Differences in the blocking and controlling effects of nine formulated soil conditioners on cadmium content in rice and soil based on experiments in middle and late rice

Heavy metal contamination has emerged as a critical constraint on agricultural productivity and food security, particularly in Cd-contaminated paddy soils. The application of low-cost soil conditioners offering passivation-based immobilization of heavy metals represents a viable remediation strategy. In this study, four amendments (lime, Ca–Mg–P fertilizer, zeolite, and sodium humate) were systematically evaluated through a uniform experimental design (U9 matrix) comprising nine treatments under field conditions. This study found that the optimal soil conditioner formulation (T3) is lime/Ca–Mg–P fertilizer/sodium humate = 1:5:0.58. Among all treatments, T3 exhibited superior remediation efficacy, achieving reductions of 33.81% and 29.75% in soil bioavailable Cd during middle and late rice cultivation, respectively. This treatment reduced grain Cd concentrations by 59.69% and 51.26% in middle and late rice crops, respectively, while simultaneously enhancing grain yields by 15.2% and 12.5%. Furthermore, T3 application significantly decreased Cd enrichment factors (BCF) and translocation factors (TF) in rice organs, particularly in roots (37.22%), stems (47.07%), leaves (52.47%), and grains (59.69%) of middle rice. Finally, soil conditioner application can reduce the available Cd content of acidic Cd-contaminated rice soils in the middle and late rice seasons and regulate the enrichment factor and the translocation factor of rice soil Cd in various rice organs. The optimized amendment formulation (lime/Ca–Mg–P fertilizer/sodium humate = 1:5:0.58) effectively regulated Cd bioavailability through pH-mediated speciation changes and competitive ion interactions, demonstrating exceptional potential for achieving dual objectives of yield enhancement and food safety assurance in acidic Cd-contaminated paddy systems.

1 Introduction

With the continuous development of industrialization and urbanization in China, soil health problems have become increasingly serious because of industrial and agricultural production. Soil heavy metal contamination was particularly a problem due to the irrational use of sewage irrigation and agricultural products as well as the discharge of large quantities of pollutants. It was reported that nearly 20 million hm² of arable land is polluted by Cd, arsenic, chromium, lead, and other heavy metals, accounting for about one-fifth of the total arable land area (Hu et al., 2014). The China National Ecological Quality Profile 2019 reports that Cd was the leading element of heavy metal contamination in agricultural soils (Pei et al., 2022). In rice-producing areas of southern China, Cd contamination was a serious constraint to the country’s grain production and security. Some studies have confirmed that rice has the most abundant Cd absorption in bulk cereal crop, and it is also the main route of Cd exposure in humans (Zhao and Wang, 2020). Cd is more toxic and has a greater level of mobility and bioavailability than other heavy metal (Li et al., 2017). Therefore, treatment and remediation of Cd-contaminated acidic rice soils are urgently needed to promote the protection and sustainable development of land resources as well as to guarantee the quality and safety of agricultural products and safeguard human health.

