Introduction

Cadmium (Cd) has been recognized as deleterious and carcinogenic to human health for a long time (Chen et al. 2022; Lv et al. 2019). It is easy to accumulate in the aboveground tissue of plants and transfer to the edible parts and could cause toxicity to human health through the food chain. Globally, nearly 2.35 × 106 km2 of cultivated fields are polluted by heavy metals (HMs), while approximately 2.79 × 103 km2 of agricultural soils are contaminated by Cd in China (Mahar et al. 2015). Cd pollution in agricultural farmlands can be mainly contributed by two aspects. Apart from a series of natural sources, e.g., atmospheric deposition, volcanic eruptions, and weathering of parent material, another reason for soil Cd pollution is the overuse of minerals, chemical fertilizers, and pesticides in agricultural activity (Byers et al. 2020; Xu et al. 2015).

As major food crops, vegetables are rich in nutrients and trace elements that are essential for the human body (Chen et al. 2014). In the last few decades, greenhouse vegetable production (GVP) has become a typical system to produce vegetables in China. It can meet the demand for vegetable growth to ensure the normal supply of vegetables throughout the year (Zhang et al. 2017). However, several reports have shown that compared with conventional vegetable cultivation, vegetables grown in the GVP system can accumulate higher levels of HMs (especially Cd) from the soil owing to excessive chemical fertilizer input, hot temperature, and high moisture (Hu et al. 2014; Yang et al. 2019). As a result, human beings will be more susceptible to various diseases such as bone fracturing, cartilage disease, and even liver or kidney cancers via dietary uptake of such vegetables and their derived products (Huang et al. 2020a). Hence, how to produce low-Cd vegetables in contaminated greenhouse soil is an urgent issue to be solved.

In situ immobilization by applying inexpensive and accessible soil amendments to reduce soil metal availability is an efficient and promising technology (Chlopecka and Adriano 1997; Shen et al. 2020). At present, many kinds of amendments have been explored to alleviate the risk of human exposure to HMs. It has been reported that inorganic (e.g., limestone, natural zeolite, sepiolite, fly ash, and hydroxyapatite) (Bashir et al. 2020; Bolan et al. 2003; Lahori et al. 2020; Sun et al. 2018) and organic materials (e.g., biochar, humid acid, mushroom residue, cow manure compost, and press mud) (Rehman et al. 2020; Sato et al. 2010; Song et al. 2014; Zeeshan et al. 2020) exhibited remarkable effects on stabilizing HMs in soil. Inorganic materials (e.g., lime) decrease metal mobility by elevating soil pH, while zeolite and sepiolite can adsorb metal ions to remediate contaminated sites due to a highly negative charged layer (Bashir et al. 2018; Garau et al. 2007). Organic materials such as biochar originated from plant residues or animal waste have long-term impacts on decreasing metal solubility and mobility because of their high pH, cation exchange capacity (CEC), and large surface area (Gonzaga et al. 2020; Khan et al. 2020a). However, only applying a single soil amendment may be inadequate to ensure the safe production of crops in contaminated farmland (Pardo et al. 2014; Qian et al. 2009). The composite amendment has been proved to possess a better effect on reducing the bioavailability and mobility of HMs in soil than its single component (Abideen et al. 2020; Mujtaba Munir et al. 2020a; Sun et al. 2016). They not only decrease HMs bioavailability via adsorption, precipitation, and ion exchange, but also improve soil fertility, thus supplying more nutrients for plants at the same time (Khan et al. 2020b; Liang et al. 2017). Therefore, quite a few studies have reported that a passivated material combined with other one or multiple materials can effectively reduce the content of Cd in cereal crops. For instance, Wu et al. (2016) found that the combined application of lime and sepiolite prominently reduced Cd concentrations in brown rice by conducting a three-year field experiment. The study of Rehman et al. (2018) showed that the mixed amendment (lignite + farmyard manure) reduced 45% Cd concentrations of wheat grains. Whereas few attempts have been made to investigate the influence of such amendment on decreasing Cd accumulation in vegetables cultivated in the GVP system.

Previously, several researchers also found that soil amendments could increase Cd absorption in crops thus raising human health risks. For instance, Khan et al. (2018) observed that the use of bagasse and maize comb waste enhanced the Cd translocation to shoots of cucumber and tomato. A similar phenomenon was also found by Pinto et al. (2005), who reported that organic matter amendment increased Cd translocation of sorghum. Hence, it is critical for food clean of toxic metals to apply the amendments to suitable vegetables. Nevertheless, so far, less attention has been paid to the influence of soil amendments on Cd absorption by multiple crops and the differences between diverse crops. Therefore, in this study, an incubation experiment was conducted to screen an optimal inorganic and organic composite amendment, which was then applied to a Cd-contaminated GVP system to investigate its effects on reducing Cd uptake by 14 stem and leaf vegetables, and the health risks from ingesting these vegetables were further evaluated. The objective of the present study is to explore a composite amendment with an excellent ability to immobilize soil Cd, and to verify whether it can be generally used on various vegetables to reduce human health risks. The results of this study can provide an important insight into the role of combined amendment in guaranteeing vegetable safe production in the polluted field.

