Introduction

Cadmium (Cd) is a toxic element with a relatively high risk of being transferred from soil to the food chain (Zhao and Wang 2020). Daily diet is the major source of Cd exposure for the nonsmoking populations (Chen et al. 2018a). Among the plant-derived foods, rice (Oryza sativa L.) is one of the most important crops for human beings, and it has a long cultivation history in China (Zou et al. 2021). Rice plants absorb essential elements from the paddy soil; meanwhile, they inevitably uptake some non-essential elements and even toxic metals like Cd. Due to the rapid modernization in recent years, parts of human activities have caused paddy soil contaminated by Cd to varying degrees, such as mining, smelting, sludge, fertilizer, and pesticide (Feng et al. 2022). In some contaminated areas, long-term intake of Cd from consumption of rice by the local residents lead to serious health problems, such as anemia, cancer, heart attack, proteinuria, lung disorder, emphysema, and osteoporosis (Mei et al. 2017). The itai-itai disease, which occurred in Japan in the 1950s, was the result of excessive ingestion of Cd through contaminated rice (Nordberg et al. 2002).

Guizhou province is located in the karst landform region of China; the paddy soil developed in carbonate rocks in Guizhou has the characteristics of high geological background value of heavy metals (Kong et al. 2018). According to China’s National Environmental Monitoring Center, the background content of Cd in Guizhou soil is 1.24 mg/kg, which is the highest value in China (CNEMC 1990). During the weathering and pedogenesis process, the easy-soluble components of carbonate rocks were leached and the insoluble components like Cd were reserved by complexation with clay or Fe and Mn oxides (Pu et al. 2015; Xia et al. 2020). Therefore, the soil formed in this natural process was with high total Cd concentration and relatively low bioavailability. However, the human activities, such as industrial production, traffic, application of fertilizers, and wastewater irrigation, have inevitably caused heavy metals activated by the rapid development in the past few decades, thereby increasing Cd bioavailability in paddy soils. Totally, Cd-contaminated paddy soils are becoming one of the critical concerns, both the natural sources and human disturbances lead to the need to search for effective ways to repair Cd-polluted soil.

As a non-essential element for plants, Cd is absorbed by rice roots and migrates to grains along with essential divalent cations like Mn2+, Zn2+, Fe2+, Mg2+, and Ca2+ (Zhao and Wang 2020). Due to its toxicity, Cd has adverse impacts on rice yields and qualities by affecting plants nutrient uptake and disturbing metabolism (Wang et al. 2021a; Yang et al. 2019). Cd uptake by plant roots depends on its bioavailability in paddy soil. The soil bioavailable Cd, an important factor influencing Cd uptake, is determined by the physical, chemical, and biological characteristics of soil (Wang et al. 2020). The physico-chemical changes in paddy soil lead to the distinctiveness in the mobility, bioavailability, and fraction of Cd (Wang et al. 2020). Therefore, reducing Cd bioavailability and its migration from soil to rice are effective measures for Cd-contaminated soil remediation.

Many measures have been used for remediating Cd pollution in paddy soils to guarantee rice security, including phytoremediation (Liu et al. 2019), water management (Huang et al. 2022), soil amendment application (Chen et al. 2018b; Hamid et al. 2020a), and microbe-assisted remediation (Li et al. 2017; Luo et al. 2019). Among them, the in situ soil amendments such as lime and biochar have received widespread attention for their relatively high remediation efficiency and cost-effectiveness. The selection of raw materials for amendments is the key for this measure. Passivation materials like lime, alkaline materials, clay minerals, and biochar are widely used in acidic soils in China (He et al. 2022; Li et al. 2022a; Yin et al. 2022). The previous studies have shown that the soil amendments exhibited a great level of improving soil pH and reducing soil Cd bioavailability by serious reactions such as adsorption, precipitation, complexation, chelation, ion exchange, and the redox reactions (Hamid et al. 2019; Lu et al. 2012; Porter et al. 2004). The CaO, SiO2, K2O, and MgO contained in passivators could react with Ca2+ as follows:

$$\mathrm{CaO}+{\mathrm{H}}_{2}\mathrm{O}+{\mathrm{Cd}}^{2+}={\mathrm{Cd}(\mathrm{OH})}_{2}+{\mathrm{Ca}}^{2+}$$
$${\mathrm{SiO}}_{2}+2{\mathrm{H}}_{2}\mathrm{O}+2{\mathrm{Cd}}^{2+}=2{\mathrm{Cd}(\mathrm{OH})}_{2}+{\mathrm{Si}}^{4+}$$
$${\mathrm{K}}_{2}\mathrm{O}+{\mathrm{H}}_{2}\mathrm{O}+{\mathrm{Cd}}^{2+}={\mathrm{Cd}(\mathrm{OH})}_{2}+2{\mathrm{K}}^{+}$$
$$\mathrm{MgO}+{\mathrm{H}}_{2}\mathrm{O}+{\mathrm{Cd}}^{2+}={\mathrm{Cd}(\mathrm{OH})}_{2}+{\mathrm{Mg}}^{2+}$$

Meanwhile, the specific surface areas, pore structures, and facilitated electron transfer of soil amendments enhance the physical adsorption of heavy metals in soil, while the functional groups and aromatic properties of amendments affect the chemisorption and precipitation of heavy metals in soil (Chen et al. 2014; Qiao et al. 2018; Tang et al. 2017).

In the past decade, a large number of studies have been carried out on the remediation of Cd pollution in paddy soil by pot experiments, and satisfactory results have been achieved. However, due to the complex external factors, the effectiveness of soil amendments in field trials remains to be verified. In this study, different soil amendments, including lime, manure compost, passivators, and their combinations, were applied in a field experiment, the effects of amendments on soil pH, organic matter (SOM), cation exchange capacity (CEC), the total content and fractionation of Cd, and the Cd accumulation in rice were analyzed to verify the effectiveness of soil amendments.

Materials and methods

Study area

Both the two experimental paddy fields were located in Zhenyuan County, Guizhou Province, China, one in Jinbao Town and the other in Qingxi Town. The regional climate was a typically humid subtropical monsoon climate, with a mean annual temperature of 18.19℃ and an annual average precipitation of about 1182.45 mm. The Jinbao site was situated in a rolling valley, where mountain streams irrigated rice fields, the soil was derived from slate and classified as the hydromorphic paddy soil series in the Chinese Soil Taxonomy. About 2 km northwest of the experimental fields was an abandoned lead–zinc mine that had been closed for about 15 years. At the Qingxi site, the research fields were located on the terrace of the Wuyang River, and the soil was derived from river alluviums, which belonged to the hydromorphic paddy soil series.

According to our previous study, 21 and 27 plots were collected in Jinbao Town and Qingxi Town, respectively, to investigate the distribution of heavy metals in paddy soils (Guo et al. 2021). The soil pH of the Jinbao site was 4.94 ± 0.19, and the average Cd content was 0.91 ± 0.48 mg/kg, while that of the Qingxi site were 5.75 ± 0.34 and 1.50 ± 0.82 mg/kg, respectively. The Cd contents of paddy soils in both study areas exceeded the Cd pollution risk screening value for soil contamination of agricultural land of China (GB 15618–2018). The variation coefficient of Cd content in Qingxi Town and Jinbao Town was 54.67% and 52.32%, respectively, reflecting large spatial variations of Cd concentrations. The Cd concentrations in irrigation water of the Jinbao and Qingxi sites were 0.009 and 0.002 mg/kg, respectively, which could be ignored.

Materials and experiment design

Three rice varieties, Zhongzheyou 1, Yixiang 2115, and Yuxiang 203, were purchased from the native seed station in Qingxi town.