Currently, soil conditioners have been widely adopted to remediate heavy-metal-contaminated soils due to their cost-effectiveness, operational simplicity, and proven efficacy in immobilizing toxic metals through adsorption, ion exchange, and precipitation mechanisms. Critically, these amendments achieve metal stabilization without compromising agricultural productivity (Zhang and Pu, 2011; Irin and Hasanuzzaman, 2024).—for example, humic acid liquid fertilizer significantly reduced the root to edible part transport coefficient of amaranth heavy metals (Li Ben et al., 2013). A new type of multifunctional cadmium-blocking fertilizer has also been developed and applied (Yu et al., 2025). Zhao et al. showed that humic acid fertilizer promoted rice tillering to increase yield (Saha et al., 2013). Hu et al. found that Ca–Mg–P fertilizer reduced the effective Cd content in the soil by increasing the soil pH (Wang et al., 2023). Lime is the most widely used soil conditioner. Lime application to the soil raises the pH value, promotes the formation of hydroxide precipitation and carbonate precipitation of ions such as Cd, Pd, Cu, and Zn, and enhances the adsorption of soil heavy metal ions, thus reducing the bioavailability of heavy metals in the soil (Yuxian et al., 2022). A field experiment study by Shao et al. found that continuing lime application on soil contaminated with Pb and Zn mine wastewater was effective in increasing corn yields and significantly reducing the Cu, Zn, Pb, and Cd levels in corn grains (Shao Le et al., 2010). Some studies have found that phosphorus fertilizer can alleviate the effect of lead on rice growth, and phosphorus can form antagonism with lead to reduce bioavailability. In addition, phosphorus may form phosphate precipitates with lead to reduce its accumulation in rice organs (Li et al., 2024). Clay minerals have been applied to soil heavy metal remediation because of their wide distribution, low price, high adsorption, and ion exchange capacity. Kang et al. studied the passivation effect of zeolite on heavy-metal-contaminated soil and found that if zeolite was applied at more than 50 g/kg, the availability of Pb, Cu, and Zn content in the soil showed a significant decreasing trend when compared with the control (Kang et al., 2015). Sun et al. found that sepiolite was able to alleviate the biotoxicity of Cd on spinach while ameliorating soil pH (Sun et al., 2016). Hu et al. demonstrated that sepiolite-based passivation exhibited superior remediation efficacy in Cd-contaminated acidic paddy soils across Hunan and Guangxi provinces. Specifically, sepiolite amendment reduced the bioavailable Cd fraction (0.1 M CaCl₂-extractable) by 38.7%–52.3% (P < 0.05) through pH-mediated immobilization and surface complexation mechanisms. This treatment concurrently decreased grain Cd accumulation by 41.2%–58.1% in brown rice, achieving concentrations below the Chinese food safety threshold (Hu et al., 2022). However, the current formulation of soil conditioner for soil Cd pollution was mainly a single modification. Single soil conditioners have drawbacks such as high application rates or repeated applications and the risk of causing soil structure consolidation, reduced fertility, and plant nutrient deprivation in later stages (Li-qun et al., 2009). Previous research has predominantly focused on single-component soil amendments, with limited investigations addressing multi-component formulations. Notably, the constituent ratios within composite conditioners have been identified as critical determinants of remediation efficacy, directly influencing metal speciation dynamics and plant bioavailability.

This study systematically evaluated four soil amendments (lime, Ca–Mg–P fertilizer, zeolite, and sodium humate) through a uniform experimental design (U9 matrix) to optimize their formulation for Cd immobilization in paddy systems. The objectives were threefold, namely: (1) to elucidate the Cd passivation mechanisms of amendment materials in rice rhizosphere soils, (2) to quantify Cd translocation patterns across rice organs, and (3) to establish an optimal amendment ratio balancing remediation efficacy with agricultural productivity. The derived formulation protocol provides a scientifically validated framework to achieve the dual goals of food safety assurance and sustainable soil management in Cd-contaminated agroecosystems.

2 Materials and methods

2.1 Study site description

The two study sites are located in Jin nan Village and Jie guan Village, Shang gao County, Jiangxi Province, China (28°07′ N, 115°10′ E), with an altitude of 25 m. This region has a subtropical monsoon climate, with an annual average temperature of 17.6°C and accumulated temperature of 5,400°C. The annual average precipitation was 1,718.4 mm. The average precipitation from April to June was 763.6 mm, accounting for 44% of the annual precipitation. The frost-free period was approximately 276 days, and the annual average sunshine hours is 1,668.2 h. The sunshine hours in July are the highest, with an average of 243 h. The region is rich in heat resources and has abundant precipitation, abundant sunshine, long frost-free period, and favorable climatic conditions for most crops. The test site soil was paddy soil, and the basic physicochemical properties are shown in Supplementary Table S1.

