1 Introduction

Soil contamination by heavy metals has received much public attention both domestically and internationally due to its toxicity, bioaccumulation and non-degradability (Zeng et al. 2019). According to the National General Survey Report on Soil Contamination, up to 16.1% of the total survey sites exceeded the soil environment quality standards, and inorganic pollutants such as Cadmium, Nickel, Arsenic and Copper were the main pollution sources (Tu et al. 2020). As one of the most toxic heavy metals, Cadmium (Cd) is identified as a priority pollutant by U.S. EPA. It can damage the liver, kidney and spleen, causing several symptoms such as osteoporosis and lung cancer (Yang et al. 2021). The main pathway of Cd entering the human body is the food chain. The bioavailable Cd in soil can be absorped by crops and then ingested by humans. Excessive accumulation of Cd in soils not only reduces the soil quality, microbial activity and crop productivity but also threatens ecological security and human health (Liu et al. 2017; Qing et al. 2015). Therefore, it is highly crucial and urgent to develop effective, low-cost and environmentally friendly technologies for Cd polluted soil remediation.

Stabilization is a promising strategy to reduce the mobility and bioavailability of heavy metals by adding proper amendments to the polluted soil, thus alleviating the health and ecological risks of toxic heavy metals in the soil (Shen et al. 2019). Previous studies have reported that soil microorganisms can stabilize heavy metals via bio-accumulation, bio-sorption, bio-transformation and bio-mineralization (Liu et al. 2020). In the last few decades, heavy metal-tolerant bacteria and their applications to the remediation of contaminated soil have been one of the research hotspots. Many bacterial species and genera including Cupriavidus taiwanensis (Siripornadulsil and Siripornadulsil 2013), Pseudomonas stutzeri (Oh et al. 2009), Halomonas (Manasi et al. 2014), Delftia (Liu et al. 2018) and Bacillus (Jiang et al. 2009) were found to be capable of immobilizing Cd in soil and decreasing Cd uptake by rice.

However, some key issues still need to be resolved before the broad application of functional microbes for Cd contaminated soil remediation. Firstly, large microbial communities are hard to form in the field because high concentrations of heavy metals in soil might hinder the survival of functional microbes (Li et al. 2017). Secondly, the polluted soil generally can’t support rapid growth of functional microbes, resulting from nutritional shortage and native species competition (Ma et al. 2020a, b; Shen et al. 2017). Therefore, numerous attempts have been made in the field of immobilized microbe technology. It has been observed that providing microorganisms with carrier materials can promote microbial population density and biological activity. (Ji et al. 2022; Qin et al. 2020).

Derived from pyrolysis of the plant, sludge and animal-based biomass under oxygen-limited conditions, biochar is an environmentally friendly functional material (El-Naggar et al. 2019). In recent years, biochar has been applied to many aspects of environment remediation due to its unique physicochemical characteristics (large specific surface area, abundant functional groups and mineral elements). One of the most essential biochar’s environmental applications is the adsorption of heavy metals (O’Connor et al. 2018). It has been demonstrated that biochar could improve the physical and chemical properties of soils and provide nutrients to corps. Besides, biochar could decrease the mobility and bioavailability of Cd in soil by electrostatic attraction, complexation, ion exchange and precipitation (Qiu et al. 2021).

Biochar immobilization microbe usually acquired by impregnating functional microorganisms with biochar together. The contaminant enrichment of biochar increases the concentration gradient of pollution between biochar and microorganisms, which increases the speed and force of mass transfer between microorganisms and biochar. Therefore the the efficiency of contaminant degradation and removal is improved. (Bharti et al. 2019; Wu et al. 2022). In addition, Biochar could not only provide carbon sources and nutrients for the soil but also offer spaces for the growth and reproduction of microorganisms (Zhang et al. 2020). It was reported that immobilized PSB strains (Leclercia adecarboxylata) with biochar could promote the formation of Pb5(PO4)3Cl and Pb5(PO4)3OH, which are the stable state of Pb (Teng et al. 2020). Tu et al. (2020) also found Pseudomonas sp. NT-2 immobilized on biochar had an excellent performance on stabilizing Cd and Cu in heavy metal polluted soil. Although there were some reports about heavy metal contaminated soil remediation using microbe immobilized on biochar, the synergetic mechanism between biochar and heavy metal (resistant) immobilizing microbes on the Cd immobilization is still not well elucidated. Furthermore, there is also a lack of knowledge about the variation of Cd bioavailability and soil properties during the heavy metal contaminated soil remediation by the biochar immobilized microbe strategy.