Materials and methods

Tested soil and amendments collection and their properties

The experimental soil was collected from acidic and moderately Cd-contaminated farmland in Hangzhou, Zhejiang Province, China (119° 53.9050′, 30° 03.060′) with an annual rainfall of 1441.5 mm, which belongs to waterloggogenic paddy soil. Topsoil (0–20 cm) was collected, air-dried, ground, and sieved to < 2 mm for the incubation experiment. This soil was contaminated with HMs due to metal-containing agricultural inputs over a long period including chemical fertilizer, pesticides, and organic manure (Ma et al. 2021). The tested soil was characterized by pH, electric conductivity (EC), organic matter (OM), available nitrogen (AN), available potassium (AP), available phosphorus (AK), cation exchange capacity (CEC), soil total contents of Cd, chromium (Cr), lead (Pb), mercury (Hg) and arsenic (As), and soil available Cd. The chemicals and reagents, the determination methods of the physicochemical analyses, and heavy metals were recorded in the supplementary materials. The physical–chemical properties and metal contents are shown in Table S1. Among these metals, Cd far exceeded the Chinese Soil Environment Quality Standard (GB 15,618–2018) for agricultural land.

Three inorganic materials (lime, L; zeolite, Z; and sepiolite, S) and two organic materials (biochar, B, and compost, C) were used to make six composite amendments including LZBC, LSBC, LZC, LZB, LSC, and LSB (Table 1), which were compared in an incubation experiment. The mix proportion and compounding of soil amendments were based on Hamid et al. (2019), and the details are given in Table 1. Materials of these amendments were purchased from different sources. Among, sources of lime, zeolite, biochar, and compost were documented by Liu et al. (2021); and sepiolite was purchased from Jinyuan Environmental Protection Company in Zhengzhou, Henan Province. The measurement methods of basic physical–chemical characteristics (pH, EC, CEC, and OM) and total Cd of mixed amendments were the same as that of the soil. The basic physic-chemical properties of the six composite amendments are shown in Table 1.

Table 1 Based physical–chemical characteristics and total Cd contents of applied amendments
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The incubation experiment

To screen the optimal soil amendment, six composite amendments (LZBC, LSBC, LZC, LZB, LSC, and LSB) were mixed with 250 g air-dried and sieved farmland soil at 1% (w/w), respectively. Then, they were put into 500-mL glass bottles, with farmland soil only as a control (CK). The unamended and amended soils were cultured for 2 months under dark conditions at 25 ℃. Per treatment was repeated three times for 21 bottles in total. During the incubation period, the water holding capacity of the soil (70%) was maintained at field capacity by adding deionized water constantly.

The field experiment

A field experiment was conducted at the same site as the “The incubation experiment” section mentioned. According to our findings in the incubation experiment, LZBC was the best one to reduce the soil Cd bioavailability. Besides, fourteen in-season vegetables which are widely cultivated in Hangzhou city were grown in the field. The names and descriptions of these vegetables are provided in Table 2. All vegetable seeds were obtained from Hangzhou seed company, Zhejiang, China. Three biological replications were carried out for each vegetable with or without the soil amendment, with a total of 84 plots. A randomized complete block design was adopted in the present research and per plot area was 5 m2 (2.5 m × 2 m). On November 1, 2019, the amendment (application rate: 1%) was fully mixed into the topsoil (0–20 cm). In the meantime, the composite fertilizer (N-P-K:15–15-15) was used as basal fertilizer with a dosage of 450 kg ha−1 to assure the normal growth of plants. After 2 weeks, seeds were directly sown or seedlings were transplanted into the respective plots. Vegetable planting density (Table 2) was defined based on the methods of local agricultural practice. During the growth period, conventional agronomic management measures were conducted in the GVP system, for example, spinetoram and zinc thiazole (450 mL ha−1) were applied to control the pests, while pendimethalin (675 mL ha−1) was sprayed for weeding.

Table 2 English name, botanical name, cultivar, planting density and growth period of experimental vegetables
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Soil sampling and analysis

In the incubation experiment, soil samples were collected every 2 weeks until the end of incubation. Sieved soil (1.0 mm) was used to determine pH, DTPA-extractable Cd, and Cd speciation. The methods for the determination of soil pH and DTPA-extractable Cd have been described in supplementary materials. Cd speciation in soil was determined in a five-step sequential extraction method referred to Tessier et al. (1979). Extraction processes were listed as follows: (1) Exchangeable, EX-Cd: extracting 1 g soil with 8 ml MgCl2 (1.0 M) at pH = 7. (2) Carbonate-bound, CB-Cd: extracting faction 1 residues with 8 ml NaOAc (1.0 M) at pH = 5. (3) Fe–Mn oxide-bound, OX-Cd: extracting faction 2 residues with 20 ml NH2OH·HCl (0.04 M) in 25% HO-Ac solution. (4) Organic matter-bound, OM-Cd: extracting faction 3 residues with 3 ml HNO3 (0.02 M) in 30% H2O2. (5) Residual, Res-Cd: digesting fraction 4 residues with HNO3-HF-HClO4 (5:1:1). The solutions were subjected to ICP-MS (PerkinElmer NexION 300X) for Cd concentration.

For the field trial, after the plants of each plot were ripe, the roots were taken out and the soils on the roots were gently shaken off to collect the rhizosphere soil. Additionally, three samples from the same plot were combined into a single representative sample. The rhizosphere soil samples were air-dried and sieved to measure pH and DTPA-extractable Cd. The determination of soil pH and DTPA-extractable Cd were the same as above.