2.2 Experimental materials and design

The tested rice varieties were the main varieties suitable for planting locally, among which the medium rice variety is Weiliangyou 7713, the late rice variety is Yongyou 1538, and the basic seedlings were 18,000 roots. Lime was provided by Shanggao Xuelin Lime Powder Factory, with calcium carbonate content ≥95%. Zeolite was provided by Zhejiang Shenshi Mining Co., Ltd., and the particle size was 40–100 mesh; Ca–Mg–P fertilizer was provided by Hubei Jinmingzhu Chemical Co., Ltd. (P₂O₅ ≥12%, CaO ≥20%, MgO ≥4%, and SiO₂ ≥20%). Sodium humate was provided by Anhua Biotechnology Co., Ltd. The sodium humate content was ≥50%, pH = 8.7. Compound fertilizer (N–P₂O₅–K₂O, 15-15-15) was provided by China Salt Anhui Hongsifang Co., Ltd. Urea (N 46%) was provided by Haoyuan Chemical Co., Ltd. Potassium chloride (K₂O 60%) was provided by Sinochem.

The experimental design was based on the uniform experimental design (UED) methodology proposed by Kaitai Fang, utilizing regression-derived models for predictive analysis and parameter optimization. Field trials employed lime, Ca-Mg-P fertilizer, zeolite, and sodium humate as experimental factors. Using a uniform design method, nine experimental groups with varying concentrations were established. The control treatment (CK) received no soil conditioners, resulting in a total of ten treatments (see Table 1). Each treatment was repeated thrice for a total of 30 plots. The area of each plot was 30 m² (5 m × 6 m), with random group arrangement. The plots, separated by ridges 0.45 m in depth and 0.5 m in width, were wrapped in plastic film and drained independently. Field management of the plots was consistent with the farmers’ customary fertilization treatments.

Table 1

Table 1. Uniform design table of U₉(9⁴).

The same amount of fertilizer and application method was applied to the middle and late rice trials. The application rates of N, P, and K were 180, 90, and 135 kg·hm^-2 as urea and compound fertilizer in the tillering stage; and 75 kg·hm^-2 of potassium chloride and 75 kg·hm^-2 of urea were applied at the stage of differentiation of young spikes. All soil conditioner materials were used as base fertilizer, and all fertilizers were applied on the surface.

2.3 Sample collection and determination

Plant samples were collected at the mature rice stage using the single-hit, single-harvest, single-count method, and the number of effective spikes in each plot was counted for 1 m². According to this effective number of spikes, five root and stem rice samples were collected randomly within the plots. A total of 20 soil samples were collected from each plot using the grid method and then mixed into one soil sample. After the collection of rice plant samples according to the root, stem, leaf, and grain, the four parts were successively washed with tap water and subjected to deionized water cleaning. The roots, stems, and leaves were dehydrated at 105°C for 30 minutes, then cooled to 40°C and left for 12 hours. Rice spikes were roasted after natural air-drying, and all plant samples were crushed, sieved, and placed in dry self-sealing bags for spare parts. The determination of total Cd from different parts of the plant refers to the determination of Cd as detailed in the National Standard for Food Safety (GB 5009.15-2014). Soil samples were collected and air-dried naturally. During the air-drying period, roots and other debris in the soil were removed, and large soil pieces were broken into small pieces, completely air-dried, crushed, ground individually to pass through 2-mm (10 mesh) and 0.149-mm (100 mesh) nylon mesh sieves, and then put into self-sealing bags for spare. The total Cd content in soil was determined by 2:2:1 (v:v:v) HNO₃/HClO₄/HF digestion, and extract solution was determined using an atomic absorption spectrophotometer. The other soil physicochemical properties determined refer to details in “Soil Agrochemical Analysis”. During sample testing and analysis, a blank control and parallel samples were used for data quality control.

For the soil pH, a soil–water ratio of 1:2.5 was adopted using the acidimeter method, soil organic matter was determined using potassium dichromate plus heat capacity method, soil total nitrogen was determined using chemical nitrogen method, and soil alkali nitrogen was determined using alkali dediffusion method. The available phosphorus was extracted by using the sodium bicarbonate leaching–molybdenum antimony colorimetric method. Soil fast potassium was determined by ammonium acetate leaching–atomic absorption spectrophotometry.