In this study, a heavy metal tolerant bacteria (Cupriavidus B-7) was first screened and isolated. Then three kinds of biochar were used as inoculum carriers for Cupriavidus B-7 during the remediation of Cd contaminated soil. Objectives of this research are to (1) shed light on the effects of microbial inoculation on the stabilization of Cd in soil; (2) investigate the variations of Cd bioavailability for plants and the properties impact on soil during the whole remediation period; and (3) explore the synergistic stabilization mechanism of Cd between biochar and Cupriavidus B-7 in soil. Results of this study are supposed to bring some insights into the green and sustainable remediation of heavy metal contaminated soil.

2 Materials and methods

2.1 Soil sampling and characterizing

The soil used in this study was collected at a depth of 0–20 cm from a lead–zinc mining area in Shangyu District, Shaoxing City, Zhejiang Province. The soil was characterized as loam. The basic physicochemical properties of the collected soil were determined, and the detailed information is presented in Additional file 1: Table S1.

2.2 Biochar preparation

The rice straw and pine wood were washed with deionized water several times and dried at 60 °C. Then the dried rice straw, pine wood and cow manure were smashed and pyrolyzed at 500 °C under nitrogen atmospheric conditions for 8 h. At last, the biochar was passed through a 0.25 mm sieve before use (Ouyang et al. 2019, 2023). The biochar derived from cow manure, rice straw and pine wood were denoted as CDB, RSB and PB, respectively.

2.3 Isolation and identification of the heavy metal strain bacteria

The bacterial strains were initially isolated from Cd contaminated soil samples collected from copper mining in Zhejiang province, China. The basic soil properties of the copper mining are presented in Additional file 1: Table S3. The Cd contaminated soil sample was firstly shaken (150 r min−1, 28 °C) in 150 mL serum bottles with 90 mL sterile water and several glass beads for 30 min. Acquired solutions were added to the fresh beef extract peptone medium (BPM) with 100 mg L−1 Cd. After incubating for 3 days at 28 °C, the enrichment culture was spread on a fresh BPM containing Cd (100 mg L−1). At last, the heavy metal tolerant strain was picked and purified after 7 times streaking method, and sixteen species of Cd resistant strains were isolated. Identification of the strain with the best cadmium passivation effect was performed by DNA extraction by a kit, and the 16S rRNA gene was amplified by PCR with primers 27F and 1492R. The PCR products were sequenced by Sangon Biotech (Shanghai) Co., Ltd. The MEGA 6.0 software was used to analyze the phylogenetic tree after the multiple alignments of data by CLUSTAL X(ver.1.8). All experiment materials were sterile in our study.

2.4 Preparation of bacteria-loaded biochar and pot experiment

Immobilization of isolated Cd resistant strain (Cupriavidus B-7) on biochar was described as follows: the Cupriavidus B-7 was cultured in BPM at 28 °C and 200 rmp. The biochar was subsequently blended with the Cupriavidus B-7 culture for 8 h with a ratio of 1.2:1 (w:v) after the culture OD600 value reached 1.0. The incubation pot experiments were carried out in a greenhouse at 25 °C to investigate the effects of Cupriavidus B-7-loaded biochar on pakchoi growth (Brassica chinensis L.) as well as its accumulation of Cd. Eight treatments of pot experiments were included: contaminated soil only (CK), contaminated soil mixed with Cupriavidus B-7 (B-7), contaminated soil mixed with three types of biochar (CDB, RSB and PB), and contaminated soil mixed with Cupriavidus B-7-loaded biochar (CDBB, RSBB and PBB). The weight-to-weight dosages of bacteria solution, biochar and bacteria-loaded biochar towards contaminated soil were 30%, 2.5% and 2.5% (w:w), respectively. The contaminated soil was incubated for 60 days after adding Cupriavidus B-7, biochar and Cupriavidus B-7 loaded biochar. Then, the pakchoi was planted in the soil. During the incubation, the soil moisture was kept at 40% of water holding capacity to facilitate equilibrium. All treatments and controls were performed in triplicate. For further analysis, the samples were air-dried and passed through a 100-mesh sieve.