Eight field treatments were designed with each rice cultivar to investigate the effects of the individual or mixed application of these amendments on paddy soil remediation (Table 1), including control (CK), lime alone (L), manure compost alone (M), lime + manure compost (LM), lime + passivator 1 (LP1), lime + passivator 2 (LP2), lime + passivator 3 (LP3), lime + passivator 4 (LP4). Two rice varieties (Zhongzheyou 1 and Yixiang 2115) were conducted in two paddy fields in Qingxi Town, and another experiment was conducted with the rice variety Yuxiang 203 in Jinbao Town. A total of 24 field plots (8 treatments × 3 rice cultivars) were conducted, with three replicates per pot. Each plot covered an area of 20m2 (5 m × 4 m), and all plots were designed as random blocks. Protective ridges were established around the plots and plastic film was covered to prevent the water seepage between the plots. Details about the main components of lime, manure compost, and the four kinds of passivators were shown in the Table 2. The application rate of amendments was determined by the dosage recommended in the instruction manuals provided by the manufacturers.

Table 1 Design of amendments in paddy soil
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Table 2 Main components of amendments
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Field management

Paddy field preparation and amendments application: From April to May, the paddy field plowing and raking were completed according to the local rice planting method. Manure compost and passivators were applied from May 10 to May 11, each of them was first applied to the surface of the paddy field and then mixed evenly by plowing. Lime was implemented at the end of tillering stage (about 1 month after transplanting).

Rice transplanting and water management: Transplanting of rice seedlings was conducted from May 18 to May 21. Rice sowing followed the local traditional methods. The fields were kept flooded until the end of tillering stage, and then drained to inhibit ineffective tillering. After tillering, flooded again until 7 days before harvest.

Sample collection

Soil samples were collected in March (before repairing) and October (after harvest) 2016, respectively. Topsoil samples from each plot were collected according to the 5-point sampling method and then air-dried after the removal of visible stones and plant residues. The soil samples were ground and sieved through 2-, 0.25-, and 0.149-mm nylon mesh to measure the soil pH, CEC, SOM, and Cd.

Rice samples were collected from each corresponding plot in October 2016, placed in nylon net bags, and taken back to the laboratory. At the same time, the number of rice effective and ineffective panicles were recorded to calculate grain yields. Rice grain yield was determined from all plants in a 5-m2 area in each plot and was adjusted to a moisture content of 14%. Rice plants were cleaned with tap water and then rinsed with deionized water at least three times. Rice plants were then separated into four parts: roots, stems, husks, and grains. Roots and stems were first dried in the oven at 110℃ for 30 min, and then dried at 70℃ to constant weight. Grains were air-dried and hilled manually. All plant samples were fully sieved through a 0.149-mm nylon mesh to measure the total Cd concentration.

Sample analysis and quality control

Soil pH was measured at a soil (m) to water (V) ratio of 1:2.5 with an acidity meter (PHS-3C, Ray Magnetic, China). The soil organic matter was determined calorimetrically by oxidation with potassium dichromate. The soil cation exchange capacity was measured using the ammonium acetate saturation method. According to the sequential extraction procedure from the modified European Union Bureau of Reference (BCR) proposed by Arain et al. (2008), Cd forms in soil were divided into four Cd fractions: the acid extractable Cd (acid-soluble fraction) extracted by 0.11 M acetic acid; the fraction bound to Fe/Mn oxides (reducible fraction) extracted by 0.5 M hydroxylamine hydrochloride (pH 2); the fraction bound to organic matter (oxidizable fraction) extracted by 30% H2O2 (pH 2–3) and 1 M NH4OAc (pH 2); and the residual fraction (residual fraction) digested with HNO3–HCl. The total Cd content of soil samples was determined using acid digestion with HCl-HNO3-HClO4. All rice samples were digested using the dry ashing method.

Cd contents were analyzed using an inductively coupled plasma optical emission spectrometer (ICP-OES, Thermo Fisher 7400). Certified reference material for soil (GBW07429) and rice (GBW10010 (GSB-1) provided by National Standard Materials Center were used for quality control, the average recovery rates of Cd in soil and rice samples were 96.8% and 95.1%, respectively.

