Study on the effect of sludge fermentation residue on soil remediation and plant growth in mining areas

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In this study, sludge fermentation residue (SFR), derived from aerobic co-composting of municipal sewage sludge and straw, was evaluated as a soil amendment for remediating heavy-metal-contaminated soils from a mining region in Tibet, China. The effects of different SFR application rates (0%, 3%, 6%) on soil properties, heavy metal stabilization, plant growth, and metal migration were comprehensively investigated using Brassica chinensis and Lolium perenne under monoculture and intercropping conditions. The results showed that SFR significantly improved soil physicochemical properties, including cation exchange capacity (CEC), total organic carbon, ammonium nitrogen, available phosphorus, and potassium. At 6% application, SFR increased the residual fractions of Cu, Zn, and Pb by over 40%, thereby reducing their mobility and bioavailability. Simultaneously, SFR enhanced plant growth, with up to 72% increase in chlorophyll content and substantial improvements in biomass and plant height. Intercropping systems effectively redistributed metal loads between species, reducing heavy metal accumulation in Chinese cabbage while promoting uptake by ryegrass. This study demonstrates that SFR offers a sustainable, cost-effective strategy for stabilizing heavy metals and promoting vegetation recovery in contaminated soils. Sludge fermentation Heavy metals Mining soil remediation Plants Figures Figure 1 Figure 2 Figure 3 Figure 4 1 Introduction With the rapid advancement of industrialization, the exploitation and processing of metal resources have resulted in widespread soil contamination by heavy metals worldwide [1, 2]. According to the National Soil Pollution Survey Bulletin of China, approximately 13.9% of soil samples exceed the regulatory thresholds for heavy metal concentrations [3]. Common contaminants such as lead (Pb), cadmium (Cd), copper (Cu), zinc (Zn), and chromium (Cr) are of particular concern due to their high toxicity, persistence, and bioaccumulation potential [4, 5]. Consequently, the development of sustainable, efficient, and cost-effective technologies for soil remediation has become a critical focus of environmental research. Among the various soil remediation strategies, biochar has attracted considerable attention owing to its high porosity, large surface area, abundant surface functional groups, and strong cation exchange capacity [6-8]. These properties enable biochar to immobilize heavy metals by reducing their mobility and bioavailability in contaminated soils [9, 10]. Specifically, sludge-derived biochar (SDB), produced by pyrolyzing municipal sewage sludge, is rich in minerals and nutrients and has demonstrated potential in heavy metal stabilization[11, 12]. However, traditional pyrolysis processes often require high energy input, yield low amounts of biochar, and may degrade functional groups critical to metal sorption, thus limiting the practical application of SDB in large-scale remediation projects [13]. As an alternative, SFR has recently emerged as a promising amendment for soil remediation. SFR is produced through aerobic fermentation of sewage sludge, a low-temperature, low-energy biological process in which microbial activity decomposes organic matter and generates stable humic substances [14]. This process retains essential nutrients and introduces active functional groups such as carboxyl and phenolic hydroxyls, which facilitate complexation or precipitation with heavy metals [15], thereby reducing their environmental risk. Compared with thermal treatment, aerobic fermentation is more energy-efficient and environmentally friendly while preserving the structural integrity and adsorptive capacity of the final product. Moreover, SFR contains abundant nitrogen, phosphorus, potassium, and organic carbon, which are essential for soil fertility. Its application can improve soil pH, increase CEC, enhance soil aggregation, and promote microbial activity, all of which contribute to soil health and plant productivity [16]. At present, research on SFR mostly focuses on the recovery of fatty acids. There is relatively little research on the remediation of soil in mining areas by SFR. This study aims to explore the potential of sludge fermentation residue as a soil amendment for ecological restoration in mining areas. A systematic evaluation was conducted to investigate its effects on (i) the transformation and stabilization of heavy metals in soil, (ii) the enhancement of soil physicochemical properties, and (iii) the growth response and metal uptake of two model plants—ryegrass ( Lolium perenne L. ) and Chinese cabbage ( Brassica chinensis L. )—under monoculture and intercropping conditions. The findings provide theoretical and practical insights into the application of low-energy sludge treatment byproducts in sustainable soil remediation and ecological restoration. 2 Materials and Methods 2.1 Material source and pre-treatment The sewage sludge used in this study was obtained from the secondary sedimentation tank of a municipal wastewater treatment plant in a town of Changzhou City, Jiangsu Province, China. SFR was prepared via aerobic composting. Specifically, the air-dried sludge was thoroughly mixed with crushed corn straw at a mass ratio of 4:1 (sludge:straw), and the mixture was placed in a 100 L forced-aeration aerobic fermenter. The composting process was carried out at a controlled temperature of 60 °C for 60 days. During fermentation, the materials were turned once every 7 days, and aeration was maintained at a flow rate of 0.3 L/(min·kg dry matter) to ensure sufficient oxygen supply. The moisture content of the composting matrix was adjusted to approximately 60% by periodic water addition to maintain optimal microbial activity. After fermentation, the residue was naturally air-dried, ground, and sieved to a particle size of <2 mm, then stored in sealed containers at room temperature (~25 °C) for later use. The experimental soil was collected from a tailings site in a metal mining area in the Tibet Autonomous Region, China. After collection, the soil was air-dried, homogenized, and passed through a 2 mm sieve after removing plant and animal debris. The processed soil was stored at room temperature prior to use. The basic physical and chemical properties of the soil are summarized in Table 1. Table 1: Soil Physical and Chemical Properties Name Sludge fermentation residue Soil Sludge treatment in urban sewage treatment plants - Soil quality for land improvement （GB/T24600—2009） Screening values for risk control standards of soil pollution in agricultural land (GB15618—2018) Standard Control Values for Soil Pollution Risk Control of Agricultural Land (GB15618—2018) CEC (cmol + /kg) 35.6 7.4 / / / SOC (%) 3.8 0.05 / / / Ammonium nitrogen (mg/kg) 450 19 / / / Available Phosphorus (mg/kg) 830 4.25 / / / Available potassium (mg/kg) 1246 42.12 / / / pH 7.4 6.3 / / / Cu (mg/kg) 257 3228 1500 100 / Zn (mg/kg) 370 490 4000 300 / Ni (mg/kg) 13 41 200 250 1300 Pb (mg/kg) 30 296 1000 170 1000 Cr (mg/kg) 40 230 1000 190 / Cd (mg/kg) 0.2 0.4 20 0.6 4 2.2 Experimental processing and methods 1 ） Experimental design SFR was incorporated into the test soil at three application rates: 0% (control), 3%, and 6% by mass. Each treatment was conducted with three replicates, and 1 kg of homogenized soil mixture was placed into plastic pots. The SFR was thoroughly mixed with the soil prior to planting. After moistening the soil with deionized water, pots were pre-incubated for 7 days at room temperature to allow stabilization. Two plant species—ryegrass ( Lolium perenne L. ) and Chinese cabbage ( Brassica chinensis L. )—were sown under both monoculture and intercropping systems. In monoculture treatments, 100 seeds of each species were sown per pot. In the intercropping treatment, 50 seeds of each species were sown simultaneously in the same pot. The experiment lasted for 60 days under natural light conditions, and no additional fertilizer was applied during the growth period. 2 ） Soil and Plant Sample Collection At harvest, the fresh weight of Chinese cabbage and ryegrass was recorded. Shoots and roots were carefully separated and rinsed with distilled water. The plant tissues were inactivated at 105 °C for 30 minutes to stop enzymatic activity, then oven-dried at 65 °C to constant weight and ground for subsequent analysis. Soil samples were collected from each pot at the end of the experiment, air-dried at room temperature, homogenized, and stored for physicochemical and heavy metal analysis. 