2.5 Analysis of heavy metal in the soil and plant

To monitor the dynamic change of heavy metal fractions in soil, Tessier method was applied (Tessier et al. 1979), and five different fractions specified by the Tessier sequential extraction method include water-soluble and exchangeable (Ex), carbonate bound (Carb), Fe–Mn oxides bound (Ox), organic matter bound (Org), and residual (Res) fractions. The bioavailable heavy metals in the soil were extracted by 0.01 M of CaCl2 solutions, considering the fitness of DTPA-extraction for neutral soil while the soil used in our research was acid soil (Ma et al. 2020b). The heavy metal concentrations in edible parts of pakchoi were determined after its digestion by a mixture of concentrated HNO3 and HClO4 (9:1, v:v) solution. Heavy metal concentrations in extracts from the soil were analyzed by an inductively coupled plasma optical emission spectrometer (ICP-OES; ICAP-7400, ThermoFisher Scientific Ltd., USA), and the heavy metal concentrations in digestion from pakchoi were measured by graphite furnace atomic absorption spectrophotometer (GFAAS).

2.6 Data analysis

Mean values with standard deviations were presented in our study. The statistical analysis was performed by SPSS Statistics 22.0 software. One-way analysis of variance (ANOVA) was applied to evaluate the effects of different amendment treatments on the analyzed parameters, and the differences between the treatment means were determined using Duncan’s test at p < 0.05. Origin 2020 was used for Pearson’s correlation analysis, and AMOS 26 was applied for the structural equation modelling (SEM).

3 Results and discussion

3.1 The isolation and identification of Cd resistant strain

We isolated 16 kinds of Cd resistant strains from the Cd polluted soil. The strains were then subjected to the fluid medium with an initial Cd concentration of 100 mg L−1. After 48 h of incubation, the Cd concentrations in the fluid medium were determined and are presented in Additional file 1: Table S2, and strain k1 showed the best immobilization ability with the Cd concentration in the fluid medium significantly decreased by 33 mg L−1. As seen from Fig. 1a, strain k1 formed round, white and opaque colonies with smooth edges. Scanning electron microscope images of strain k1 illustrated that the cells of bacteria were rod-like with a size of about 1.0–2.0 μm (Fig. 1b and c). In addition, We conducted sequencing of the 16S rRNA genes of strain k1 to determine its taxonomy. The NCBI BLAST search program revealed that strain k1 exhibited a sequence identity ranging from 98 to 100% with species from the genera Cupriavidus sp., Ralstonia sp., and Arthrobacter sp., all of which are part of the Burkholderiaceae family. Based on the phylogenetic analysis (Fig. 1d), the Cd resistant strain k1 is primarily similar to the genus of Cupriavidus sp. and thereafter denoted as Cupriavidus B-7. Similarly, previous studies have also reported a high heavy metal accumulating soil bacterium, Cupriavidus sp. W2 (Shi et al. 2020). The Cupriavidus sp. were previously reported as kinds of heavy metal tolerant bacteria. For instance, the Cupriavidus taiwanensis could grow well in the culture medium with approximately 3100 mg L−1, 490 mg L−1, 318 mg L−1 and 281 mg L−1 of Pb, Zn, Cu, and Cd (Chen et al. 2008). Although the the maximum inhibitory concentration of Cd was not explored in this study, it was found that the Cupriavidus B-7 survived well with at lest 100 mg L−1 of Cd.