2.4 Data analysis

The enrichment coefficient (BCF) was derived as Cd content of organ/total Cd of soil and the transport coefficient (TFA-B) as Cd content of organ B/Cd content of organ A. The enrichment coefficient of Cd was divided into BCF root, BCF stem, BCF leaf, and BCF grain.

The reduction Cd rate was calculated as (mean Cd content of control group - mean Cd content of treated group)/mean Cd content of control group * 100.

Test data and charts were processed by using Excel 2019 and Origin Pro software, and the correlation analysis (Pearson correlation) and significant difference test (Duncan method) were conducted by using SPSS 22.0 software.

3 Results

3.1 Effects on Cd, pH, EC, and organic matter of different treatments

Figure 1 shows that the soil conditioner significantly reduced the available Cd content in the soil (P < 0.05), and the passivation effect of each treatment on soil Cd in middle and late rice was T3 > T9 > T6 > T5 > T2 > T1 > T4 > T7 > CK treatments, in which T3 and T9 had the most significant passivation effect on organic Cd in the soil. In the Jin nan sample (medium rice), the available Cd content was 33.81% (T3) and 29.89% (T9) compared with the control. In the Jie guan sample (late rice), the soil Cd content decreased by 29.75% (T3) and 20.66% (T9) compared with the control. From the effect of each treatment, the passivation effect of soil conditioner on medium rice soil was higher than that of late rice soil. In the Jin nan sample (medium rice), the soil conditioner did not change the soil organic matter content (P > 0.05). In the Jie guan sample (late rice), except for T1 and T2, the soil organic matter content was not significantly different from the control (P > 0.05). Soil organic matter content was not a major factor affecting the soil available Cd content.

Figure 1

Figure 1. Effect of different treatments on available Cd and organic matter in rice soil at middle and late seasons. (a) Jin nan (middle rice) soil available Cd condition; (b) Jin nan (middle rice) soil organic matter condition; (c) Jie guan (late rice) soil available Cd condition; (d) Jie guan (late rice) soil organic matter condition. Different lowercase letters indicate significant differences between different groups according to one-way ANOVA with Duncan tests (P < 0.05). n = 3 per group.

Compared with the control treatment, some treatments (except T1) significantly increased the soil pH value (Figure 2; P < 0.05). T8 and T9 increased the most by 0.6 pH units, and T9 increased to 1.01 pH units. This showed that the addition or non-addition of lime to the soil conditioner was a key factor influencing soil pH. Compared with the control, T9 treatment significantly increased the soil EC (P < 0.05), while the other treatments were not significantly different.

Figure 2

Figure 2. Effect of different treatments on pH and EC in rice soil at middle and late seasons. (a) Jin nan (middle rice) soil pH; (b) Jin nan (middle rice) soil EC; (c) Jie guan (late rice) soil pH; (d) Jie guan (late rice) soil EC. Different lowercase letters indicate significant differences between different groups according to one-way ANOVA with Duncan tests (P < 0.05). n = 3 per group.

3.2 Effect on rice organs’ Cd content of different treatments

In the Jin nan (medium rice) and the Jie guan samples (late rice), the soil conditioner (except T7) reduced the Cd accumulation in rice organs (Figure 3). In terms of the passivation effect of applied soil conditioners on rice Cd, the result was T3 > T9 > T5 > T6 > T8 > T2 > T1 > T4 > T7 > CK. The most significant effect was in the T3 treatment. The T7 treatment had the worst passivation effect on rice soil Cd. This indicated that zeolite was not effective in reducing rice organ Cd content. Soil conditioners also had significant effects on various nutrient organs at the maturity stage of rice. In the Jin nan samples (medium rice), all soil conditioners significantly reduced the Cd content of stems (24.24%–47.07%) and leaves (5.77%–52.47%) compared with the control. The T4 and T7 treatments had the worst passivation effect Cd in rice stems and leaves. The T3 treatment was the best for the passivation of Cd in rice stems. The T3 and T9 treatments were the best for the passivation of Cd in rice leaves. In rice roots, the T3, T5, T6, T8, and T9 treatments significantly reduced Cd content in rice roots (P < 0.05), but the T1, T2, T4, and T7 treatments had no significant effect on Cd content in rice roots (P > 0.05). Except for the T4 treatment, the other soil conditioner treatments could effectively reduce the Cd content in rice grains (P < 0.05). Compared with the control, the T3 treatment reduced the Cd content in rice roots, stems, leaves, and grains by 37.22%, 47.07%, 52.47%, and 59.69%, respectively.