Plant sampling and analysis

The growth period of per plant is exhibited in Table 2. When plants were mature, 1 m2 of field plot was randomly harvested to estimate the plant yield. Afterward, the remaining plants in each plot were used to measure other parameters. Plant samples including roots and shoots were rinsed with topwater to remove the dust or soil, then were washed with deionized water by three times. After fresh weights were determined, they were oven-dried for 30 min at 105 °C, and then placed in a 65 ℃ until constant weight for dry weight. Dried plants were digested with HNO3/H2O2 (5:1) for 4 h to determine Cd concentration. Plant standard samples (GBW (E) 100,495 Chinese) and three blanks were applied for quality control and assurance. The reference value was 0.36 ± 0.02 mg kg−1 and obtained value for standard reference material was 0.35 ± 0.08 mg kg−1. The recovery ranged between 90 and 110%, indicating that all the measured data were within the range of standard referred values.

Morphological assessment of LZBC used in the field experiment

Composite amendment (LZBC) was characterized via scanning electron microscopy (SEM, FEI QUANTA FEG 650) equipped with energy-dispersive X-ray spectroscopy (EDS, EDAX Inc. Genesis XM) to evaluate its surface morphology and microstructure. The test procedure was to directly stick the sample powder on the sample table with conductive adhesive. Fourier transform infrared (FTIR) spectroscopy (Nicolet 6700) of the applied amendment was examined via using a 2 mg of grounded sample, in a KBr pellet by taking 10 scans at 1 cm−1 interval on average in a range of 500 and 4000 cm−1. The crystalline properties of the soil amendment were characterized by X-ray diffraction (XRD, Bruker D8 Advance), using a target voltage of 40 kV, and an emission current of 30 mA with a scanning angle of 5–70° 2θ at a rate of 0.1° per second.

Bioaccumulation factor and translocation factor

Bioaccumulation factor (BAF) from soil to shoot or the edible part was calculated using the following formula:

$$BAF=\frac{{C}_{a}}{{C}_{soil}}$$
(1)

Translocation factor (TF) from root to shoot or the edible part was calculated using the following formula:

$$TF=\frac{{C}_{a}}{{C}_{root}}$$
(2)

where Ca refers to the Cd concentration of the plant shoot or the edible part, Croot refers to the Cd concentration of plant root and Csoil refers to the Cd concentration of soil.

Health risks assessment

  1. (1)

    Dietary intake of metal (DIM)

The impacts on daily dietary intake of metal by the combined amendment were calculated using the Eq. (3).

$$DIM=\frac{{C}_{metal }{\times C}_{factor} \times {D}_{food\;intakemetals}}{{BW}_{average\;weight}}$$
(3)

where Cmetal is the Cd content of vegetable, Cfactor is the conversion coefficient to convert dry weight (DW) into fresh weight (FW). Cfactor of experimental vegetables is listed in Table S2. Dfood intake is the daily intake of vegetables. BW is the mean body weight (65 kg for adults). The average daily consumption of vegetables for adults was 345 g person−1 day−1 referred to Khan et al. (2018).

  1. (2)

    HRI and THQ

Health risk index (HRI) and target hazard quotient (THQ) are used to assess the human health risk for the public consume these vegetables planted in polluted soil treated with the combined amendment.

$$HRI=\frac{DIM}{RfD}$$
(4)
$$THQ=\frac{MC\times FI\times {EF}_{r}\times ED}{RfD\times BW\times AT} \times {10}^{-3}$$
(5)

In Eq. (4), the value of RfD for Cd is 0.0010 mg kg−1 d represents reference oral dose (USEPA 2015). In Eq. (5), MC is metal content, the value of FI is 345 g person−1 day−1 represents frequency of ingestion, the value of EFr is 365 days year−1 represents exposure frequency, the value of ED is 70 years represents total exposure duration, and AT is the average exposure duration and exposure frequency noncarcinogens (25,550 days for adult) (Fathabad et al. 2018; Wang et al. 2019).

Statistical analysis

The values in this study were shown as mean ± standard error, with three biological replicates conducted. Statistical analysis was handled using IBM SPSS Statistic 20.0, and graphical data was completed by Origin 2016 version. Significant differences among treatments were evaluated using one-way ANOVA, followed by Duncan’s multiple range tests at p < 0.05.

Results and discussions

Screening the optimal composite amendment by an incubation experiment

Soil pH and DTPA-extractable Cd

During the 8-week incubation period, an obvious (p < 0.05) rise in soil pH was found in the six composite amendments treatments (Fig. 1a). On the 56th day, the entire change tendency of soil pH followed the order of LZBC (8.73 units) > LZC (8.44 units) > LSBC (8.21 units) > LSC (8.10 units) > LSB (7.78 units) > LZB (7.66 units), as compared to the CK. The addition of stabilizing agents obviously boosted soil pH could be mainly due to the alkaline nature of materials, which resulted in a large number of hydrogen ions in the soil environment being neutralized (Mukome et al. 2020). Additionally, hydrolysis of CaCO3 in lime and zeolite to hydroxyl ions may be responsible for elevating soil pH in composite amendments-treated samples (Rehaman et al. 2017; Vrînceanu et al. 2019). In the meantime, the formation of hydroxyl ions, mineralization of carbon source, and the release of basic cations were important reasons for the increase of pH value of organic materials such as biochar and compost (Sato et al. 2010; Shen et al. 2016). A similar phenomenon was found by Tang et al. (2015), who observed that the application of inorganic amendments (calcium magnesium phosphate and phosphate rock) and organic amendments (peat and straw manure) significantly increased the soil pH. Besides, LZBC was more effective in improving soil pH than other amendments, which may be owing to a higher presence of basic cations (e.g., Ca2+ and Mg2+), functional groups, and hydroxyl ions (Gonzaga et al. 2020; Hwidi et al. 2018). The combination of lime, zeolite, biochar and compost application may better promote the hydrolysis of alkaline substances such as CaCO3.