3 ） Simulate soil erosion To evaluate the effect of SFR on soil erosion resistance, a sloped container (30 × 10 × 5 cm, length × width × height) was used to simulate soil loss (Figure 1). Each container was filled with 1 kg of soil treated with 0%, 3%, or 6% SFR by weight, with three replicates per treatment. A rainfall simulation experiment was conducted by applying 900 mL of deionized water evenly over 2 hours, mimicking the historical maximum daily rainfall (48.7 mm) observed in a region of Tibet. The simulation was repeated twice per week for a total of 10 cycles. Runoff and eroded soil were collected, oven-dried, and weighed to determine soil loss. 2.3 Analytical Methods and Instruments All analytical procedures in this study were carried out following the Chinese national environmental testing standards. The CEC of the soil was determined using a 1.66 cmol/L hexamminecobalt (III) chloride solution as extractant. After 60 minutes of extraction, samples were centrifuged at 400 rpm, and the absorbance was measured at wavelengths of 475 nm and 375 nm using a UV spectrophotometer (UV-756, Shanghai Precision Scientific Instruments Co., China). Soil total organic carbon was analyzed by the potassium dichromate oxidation method. In this procedure, 5 mL of 0.27 mol/L potassium dichromate, 0.1 g of mercury(II) sulfate, and 7.5 mL of concentrated sulfuric acid (1.84 g/mL) were added to the sample, followed by constant-temperature heating at 135 °C for 30 minutes. The resulting absorbance was measured at 585 nm using the same spectrophotometer. Ammonium nitrogen was quantified using the indophenol blue colorimetric method. Phenol and sodium hypochlorite solutions were added to the samples, which were then kept at room temperature for 60 minutes before adding a masking agent, after which the absorbance was recorded at 352 nm. Available phosphorus was extracted with sodium bicarbonate solution, shaken for 30 minutes, and then filtered, with the absorbance measured at 660 nm using UV spectrophotometry. Chlorophyll in plant tissues was extracted using a mixture of absolute ethanol and acetone (1:1, v/v), and its absorbance was measured at 645 nm and 663 nm to calculate total chlorophyll content. The concentrations of heavy metals (Cu, Zn, Pb, and Cr) in soil samples were determined after digestion with aqua regia (HNO₃:HCl = 1:3, v/v), while plant samples were digested with a nitric acid–hydrogen peroxide mixture. Both types of samples were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 8000, PerkinElmer, USA). Soil pH was measured in a 1:2.5 (w/v) soil-to-water suspension using a calibrated pH meter (PHS-3C, INESA Scientific Instrument Co., China). All instruments were calibrated before use, and all procedures were performed in triplicate to ensure accuracy and repeatability. 3 Results and Discussion 3.1 Effect of SFR on the Fractional Distribution of Heavy Metals in Soil The influence of SFR addition on the chemical forms of heavy metals (Cu, Zn, Pb, Cr) in soil is illustrated in Fig. 2 . All four metals exhibited significant immobilization responses following the application of SFR. Compared to the control (0% addition), increasing the SFR dosage enhanced the proportion of stable fractions (F3 + F4), corresponding to organically bound and residual forms. At the 6% addition level, the proportion of stable forms reached 75% for Cu, 87% for Zn, 75% for Pb, and 98% for Cr. The transformation pattern for all metals was characterized by a shift from labile (F1 + F2) to stable (F3 + F4) fractions. Notably, the immobilization of Pb showed the greatest enhancement (~ 6%), whereas Cr showed minimal change (~ 1%) due to its already high baseline stability (97%). The enhanced Pb stabilization can be attributed to multiple interacting mechanisms. Sewage sludge is rich in phosphorus, and phosphate species are largely retained in the SFR. These compounds contribute to the formation of low-solubility lead minerals, such as pyromorphite [Pb₅(PO₄)₃X; X = F, Cl, OH] through ion exchange and mineral precipitation reactions (Huang et al., 2017 ). Furthermore, co-fermentation with straw significantly increases the specific surface area and porosity of the residue, providing additional Pb adsorption sites associated with phosphorus-based functional groups (Kumpiene et al., 2008 ). The elevated total organic carbon content introduced by SFR also enhances the formation of stable Pb–organic complexes in soil, further reducing Pb bioavailability (Boughattas et al., 2025 ). Additionally, an increase in soil pH induced by SFR application contributes to decreased Pb mobility, as lead bioavailability generally declines under more alkaline conditions. Cu demonstrated a strong affinity for organic matter, primarily due to its tendency to complex with high molecular weight humic substances and functional groups such as carboxyl and phenolic hydroxyls (Li, S. et al., 2024 ). The surface of SFR is enriched with such groups, which can form stable Cu–organic complexes, leading to its effective immobilization(Lee et al., 2020 ). In addition,the porous structure of SFR supports microbial colonization and nutrient retention, indirectly increasing soil organic matter levels and enhancing microbial-mediated Cu stabilization. The immobilization of Zn is primarily governed by precipitation with anions (hydroxides, carbonates, phosphates) and complexation with organic ligands (Beesley et al., 2011 ). The combined effect of cation exchange and ligand binding is considered the dominant mechanism of Zn stabilization in the presence of SFR (Zhang et al., 2017 ). Additionally, the rise in soil pH further facilitates the formation of insoluble Zn precipitates, thereby contributing to its decreased mobility (Lu et al., 2017 ). Although Cr showed relatively minor changes in its fractional distribution following SFR treatment, this can be attributed to its already high stability in the initial soil matrix. Nonetheless, the presence of organic matter and elevated pH conditions introduced by SFR may further suppress its potential remobilization. 3.2 The effect of sludge fermentation residue on soil physicochemical properties The effects of SFR application on soil physicochemical properties are illustrated in Fig. 3 and Table 2 . Increasing the addition rate of SFR markedly improved several key indicators of soil fertility. Compared to the control (0% SFR), the 6% treatment resulted in significant increases in available potassium (by 176 mg/kg, + 419%), available phosphorus (by 62 mg/kg, + 1476%), ammonium nitrogen (by 21 mg/kg, + 112%), and total organic carbon (by 1.3%, + 582%). Additionally, the soil CEC increased by 9.4 cmol⁺/kg (+ 127%). These improvements can be primarily attributed to two synergistic mechanisms. First, SFR itself is rich in essential nutrients including nitrogen, phosphorus, potassium, and organic carbon, which directly contribute to the replenishment of soil nutrient pools. Second, the porous structure and surface functional groups (e.g., carboxyl and hydroxyl groups) of SFR exhibit strong cation adsorption capacity, enhancing nutrient retention and reducing nutrient leaching. Previous studies have shown that biochar derived from sludge exhibits high affinity for NH 3 /NH 4 + , thereby reducing gaseous nitrogen losses and improving soil nitrogen use efficiency (Rens et al., 2018 ). Moreover, SFR application promotes microbial activity and soil organic matter content, further enhancing the retention of ammonium nitrogen in the soil matrix (Liu et al., 2025 ). The notable increase in available phosphorus can also be explained by several chemical interactions. Carboxyl groups on the SFR surface can adsorb phosphate ions, thereby inhibiting their fixation by iron and aluminum oxides in acidic soils. In addition, organic acids released during fermentation may interact with phosphate minerals, promoting phosphorus solubilization (Lu et al., 2023 ). Furthermore, pyrolyzed organic matter has been reported to enhance the mineralization of organic phosphorus by adsorbing and concentrating phosphate ions in the rhizosphere (Hua et al., 2008 ). In degraded soils from mining areas, low cohesion and poor aggregation often result in high erosion risk, which can lead to severe outcomes such as surface runoff and mudslides. Application of SFR significantly mitigated this problem. At a 6% addition rate, soil bulk density increased by 44%, while simulated soil loss decreased by 36%. This can be explained by the ability of SFR to enhance the aggregation stability of soil particles and reduce their detachment during rainfall events. There is a research report (Heikkinen et al., 2019 ) that the relatively coarse particle size of fermentation residue increases surface roughness, which disrupts the lateral flow of runoff and physically intercepts soil particles, thereby reducing erosion. Table 2 Effect of adding amount of SFR on soil properties Addlication Amount (%) Soil bulk density (g/cm 3 ) Soil loss (g/week) Organic carbon (%) pH CEC 0 0.86 80.45 0.234 6.3 7.47 3 1.13 60.73 1.009 7.8 12.11 6 1.24 50.44 1.571 8.2 16.83 3.3 The impact of SFR on plants The application of sludge SFR significantly enhanced the growth performance of both ryegrass and Chinese cabbage, as shown in Fig. 4 . In monoculture treatments, the application of 6% SFR resulted in substantial increases in chlorophyll content, fresh weight, and plant height. Specifically, chlorophyll content increased by 72% in Chinese cabbage and 19% in ryegrass, while fresh weight increased by 10% and 50%, and plant height increased by 62% and 33%, respectively, compared to the control group without SFR application. Under intercropping conditions, the growth-promoting effects of SFR were also evident. At the 6% application rate, chlorophyll content in Chinese cabbage and ryegrass increased by 71% and 19%, respectively. Fresh weight increased by 32% for Chinese cabbage and 51% for ryegrass, while plant height increased by 52% and 40%, respectively. These results indicate that SFR application is effective in promoting photosynthetic pigment accumulation and biomass production in both species, under both monoculture and intercropping systems. Interestingly, intercropping appeared to further enhance the chlorophyll content and biomass accumulation of Chinese cabbage, whereas it slightly reduced those of ryegrass. This differential response may be related to the interspecific interactions in metal-contaminated soil. It is hypothesized that ryegrass, which exhibits higher heavy metal tolerance and accumulation capacity, may preferentially absorb and translocate heavy metals in the rhizosphere. This mitigates metal-induced stress on Chinese cabbage and promotes its growth. A detailed analysis of heavy metal enrichment and transport mechanisms in both plants is provided in section 3.4 . 