Fig. 1
figure 1

Formed bacterial colonies (a), scanning electron microscope images (b–c), and phylogenetic analysis (d) of the isolated heavy metal resistant strain k1(Cupriavidus B-7)

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3.2 Characterization of biochar and Cupriavidus B-7 loaded biochar

Table 1 shows the basic properties of biochars, and the pH values of CDB, RSB, and PB were 9.26, 10.08 and 9.68, respectively. Thus, adding biochar or Cupriavidus B-7-loaded biochar would effectively enhance the soil pH (Yang et al. 2021). With relatively high surface areas and porosity, CDB, RSB, and PB could serve as a safe habitat for Cd resistant bacteria immobilization (Song et al. 2022; Wahla et al. 2020). Besides, the three biochars contained nitrogen, phosphorus and potassium elements, which could serve as nutrient sources for the growth of Cd resistant bacteria.

Table 1 Basic properties of the biochar
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The colonization of bacterium on biochar depends on the biochar properties as well as the physiological features of the bacterium (Zhu et al. 2017). Quilliam et al. (2013) found that the high mineral salt content could inhibit the colonization of microbes on biochar surfaces. However, Tu et al. (2020) also reported a heavy metal-tolerant strain Pseudomonas sp. NT-2 which could colonise well on the biochar surface within only 8 h. In our present study, the scanning electron microscope was used to observe the morphology of three biochars and bacteria-loaded biochars. As illustrated in Fig. 2, the Cupriavidus B-7 successfully colonized the surface and pores of biochars in a short time (48 h). The results suggested that the Cupriavidus B-7 has great potential to survive on biochar surface and synergistically immobilize Cd in soil with biochar.

Fig. 2
figure 2

Scanning electron microscope images of CDB (a), CDBB (b), PCB (c), PCBB (d), RSB (e) and RSBB (f)

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3.3 Influence of Cupriavidus B-7 loaded biochar on soil properties

The changes in soil pH after the treatment with biochar and Cupriavidus B-7-loaded biochar are shown in Fig. 3a. Compared with CK (5.97), soil pH increased to 6.46, 6.5, 6.25, 6.3, 6.22, 6.37 and 6.02 after five days of incubation with RSB, RSBB, CDB, CDBB, PB, PBB and Cupriavidus B-7, respectively. Then, the pH of soil declined slightly after 60 days of incubation. However, final pH of all treatment groups was still higher than that of the CK treatment. During the pyrolysis process, the base cations in biomass could produce alkaline substances, such as oxides, hydroxides, and carbonates. Therefore, biochar can effectively enhance soil pH (Qi et al. 2021). The soil pH of all the treatment presented a quick increase by biochar application, and then showed a slight declining trend with the increasing incubation. Tu et al. (2020) also observed a similar phenomenon and suggested that the rapid increase in soil pH was attributed to the dissolution of alkaline substances in the biochar. Subsequently, the soil pH slightly decreased once these readily released alkaline substances were depleted.

Fig. 3
figure 3

The variation of soil pH during 60 days of incubation with biochar, Cupriavidus B-7 and Cupriavidus B-7-loaded biochar (a); The organic matter (b), alkaline nitrogen (c), available phosphorus (d) and potassium (e) values of soil after 60 days of incubation with biochar, Cupriavidus B-7 and Cupriavidus B-7-loaded biochar. Different letters indicate significant differences among treatments (p < 0.05; Duncan’s test)

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The soil organic matter, alkaline nitrogen, available phosphorus and potassium were determined after 60 days of incubation with Cupriavidus B-7, biochars or Cupriavidus B-7-loaded biochars. We have found that applying Cupriavidus B-7, biochar and Cupriavidus B-7-loaded biochar could significantly enhance the organic matter in the soil, and the biochar or Cupriavidus B-7- loaded biochar treatment was significantly better than that of Cupriavidus B-7 alone (Fig. 3b). Similarly, applying biochar or Cupriavidus B-7-loaded biochar increased the alkaline nitrogen (Fig. 3c), available phosphorus (Fig. 3d) and potassium (Fig. 3e) in the soil compared with the CK treatment. In addition, according to our results, the Cupriavidus B-7-loaded biochar presented a better effect on organic matter, alkaline nitrogen, available phosphorus and potassium increase in the soil.