Data analysis

Assessment of the repairing effect of the amendments on soil and rice plant

Due to this field experiment was conducted in two towns with 24 paddy pots, the various external environments and Cd background content affected the quantitative value and caused large deviations of soil pH, SOM, CEC, and Cd content, then influenced the accuracy of effectiveness evaluation. Therefore, we chose the change percentage of soil pH, SOM, CEC, and Cd content as the evaluation indexes. The calculation method was as follows:

$${\mathrm{Index}}_{\mathrm{red}}=\frac{{C}_{\mathrm{pre}}-{C}_{\mathrm{post}}}{{C}_{\mathrm{pre}}}\times 100\mathrm{\%}$$
$${\mathrm{Index}}_{\mathrm{imp}}=\frac{{C}_{\mathrm{post}}-{C}_{\mathrm{pre}}}{{C}_{\mathrm{pre}}}\times 100\%$$

The Indexred and Indeximp represented the reduction percentages and improvement percentages of the soil indexes (i.e., soil pH, SOM, CEC, and Cd concentration), respectively. Cpre represented the concentration (value) of the indexes before reparing. Cpost represented the concentration (value) of the indexes after rice grain harvest.

Considering the different Cd concentration in different fields, it was not feasible to calculate Cd content in rice alone. We considered the Cd contents of soil and rice plants together, so we used the bioconcentration factor (BCF) as the assessment index. The BCF was the ratio of the Cd concentration in plant organs to that in soil (Peng et al. 2020), and it was calculated as follows:

$${\mathrm{BCF}}_{\mathrm{plant}}=\frac{{C}_{\mathrm{plant}}}{{C}_{\mathrm{soil}}}$$

where Cplant and Csoil represented the Cd concentrations of rice plant and soil, respectively. The BCFroot, BCFstem, BCFhusk, and BCFgrain were also calculated.

Statistical analysis

The statistical relationship among all data was analyzed using one-way analysis of variance (ANOVA), and the least significant difference (LSD) was utilized to determine if significant differences existed between the mean values (P < 0.05) with SPSS 19.0 (SPSS Inc., Chicago, IL, USA). All data was expressed as the mean ± SD (n = 3). All of the figures were generated utilizing Origin 9.2 (OriginLab Corporation, Northampton, MA, USA), CorelDRAW X8 (Corel Corporation, Ottawa, Canada), and Adobe Photoshop 2020 and Adobe Illustrator 2021 software (Adobe Systems Inc., Mountain View, California, USA).

Results

Effects of amendments on soil pH, organic matter, and CEC

As shown in Fig. 1, the application of different amendments increased soil pH to varying degrees, among which CK treatment had the lowest pH value. The pH increase rate of manure compost treatment was only 1.1% higher than CK, and their difference did not reach a significant level. Lime, LM, LP1, LP2, and LP3 treatments increased soil pH by 7.6 ± 2.9%, 8.5 ± 2.8%, 8.0 ± 1.2%, 9.4 ± 0.3%, 9.3 ± 3.5%, respectively. What was noteworthy was that LP4 treatment increased soil pH by 13.7 ± 1.5%, which was significantly higher than others. It could be seen that compared with the lime treatment and passivators treatments, the manure compost treatment had relatively less impact on soil pH. Lime and passivators consisted of alkaline materials, especially passivator 4, which could neutralize hydrogen ions, then improve soil pH.

Fig. 1
figure 1

Effect of amendments on the increase percentages of soil pH (a), SOM (b), and CEC (c). Error bars indicate standard deviation. Different letters above the bars indicate a significant difference (p < 0.05) among amendments

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SOM contained various organic acids and soil colloid, and it had a two-sided effect on Cd ions. Among them, organic acids could act as chelators that increased the mobilization of Cd, meanwhile, the ligands and functional groups in SOM could complex with Cd ions to reduce Cd bioavailability in paddy soils. As shown in Fig. 1, all treatments improved SOM by different degrees. The L, LP1 LP2, LP3, and LP4 treatments had no significant difference with CK treatment. The increase rate of SOM in M treatment was the highest (8.2 ± 2.3%), followed by LM treatment (8.1 ± 4.1%), both of them were significantly higher than other treatments. The manure compost contained organic humus and other organic materials, which was important to improve soil fertility and adsorb heavy metals in soil.