Figure 3

Figure 3. Effect of different treatments on cadmium content in various organs of rice at maturity in middle and late seasons. (a) cadmium content in various organs of rice in Jin nan (middle rice); (b) cadmium content in various organs of rice in Jie guan (late rice). Different lowercase letters indicate significant differences between different groups according to one-way ANOVA with Duncan tests (P < 0.05). n = 3 per group.

In the Jie guan samples (late rice), all soil conditioners significantly reduced the Cd content in rice stems, leaves, and grains (P < 0.05). The T7 and T8 treatments showed the worst Cd passivation in rice stems, leaves, and grains. The T3 treatment showed the best Cd passivation in rice stems, leaves, and grains. The results showed that the T3 treatment was the most effective in reducing Cd in all rice organs, while the zeolite-based T4 and T7 treatments were the least effective.

3.3 Correlation between soil Cd content and rice organs’ Cd content in the mature stage

There were different degrees of correlation between the effective Cd content in the soil and the Cd content in different mature rice organs (Supplementary Table S2). In both of the Jin nan samples (medium rice) and the Jie guan samples (late rice), the soil available Cd content was positively correlated with rice organs, and the correlation reached a highly significant level (P < 0.01). In the Jin nan (medium rice) samples, there was a positive correlation between different rice organs, with a significant correlation between roots and grains (P < 0.05) and a highly significant correlation between other organs (P < 0.01). In the Jie guan (late rice) samples, there was a highly significant positive correlation between all of the organs of rice (P < 0.01). This indicated that reducing the soil available Cd content could effectively reduce Cd accumulation in various rice organs. Therefore, reducing the soil available Cd content can be an important means of remediating Cd-contaminated farmland and achieving safe rice production.

3.4 Effect of different treatments on rice Cd transport coefficient and enrichment coefficient

3.4.1 Cd transport coefficient

The transport coefficient indicated the ability of heavy metals between different organs, and the larger the transport coefficient, the easier the heavy metal transport between plant organs. Table 2 shows the effects of soil conditioner application on the various organs’ Cd transport coefficient. In the Jin nan (middle rice) samples, the soil conditioner had not affected the TF (stem–root), TF (grain–root), TF (grain–stem), and TF (grain–leaf) (P > 0.05). Compared with the control, the soil conditioner reduced the Cd transport coefficient of TF (stem–root), TF (leaf–root), and TF (grain–root). The T2, T3, and T9 treatments significantly reduced the Cd transport coefficient of TF (leaf–root) (P < 0.05). The T3 and T9 treatments significantly reduced the Cd transport coefficient of TF (leaf–stem) (P < 0.05).

Table 2

Table 2. Effects of soil conditioners on cadmium transport coefficients of rice organs in middle and late seasons.

In the Jie guan (late rice) samples, the Cd transport factor of TF (grain–stem) and TF (grain–leaf) was not affected by the soil conditioner (P > 0.05). Compared with the control, the T6 treatment significantly reduced the Cd transport coefficient of TF (stem–root) (P < 0.05). The T7 and T8 treatments decreased the TF (grain–root) (P < 0.05). The T5 treatment increased the TF (leaf–root) (P < 0.05), and the T6 treatment increased the TF (leaf–stem) (P < 0.05). There was no significant difference in the Cd transport coefficient between TF (stem–root), TF (leaf–root), TF (grain–root), and TF (leaf–stem) (P > 0.05). This indicated that the soil conditioner mainly reduces the transport coefficient between roots and shoot organs, but the transport coefficient between shoot organs was not affected.