3.4 Migration and enrichment of heavy metals in plants The effects of SFR on the migration and enrichment of heavy metals in plants are summarized in Table 3 . Overall, the application of SFR significantly reduced the translocation factor (TF) and enrichment coefficient (EC) for Cu, Zn, Pb, and Cr in both Chinese cabbage and ryegrass, indicating a strong immobilization effect in the plant–soil system. In monoculture systems, the application of 6% SFR reduced the TF of Cu, Zn, Pb, and Cr in Chinese cabbage by 52%, 50%, 60%, and 60%, respectively, while reductions in ryegrass were 53%, 61%, 56%, and 66%, respectively. Correspondingly, the EC of these metals was also decreased—by 42%, 41%, 46%, and 45% in Chinese cabbage, and by 33%, 35%, 37%, and 36% in ryegrass. Under intercropping conditions, SFR exerted a similar but more differentiated effect. At the same 6% application rate, TF values in Chinese cabbage were reduced by 58% (Cu), 40% (Zn), 71% (Pb), and 66% (Cr), while ryegrass exhibited TF reductions of 53%, 61%, 56%, and 66%, respectively. Reductions in EC values under intercropping were also observed, with declines of 38%, 38%, 43%, and 41% in Chinese cabbage, and 33%, 35%, 37%, and 36% in ryegrass. These results suggest that Chinese cabbage consistently exhibits lower TF and EC values compared to ryegrass, indicating a lower tendency for heavy metal uptake and translocation. Notably, the intercropping system appears to further suppress heavy metal accumulation in Chinese cabbage while slightly enhancing it in ryegrass. This asymmetry may be attributed to rhizospheric interactions between the two species. Ryegrass, with its stronger metal tolerance and uptake capacity, likely acts as a “metal sink,” preferentially absorbing mobile metal ions from the shared rhizosphere, thereby alleviating metal stress on Chinese cabbage. This phenomenon is consistent with previous findings in intercropping systems such as soybean–corn, where the nutrient and signaling exchanges between species altered heavy metal partitioning. In that case, soybean's higher metal uptake helped protect corn from heavy metal toxicity, possibly through nutrient and nitrogen transfer from root exudates (Abrol et al., 2016 ). Similar mechanisms may operate here, where signal-induced exudation of organic acids or chelators by ryegrass modulates the rhizosphere microenvironment and limits metal transport to Chinese cabbage (Hussein et al., 2019 ). As (Liu et al., 2023 ) emphasized, the effects of intercropping on metal migration are strongly dependent on plant species interactions, root secretion profiles, and the specific metal species involved. Table 3 Migration and enrichment of heavy metals in plants TF Monoculture-Chinese cabbage Intercropping-Chinese cabbage Monoculture- ryegrass Intercropping- ryegrass Addlication Amount 0% 3% 6% 0% 3% 6% 0% 3% 6% 0% 3% 6% Cu 0.23 ± 0.02 0.19 ± 0.01 0.11 ± 0.01 0.17 ± 0.01 0.14 ± 0.02 0.07 ± 0.01 0.28 ± 0.02 0.14 ± 0.03 0.13 ± 0.01 0.33 ± 0.01 0.24 ± 0.03 0.19 ± 0.01 Zn 0.30 ± 0.02 0.25 ± 0.01 0.15 ± 0.01 0.22 ± 0.01 0.18 ± 0.02 0.13 ± 0.02 0.36 ± 0.02 0.26 ± 0.03 0.14 ± 0.01 0.43 ± 0.01 0.31 ± 0.02 0.25 ± 0.02 Pb 0.20 ± 0.02 0.16 ± 0.01 0.08 ± 0.02 0.14 ± 0.03 0.11 ± 0.03 0.04 ± 0.02 0.25 ± 0.01 0.24 ± 0.02 0.11 ± 0.03 0.30 ± 0.03 0.21 ± 0.02 0.16 ± 0.02 Cr 0.23 ± 0.02 0.18 ± 0.02 0.09 ± 0.01 0.15 ± 0.01 0.13 ± 0.02 0.05 ± 0.02 0.27 ± 0.01 0.22 ± 0.01 0.09 ± 0.02 0.31 ± 0.02 0.22 ± 0.03 0.17 ± 0.02 EC Monoculture-Chinese cabbage Intercropping-Chinese cabbage Monoculture- ryegrass Intercropping- ryegrass Addlication Amount 0% 3% 6% 0% 3% 6% 0% 3% 6% 0% 3% 6% Cu 0.33 ± 0.04 0.24 ± 0.02 0.19 ± 0.03 0.26 ± 0.02 0.23 ± 0.02 0.16 ± 0.03 0.37 ± 0.02 0.23 ± 0.02 0.20 ± 0.03 0.42 ± 0.02 0.33 ± 0.01 0.28 ± 0.02 Zn 0.43 ± 0.02 0.31 ± 0.01 0.25 ± 0.01 0.34 ± 0.03 0.30 ± 0.02 0.21 ± 0.02 0.48 ± 0.04 0.30 ± 0.02 0.26 ± 0.02 0.55 ± 0.01 0.43 ± 0.04 0.36 ± 0.02 Pb 0.30 ± 0.02 0.21 ± 0.01 0.16 ± 0.02 0.23 ± 0.04 0.20 ± 0.03 0.13 ± 0.02 0.34 ± 0.03 0.20 ± 0.02 0.17 ± 0.02 0.39 ± 0.03 0.30 ± 0.02 0.25 ± 0.02 Cd 0.31 ± 0.02 0.22 ± 0.04 0.17 ± 0.02 0.24 ± 0.03 0.21 ± 0.02 0.14 ± 0.01 0.36 ± 0.04 0.21 ± 0.01 0.18 ± 0.02 0.41 ± 0.03 0.31 ± 0.03 0.26 ± 0.02 4 Conclusion The application of SFR significantly enhanced several key soil physicochemical properties, including CEC, total organic carbon, ammonium nitrogen, available phosphorus, and available potassium. Moreover, SFR increased the proportion of heavy metals (Cu, Zn, and Pb) present in stable residual fractions, thereby reducing their mobility and bioavailability in the soil. SFR also promoted the growth of Chinese cabbage and ryegrass, as evidenced by increased chlorophyll content, fresh biomass, and plant height. In intercropping systems, Chinese cabbage exhibited improved growth performance compared to monoculture, whereas ryegrass growth was slightly suppressed. This phenomenon may be attributed to differential uptake of heavy metals, where intercropping reduced the translocation and accumulation of heavy metals in Chinese cabbage but enhanced those in ryegrass, suggesting a complementary metal partitioning strategy between the two species. Declarations Authors’ contributions Shijie Zhang contributed to the experiment and wrote the original draft of the manuscript. Weihua Gu was responsible for reviewing and editing the manuscript. Jing Zhao performed formal analysis and data interpretation for the study. Jianfeng Bai contributed to the conceptualization, methodology design, and supervised the entire study. Funding This work was partially supported by Results incorporated in this paper received funding from the Local College Capacity Building Project (Grant number 23010500500) and the Shanghai Pudong New Area Livelihood Research Project (Grant number PKJ2023-C07, PKJ2024-C02). Data availability All data generated or analyzed during this study are included in this article. Competing interests The authors declare that they have no competing interests. Ethics approval and consent to participate Not applicable. Consent for publication Not applicable. References Kumar S, Islam ARMT, Islam HMT, et al. Water resources pollution associated with risks of heavy metals from Vatukoula Goldmine region, Fiji. Journal of Environmental Management 2021;293:112868-112881; doi: https://doi.org/10.1016/j.jenvman.2021.112868. 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Metal immobilization by sludge-derived biochar: roles of mineral oxides and carbonized organic compartment. Environmental Geochemistry and Health 2017;39(2):379-389; doi: 10.1007/s10653-016-9851-z. Lu K, Yang X, Gielen G, et al. Effect of bamboo and rice straw biochars on the mobility and redistribution of heavy metals (Cd, Cu, Pb and Zn) in contaminated soil. Journal of Environmental Management 2017;186:285-292; doi: https://doi.org/10.1016/j.jenvman.2016.05.068. Rens H, Bera T, Alva AK. Effects of Biochar and Biosolid on Adsorption of Nitrogen, Phosphorus, and Potassium in Two Soils. Water, Air, & Soil Pollution 2018;229(8):281-293; doi: 10.1007/s11270-018-3925-8. Liu W, Luo Y, Zhu X, et al. Optimizing tea plantation productivity: Magnesium-modified tea pruning litter biochar enhances soil quality and tea aroma profiles. Environmental Technology & Innovation 2025:104375-104393; doi: https://doi.org/10.1016/j.eti.2025.104375. Lu H, Yang Y, Huang K, et al. Transformation kinetics of exogenous lead in an acidic soil during anoxic-oxic alteration: Important roles of phosphorus and organic matter. Environmental Pollution 2023;335:122271-122281; doi: https://doi.org/10.1016/j.envpol.2023.122271. Hua Q-X, Li J-Y, Zhou J-M, et al. Enhancement of Phosphorus Solubility by Humic Substances in Ferrosols1 1Project supported by the National Natural Science Foundation of China (No. 30400273) and the Potash and Phosphate Institute/Potash and Phosphate Institute of Canada (PPI/PPIC). Pedosphere 2008;18(4):533-538; doi: https://doi.org/10.1016/S1002-0160(08)60044-2. Heikkinen J, Keskinen R, Soinne H, et al. Possibilities to improve soil aggregate stability using biochars derived from various biomasses through slow pyrolysis, hydrothermal carbonization, or torrefaction. Geoderma 2019;344:40-49; doi: https://doi.org/10.1016/j.geoderma.2019.02.028. Abrol V, Ben-Hur M, Verheijen FGA, et al. Biochar effects on soil water infiltration and erosion under seal formation conditions: rainfall simulation experiment. Journal of Soils and Sediments 2016;16(12):2709-2719; doi: 10.1007/s11368-016-1448-8. Hussein YHA, Amin G, Askora A, et al. Phytotoxicity remediation in wheat (Triticum aestivum L.) cultivated in Cadmium- contaminated soil by intercropping design. Bioscience Research 2019;16:2678-2689. Liu Y, Huang L, Wen Z, et al. Effects of intercropping on safe agricultural production and phytoremediation of heavy metal-contaminated soils. Science of The Total Environment 2023;875:162700-162709; doi: https://doi.org/10.1016/j.scitotenv.2023.162700. Additional Declarations No competing interests reported. Supplementary Files Graphicalabstract.docx Cite Share Download PDF Status: Published Journal Publication published 05 Mar, 2026 Read the published version in Environmental Monitoring and Assessment → Version 1 posted Editorial decision: Revision requested 01 Sep, 2025 Editor assigned by journal 28 Aug, 2025 Submission checks completed at journal 28 Aug, 2025 