The cation exchange capacity was a critical index representing the Cd adsorption capacity of soils. It could be seen from Fig. 1 that the improvement percentage of CEC in CK treatment was only 1.8 ± 2.0%, which was the least among all treatments. The M treatment increased CEC by 5.7 ± 5.6%, but it had no significant difference with CK treatment. Each of the lime and passivators treatments improved CEC by more than 19%, and the LP4 treatment made the biggest increase (38.9 ± 5.0%). These results indicated both lime and passivators could effectively improve soil CEC, while manure compost had a limited effect.

Effects of amendments on soil Cd content and fraction

The amendments had little effect on the soil total Cd content. Apart from the lime treatment, the change rates of total Cd in other treatments were less than 5%, and there was no significant difference among treatments.

The acid-extractable Cd was considered the most active fraction of Cd in soil, while the residual Cd is stable and had a low bioavailability. It could be seen from Fig. 2 that the acid-extractable fraction accounted for almost half of the total Cd, indicating the paddy soils had a certain risk of Cd migration to rice plants. The amendments promoted acid-extractable Cd transformed to reducible fraction and residual fraction. Compared with CK, lime alone and combined with manure or passivators significantly improved the reduction effect on soil acid-extractable Cd. LP4 treatment reduced the acid-extractable Cd most, with an average reduction percentage of 13.9 ± 0.3%, followed by LP2 treatment with an average reduction of 11.3 ± 3.4%. There were no significant differences in the reduction rate of exchangeable Cd between the M treatment and the control (CK). These results indicated lime alone and its combinations with passivators could effectively reduce acid-extractable Cd content, while manure compost treatment had relatively few effects on acid-extractable Cd.

Fig. 2
figure 2

Effect of amendments on the distributions of Cd in the acid-soluble, reducible, oxidizable, residual fractions of paddy soil

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Compared with before repairing, the reducible Cd (Fe–Mn oxide-bound Cd) was improved in all amendment treatments, whereas that in CK was decreased. In CK, the organic acids secreted by rice root possibly caused the release of iron and manganese oxide-bound Cd and promote its transformation to other forms. Manure compost, lime, and passivators combinations increased reducible Cd by 5.4–28.1%, with LP3 treatment having the most significant effect.

No marked changes in the oxidizable Cd contents were observed after treatment in CK, LP3, and LP4 groups. L and M treatments increased oxidizable Cd by 26.1% and 26.7%, respectively. All amendments promoted the increase of residual fraction, except for the manure compost alone treatment. The LP4 treatment improved residual fraction by 66.2%, these results indicated applying amendments enhanced the role of residual fractions in Cd forms.

Effects of combinations of amendments on Cd accumulation and translocation in rice

As shown in Fig. 3 that compared with CK treatment, the amendments decreased the BCF of rice root, stem, rice husk, and grain by 7.3–36.4%, 12.5–55.8%, 12.1–9.0%, and 32.5–71.3% respectively. Among all treatments, the LP4 treatment had the best effect on decreasing Cd BCF in each rice organ. The grain BCF of all amendment treatments were lower than CK, while the LP1, LP2, LP3, and LP4 treatments showed a significant level. The decrease of root BCF in LP4 treatment was largest, implying amendments inhibited the migration of Cd from soil to roots, thus reducing the Cd translocation from underground parts to edible parts. The order of the decrease of rice grain Cd was as the following: LP4 > LP1, LP2, LP3 > L > L, LM > M > CK. It could be seen that the combination of lime and passivator 4 had a great effect on reducing Cd enrichment in each part of rice, followed by lime combined with other passivators. Translocation factors were calculated at the same time, there was no significant difference between CK and other treatments for Cd translocation in rice plants.