Fig. 1
figure 1

Effect of different composite amendments on incubated soil pH and DTPA-Cd concentration. Bars represent standard error of the mean (n = 3). Different letters indicate statistically significant differences at p < 0.05. CK, control; LZBC, lime + zeolite + biochar + compost; LSBC, lime + sepiolite + biochar + compost; LZC, lime + zeolite + compost; LZB, lime + zeolite + biochar; LSC, lime + sepiolite + compost; and LSB, lime + sepiolite + biochar

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In contrast to the CK, the whole combined amendments dramatically (p < 0.05) decreased the DTPA-extractable Cd concentration throughout the incubation period (Fig. 1b). The reduction of soil available Cd could be attributed to the direct or indirect influences of stabilizing agents in soil (Chlopecka and Adriano 1997; Liu et al. 2021; Ondrasek et al. 2021). For example, biochar was able to directly adsorb Cd2+ in soil solution due to its fine pore structure and large specific surface (Rizwan et al. 2017). It was reported that zeolite possessed various large or small holes and channels, which made it perform well in the decrease of soil Cd content (Lahori et al. 2020). Whereas, the changes of soil pH, EC, CEC, OM, or soil available nutrients induced by multiple amendments could indirectly promote a reduction in the number of soil Cd2+ (Liang et al. 2017). According to Walker et al. (2004) and Bian et al. (2014), the varieties of soil physical and chemical characteristics had a significant impact on the distribution of HMs. Consistent with the results of soil pH values, LZBC had the best impact on reducing soil Cd availability which showed a decrease of 40.96% compared with the CK at the end of the experiment. Meanwhile, there was an obvious negative correlation between soil pH and DTPA-extractable Cd in the whole incubated time (Fig. S1), with a correlation coefficient of − 0.0254***. Thus, LZBC had a lower available Cd content may be attributed to the higher soil pH during the incubation experiment. The rapid increase of pH induced more precipitation of Cd2+ in soil solution after the addition of LZBC. Previous studies have indicated that pH as a soil parameter played an essential role in the bioavailability and immobility of toxic metals (Shen et al. 2016; Zeng et al. 2011).

Cd fractionation

Heavy metal fractionation can predict its bioavailability in soil, which is related to the absorption of plants. Figure S2 presents Cd fractions in the treated and control soils on the 56th day. The maximum EX-Cd in incubation soil was observed in the CK treatment with a 29.20% proportion. Six composite amendments converted EX-Cd into less available forms such as CB-Cd, OX-Cd, OM-Cd, and RS-Cd fractions. The lowest EX fraction was observed in soil amended with LZBC amendment (11.13%) followed by LSB (13.17%) and LZC (17.40%). Compared with the CK, there was a notable (p < 0.05) increased CB-Cd under LZBC, LZC, LZB, and LSB, increased OX-Cd under LZBC, LZC, and LSC, and increased OM-Cd under LZBC and LZB. Such varieties in Cd fractionation owing to the applied amendments may be linked with a series of biological or chemical reactions such as adsorption, precipitation, ion exchange, and chelation (Pardo et al. 2011). Several studies revealed that Cd has strong mobility in the soil, and the proportion of EX-Cd could be notably decreased by adding organic and inorganic mixed amendments. For instance, Hamid et al. (2020a) observed that the use of manure with lime and sepiolite (2400 kg acre−1) significantly reduced EX-Cd from 45 to 19% and improved OM-Cd. Mujtaba Munir et al. (2020b) indicated that the mixed application of coal gangue with biochar caused a decrease of 63.60% in EX-Cd and elevation in RS-Cd. These findings suggested that immobilizing agents can promote the immobilization of Cd, which is chiefly associated with the electrostatic interaction between the surface functional groups of immobilizing agents (-OH, C = O, and C–O–C) and Cd2+, causing the production of hydroxide or carbonate precipitates by Cd2+ and various activated functional groups (Garau et al., 2007). Besides, compared with the control, all the composite amendments did not increase the RS-Cd and even reduce the value after the addition of immobilizing agents (Fig. S2). This phenomenon may be because these amendments redistributed the fractionations of soil Cd with the varieties of soil environment after transferring EX-Cd into other stable forms of Cd. A similar result was reported by He et al. (2019), who observed PL (apatite + boricfertilizer) composite amendment significantly reduced EX-Cd and RS-Cd simultaneously, but increased other Cd fractionations, as compared to unamended soil.