First submitted to journal 26 Aug, 2025 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-7458907","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":508719011,"identity":"a981dd34-b9ee-496f-84f1-f063b906c933","order_by":0,"name":"Shijie Zhang","email":"","orcid":"","institution":"Shanghai University","correspondingAuthor":false,"prefix":"","firstName":"Shijie","middleName":"","lastName":"Zhang","suffix":""},{"id":508719012,"identity":"7e6a7c24-f0a8-4145-a20e-fbd60d985ba3","order_by":1,"name":"Weihua Gu","email":"","orcid":"","institution":"Shanghai Polytechnic University","correspondingAuthor":false,"prefix":"","firstName":"Weihua","middleName":"","lastName":"Gu","suffix":""},{"id":508719013,"identity":"3197dc03-347c-4ddc-bd30-fa1b3f945288","order_by":2,"name":"Jing Zhao","email":"","orcid":"","institution":"Shanghai Polytechnic University","correspondingAuthor":false,"prefix":"","firstName":"Jing","middleName":"","lastName":"Zhao","suffix":""},{"id":508719014,"identity":"bef26c30-7e41-4b66-926a-0fc747757cdc","order_by":3,"name":"Jianfeng Bai","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAyElEQVRIiWNgGAWjYBACxuYzDAwJFVAeD1Fa2nKAWs6QooWBLQekjxQtzG28Bz88nFdnby6RwPjgbRuDvDlhh/ElSyRuO5y4c0YCs+HcNgbDnQ2EtMzvMQBqOZBgcCOBTZq3jSHB4ABBW3iMfyTOqbMHamH/TawWM4nEBmbGDUBbmInUwpdmkXAM6Jeeh82Sc85JGG4gpMWwjffwzR81wBBjTz744U2ZjTxBWwwboAwDBkYQU4KAeiCQhzEMCKsdBaNgFIyCkQoAMFY+LykKL6EAAAAASUVORK5CYII=","orcid":"","institution":"Shanghai Polytechnic University","correspondingAuthor":true,"prefix":"","firstName":"Jianfeng","middleName":"","lastName":"Bai","suffix":""}],"badges":[],"createdAt":"2025-08-26 05:23:40","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7458907/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7458907/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10661-026-15007-8","type":"published","date":"2026-03-05T15:59:42+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":93455747,"identity":"d5e24fbd-aa40-44ec-9296-80d90589e0e4","added_by":"auto","created_at":"2025-10-14 05:05:52","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":46589,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSoil Loss Simulation (Brown represents soil )\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7458907/v1/c8070242ddd31f28bd51d883.png"},{"id":93455581,"identity":"1dcda824-0f33-4240-8d94-2bf6f60eb8d7","added_by":"auto","created_at":"2025-10-14 04:57:52","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":56187,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEffect of adding amount of SFR on the form of heavy metals in soil\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7458907/v1/eb7efc0ccd637ba09ceb40a8.png"},{"id":93455584,"identity":"d9b120c7-26c4-498a-9bc8-2b91e7ab220f","added_by":"auto","created_at":"2025-10-14 04:57:53","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":48948,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEffect of adding amount of SFR on soil nutrients\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7458907/v1/86f1774d788e19e2d1375a18.png"},{"id":93455748,"identity":"cdea7a5f-0e46-4336-8e86-0b38ae866827","added_by":"auto","created_at":"2025-10-14 05:05:53","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":102030,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePhysical and Chemical Properties of Plants\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7458907/v1/63c02fd9e729aa08a4c9fad2.png"},{"id":104250803,"identity":"b946cc21-3059-42f2-8fd6-d9a8c9c67d5f","added_by":"auto","created_at":"2026-03-09 16:08:58","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1277679,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7458907/v1/58aa96f9-840e-4d21-8e55-34a44e8655e0.pdf"},{"id":93455582,"identity":"74277b63-3978-405b-af12-95ab8009331a","added_by":"auto","created_at":"2025-10-14 04:57:53","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":277796,"visible":true,"origin":"","legend":"","description":"","filename":"Graphicalabstract.docx","url":"https://assets-eu.researchsquare.com/files/rs-7458907/v1/3b855c0a17d9261fc781d020.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Study on the effect of sludge fermentation residue on soil remediation and plant growth in mining areas","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eWith the rapid advancement of industrialization, the exploitation and processing of metal resources have resulted in widespread soil contamination by heavy metals worldwide [1, 2]. According to the National Soil Pollution Survey Bulletin of China, approximately 13.9% of soil samples exceed the regulatory thresholds for heavy metal concentrations [3]. Common contaminants such as lead (Pb), cadmium (Cd), copper (Cu), zinc (Zn), and chromium (Cr) are of particular concern due to their high toxicity, persistence, and bioaccumulation potential [4, 5]. Consequently, the development of sustainable, efficient, and cost-effective technologies for soil remediation has become a critical focus of environmental research.\u003c/p\u003e\n\u003cp\u003eAmong the various soil remediation strategies, biochar has attracted considerable attention owing to its high porosity, large surface area, abundant surface functional groups, and strong cation exchange capacity [6-8]. These properties enable biochar to immobilize heavy metals by reducing their mobility and bioavailability in contaminated soils [9, 10]. Specifically, sludge-derived biochar (SDB), produced by pyrolyzing municipal sewage sludge, is rich in minerals and nutrients and has demonstrated potential in heavy metal stabilization[11, 12]. However, traditional pyrolysis processes often require high energy input, yield low amounts of biochar, and may degrade functional groups critical to metal sorption, thus limiting the practical application of SDB in large-scale remediation projects [13].\u003c/p\u003e\n\u003cp\u003eAs an alternative,\u0026nbsp;\u003cstrong\u003eSFR\u003c/strong\u003e has recently emerged as a promising amendment for soil remediation. SFR is produced through aerobic fermentation of sewage sludge, a low-temperature, low-energy biological process in which microbial activity decomposes organic matter and generates stable humic substances [14]. This process retains essential nutrients and introduces active functional groups such as carboxyl and phenolic hydroxyls, which facilitate complexation or precipitation with heavy metals [15], thereby reducing their environmental risk. Compared with thermal treatment, aerobic fermentation is more energy-efficient and environmentally friendly while preserving the structural integrity and adsorptive capacity of the final product.\u003c/p\u003e\n\u003cp\u003eMoreover, SFR contains abundant nitrogen, phosphorus, potassium, and organic carbon, which are essential for soil fertility. Its application can improve soil pH, increase CEC, enhance soil aggregation, and promote microbial activity, all of which contribute to soil health and plant productivity [16]. At present, research on SFR mostly focuses on the recovery of fatty acids. There is relatively little research on the remediation of soil in mining areas by SFR.\u003c/p\u003e\n\u003cp\u003eThis study aims to explore the potential of sludge fermentation residue as a soil amendment for ecological restoration in mining areas. A systematic evaluation was conducted to investigate its effects on (i) the transformation and stabilization of heavy metals in soil, (ii) the enhancement of soil physicochemical properties, and (iii) the growth response and metal uptake of two model plants\u0026mdash;ryegrass (\u003cem\u003eLolium perenne L.\u003c/em\u003e) and Chinese cabbage (\u003cem\u003eBrassica chinensis L.\u003c/em\u003e)\u0026mdash;under monoculture and intercropping conditions. The findings provide theoretical and practical insights into the application of low-energy sludge treatment byproducts in sustainable soil remediation and ecological restoration.\u003c/p\u003e"},{"header":"2 Materials and Methods","content":"\u003ch3\u003e\u003cstrong\u003e2.1\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eMaterial source and pre-treatment\u003c/strong\u003e\u003c/h3\u003e\n\u003cp\u003eThe sewage sludge used in this study was obtained from the secondary sedimentation tank of a municipal wastewater treatment plant in a town of Changzhou City, Jiangsu Province, China. SFR was prepared via aerobic composting. Specifically, the air-dried sludge was thoroughly mixed with crushed corn straw at a mass ratio of 4:1 (sludge:straw), and the mixture was placed in a 100 L forced-aeration aerobic fermenter. The composting process was carried out at a controlled temperature of 60 \u0026deg;C for 60 days. During fermentation, the materials were turned once every 7 days, and aeration was maintained at a flow rate of 0.3 L/(min\u0026middot;kg dry matter) to ensure sufficient oxygen supply. The moisture content of the composting matrix was adjusted to approximately 60% by periodic water addition to maintain optimal microbial activity. After fermentation, the residue was naturally air-dried, ground, and sieved to a particle size of \u0026lt;2 mm, then stored in sealed containers at room temperature (~25 \u0026deg;C) for later use.\u003c/p\u003e\n\u003cp\u003eThe experimental soil was collected from a tailings site in a metal mining area in the Tibet Autonomous Region, China. After collection, the soil was air-dried, homogenized, and passed through a 2 mm sieve after removing plant and animal debris. The processed soil was stored at room temperature prior to use. The basic physical and chemical properties of the soil are summarized in Table 1.