Fig. 3
figure 3

Effect of amendments on the BCF of rice root (a), stem (b), husk (c), and grain (d). Error bars indicate standard deviation. Different letters above the bars indicate a significant difference (p < 0.05) among amendments

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Effects of amendments on rice grain yields

As shown in Fig. 4, compared with CK, the application of amendments increased rice grain yields of Zhongzheyou 1, Yixiangyou 2115, and Yuxiang203 cultivars by 15.8–27.6%, 8.1–15.9%, and 5.1–18.4%, respectively. Among the various amendments, the LM treatment obtained the highest yields in Yixiangyou 2115, and Yuxiang203 varieties, and that was M treatment in Zhongzheyou 1 variety. Despite the combined application of passivators and lime having a positive effect on rice yield, the increase of grain yield did not reach a significant level. The L and LM treatments in Zhongzheyou 1 rice cultivar were significantly higher than CK. These results showed that the manure compost could effectively improve rice grain yield, which might be due to the rich nutrient elements in it, alleviating the phytotoxicity of Cd to rice.

Fig. 4
figure 4

Effect of amendments on the rice grain yield. Error bars indicate standard deviation. Different letters above the bars indicate a significant difference (p < 0.05) among rice cultivars

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Discussion

Effects of amendments on the distributions of Cd in soil

The results showed that amendments effectively reduced soil acid–extractable Cd contents and Cd accumulation in rice, meanwhile increased soil pH, CEC, reducible Cd, and residual Cd. Lime, manure compost, and the four kinds of passivators used in this experiment contained organic anions and mineral components, such as carbonates, oxides, and hydroxides. These components are alkaline materials, which could neutralize soil hydrogen ions and increase pH, promoting the conversion of Cd from acid-solvable fraction to immobile fraction (Chen et al. 2018b). At the same time, the increasing pH enhanced the negative charge of paddy soil, which could be complexed with Cd2+ ions to Cd (OH)2 or form precipitation of CdCO3, thus binding Cd to soil colloids and reducing Cd bioavailability (El-eswed 2020). In this study, the LP4 treatment resulted in the biggest increase in soil pH, it was related to the high contents of CaO in passivator 4, causing the decrease of soil acid–extractable Cd and reducing Cd accumulation.

Except for lime, the manure compost and passivators also contained a certain content of organic matter. The dissociation of carboxyl and phenolic groups in organics also improved paddy soil pH (Hamid et al. 2020b). Meanwhile, ligands and functional groups in organics could bind Cd by metal-matter complex (Laurent et al. 2009). The carbonates, sulfides, and hydroxides of humus in organics formed precipitation with Cd ions and reduced Cd mobility (Thanh et al. 2016). However, the dissolved organic matter of organics could compete with Cd2+ ions on soil surface adsorption sites, and the organics released the low molecular organic acids, which could make chelates with Cd, enhancing Cd absorption by rice (Qiao et al. 2022; Xu et al. 2022; Zhu et al. 2019). It might be one of the reasons why the effects of LM and parts of passivators treatments were inferior to the lime alone treatment.

The cation exchange capacity was an indicator of good adsorptive capabilities via ion exchange which affected the translocation of heavy metals in the soil-rice system (Harvey et al. 2011). The dissolved Cd ions were exchanged with other divalent cation ions with similar charges and bond characteristics (Hamid et al. 2020b). Amendments with high CEC, which promoted the ion exchange process, had a strong sorption ability due to hydrogen ion deprotonation and subsequent exchange (Pan et al. 2009). In this study, the calcium, magnesium, silicon ions, and organic functional groups in amendments enhanced soil CEC, competing with Cd ions for uptake site in rice roots.

The amendments consisted of various essential elements, such as calcium and magnesium, which were antagonistic to cadmium. Moreover, these essential elements improved the rice growth conditions and reduced Cd toxicity by adjusting rice physiological functions and alleviating reactive oxide species caused by Cd stresses, improving photosynthesis and enzyme activity in rice plants (Kanu et al. 2019).