Characteristics of LZBC amendment used in the field experiment

LZBC showed the highest soil pH, the lowest available Cd concentration and EX-Cd among six amendments in the incubation experiment. Therefore, the soil amendment was further used in the field experiment. Figure 2 presents SEM, EDS, FTIR, and XRD characteristics of LZBC which illustrate its surface morphology, chemical composition, and crystallographic structure, respectively. The SEM images (Fig. 2a) depicted an irregular and porous structure of LZBC, while the EDS spectrum (Fig. 2b) of the exterior surface of LZBC presented C, O, and Ca in the highest amount with values of 28.69%, 30.43%, and 34.20%, with lower values of Mg (1.08%), Al (0.62%), and Si (4.98%). The high values of C, O, and Ca in soil additive indicated the significance of the carbonated phase in immobilizing agents (Hwidi et al., 2018). In Fig. 2c, in the FTIR pattern, there were many strong absorption peaks of LZBC amendment, corresponding to the peak of stretching of structural -OH group at 3440 cm−1, the broad peak of carbonate group (C-O) at 1030 cm−1, and the weak peak of aromatic C = C at 1630 cm−1 (Wan et al. 2019). Moreover, the peaks at 1400–800 cm−1 indicated the active C-N, C-H, and Si–O groups (Jeon et al. 2018). Figure 2d shows XRD pattern had lots of impurity peaks. These characteristic peaks mainly were Ca(OH)2 (2θ = 17.89°, 28.90°, 33.99°, 47.00°, 50.78°, 54.40°, 59.50°, 62.29°, 64.26°, 72.00°, 84.81°) and some low content crystals including CaCO3 (2θ = 29.40°, 39.59°, 42.71°), SiO2 (2θ = 22.81°, 78.74°, 82.02°), and Al12Mo2O32Sr8 (2θ = 20.68°, 22.17°, 26.43°) were detected in LZBC sample. In the XRD spectrum, Hannan et al. (2021) reported that lime was mostly composed of calcium, whereas zeolite exhibited its enrichment with quartz and other minerals related to the absorption of metals. This could explain why the XRD pattern of LZBC exhibited Ca(OH)2, CaCO3, and SiO2 minerals. Besides, several studies found that immobilizing agents biochar and compost contained lots of mineral elements required by plants, which could form the complex mineral crystals in LZBC (Abideen et al. 2020; Ghasemzadeh and Bostani 2017; Kim et al. 2012). The finding of mineral species was consistent with the results of elemental analysis (Fig. 2b), while it was proved that the above chemical compounds were associated with the adsorption of metal species (Chandra et al. 2020; Vrînceanu et al. 2019).

Fig. 2
figure 2

SEM images (a), EDS (b), FTIR (c), and XRD spectra (d) of LZBC

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The effects of LZBC in the Cd-contaminated GVP system

Rhizosphere soil pH and DTPA-extractable Cd

As expected, the addition of LZBC improved the rhizosphere soil pH of 14 vegetables (Fig. 3a). The pH values of rhizosphere soil amended by LZBC were significantly (p < 0.01) higher than un-amended soils in most vegetables. Compared with no treatment soil, the adding values were displayed in descending order: A. tricolor (2.01 units) > S. oleracea (0.97 units) > M. crystallinum (0.81 units) > B. pekinensis2 (0.75 units) > B. pekinensis1 (0.64 units) > L. sativa1 (0.62 units) > L. sativa3 (0.45 units) > A. fistulosum (0.44 units) > B. chinensis (0.42 units) > C. sativum (0.41 units) > B. oleracea1 (0.38 units) = B. oleracea2 (0.38 units). LZBC elevated rhizosphere soil pH owing to its alkaline characteristic, high cation content (e.g., Ca), and the dissolution of its respective carbonates and oxides (Fig. 2). The alkaline reaction between hydrogen ions in soil and stabilizing agents is the reason for the improvement of soil pH. A similar result of the increased soil pH due to adding combined amendments was found by Zhou et al. (2014). Besides, Zhao et al. (2021) found that the soil pH of gangue and zeolite with different application ratios increased the soil pH by 7.7–10.5% and 3.6–5.8%. Yang et al. (2018) reported that the addition increased the soil pH to 7.3 with an increase of 0.94–0.96 units in a pot experiment. But here, for A. graveolens and L. sativa2, there were no significant (p > 0.05) differences in rhizosphere soil pH between the treated and untreated soils (Fig. 3a). This phenomenon could be due to the changes in the soil environment by the use of LZBC, which led to the secretion of more organic acids in the plant rhizosphere to maintain the acid–base balance of soil conditions (Bolan et al. 2014).

Fig. 3
figure 3

Rhizosphere soil pH (a), DTPA-extractable Cd concentrations (b) of fourteen vegetables grown in LZBC amended and unamended soils. Bars represent standard error of the mean (n = 3). *Significant at the level 0.01 < p ≤ 0.05, **significant at the level p ≤ 0.01

Full size image

The application of LZBC decreased DTPA-extractable Cd contents in all vegetable soils (Fig. 3b). In comparison with untreated soils, DTPA-extractable Cd contents were remarkedly (p < 0.01) decreased after the addition of LZBC, with the maximum decrease of 37.99% in L. sativa3, then 15.28% in A. fistulosum, 12.98% in A. tricolor, 11.31% in C. sativum, 8.23% in B. pekinensis1, and 7.09% in B. oleracea1. Previous studies proved that alkaline materials, e.g., lime, Fe-biochar, fly ash, red mud and sepiolite, etc., could reduce metal mobility from artificially or historically polluted soils by improving the soil pH (Tang et al. 2020; Wang et al. 2012; Wu et al. 2016). Here, LZBC composed of different alkaline materials possessed active functional groups and crystal structures (Fig. 2c and d) which caused precipitation of hydroxides or carbonates, and the special pore structure of LZBC (Fig. 2a) was able to further enhance the metal immobilization in polluted soil. The findings were agreeable with Taghipour and Jalali (2016) and Rehman et al. (2017) in which the combination of organic and inorganic materials increased HMs immobilization. However, LZBC did not decrease significantly in soil DTPA-extractable Cd contents of the other vegetables including A. graveolens, B. chinensis, B. oleracea2, B. pekinensis2, L. sativa1, L. sativa2, M. crystallinum, and S. oleracea. Such results have occurred may be because only a small amount of Cd2+ in soil was precipitated in the form of Cd3(PO4)2, CdS, or Cd(OH)2 after the use of LZBC (Rizwan et al. 2017; Shahkolaie et al. 2020). Besides, LZBC occurred the aggregation in the rhizosphere of these vegetables may be another reason for reducing its capacity for metal adsorption (Lebrun et al. 2019). A similar phenomenon was reported by Yang et al. (2020), who observed no significant decrease of CaCl2-extractable Cd content in celery rhizosphere soil after the use of hydroxyapatite (< 60 nm). It indicated that the immobilization efficiency or mechanism was dominated by the comprehensive effect of soil-amendment-plant and environmental characteristics rather than barely depending upon amendments.

