble and exchangeable Cd faster than the rate Cd is supplemented from other chemical fractions, and \u003cem\u003eS. alfredii\u003c/em\u003e, particularly the magnetically treated species, can mobilize Cd that is not initially bioavailable. The mobilizing ability of \u003cem\u003eS. alfredii\u003c/em\u003e increased with increasing magnetic field intensity.\u003c/p\u003e \u003cp\u003eIncreased DOM in the rhizosphere of the species can regulate the Cd mobilization process. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the proportion of organic matter Cd increased significantly from 11.0 % in the control to 25.7 % and 25.9 % in the 100 and 150 mT treatments, respectively, while no significant changes in the carbonate, Fe-Mn oxide, and residual fraction proportions were observed. This finding is consistent with the results suggested by Krishnamurti et al. (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) who suggested that Cd-DOM complexes can be taken up intact by \u003cem\u003eChlorococcum\u003c/em\u003e sp. and result in increased Cd toxicity. Generally, soil pH and DOM concentration are the two dominant factors controlling bioavailability and chemical activity in soil (Christensen and Christensen, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). In this study, soil pH did not vary after plant growth; therefore, the variation in the chemical fraction of Cd in the soil is mainly driven by the increased DOM in the rhizosphere.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Cd extraction capacity of DOM from the rhizosphere\u003c/h2\u003e \u003cp\u003eThe DOM in the magnetically treated soils of \u003cem\u003eS. alfredii\u003c/em\u003e significantly enhanced the Cd solubility in the soils relative to deionized water (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). For example, the Cd concentration extracted using 100 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e DOM from the control and the 50, 100, and 150 mT treatments were 48.3 %, 58.6 %, 137.9 %, and 151.7 % higher, respectively, than that extracted using deionized water, indicating that the Cd extraction capacity of DOM improved with increasing magnetic field intensity.\u003c/p\u003e\u003cp\u003eBorggaard et al. (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) reported that increased DOM concentration improved the metal solubility in the soil and attributed the metal extractability to hydrophilic fractions. Chen et al. (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) studied the influence of DOM on Zn extractability in calcareous soil and suggested that the addition of maize straw resulted in increased amounts of applied Zn recovered by the DTPA-extractable fraction, which is attributed to the increased DOM concentration and consequently, increased formation of Zn-fulvic acid complexes. Additionally, the DOM from the soils of the 100 and 150 mT treated \u003cem\u003eS. alfredii\u003c/em\u003e had a greater ability to mobilize Cd in the rhizosphere compared with the control and 50 mT treatment. For instance, the Cd concentration extracted by the DOM in the 150 mT treatment was 0.73 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which was 0.70 and 0.59 times greater than that of the control and 50 mT treatments, respectively. This indicates that the DOM fractions under the different magnetic field treatments varied and potentially influenced Cd mobilization. Furthermore, the magnetically treated \u003cem\u003eS. alfredii\u003c/em\u003e had a greater capacity to generate chelates with Cd compared with the control.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Phytoextraction efficiency\u003c/h2\u003e \u003cp\u003eThe average soil Cd concentration difference before and after transplantation could not be used to estimate the phytoextraction efficiency of \u003cem\u003eS. alfredii\u003c/em\u003e because the duration of the experiment was not long enough for the \u003cem\u003eS. alfredii\u003c/em\u003e to change the total Cd concentration in its rhizosphere. Therefore, the Cd uptake in the harvested tissues was calculated to evaluate the phytoremediation effect of \u003cem\u003eS. alfredii\u003c/em\u003e.\u003c/p\u003e \u003cp\u003eThe Cd uptake of \u003cem\u003eS. alfredii\u003c/em\u003e was defined as the product of the plant\u0026rsquo;s dry weight (belowground and aerial plant parts) and Cd concentration. As shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the 150 mT treated \u003cem\u003eS. alfredii\u003c/em\u003e had the largest dry weight, and that of the 100 and 50 mT treatments and the control were successively lower. Except for the 50 mT treatment, magnetic field treatments significantly improved the dry weight. Similarly, Cd concentrations in the belowground and aerial plant parts increased with increasing magnetic field intensity (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), consistent with the DOM concentration and fraction variation trends in the \u003cem\u003eS. alfredii\u003c/em\u003e rhizosphere.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDry weight, Cd concentrations, and Cd decontamination capacity of \u003cem\u003eS. alfredii\u003c/em\u003e in different treatments\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eDry weight (g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003eCd content\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eCd accumulation\u003c/p\u003e \u003cp\u003e(mg pot\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRoots\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eShoots\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRoots\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eShoots\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003econtrol\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.73\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e8.21\u0026thinsp;\u0026plusmn;\u0026thinsp;1.