3.4 Influence of Cupriavidus B-7 loaded biochar on Cd bioavailability in soil

The CaCl2-extractable of Cd could be used to evaluate the mobility and bioavailability of heavy metals in acidic and neutral soils. To determine the immobilization effects of different treatments on Cd, the contents of CaCl2-extractable Cd in soil were analyzed after incubation with Cupriavidus B-7, biochars or Cupriavidus B-7-loaded biochars, respectively (Fig. 4a). The initial CaCl2-extractable Cd was 2.34 mg kg−1, and it decreased to 1.63 mg kg−1 after 60 days of incubation with Cupriavidus B-7. The stabilization of Cd by Cupriavidus B-7 could be mainly due to the cell surface adsorption by electrostatic attraction or ionic bonding (Li et al. 2021b). Cd2+ likely has the potential to engage in interactions with the surface ligands found in extracellular polymers and cell wall surface molecules, including –OH, –NH, –SO3, –HPO3, –C–O, –SH, and –COOH, during the process of surface biosorption (Chi et al. 2020; Qi et al. 2023). Furthermore, Li et al. (2019) also reported an available Cd reduction mechanism of Cupriavidus sp. through extracellular bio-precipitation of Cd2+ in the form of cadmium carbonate. In this study, the RSBB treatment showed the best effect on CaCl2-extartable Cd decrease. It was because the Cupriavidus B-7 exhibits the best survival on RSB (Fig. 4b) and the Cd passivation effect by Cupriavidus B-7 was significantly increased.

Fig. 4
figure 4

Changes of Cd CaCl2-exctractable in soil after 30 days of incubation (a), the number of viable cells on biochar (b) and percentage of fractions distribution in soil after 30 (c) and 60 days (d) of incubation. Different letters indicate significant differences among treatments (p < 0.05; Duncan’s test)

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Biochar has been proven to have immobilization effects on heavy metals in the soil, including direct adsorption via electrostatic attraction, ion exchange, precipitation and physical adsorption. Additionally, biochar can alter the soil’s physicochemical properties, particularly its pH, leading to a decrease in the mobility of heavy metals in the soil (Wang et al. 2021). In this study, the CaCl2-extractable Cd exhibited reductions of 39.3%, 36.3%, and 49.7%, respectively, following the addition of CDB, PB, and RSB to the soil. Among these amendments, RSB proved to be the most effective in Cd stabilization, largely due to its ability to significantly increase soil pH (Fig. 3a). Furthermore, as evidenced by BET surface area analysis (Table 1), RSB had a higher specific surface area and a porous structure compared with CDB and PB, enhancing its ability to efficiently adsorb Cd2+.

As anticipated, immobilizing Cupriavidus B-7 onto biochar enhanced the passivation effect of biochar on Cd in the soil. The RSBB showed the best effect on Cd stabilization, with a decrease of CaCl2-extractable Cd by 59.8%, compared with CK treatment. Compared with CDB, PB and RSB, the CaCl2-extractable Cd in the soil after the application of CDBB, PBB and RSBB decreased by 9.7%, 10.6% and 20.1%, respectively. The extent to which Cupriavidus B-7 enhances Cd passivation in soil differs when immobilized on various biochars. This variation is primarily attributed to differences in the physicochemical properties of the biochars and their effectiveness in immobilizing Cupriavidus B-7 (Wei et al. 2022). In this study, the number of viable cells on CDBB, PBB, and RSBB was determined to be 1.47 × 109, 1.06 × 109 and 2.29 × 109 CFU g−1 (Fig. 4b), respectively. The viable cells on RSBB were approximately two times more than those on CDBB and PBB, suggesting that RSB is more suitable for the colonization of Cupriavidus B-7. Therefore, RSB exhibited the greatest enhancement in the CaCl2-extractable Cd decrease in the soil after being loaded with Cupriavidus B-7.