Biomass and yield of 14 vegetables

Biomass and yield are the most intuitive characteristics to reflect the influences of harmful substances in soil on crop growth and development (Wu et al. 2019). In the present study, all the plants with different treatments were not threatened by diseases and pests and grew normally during the growth. The shoots and roots biomass of 14 vegetables grown in plots with the application of LZBC was generally higher than those in un-amended plots (Fig. 4a). Specifically, the application of LZBC significantly boosted the biomass of all vegetables (p < 0.05), except for the shoot biomass of 3 vegetables (A. tricolor, L. sativa1, and S. oleracea). Compared with the CK, the maximum biomass increment was observed in B. chinensis (49.38%) followed by B. oleracea2 (36.91%) and B. oleracea1 (32.24%). In terms of root biomass, four vegetables including A. fistulosum, A. tricolor, B. chinensis, and B. pekinensis1 with the LZBC treatment were remarkedly (p < 0.05) higher than CK by 49.0%, 48.1%, 41.2%, and 25.5%, respectively. Additionally, as compared to the CK (Fig. 4b), LZBC dramatically (p < 0.05) improved the yield of edible parts in A. fistulosum, A. graveolens, B. chinensis, B. oleracea2, B. pekinensis1, B. pekinensis2, C. sativum, L. sativa2, L. sativa3, and M. crystallinum. To the best of our knowledge, organically enhanced crop yield is owed to the enhancement of soil nutrient availability and physiochemical properties. Organic amendments could promote plant growth and enhance the quality of crops by providing essential nutrients and decreasing the availability of toxic metals (Lin et al. 1998; Mukome et al. 2020). For instance, Agegnehu et al. (2016) showed biochar, compost, and mixtures of the two could increase maize grain yield due to higher soil organic carbon, soil water content, total N, and available P. In the meantime, the improvement of soil enzyme activities and microbial diversity with the use of amendments plays an indispensable role in promoting plant growth (Beesley et al. 2014). Previous studies also reported that the addition of inorganic materials such as lime and zeolite can improve soil conditions, thus strengthening soil’s capacity to retain nutrients (Huang et al. 2020b; Pardo et al. 2011). Additionally, it has been reported that the grain weight of Oryza sativa L. under the sepiolite and phosphate soil treatments improved by 17.1% and 39.5%, respectively, compared with the control (Sun et al. 2016). Another possible reason for enhancing plant growth could be the reduction of the bioaccumulation of toxic metals in crops (Qian et al. 2009). Cd toxicity can depress the net photosynthetic rate of plants and reduce chlorophyll content, thereby having a negative impact on plant growth and biomass (Zeeshan et al. 2020). It has been reported that the interaction between Cd2+ and cations contained in amendments affected the phytotoxicity imposed by the large accumulation of Cd in plant roots and reduced the adsorption of Cd in the root. Mehdizadeh et al. (2021) found that biochar application increased morphological traits and photosynthetic pigments activity of O. ciliatum and decreased Cd content under Cd stress. A similar result was observed by Lahori et al. (2020) indicated that zeolite as an immobilizing agent significantly promoted the production of the cabbage and corn and reduced Cd absorption in all tissues of plants.

Fig. 4
figure 4

Shoot and root dry biomass (a) and yield of edible part (b) in fourteen vegetables grown in LZBC amended and unamended soils. Bars represent standard error of the mean (n = 3). *Significant at the level 0.01 < p ≤ 0.05, **significant at the level p ≤ 0.01

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Cd absorption, bioaccumulation, and translocation in 14 vegetables