The passivator 1 contained more than 45% of silicon-sulfhydryl groups. Previous research showed sulfhydryl grafted palygorskite had an excellent immobilization effect and effectively reduced gain Cd contents (Wu et al. 2022). The soil available sulfur contents could be increased with the application of silicon-sulfhydryl groups. The more sulfur contents facilitated the formation of cadmium sulfides (CdS) during the flooding period, thus decreasing Cd bioavailability. Researchers put forward novel mechanisms: the voltaic effect among chalcophile trace metals possibly promotes the oxidative dissolution of Cd sulfides subsequently drained; meanwhile, the free radicals produced from the oxidation of ferrous sulfides could promote the remobilization of cadmium (Huang et al. 2021a, b). It might be the reason that the effect of LP1 treatment on acid-extractable Cd was not apparent as others.

The passivator 4 contained a small amount of Fe (OH)2, previous studies found that the mobilization of Cd upon soil drainage was related to the release of Cd from Fe–Mn (oxyhydro)oxides to exchangeable fraction, Fe–Mn (oxyhydro)oxides played important roles in controlling the mobilization of Cd in paddy system (Wang et al. 2019). The iron oxides in passivator 4 provided more absorption sites, improving the transformation of Cd from exchangeable fraction to Fe–Mn oxide fraction. Therefore, the LP4 treatment obtained the best passivating effect among the eight treatments.

Effects of amendments combinations on soil properties and rice yields

All treatments displayed an increase in grain yield. One of the reasons was the amendments improved soil fertility for rice plants to respond to Cd stress. The amendments application in soil increased the SOM and other mineral nutrients such as Ca, Mg, and Si. Moreover, manure compost and the four kinds of passivators possibly contained abundant microorganisms, which might improve the abundance of soil microorganisms and enzyme activities. Studies found that the combined use of lime and biochars increased the activities of soil urease and invertase (Lu et al. 2015). With these microbes and enzymes, rice plants could effectively absorb nutrients from the soil and increase grain yields.

Another reason was the application of amendments reduced the Cd toxic effect on rice. Previous studies found that Ca, Mg, and Si ions could compete with Cd for transporters and channels owing to the chemical similarity, thus reducing Cd absorption by root and translocation in rice plants (Wang et al. 2021b; Zhang et al. 2019). Our previous study found both Ca and Mg had an apparent antagonism with Cd in different parts of the rice plant, and the antagonism was more obvious in the high Cd stress treatments (Li et al. 2022b). With the similar surface charge and ionic radius, Ca and Mg competed with Cd for the transport channels of plant cell membranes (Treesubsuntorn and Thiravetyan 2019; Wang et al. 2021b). Despite contained in passivator 4, Si was not an essential element for rice plants, a previous study found it could ameliorate the toxic effects of excessive heavy metals and essential elements on plants (Vaculik et al. 2020). Meanwhile, Si could decrease Cd concentrations in the symplast and apoplast sap of rice root by enhancing Cd binding to the cell walls, thus inhibiting Cd translocation to the aboveground parts (Wu et al. 2018). The organic functional groups contained in organic materials could be complexed with Cd, leading to the decrease of Cd phytoavailabe fraction and Cd toxicity. The Ca, Si, Mg, and organic carbon in passivators apparently inhibit the translocation of Cd in rice, but the interaction of the three elements should be further studied.

Conclusion

The use of lime, manure compost, and the combination of lime and manure or passivators effectively improved soil pH and CEC. The lime and passivator treatments significantly decreased the soil acid-extractable Cd content. Moreover, amendments reduced the absorption and accumulation of Cd in each part of the rice plants. Amendments also increased grain yield, which was caused by the enhancement of soil fertility and the reduction of negative effects of Cd stress. Totally, the high pH and some nutrients contained in amendments were important factors to reduce bioavailable Cd and decrease Cd content in rice. This field study verified the feasibility of remediating Cd-contaminated paddy field by the amendments combination of lime and passivators.