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e22.3\u0026thinsp;\u0026plusmn;\u0026thinsp;2.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e71.2\u0026thinsp;\u0026plusmn;\u0026thinsp;5.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e50 mT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.69\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e9.22\u0026thinsp;\u0026plusmn;\u0026thinsp;1.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e25.6\u0026thinsp;\u0026plusmn;\u0026thinsp;2.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e66.5\u0026thinsp;\u0026plusmn;\u0026thinsp;6.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e100 mT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e1.02\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e10.63\u0026thinsp;\u0026plusmn;\u0026thinsp;1.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e29.7\u0026thinsp;\u0026plusmn;\u0026thinsp;3.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e90.5\u0026thinsp;\u0026plusmn;\u0026thinsp;9.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e150 mT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e1.12\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e12.39\u0026thinsp;\u0026plusmn;\u0026thinsp;2.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e38.1\u0026thinsp;\u0026plusmn;\u0026thinsp;2.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e95.7\u0026thinsp;\u0026plusmn;\u0026thinsp;7.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eBased on the dry weight and Cd content, 2.4, 2.5, 4.0, and 4.9 mg of Cd were extracted from the control, 50, 100, and 150 mT treatment pots. The excess Cd in each pot that required removal was calculated as the product of the difference between the initial soil Cd concentration (3.83 mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and its safe threshold (0.30 mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and the soil mass (6 kg). The required number of planting cycles to clean the soil was determined by the excess Cd divided by the Cd uptake capacity of \u003cem\u003eS. alfredii\u003c/em\u003e; therefore, 9, 9, 6, and 5 planting cycles are required for \u003cem\u003eS. alfredii\u003c/em\u003e to decontaminate the soil in the control, 50, 100, and 150 mT treatments, respectively.\u003c/p\u003e \u003cp\u003eA previous study concluded that an appropriate magnetic field improves the plant dry weight by increasing antioxidant enzyme activity that scavenges oxyradicals resulting from stresses, balancing the internal magnetic field in plants, and increasing cytomembrane permeability to enhance nutrient pass through (Luo et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The study focused on the plant improvement effects of magnetic fields rather than effects on the rhizosphere. Previous studies have shown that many plants can modulate the chemical properties of their rhizosphere to improve pollutant extraction via various pathways, including acidification, stimulating microbial proliferation, and DOM excretion (Cao et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Gattullo et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). This study found that the increased Cd uptake capacity of magnetically treated \u003cem\u003eS. alfredii\u003c/em\u003e was related to increased DOM concentrations and its acid and hydrophilic fractions. Furthermore, higher Cd extractability was observed for DOM under magnetic field treatments, particularly when exposed to 150 mT, indicating that DOM from magnetically treated soils exhibits a greater capacity to mobilize Cd in the soil by generating a complex.\u003c/p\u003e \u003cp\u003eThe Cd extraction capacity of magnetically treated \u003cem\u003eS. alfredii\u003c/em\u003e, particularly at 150 mT, was much greater than that of the control and decreased the required remediation time by half. This study demonstrated that applying an appropriate magnetic field is an effective method to enhance the phytoextraction efficiency of \u003cem\u003eS. alfredii\u003c/em\u003e based on the variation in DOM concentrations and its fractions in the soil.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThis study determined that magnetic field treatment, particularly a 150 mT treatment, improves the phytoextraction of \u003cem\u003eS. alfredii\u003c/em\u003e by increasing its dry weight and Cd uptake capacity. Compared to the control, the DOM concentrations and acid fractions were significantly higher in the 100 and 150 mT treatments. Additionally, the hydrophilic fraction proportions significantly influence metal mobilization in the soil, which increases with increasing magnetic intensity. Compared with the initial state, water-soluble and exchangeable Cd in the rhizosphere decreased significantly after the \u003cem\u003eS. alfredii\u003c/em\u003e growth, and the decreases were significantly higher in the magnetically treated plants than in the control. The Cd extraction capacity of DOM from the rhizosphere of magnetically treated \u003cem\u003eS. alfredii\u003c/em\u003e was significantly higher than that of the control, thus increasing the Cd concentrations in the plant tissues. This study reveals that the primary mechanism to increase Cd extraction in \u003cem\u003eS. alfredii\u003c/em\u003e is the DOM in magnetically treated \u003cem\u003eS. alfredii\u003c/em\u003e, which increases the Cd mobilization capacity in the rhizosphere by generating DOM-Cd (organic matter Cd fraction) and consequently increases the phytoextraction efficiency of \u003cem\u003eS. alfredii\u003c/em\u003e in Cd-polluted soil.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCorresponding author\u0026apos;s email address\u003c/strong\u003e: [email protected]\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u003c/strong\u003e: Not applicable. This work did not describe experiments with animals, human subjects, or human tissue samples.