3.5 Influence of bacteria-loaded biochar on Cd fractions in soil

The migration and biological toxicity of heavy metals are strongly affected by their chemical fractions in soil. After the introduction of Cupriavidus B-7, biochars, or Cupriavidus B-7-loaded biochars, the changes in Cd fractions within the soil were assessed through Tessier sequential extraction (Fig. 4c and d). For the original soil (CK treatment), the dominant Cd fraction was FeMn oxide bound fraction (42.16%), followed by exchangeable Cd (33.77%), while the residual, organic matter and carbonate bound Cd accounted for less (12.66%, 5.84% and 5.57%, respectively). A higher concentration of exchangeable Cd in the untreated soil indicates greater Cd effectiveness and mobility, thereby posing a higher ecological environmental risk. Besides, the FeMn oxide bound fraction of Cd could be released from the soil and be taken up by corps when the soil transforms into a reductive environment (Li et al. 2021a).

After 30 days of incubation, the proportion of exchangeable Cd decreased from 33.77% to 30.10%, 30.36% and 24.54%, respectively, with the addition of CDB, PB and RSB. The FeMn oxide bound fraction of Cd decreased from 42.16% to 36.32%, 41.73%, and 40.75%, respectively. Accordingly, the residual Cd fraction increased from 12.66% to 16.57–20.80%, and 20.47%, respectively. Compared with biochars, the Cupriavidus B-7 presented a much better effect on the transformation of exchangeable and FeMn oxide bound fraction Cd into residual fraction, resulting in an increase in the proportion of residual Cd from 12.66% to 31.66%. As reported by previous study, biochar could release alkaine substance, facilitating the formation of (oxy)hydroxide and carbonate bound Cd (Cui et al. 2019). However, microbial cells could tend to transport heavy metal ions into the cell and change the heavy metals from exchangeable fraction to residual fraction through respiration (Ji et al. 2022). As the incubation period extended to 60 days, the residual fraction of Cd in the soil showed a slight increase, rising from 31.66% to 36.40% in the Cupriavidus B-7 treatment. However, the residual Cd increased pronouncedly from 16.57% to 20.80% (30 days of incubation) to 23.64–33.46% in the biochar treatments. Remarkably, the RSB treatment demonstrated the most effective transformation of Cd from the exchangeable and FeMn oxide-bound fractions to the residual fraction.

Upon Cupriavidus B-7 was immobilized onto biochar, the CDBB, PBB and RSBB increased the soil residual Cd fraction to 35.38%, 33.51%, 37.62% and 39.63%, 38.17%, 43.29% within 30 and 60 days, respectively. As a result, the exchangeable in the soil was decreased to 21.67%, 23.29%, 20.29% and 17.96%, 18.26%, 17.78% within 30 and 60 days, respectively. As compared to their separate applications, the Cupriavidus B-7-loaded biochars showed an improvement in reducing the exchangeable Cd and increasing the residual Cd fractions in the soil. These results were consistent with previous studies (Ji et al. 2022; Song et al. 2022; Zhang et al. 2023), demonstrating that immobilizing heavy metal-tolerant (or immobilizing) bacteria onto biochar can effectively enhance the stabilization and immobilization of heavy metals in soil, both in terms of effectiveness and rate. This may result not only from the immobilizing mechanisms by the biochar and bacteria as discussed above, but also from their synergistic effects. For instance, the larger specific surface area of biochar increased the contact area between Cupriavidus B-7 and soil, leading to the exposure of more metal ion binding sites (Zhang et al. 2023) and thus enhanced the immobilizing effect on heavy metals of Cupriavidus B-7-loaded biochars.