\u003c/p\u003e\n\u003ch4\u003eTable 1: Soil Physical and Chemical Properties\u003c/h4\u003e\n\u003cdiv align=\"center\"\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"661\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 17.2466%;\"\u003e\n \u003cp\u003eName\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.8593%;\"\u003e\n \u003cp\u003eSludge fermentation residue\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.01815%;\"\u003e\n \u003cp\u003eSoil\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003eSludge treatment in urban sewage treatment plants - Soil quality for land improvement\u0026nbsp;（GB/T24600\u0026mdash;2009）\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003eScreening values for risk control standards of soil pollution in agricultural land\u003c/p\u003e\n \u003cp\u003e(GB15618\u0026mdash;2018)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003eStandard Control Values for Soil Pollution Risk Control of Agricultural Land\u003c/p\u003e\n \u003cp\u003e(GB15618\u0026mdash;2018)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 17.2466%;\"\u003e\n \u003cp\u003eCEC (cmol\u003csup\u003e+\u003c/sup\u003e/kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.8593%;\"\u003e\n \u003cp\u003e35.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 8.01815%;\"\u003e\n \u003cp\u003e7.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 17.2466%;\"\u003e\n \u003cp\u003eSOC (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.8593%;\"\u003e\n \u003cp\u003e3.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 8.01815%;\"\u003e\n \u003cp\u003e0.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 17.2466%;\"\u003e\n \u003cp\u003eAmmonium nitrogen (mg/kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.8593%;\"\u003e\n \u003cp\u003e450\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 8.01815%;\"\u003e\n \u003cp\u003e19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 17.2466%;\"\u003e\n \u003cp\u003eAvailable Phosphorus (mg/kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.8593%;\"\u003e\n \u003cp\u003e830\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 8.01815%;\"\u003e\n \u003cp\u003e4.25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 17.2466%;\"\u003e\n \u003cp\u003eAvailable potassium (mg/kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.8593%;\"\u003e\n \u003cp\u003e1246\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 8.01815%;\"\u003e\n \u003cp\u003e42.12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 17.2466%;\"\u003e\n \u003cp\u003epH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.8593%;\"\u003e\n \u003cp\u003e7.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 8.01815%;\"\u003e\n \u003cp\u003e6.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 17.2466%;\"\u003e\n \u003cp\u003eCu (mg/kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.8593%;\"\u003e\n \u003cp\u003e257\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 8.01815%;\"\u003e\n \u003cp\u003e3228\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003e1500\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 17.2466%;\"\u003e\n \u003cp\u003eZn (mg/kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.8593%;\"\u003e\n \u003cp\u003e370\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 8.01815%;\"\u003e\n \u003cp\u003e490\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003e4000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003e300\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 17.2466%;\"\u003e\n \u003cp\u003eNi (mg/kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.8593%;\"\u003e\n \u003cp\u003e13\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 8.01815%;\"\u003e\n \u003cp\u003e41\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003e200\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003e250\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003e1300\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 17.2466%;\"\u003e\n \u003cp\u003ePb (mg/kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.8593%;\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 8.01815%;\"\u003e\n \u003cp\u003e296\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003e1000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003e170\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003e1000\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 17.2466%;\"\u003e\n \u003cp\u003eCr (mg/kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.8593%;\"\u003e\n \u003cp\u003e40\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 8.01815%;\"\u003e\n \u003cp\u003e230\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003e1000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003e190\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 17.2466%;\"\u003e\n \u003cp\u003eCd (mg/kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 12.8593%;\"\u003e\n \u003cp\u003e0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 8.01815%;\"\u003e\n \u003cp\u003e0.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 21.3313%;\"\u003e\n \u003cp\u003e20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 19.6672%;\"\u003e\n \u003cp\u003e0.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 20.8775%;\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003ch3 id=\"_Toc4701\"\u003e\u003cstrong\u003e2.2\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eExperimental processing and methods\u003c/strong\u003e\u003c/h3\u003e\n\u003cp\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003cstrong\u003e）\u003c/strong\u003e\u003cstrong\u003eExperimental design\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSFR was incorporated into the test soil at three application rates: 0% (control), 3%, and 6% by mass. Each treatment was conducted with three replicates, and 1 kg of homogenized soil mixture was placed into plastic pots. The SFR was thoroughly mixed with the soil prior to planting. After moistening the soil with deionized water, pots were pre-incubated for 7 days at room temperature to allow stabilization.\u003c/p\u003e\n\u003cp\u003eTwo plant species\u0026mdash;ryegrass (\u003cem\u003eLolium perenne L.\u003c/em\u003e) and Chinese cabbage (\u003cem\u003eBrassica chinensis L.\u003c/em\u003e)\u0026mdash;were sown under both monoculture and intercropping systems. In monoculture treatments, 100 seeds of each species were sown per pot. In the intercropping treatment, 50 seeds of each species were sown simultaneously in the same pot. The experiment lasted for 60 days under natural light conditions, and no additional fertilizer was applied during the growth period.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003cstrong\u003e）\u003c/strong\u003e\u003cstrong\u003eSoil and Plant Sample Collection\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAt harvest, the fresh weight of Chinese cabbage and ryegrass was recorded. Shoots and roots were carefully separated and rinsed with distilled water. The plant tissues were inactivated at 105 \u0026deg;C for 30 minutes to stop enzymatic activity, then oven-dried at 65 \u0026deg;C to constant weight and ground for subsequent analysis. Soil samples were collected from each pot at the end of the experiment, air-dried at room temperature, homogenized, and stored for physicochemical and heavy metal analysis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003cstrong\u003e）\u003c/strong\u003e\u003cstrong\u003eSimulate soil erosion\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo evaluate the effect of SFR on soil erosion resistance, a sloped container (30 \u0026times; 10 \u0026times; 5 cm, length \u0026times; width \u0026times; height) was used to simulate soil loss (Figure 1). Each container was filled with 1 kg of soil treated with 0%, 3%, or 6% SFR by weight, with three replicates per treatment. A rainfall simulation experiment was conducted by applying 900 mL of deionized water evenly over 2 hours, mimicking the historical maximum daily rainfall (48.7 mm) observed in a region of Tibet. The simulation was repeated twice per week for a total of 10 cycles. Runoff and eroded soil were collected, oven-dried, and weighed to determine soil loss.\u003c/p\u003e\n\u003ch3\u003e\u003cstrong\u003e2.3 \u0026nbsp;Analytical Methods and Instruments\u003c/strong\u003e\u003c/h3\u003e\n\u003cp\u003eAll analytical procedures in this study were carried out following the Chinese national environmental testing standards. The CEC of the soil was determined using a 1.66 cmol/L hexamminecobalt (III) chloride solution as extractant. After 60 minutes of extraction, samples were centrifuged at 400 rpm, and the absorbance was measured at wavelengths of 475 nm and 375 nm using a UV spectrophotometer (UV-756, Shanghai Precision Scientific Instruments Co., China). Soil total organic carbon was analyzed by the potassium dichromate oxidation method. In this procedure, 5 mL of 0.27 mol/L potassium dichromate, 0.1 g of mercury(II) sulfate, and 7.5 mL of concentrated sulfuric acid (1.84 g/mL) were added to the sample, followed by constant-temperature heating at 135 \u0026deg;C for 30 minutes. The resulting absorbance was measured at 585 nm using the same spectrophotometer.\u003c/p\u003e\n\u003cp\u003eAmmonium nitrogen was quantified using the indophenol blue colorimetric method. Phenol and sodium hypochlorite solutions were added to the samples, which were then kept at room temperature for 60 minutes before adding a masking agent, after which the absorbance was recorded at 352 nm. Available phosphorus was extracted with sodium bicarbonate solution, shaken for 30 minutes, and then filtered, with the absorbance measured at 660 nm using UV spectrophotometry. Chlorophyll in plant tissues was extracted using a mixture of absolute ethanol and acetone (1:1, v/v), and its absorbance was measured at 645 nm and 663 nm to calculate total chlorophyll content.