Different plant species or even different cultivars can regulate HM absorption at the interface between soil and root to different extents (Dermatas and Meng 2003). To our surprise, not all Cd levels in edible parts of vegetables showed a downward trend after applying LZBC (Fig. 5), which revealed that the adaptabilities of different vegetables to soil amendments are diverse. Firstly, as shown in Fig. 5, Cd concentration in edible parts of A. fistulosum, A. tricolor, and C. sativum were significantly (p < 0.01) reduced by 50.72%, 46.61% and 31.88% by LZBC amended. For these crops, the lowered Cd accumulation in edible parts may be principally attributed to the improvement in soil pH (Fig. 3a), which led to an accompanying decline in their phytoavailable pools (Fig. 3b) after the addition of LZBC. Many studies have illustrated that improvement of soil pH is capable of reducing toxic metals bioavailability, consequently decreasing metals absorption by crops (Feng et al. 2013; He et al. 2019; Lebrun et al. 2019). Besides, it may be related to the competition between cation anions (e.g., Ca2+, Mg2+, Al3+ contained in LZBC) and soil Cd2+, which inhibited the translocation of Cd to edible parts. Similar findings were observed by Kim et al. (2012), who reported that Cd in spring onion could be decreased below the permissible limit by the addition of dolomite, steel slag, or agri-lime. Khan et al. (2020c) also observed that the use of hardwood biochar restricted Cd uptake in Cilantro and spinach vegetables. However, in an opposite trend, LZBC addition dramatically (p < 0.05) enhanced Cd concentration in edible parts of L. sativa3, L. sativa2, and L. sativa1 by up to 28.26%, 29.96%, and 10.53% (Fig. 5). It should be mentioned that the three crops belong to the same genus Lactuca of Compositae. The increase in Cd translocation to edible parts was consistent with Ondrasek et al. (2021), who reported that biomass bottom and dolomite (whether combined or single) elevated Cd accumulation in the whole radish tissues. They considered that the alkaline matrix of bio-ash or dolomite may contribute to the retention and immobilization of toxic Cd. Similarly, in the present study, LZBC enhanced the rhizosphere soil pH of the three crops (Fig. 3a). Therefore, the changes in soil pH could be a key factor affecting Cd transfer from soil to plant. A similar result was reported in our previous study (Liu et al. 2021), we found that LZBC with different application techniques significantly increased Cd accumulation of cucumber under a rotation system. Moreover, Cd contents in edible parts of A. graveolens, B. chinensis, B. oleracea1, B. oleracea2, B. pekinensis1, B. pekinensis2, M. crystallinum, and S. oleracea grown in the amended plots had no obvious (p > 0.05) differences compared with the control (Fig. 5). This phenomenon could be explained from two aspects. On the one hand, no obvious decrease was found in soil available Cd of crops after LZBC application (Fig. 3b), indicating that Cd transport in crops was not significantly affected. On the other hand, the variation of soil environment (e.g., rhizosphere soil pH) enhanced the ability of crops to gain Cd2+ in soil solution when soil available Cd content was at a lower level.

Fig. 5
figure 5

Cd concentration in edible parts of fourteen vegetables grown in LZBC amended and unamended soils. Bars represent standard error of the mean (n = 3). *Significant at the level 0.01 < p ≤ 0.05, **significant at the level p ≤ 0.01

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BAF and TF in plants are important parameters in HMs research, which can reflect the absorptive capacity of crops to metals. Previous studies defined low accumulated cultivars of Cd by the standard that bioaccumulation rates < 1 and transfer rates < 1 (Wang et al. 2019). As shown in Fig. S3, the BAF and TF values in A. fistulosum, B. oleracea1, B. oleracea2, L. sativa1, and L. sativa2 were lower than 1.0 in unamended soils; therefore, these vegetables could be selected as Cd low accumulators. Besides, after the application of LZBC, the BAF and TF of Cd in A. fistulosum (from 0.26 to 1.33, 0.25 to 0.15) and C. sativum (from 1.43 to 0.97, 0.61 to 0.36) were markedly (p < 0.01) reduced, indicating LZBC efficiently inhibited Cd uptake in A. fistulosum and C. sativum. The finding could be agreed with the work conducted by Hamid et al. (2020b), who reported shoot to grain Cd translocation in wheat was reduced by 29–32% with the addition of composited material deriving from manure, lime, and sepiolite at the rate of 6000 and 3000 kg ha−1. Meanwhile, this reduction suggested that LZBC helped decrease the content of Cd2+ around the rhizosphere environment. However, contrary to the expectation, the use of LZBC significantly (p < 0.01) increased BAF and TF values of Cd in L. sativa1 (from 0.91 to 1.07, 0.57 to 1.08), L. sativa2 (from 0.83 to 1.05, 0.50 to 0.82), and L. sativa3 (from 3.02 to 3.78, 0.92 to 1.75). This phenomenon could be explained via the increase of Cd content in the shoots of vegetables with the use of LZBC, while the limited changes in soil total Cd exacerbated the enhancement of Cd bioaccumulation and transfer in vegetables. A similar result was found in Mujtaba Munir et al. (2020c) reported the 1–2% raw coal treatments increased Cd, Cr, and Pb bioconcentration factor (BCF) and translocation factor (TF) in maize owing to the higher HMs bioavailability in the soil solution. Besides, there were no evident differences (p > 0.05) in TF and BAF values of Cd for the other vegetables ( A. graveolens, B. oleracea1, B. oleracea2, B. pekinensis1, B. pekinensis2, M. crystallinum, S. oleracea) between the control and treated soils, indicating the composite amendment was not suitable for the safe production of these vegetables.