3.6 The growth promotion and Cd uptake alleviation of pakchoi (Brassica chinensis L.) by bacteria-loaded biochar

To evaluate the effectiveness of Cupriavidus B-7, biochar, and Cupriavidus B-7 loaded biochar on pakchoi growth improvement, the dry weight of pakchoi was determined by the end of incubation (Fig. 5). As shown in Fig. 5a, the growth of pakchoi was inhibited in the CK treatment with only 0.30 g and 0.055 g dry weight of shoots (edible parts) and roots, respectively. Prior studies also documented growth inhibition of pakchoi in soil contaminated with Cd (II) (Huang et al. 2021; Qi et al. 2023). However, many studies reported that, under the heavy metal stress, certain heavy metal-resistant bacteria would produce amino acids and amines such as polyamines, which could facilitate plant growth (Rady and Hemida 2015; Sang et al. 2017; Wang et al. 2022). Additionally, Zheng et al. (2023) discovered that a heavy metal-resistant strain identified as Cupriavidus sp. could produce phytohormones, such as indole acetic acid (IAA), when exposed to heavy metal stress, thereby promoting the growth of pakchoi. In our present study, the dry weight of shoots and roots of pakchoi significantly increased to 4.75 g and 0.22 g, respectively, with the addition of Cupriavidus B-7. Likewise, all three types of biochar could effectively improve the growth of pakchoi, with an increase in the dry weight of shoots and roots to 1.32–1.76 g and 0.065–0.126 g, respectively. Furthermore, loading the Cupriavidus B-7 on biochar could significantly strengthen the effectiveness of biochar on plant growth promotion. Specifically, the dry weight of shoots of pakchoi in the CDBB, PBB, and RSBB treatments were 1.73, 2.96, and 1.99 times that of the CBD, PB, and RSB treatments, respectively. However, the plant growth promotion by Cupriavidus B-7 immobilized biochars was less than that of Cupriavidus B-7 alone. It was because when immobilized on biochars, the toxic effect of heavy metal on Cupriavidus B-7 was siginificantly alleviated, and less phytohormones would be secreted by Cupriavidus B-7 to enhance the growth of pakchoi.

Fig. 5
figure 5

The effect of biochars and Cupriavidus B-7-loaded biochars on pakchoi growth (a), soil enzyme (b–d) and the Cd accumulations in shoots of pakchoi (e). Different letters indicate significant differences among treatments (p < 0.05; Duncan’s test)

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The application of Cupriavidus B-7 and biochar could improve the physicochemical properties of soil as discussed in Sect. 3.3, thereby improving the soil health and pakchoi growth (Ma et al. 2020a). Soil enzyme activity is a sensitive indicator of environmental stress and is widely recognized as an integrated bio-indicator of soil health (Gao et al. 2010). As presented in Fig. 5b–d, all applied Cupriavidus B-7 and biochar treatments increased the acid phosphatase, urease and catalase activities. The maximum of catalase activities were found in RSBB treatment while there was no significant difference in the phosphatase and urease activities between RSBB and RSB treatments, which were higher than those in the other treatments. In comparison to CK treatment, the activities of phosphatase, urease, and catalase in the RSBB treatment increased by 30.76%, 78.38%, and 94.93%, respectively. The results of soil enzyme activity improvement by Cupriavidus B-7, biochar and Cupriavidus B-7-loaded biochar were in accordance with the available soil nutrient elements and organic matters (Fig. 3). As shown in Fig. 6, the enzyme activities presented a significant correlation with available nutrient elements, pH, organic matter and the residual Cd in soil. Biochar mainly consists of carbon (C), which could partly serve as a carbon source for microorganisms. At the same time, the macronutrients (N, P, and K) in biochar supply extra nutrients for microbe growth (Ali et al. 2019). In addition, biochar with many porous structures can offer a habitat for microorganisms, facilitating their proliferation and enhancing soil enzyme activities (Nie et al. 2018).