\u003c/p\u003e\n\u003cp\u003eThe concentrations of heavy metals (Cu, Zn, Pb, and Cr) in soil samples were determined after digestion with aqua regia (HNO₃:HCl = 1:3, v/v), while plant samples were digested with a nitric acid\u0026ndash;hydrogen peroxide mixture. Both types of samples were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 8000, PerkinElmer, USA). Soil pH was measured in a 1:2.5 (w/v) soil-to-water suspension using a calibrated pH meter (PHS-3C, INESA Scientific Instrument Co., China). All instruments were calibrated before use, and all procedures were performed in triplicate to ensure accuracy and repeatability.\u003c/p\u003e"},{"header":"3 Results and Discussion","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Effect of SFR on the Fractional Distribution of Heavy Metals in Soil\u003c/h2\u003e\u003cp\u003eThe influence of SFR addition on the chemical forms of heavy metals (Cu, Zn, Pb, Cr) in soil is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. All four metals exhibited significant immobilization responses following the application of SFR. Compared to the control (0% addition), increasing the SFR dosage enhanced the proportion of stable fractions (F3\u0026thinsp;+\u0026thinsp;F4), corresponding to organically bound and residual forms. At the 6% addition level, the proportion of stable forms reached 75% for Cu, 87% for Zn, 75% for Pb, and 98% for Cr. The transformation pattern for all metals was characterized by a shift from labile (F1\u0026thinsp;+\u0026thinsp;F2) to stable (F3\u0026thinsp;+\u0026thinsp;F4) fractions. Notably, the immobilization of Pb showed the greatest enhancement (~\u0026thinsp;6%), whereas Cr showed minimal change (~\u0026thinsp;1%) due to its already high baseline stability (97%).\u003c/p\u003e\u003cp\u003eThe enhanced Pb stabilization can be attributed to multiple interacting mechanisms. Sewage sludge is rich in phosphorus, and phosphate species are largely retained in the SFR. These compounds contribute to the formation of low-solubility lead minerals, such as pyromorphite [Pb₅(PO₄)₃X; X\u0026thinsp;=\u0026thinsp;F, Cl, OH] through ion exchange and mineral precipitation reactions (Huang et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Furthermore, co-fermentation with straw significantly increases the specific surface area and porosity of the residue, providing additional Pb adsorption sites associated with phosphorus-based functional groups (Kumpiene et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). The elevated total organic carbon content introduced by SFR also enhances the formation of stable Pb\u0026ndash;organic complexes in soil, further reducing Pb bioavailability (Boughattas et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Additionally, an increase in soil pH induced by SFR application contributes to decreased Pb mobility, as lead bioavailability generally declines under more alkaline conditions.\u003c/p\u003e\u003cp\u003eCu demonstrated a strong affinity for organic matter, primarily due to its tendency to complex with high molecular weight humic substances and functional groups such as carboxyl and phenolic hydroxyls (Li, S. et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The surface of SFR is enriched with such groups, which can form stable Cu\u0026ndash;organic complexes, leading to its effective immobilization(Lee et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In addition,the porous structure of SFR supports microbial colonization and nutrient retention, indirectly increasing soil organic matter levels and enhancing microbial-mediated Cu stabilization.\u003c/p\u003e\u003cp\u003eThe immobilization of Zn is primarily governed by precipitation with anions (hydroxides, carbonates, phosphates) and complexation with organic ligands (Beesley et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). The combined effect of cation exchange and ligand binding is considered the dominant mechanism of Zn stabilization in the presence of SFR (Zhang et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Additionally, the rise in soil pH further facilitates the formation of insoluble Zn precipitates, thereby contributing to its decreased mobility (Lu et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Although Cr showed relatively minor changes in its fractional distribution following SFR treatment, this can be attributed to its already high stability in the initial soil matrix. Nonetheless, the presence of organic matter and elevated pH conditions introduced by SFR may further suppress its potential remobilization.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.2 The effect of sludge fermentation residue on soil physicochemical properties\u003c/h2\u003e\u003cp\u003eThe effects of SFR application on soil physicochemical properties are illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Increasing the addition rate of SFR markedly improved several key indicators of soil fertility. Compared to the control (0% SFR), the 6% treatment resulted in significant increases in available potassium (by 176 mg/kg, +\u0026thinsp;419%), available phosphorus (by 62 mg/kg, +\u0026thinsp;1476%), ammonium nitrogen (by 21 mg/kg, +\u0026thinsp;112%), and total organic carbon (by 1.3%, +\u0026thinsp;582%). Additionally, the soil CEC increased by 9.4 cmol⁺/kg (+\u0026thinsp;127%).\u003c/p\u003e\u003cp\u003eThese improvements can be primarily attributed to two synergistic mechanisms. First, SFR itself is rich in essential nutrients including nitrogen, phosphorus, potassium, and organic carbon, which directly contribute to the replenishment of soil nutrient pools. Second, the porous structure and surface functional groups (e.g., carboxyl and hydroxyl groups) of SFR exhibit strong cation adsorption capacity, enhancing nutrient retention and reducing nutrient leaching. Previous studies have shown that biochar derived from sludge exhibits high affinity for NH\u003csub\u003e3\u003c/sub\u003e/NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, thereby reducing gaseous nitrogen losses and improving soil nitrogen use efficiency (Rens et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Moreover, SFR application promotes microbial activity and soil organic matter content, further enhancing the retention of ammonium nitrogen in the soil matrix (Liu et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe notable increase in available phosphorus can also be explained by several chemical interactions. Carboxyl groups on the SFR surface can adsorb phosphate ions, thereby inhibiting their fixation by iron and aluminum oxides in acidic soils. In addition, organic acids released during fermentation may interact with phosphate minerals, promoting phosphorus solubilization (Lu et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Furthermore, pyrolyzed organic matter has been reported to enhance the mineralization of organic phosphorus by adsorbing and concentrating phosphate ions in the rhizosphere (Hua et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2008\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIn degraded soils from mining areas, low cohesion and poor aggregation often result in high erosion risk, which can lead to severe outcomes such as surface runoff and mudslides. Application of SFR significantly mitigated this problem. At a 6% addition rate, soil bulk density increased by 44%, while simulated soil loss decreased by 36%. This can be explained by the ability of SFR to enhance the aggregation stability of soil particles and reduce their detachment during rainfall events. There is a research report (Heikkinen et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) that the relatively coarse particle size of fermentation residue increases surface roughness, which disrupts the lateral flow of runoff and physically intercepts soil particles, thereby reducing erosion.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eEffect of adding amount of SFR on soil properties\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAddlication\u003c/p\u003e\u003cp\u003eAmount (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSoil bulk density (g/cm\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSoil loss (g/week)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eOrganic carbon (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003epH\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eCEC\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e0.86\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e80.45\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.234\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e6.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e7.47\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1.13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e60.73\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e1.009\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e7.