3.4.2 Enrichment coefficient

The enrichment coefficient was an important indicator used to describe the ability of the plant organs to absorb soil heavy metals (Table 3). Soil conditioners have different effects on Cd enrichment coefficient in rice organs. In the Jin nan (middle rice) samples, the soil conditioner application reduced the Cd enrichment coefficient of BCF roots, BCF stems, BCF leaves, and BCF grains. Compared with the controls, the T3, T5, T6, T8, and T9 treatments significantly reduced the Cd enrichment coefficient of BCF roots (P < 0.05). All treatments significantly decreased the Cd enrichment coefficient of the BCF stems compared with the control (P < 0.05). The T4 treatment had no effect on the Cd enrichment coefficient of rice BCF grains, while other treatments significantly reduced the Cd enrichment coefficient of BCF grains (P < 0.05).

Table 3

Table 3. Effect of soil conditioners on cadmium enrichment coefficients of various organs of rice in middle and late seasons.

In the Jie guan (late rice) samples, all treatments significantly reduced the Cd enrichment coefficient of BCF stems and BCF grains (P < 0.05). The T7 treatment had no effect on the Cd enrichment coefficient of BCF roots (P > 0.05), while other treatments significantly reduced the Cd enrichment coefficient of BCF roots (P < 0.05). In addition, the T4, T5, and T7 treatments similarly affected the Cd enrichment coefficient of BCF leaves (P < 0.05). This showed that the soil conditioner effectively reduced the enrichment coefficient of the aboveground rice organs.

3.5 Fit model

Based on the soil conditioner composition and the experimental results of Cd content in grains, a multiple one-step regression analysis was conducted using the experimental data to derive the equations for the relationship between Cd reduction rate of grains under different soil conditioner treatments and the factors as follows:

Middle rice Y = 43.60 - 0.081 * X1 + 0.062 * X2 - 0.047 * X4 (F-value: 13.1915, R² = 0.887828, P = 0.0082)

Late rice Y = 66.13 - 0.17 * X1 + 0.038 * X2 - 0.075 * X4 (F-value: 27.6375, R² = 0.943119, P = 0.0015)

The optimized formula is as follows: lime—1,500 kg·hm^-2, Ca–Mg–P fertilizer—7,500 kg·hm^-2, and sodium humate—870 kg·hm^-2. The lime/Ca–Mg–P fertilizer/sodium humate ratio was 1:5:0.58. The regression equations for middle and late rice were significant. The errors between the observed and fitted values were very small and accurate, as shown in Supplementary Table S3.

3.6 Influence of different treatments on rice yield and its components

The soil conditioner application had a significant effect on the rice yield (Supplementary Table S4). In the Jin nan (medium rice) sample, there were no significant differences (P > 0.05) in the number of effective grains, number of empty grains, and thousand grain weight of rice between the soil conditioner and control treatment. Compared with the control, the T3 and T9 treatments significantly increased the rice solid grain number (P < 0.05), where T3 and T9 increased by 15.3% and 13.5%, respectively. The T1, T2, T3, T4, and T5 treatments increased the rice fruiting percentage (P < 0.05), with T5 being the most effective with an increase of approximately 3.48%. Theoretical yield and actual yield have the same variation trend. The soil conditioner application could improve the rice theoretical and actual yield but showed no difference in T1, T2, T4, T5, T6, T7, and CK (P > 0.05), and T3, T8, and T9 significantly improve the rice theoretical and actual yield (P < 0.05). The T3 treatment had the highest actual yield (15.2%), followed by T9 and T8 at approximately 10.8% and 8.0%. In the Jie guan (late rice) samples, there was no significant difference (P > 0.05) in the number of empty grains, fruiting rate, and thousand grain weight between the soil conditioner treatments and the control. Both T3 and T9 treatments increased the rice effective grain, with an average increase of 5.33 and 3.67 compared with the control (P < 0.05). Compared with the control, the T3 and T9 treatments significantly increased the rice real grain number and theoretical yield (P < 0.05).