Fig. 6
figure 6

Correlation analysis among soil physiochemical properties, Cd fractions, enzymes and Cd in the shoot of pakchoi (a), and the structural equation model of Cd accumulation of the pakchoi under Cupriavidus B-7 immobilized biochar application condition (b). Green and red arrows indicate positive and negative relationships, respectively. Continuous and dashed arrows indicate significant and nonsignificant relationships, respectively (*p < 0.05 and **p < 0.001)

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Available heavy metals in the soil are readily taken up by crops, and our research revealed that application of Cupriavidus B-7, biochar and Cupriavidus B-7-loaded biochar significantly decreased the Cd content in shoots of pakchoi by the end of incubation (Fig. 5e). Among them, the RSBB treatment was optimal, decreasing the Cd content in shoots by 50.75% compared with the CK treatment. In general, the application of Cupriavidus B-7 loaded biochar (CDBB, PBB and RSBB) exhibited a more favourable effect compared to the individual applications of Cupriavidus B-7 and biochar, indicating a synergistic effect between Cupriavidus B-7 and biochar on pakchoi Cd accumulation reduction. Similar to the present work, Chen et al. (2023) discovered that biochar, on one hand, could alleviate the toxic effects of heavy metals on microbes. On the other hand, it could assist the microbes in secreting organic substances and facilitating extracellular electron transfer, thereby enhancing the Cd passivation of microbes.

The structural equation model (SEM) was used to evaluate the impact of biochar and Cupriavidus B-7 on pakchoi Cd uptake (Fig. 6b). The SEM results suggested that the bioavailable Cd and soil nutrients were two critical factors for the Cd accumulation of pakchoi. Biochar had a significant positive correlation with the soil pH, organic matter and nutrients (p < 0.001). Among these, the soil pH (p < 0.001) and organic matter could decrease the bioavailable Cd concentration and therefore decrease the Cd uptake of pakchoi. At the same time, biochar would promote the growth of pakchoi by increasing the soil nutrients, and the larger biomass could decrease the Cd concentration in plants. As for Cupriavidus B-7, it could directly decrease the bioavailable Cd concentration (p < 0.001) and indirectly decrease the bioavailable Cd concentration by increasing the soil organic matter (p < 0.001).

Although the application of Cupriavidus B-7 alone showed a more pronounced effect on enhancing the pakchoi growth compared to CDBB, PBB, and RSBB, the Cd accumulation in pakchoi shoot was reduced by 28.44%, 30.17%, and 43.1% in the CDBB, PBB, and RSBB treatments, respectively. This was significantly better than the application of Cupriavidus B-7 alone. Based on the above results, the Cupriavidus B-7-loaded biochar treatment, especially, the RSBB treatment not only improved soil health and increased crop yields but also significantly alleviated the accumulation of Cd in crops. It is a promising technology for the safe utilization of heavy metal-contaminated farmland.

3.7 Environmental implication

The farmland Cadmium (Cd) contamination poses a great threat to human health and ecosystem function due to its carcinogenic, teratogenic and mutagenic properties. In this study, we screened a Cd resistant strain (Cupriavidus B-7) and successfully loaded the Cupriavidus B-7 onto biochar. The effectiveness of Cupriavidus B-7-loaded biochar in reducing Cd uptake by pakchoi was demonstrated. Additionally, the growth of pakchoi was significantly improved due to the enhancement of soil health by Cupriavidus B-7-loaded biochar. Furthermore, the Cupriavidus B-7-loaded biochar is a low-cost and environmentally friendly strategy for farmland in-situ remediation. Therefore, it meets the requirement of pollution reduction and carbon reduction when addressing environmental issues. However, it is necessary to carry out field experiments in further steps to evaluate the efficiency and its long-term effectiveness before practice application.

4 Conclusions

The Cupriavidus B-7 and biochar effectively converted the exchangeable Cd to a residual fraction in the soil and reduced the bioavailability of Cd in the soil. Cupriavidus B-7 and biochar could significantly improve the soil’s physicochemical properties and provide nutrients for microbes and plants, enhancing the soil health, Therefore, the Cupriavidus B-7 and biochar could not only alleviate the accumulation of Cd in pakchoi but also increased its growth. In addition, Cupriavidus B-7-loaded biochar showed a better effect on Cd immobilization and pakchoi growth increase, indicating a synergistic effect between Cupriavidus B-7 and biochar. This research suggested that the Cupriavidus B-7-loaded biochar is a promising and sustainable technology for in-situ Cd contaminated farmland soil.