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e12.11\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1.24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e50.44\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e1.571\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e16.83\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.3 The impact of SFR on plants\u003c/h2\u003e\u003cp\u003eThe application of sludge SFR significantly enhanced the growth performance of both ryegrass and Chinese cabbage, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. In monoculture treatments, the application of 6% SFR resulted in substantial increases in chlorophyll content, fresh weight, and plant height. Specifically, chlorophyll content increased by 72% in Chinese cabbage and 19% in ryegrass, while fresh weight increased by 10% and 50%, and plant height increased by 62% and 33%, respectively, compared to the control group without SFR application.\u003c/p\u003e\u003cp\u003eUnder intercropping conditions, the growth-promoting effects of SFR were also evident. At the 6% application rate, chlorophyll content in Chinese cabbage and ryegrass increased by 71% and 19%, respectively. Fresh weight increased by 32% for Chinese cabbage and 51% for ryegrass, while plant height increased by 52% and 40%, respectively. These results indicate that SFR application is effective in promoting photosynthetic pigment accumulation and biomass production in both species, under both monoculture and intercropping systems.\u003c/p\u003e\u003cp\u003eInterestingly, intercropping appeared to further enhance the chlorophyll content and biomass accumulation of Chinese cabbage, whereas it slightly reduced those of ryegrass. This differential response may be related to the interspecific interactions in metal-contaminated soil. It is hypothesized that ryegrass, which exhibits higher heavy metal tolerance and accumulation capacity, may preferentially absorb and translocate heavy metals in the rhizosphere. This mitigates metal-induced stress on Chinese cabbage and promotes its growth. A detailed analysis of heavy metal enrichment and transport mechanisms in both plants is provided in section \u003cspan refid=\"Sec13\" class=\"InternalRef\"\u003e3.4\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Migration and enrichment of heavy metals in plants\u003c/h2\u003e\u003cp\u003eThe effects of SFR on the migration and enrichment of heavy metals in plants are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Overall, the application of SFR significantly reduced the translocation factor (TF) and enrichment coefficient (EC) for Cu, Zn, Pb, and Cr in both Chinese cabbage and ryegrass, indicating a strong immobilization effect in the plant\u0026ndash;soil system.\u003c/p\u003e\u003cp\u003eIn monoculture systems, the application of 6% SFR reduced the TF of Cu, Zn, Pb, and Cr in Chinese cabbage by 52%, 50%, 60%, and 60%, respectively, while reductions in ryegrass were 53%, 61%, 56%, and 66%, respectively. Correspondingly, the EC of these metals was also decreased\u0026mdash;by 42%, 41%, 46%, and 45% in Chinese cabbage, and by 33%, 35%, 37%, and 36% in ryegrass. Under intercropping conditions, SFR exerted a similar but more differentiated effect. At the same 6% application rate, TF values in Chinese cabbage were reduced by 58% (Cu), 40% (Zn), 71% (Pb), and 66% (Cr), while ryegrass exhibited TF reductions of 53%, 61%, 56%, and 66%, respectively. Reductions in EC values under intercropping were also observed, with declines of 38%, 38%, 43%, and 41% in Chinese cabbage, and 33%, 35%, 37%, and 36% in ryegrass.\u003c/p\u003e\u003cp\u003eThese results suggest that Chinese cabbage consistently exhibits lower TF and EC values compared to ryegrass, indicating a lower tendency for heavy metal uptake and translocation. Notably, the intercropping system appears to further suppress heavy metal accumulation in Chinese cabbage while slightly enhancing it in ryegrass. This asymmetry may be attributed to rhizospheric interactions between the two species. Ryegrass, with its stronger metal tolerance and uptake capacity, likely acts as a \u0026ldquo;metal sink,\u0026rdquo; preferentially absorbing mobile metal ions from the shared rhizosphere, thereby alleviating metal stress on Chinese cabbage.\u003c/p\u003e\u003cp\u003eThis phenomenon is consistent with previous findings in intercropping systems such as soybean\u0026ndash;corn, where the nutrient and signaling exchanges between species altered heavy metal partitioning. In that case, soybean's higher metal uptake helped protect corn from heavy metal toxicity, possibly through nutrient and nitrogen transfer from root exudates (Abrol et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Similar mechanisms may operate here, where signal-induced exudation of organic acids or chelators by ryegrass modulates the rhizosphere microenvironment and limits metal transport to Chinese cabbage (Hussein et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). As (Liu et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) emphasized, the effects of intercropping on metal migration are strongly dependent on plant species interactions, root secretion profiles, and the specific metal species involved.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eMigration and enrichment of heavy metals in plants\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"13\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c12\" colnum=\"12\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c13\" colnum=\"13\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTF\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u003cp\u003eMonoculture-Chinese cabbage\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e\u003cp\u003eIntercropping-Chinese cabbage\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c10\" namest=\"c8\"\u003e\u003cp\u003eMonoculture- ryegrass\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c13\" namest=\"c11\"\u003e\u003cp\u003eIntercropping- ryegrass\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAddlication Amount\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e6%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e3%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e6%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e3%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e6%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003e0%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003e3%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003e6%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCu\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.23\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.19\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.11\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.17\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.14\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.07\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.28\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.14\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.13\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003e0.33\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003e0.24\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003e0.19\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eZn\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.30\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.25\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.15\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.22\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.18\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.13\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.36\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.26\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.14\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003e0.43\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003e0.31\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePb\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.20\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.16\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.08\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.14\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.04\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.24\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.11\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003e0.30\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003e0.21\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003e0.16\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCr\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.23\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.18\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.09\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.15\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.13\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.05\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.27\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.22\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.09\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003e0.31\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003e0.22\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003e0.