Health risk assessment

Table 3 shows the values of DIM for adults by eating selected vegetables planted in plots amended with the composite amendment. DIM values with LZBC exhibited different effects dependent upon crop species, and all the values were far below 1. Among 14 vegetables, the application of LZBC did not obviously change the amount of dietary Cd intake from the consumption of A. graveolens, B. oleracea1, B. oleracea2, B. pekinensis1, M. crystallinum, and S. oleracea; however, DIM values in L. sativa1 (1.84E − 04 to 2.03E − 04), L. sativa2 (1.74E − 04 to 2.26E − 04), and L. sativa3 (from 6.32E − 04 to 8.10E − 04) were significantly (p < 0.05) increased by LZBC. Besides, LZBC significantly decreased the dietary uptake of A. fistulosum, A. tricolor, and C. sativum, HRI values under the control and LZBC amendment for adults are presented in Table 3. The highest HRI value for Cd in the control plot was observed in A. tricolor (1.21E + 00), which was greater than 1 indicating that adults were exposed to substantial health risk upon consuming this vegetable grown in a moderately Cd-polluted field. While the HIR value of Cd in A. tricolor was lower than 1 with LZBC treatment, indicating LZBC usage in moderately Cd-contaminated soil helped the public to stay risk-free. Other vegetables had HIR < 1 for adults both in treated and untreated plots. The THQ values of Cd for 14 vegetables in control and under LZBC are given in Table 3. THQ values for these vegetables were lower than 1 in unamended and amended plots, stating low health risk for the public. However, THQ values of L. sativa1, L. sativa2, and L. sativa3 were significantly (p < 0.05) enhanced by up to 2.03E − 02, 2.26E − 02, and 8.10E − 02 after the addition of LZBC. Additionally, LZBC was capable of reducing obviously THQ values in A. fistulosum from 9.70E − 03 to 4.78E − 03, A. tricolor from 1.21E − 01 to 6.48E − 02, and C. sativum from 4.86E − 02 to 3.31E − 02.

Table 3 The influences of LZBC amendment on daily dietary intake of metal Cd (DIM), health risk index (HRI), and target hazard quotient (THQ) for adults in 14 vegetables
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Although previous results proved that soil amendments decreased the accumulation of HMs in different vegetable species, so as to reduce human health risks (Tian et al. 2016; Wang et al. 2019), our results showed that applying the LZBC amendment to 14 vegetables had different changes on human risks due to the increasing or decreasing Cd levels of the crops. According to the WHO recommendation, Cd consumption in vegetables is limited to 6 µg kg−1 per body weight. Hence here the low daily dietary intake of Cd values indicated no risk to human health. Further, the application of LZBC was capable of reducing DIM of Cd in A. fistulosum, A. tricolor, and C. sativum, suggesting that LZBC was better for human health when the public eat such vegetables cultivated in the LZBC-amended GVP system. Previously Nawab et al. (2018) reported the use of biochar amendment reduced daily dietary intake of HMs by 21–36% via the intake of pea fruits, which was in coordination with our study. However, for HRI of Cd, the results showed that in all the vegetables, the consumption of A. tricolor planted in this soil should be avoided by adults as its HRI value was higher than 1. Fortunately, we observed HRI value of A. tricolor was decreased to less than 1 by adding LZBC to the soil, representing the amendment that could be recommended for the soil of A. tricolor vegetable. Results of HRI of this vegetable were consistent with Muhammad et al. (2020). Yousaf et al. (2018) reported the application of pinewood saw dust derived biochar obviously reduced HRI of Cd (77%), Cr (73%), Ni (68%), and Pb (78%) in wheat. Additionally, THQ is usually applied as an index for assessment of potential human health risks as a result of consumption of toxic metal-polluted food (Adel et al. 2016). For the public health consumption, THQ not higher than 1 exhibits no risk. The THQ values in Table 3 further indicated that the health risk of the public was reduced by consuming A. fistulosum, A. tricolor and C. sativum growing LZBC amended soils. In agreement with the present study, Wang et al. (2019) also reported Pb THQ value of pakchoi for adults was reduced to < 1 by the use of the mixed amendment (biochar + calcium superphosphate + compost). Based on the above results, the accumulation of Cd in A. fistulosum, A. tricolor, and C. sativum planted in moderately Cd-polluted soil can be reduced by the use of LZBC, so as to reduce the daily intake of Cd, HRI and THQ, while LZBC should be forbidden to apply in Lactuca of Compositae vegetable fields.

Conclusions

The present study based on incubation and field experiments investigated the effects of organic and inorganic mixed amendments on Cd immobilization efficiency and vegetable consumption risks in a Cd-contaminated GVP system. Results indicated that six combined amendments (LZBC, LSBC, LZC, LZB, LSC, and LSB) composed of inorganic materials (Lime: L, Zeolite: Z, and Sepiolite: S) and organic materials (Biochar: B and Compost: C) significantly improved soil pH and decreased Cd bioavailability, with LZBC exhibited the highest stabilization efficiency at the end of the incubation experiment. LZBC was further employed in field conditions to explore its effects on Cd uptake and consumption risks of 14 vegetables. On the whole, the application of LZBC at 1% w/w showed considerable efficiency in increasing rhizosphere soil pH and reducing DTPA-extractable Cd as compared to the control. Additionally, the use of LZBC revealed an excellent performance on the biomass and the yields of edible parts of 14 vegetables. LZBC significantly reduced the accumulation of Cd in edible parts of Allium fistulosum L., Amaranthus tricolor L., and Coriandrum sativum Linn. as well as their bioaccumulation factor (BAF) and translocation factor (TF). Conversely, LZBC increased Cd contents in edible parts, BAF and TF in Lactuca sativa var. ramosa Hort., Lactuca sativa var. asparagina, and Lactuca sativa var. angustana Irish. Similarly, the health risk assessment indicated that LZBC decreased the daily intake of metal (DIM), health risk index (HRI), and target hazard quotient (THQ) for Cd in Allium fistulosum L., Amaranthus tricolor L., and Coriandrum sativum Linn., but significantly increased in Lactuca. sativa. Overall, these findings indicate that the mixed amendment needs to be applied to specific vegetables in Cd-contaminated fields to reduce human health risks.