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Participate\u003c/strong\u003e: Not applicable. This work did not describe experiments with animals, human subjects, or human tissue samples.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Publish\u003c/strong\u003e: The manuscript entitled, \u0026ldquo;Effects of magnetically treated \u003cem\u003eSedum alfredii\u003c/em\u003e seeds on the dissolved organic matter characteristics of Cd-contaminated soil during phytoextraction\u0026rdquo; is prepared in accordance with the Guide for authors available on the journal\u0026rsquo;s website and it has not been published elsewhere in part or in its entirety. All authors attest to the validity of its contents, and agree to its submission in Environmental Science and Pollution Research.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors Contributions\u003c/strong\u003e: Conceptualization, Youjun Tang, Jie Luo; Data curation, Shuaizhi Ji, Jiawei Wang; Methodology, Jie Luo, Dan Chen; Funding acquisition, Jie Luo; Writing \u0026ndash; original draft, Youjun Tang, Shuaizhi Ji, Jie Luo; Writing \u0026ndash; review \u0026amp; editing, Min Cao, Dan Chen. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e: The authors thank the National Natural Science Foundation of China (Project No. 21876014) for financial support to carry out this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e: None of the authors have any competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e: The datasets used or analyzed during the current study are available from the corresponding author on reasonable request. All data generated or analyzed during this study are included in this published article.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBeiyuan J, Lau AYT, Tsang DCW, Zhang W, Kao CM, Baek K, Ok YS, Li XD (2018) Chelant-enhanced washing of CCA-contaminated soil: Coupled with selective dissolution or soil stabilization. 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Water Res 158:171\u0026ndash;181\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Phytoextraction, Sedum alfredii, Magnetic field, Dissolved organic matter, Rhizosphere","lastPublishedDoi":"10.21203/rs.3.rs-560215/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-560215/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe effects of magnetic field treatments on the two determining factors of phytoremediation, growth status and element uptake capacity, of \u003cem\u003eSedum alfredii\u003c/em\u003e have been thoroughly studied; however, minimal studies have been performed to determine the influence of the Cd hyperaccumulator \u003cem\u003eSedum alfredii\u003c/em\u003e, grown from magnetically treated seeds, on the dissolved organic matter (DOM) characteristics in its rhizosphere. A series of pot experiments were conducted to evaluate the variations in the DOM concentration and fractionations in the rhizosphere of \u003cem\u003eS. alfredii\u003c/em\u003e treated with external magnetic fields. Compared with the untreated seeds, \u003cem\u003eS. alfredii\u003c/em\u003e grown from magnetically treated seeds excreted more DOM in its rhizosphere. Additionally, the hydrophilic DOM fractionation proportion, which presented a greater capacity to mobilize Cd in the soil, increased from 42.7 % in the control sample to 47.2 % in the 150 mT magnetically treated \u003cem\u003eS. alfredii\u003c/em\u003e sample. The water-soluble and exchangeable forms of Cd (extracted using deionized water and NH\u003csub\u003e4\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e, respectively) in the rhizosphere of the magnetically treated \u003cem\u003eS. alfredii\u003c/em\u003e were significantly lower than those of the control sample. Furthermore, the Cd extraction capacity of DOM from the rhizosphere of the magnetically treated \u003cem\u003eS. alfredii\u003c/em\u003e was greater than that of the control sample, thereby increasing the Cd uptake ability of the magnetically treated species. These results suggest that the secretion of large amounts of DOM, especially acid and hydrophilic fractionations, is an essential mechanism of magnetically treated \u003cem\u003eS. alfredii\u003c/em\u003e to mobilize Cd in the soil.\u003c/p\u003e","manuscriptTitle":"Effects of Magnetically Treated Sedum Alfredii Seeds On The Dissolved Organic Matter Characteristics of Cd-Contaminated Soil During Phytoextraction","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2021-06-22 14:58:42","doi":"10.21203/rs.3.rs-560215/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2021-06-22T00:00:00+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2021-06-19T00:00:00+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Environmental Science and Pollution Research","date":"2021-06-03T14:05:00+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2021-05-26T02:04:00+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Science and Pollution Research","date":"2021-05-25T01:21:45+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"63a94a6f-abea-4d91-b8cf-864c77d4cd0c","owner":[],"postedDate":"June 22nd, 2021","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":5183072,"name":"Environmental Engineering"},{"id":5183073,"name":"Environmental Policy"}],"tags":[],"updatedAt":"2022-03-05T06:59:02+00:00","versionOfRecord":{"articleIdentity":"rs-560215","link":"https://doi.org/10.1007/s11356-021-17312-w","journal":{"identity":"environmental-science-and-pollution-research","isVorOnly":false,"title":"Environmental Science and Pollution Research"},"publishedOn":"2021-11-06 06:59:02","publishedOnDateReadable":"November 6th, 2021"},"versionCreatedAt":"2021-06-22 14:58:42","video":"","vorDoi":"10.1007/s11356-021-17312-w","vorDoiUrl":"https://doi.org/10.1007/s11356-021-17312-w","workflowStages":[]},"version":"v1","identity":"rs-560215","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-560215","identity":"rs-560215","version":["v1"]},"buildId":"cBFmMYwuxLRRLfASyISRj","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}