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\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\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e\u003ccolgroup cols=\"13\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c12\" colnum=\"12\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c13\" colnum=\"13\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eEC\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u003cp\u003eMonoculture-Chinese cabbage\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e\u003cp\u003eIntercropping-Chinese cabbage\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c10\" namest=\"c8\"\u003e\u003cp\u003eMonoculture- ryegrass\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c13\" namest=\"c11\"\u003e\u003cp\u003eIntercropping- ryegrass\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAddlication Amount\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e6%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e3%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e6%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e3%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e6%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003e0%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003e3%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003e6%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCu\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.33\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.24\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.19\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.26\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.16\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.37\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.20\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003e0.42\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003e0.33\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003e0.28\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eZn\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.43\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.31\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.30\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.21\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.48\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.30\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.26\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003e0.55\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003e0.43\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003e0.36\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePb\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.30\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.21\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.16\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.23\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.20\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.13\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.20\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003e0.39\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003e0.30\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCd\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.31\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.22\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.24\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.21\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.14\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.36\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.21\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.18\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003e0.41\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003e0.31\u003c/p\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003e0.26\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eThe application of SFR significantly enhanced several key soil physicochemical properties, including CEC, total organic carbon, ammonium nitrogen, available phosphorus, and available potassium. Moreover, SFR increased the proportion of heavy metals (Cu, Zn, and Pb) present in stable residual fractions, thereby reducing their mobility and bioavailability in the soil.\u003c/p\u003e\u003cp\u003eSFR also promoted the growth of Chinese cabbage and ryegrass, as evidenced by increased chlorophyll content, fresh biomass, and plant height. In intercropping systems, Chinese cabbage exhibited improved growth performance compared to monoculture, whereas ryegrass growth was slightly suppressed. This phenomenon may be attributed to differential uptake of heavy metals, where intercropping reduced the translocation and accumulation of heavy metals in Chinese cabbage but enhanced those in ryegrass, suggesting a complementary metal partitioning strategy between the two species.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthors’ contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eShijie Zhang\u003c/strong\u003e contributed to the experiment and wrote the original draft of the manuscript. \u003cstrong\u003eWeihua Gu\u003c/strong\u003e was responsible for reviewing and editing the manuscript. \u003cstrong\u003eJing Zhao\u003c/strong\u003e performed formal analysis and data interpretation for the study. \u003cstrong\u003eJianfeng Bai\u003c/strong\u003e contributed to the conceptualization, methodology design, and supervised the entire study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was partially supported by Results incorporated in this paper received funding from the Local College Capacity Building Project (Grant number 23010500500) and the Shanghai Pudong New Area Livelihood Research Project (Grant number PKJ2023-C07, PKJ2024-C02).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data generated or analyzed during this study are included in this article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eKumar S, Islam ARMT, Islam HMT, et al. 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Effects of intercropping on safe agricultural production and phytoremediation of heavy metal-contaminated soils. Science of The Total Environment 2023;875:162700-162709; doi: https://doi.org/10.1016/j.scitotenv.2023.162700.\u003c/li\u003e\n\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":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-monitoring-and-assessment","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"emas","sideBox":"Learn more about [Environmental Monitoring and Assessment](http://link.springer.com/journal/10661)","snPcode":"10661","submissionUrl":"https://submission.nature.com/new-submission/10661/3","title":"Environmental Monitoring and Assessment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Sludge fermentation, Heavy metals, Mining soil remediation, Plants","lastPublishedDoi":"10.21203/rs.3.rs-7458907/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7458907/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHeavy metal contamination in mining-affected soils poses severe risks to ecosystems and crop productivity. In this study, sludge fermentation residue (SFR), derived from aerobic co-composting of municipal sewage sludge and straw, was evaluated as a soil amendment for remediating heavy-metal-contaminated soils from a mining region in Tibet, China. The effects of different SFR application rates (0%, 3%, 6%) on soil properties, heavy metal stabilization, plant growth, and metal migration were comprehensively investigated using \u003cem\u003eBrassica chinensis\u003c/em\u003e and \u003cem\u003eLolium perenne\u003c/em\u003eunder monoculture and intercropping conditions. The results showed that SFR significantly improved soil physicochemical properties, including cation exchange capacity (CEC), total organic carbon, ammonium nitrogen, available phosphorus, and potassium. At 6% application, SFR increased the residual fractions of Cu, Zn, and Pb by over 40%, thereby reducing their mobility and bioavailability. Simultaneously, SFR enhanced plant growth, with up to 72% increase in chlorophyll content and substantial improvements in biomass and plant height. Intercropping systems effectively redistributed metal loads between species, reducing heavy metal accumulation in Chinese cabbage while promoting uptake by ryegrass. This study demonstrates that SFR offers a sustainable, cost-effective strategy for stabilizing heavy metals and promoting vegetation recovery in contaminated soils.\u003c/p\u003e","manuscriptTitle":"Study on the effect of sludge fermentation residue on soil remediation and plant growth in mining areas","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-14 04:57:48","doi":"10.21203/rs.3.rs-7458907/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-01T20:12:14+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-28T23:49:17+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-28T23:48:38+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Monitoring and Assessment","date":"2025-08-26T05:13:31+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-monitoring-and-assessment","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"emas","sideBox":"Learn more about [Environmental Monitoring and Assessment](http://link.springer.com/journal/10661)","snPcode":"10661","submissionUrl":"https://submission.nature.com/new-submission/10661/3","title":"Environmental Monitoring and Assessment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"c5566462-0bc7-40d6-b78b-75eed86d3642","owner":[],"postedDate":"October 14th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-03-09T16:05:32+00:00","versionOfRecord":{"articleIdentity":"rs-7458907","link":"https://doi.org/10.1007/s10661-026-15007-8","journal":{"identity":"environmental-monitoring-and-assessment","isVorOnly":false,"title":"Environmental Monitoring and Assessment"},"publishedOn":"2026-03-05 15:59:42","publishedOnDateReadable":"March 5th, 2026"},"versionCreatedAt":"2025-10-14 04:57:48","video":"","vorDoi":"10.1007/s10661-026-15007-8","vorDoiUrl":"https://doi.org/10.1007/s10661-026-15007-8","workflowStages":[]},"version":"v1","identity":"rs-7458907","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7458907","identity":"rs-7458907","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}