4 Discussion

4.1 Regulatory effects of soil conditioner application on soil available Cd and rice organ Cd content

The application of soil conditioner significantly reduced soil available Cd concentrations by promoting the transformation of Cd from labile to stable fraction (Li et al., 2018). This reduction may be attributed to the decreased exchangeable acidity and aluminum content, which altered Cd speciation in soils (Tong et al., 2023). Furthermore, root pressure and plant transpiration facilitated Cd translocation from rhizosphere soil to aerial plant organs, underscoring the critical role of available Cd reduction in mitigating Cd accumulation in crops (Yang et al., 2020). Our results demonstrated that soil conditioner application elevated soil pH (Figure 2), consequently decreasing the available Cd content. Elevated pH enhances the hydroxide precipitation of heavy metals and increases negative surface charges on clay minerals, metal oxides, and organic matter, thereby improving metal ion adsorption capacity while reducing metal mobility (Hamid et al., 2020). Among all treatments, T3 exhibited the highest efficacy in reducing both soil available Cd and Cd accumulation in rice organs (Table 2). Notably, T3 did not contain the zeolite content (0%) compared with T7 (61.5% zeolite), indicating zeolite’s inferior Cd immobilization performance relative to other components. Although zeolite (pH 6–8) can moderately raise soil pH in acidic conditions, its pH adjustment and Cd remediation capacities were substantially weaker than those of lime, Ca–Mg–P fertilizer, and sodium humate. Zeolite reduces the available cadmium content in soil by adsorption. We speculate that adsorption sites on the zeolite surface are occupied by H⁺ in acidified soil environments, preventing Cd²⁺ from being adsorbed by the zeolite. Additionally, under flooded conditions, Cd ions adsorbed by zeolite may be released into the soil environment. Periodic adsorption–desorption cycles may lead to a decline in the adsorption capacity of zeolite (Nuić et al., 2019; Ma et al., 2022). In this study, the highest proportion of Ca–Mg–P fertilizer was highest in the T3, T6, and T9 treatments, which indicated that it plays an important role in reducing Cd content and increasing rice yields. On the one hand, Ca–Mg–P fertilizer was chemical alkaline fertilizer (main components: Ca₃(PO₄)₂, CaSiO₃, and MgSiO₃). Application of Ca–Mg–P fertilizer increased the soil pH while introducing elements such as Ca, Mg, Si, and P to promote rice growth and development (Vaca et al., 2011). On the other hand, the introduced elements will compete, antagonize, and passivate with Cd, thus significantly reducing the accumulation of Cd in rice organs (Tan et al., 2020).

Using the uniform design method, the effects of lime, Ca–Mg–P fertilizer, zeolite, and sodium humate addition on the Cd reduction rate in rice grains were investigated, and the data were analyzed by stepwise regression method to obtain a multinomial primary regression model of the relationship between them, which was verified to have a high degree of fit. According to the design of nine soil conditioners, T3 had the optimal grain Cd reduction formula in early rice and late rice, and the materials were as follows: lime 750 kg·hm^-2, Ca–Mg–P fertilizer 9,000 kg·hm^-2, zeolite 0 kg·hm^-2, and sodium humate 4,500 kg·hm^-2. This study proved that it is feasible to optimize the formulation of Cd-reducing soil conditioners for rice grains by using the uniform design method, which can provide a theoretical basis for the production of Cd-reducing soil conditioners.

4.2 Mechanisms of soil conditioner effects on Cd enrichment and transport in rice

It has been shown that rice organs differ in their ability to enrich and transport Cd. Soil conditioner application can reduce the transport coefficient and enrichment coefficient of rice organs. Soil conditioners contain a variety of foreign elements that can form an antagonistic effect with soil Cd, further reducing the uptake and Cd translocation by plants (Wang et al., 2021). In this study, the T3 treatment was effective in reducing the soil Cd transport coefficients and enrichment coefficients among rice organs. The high proportion of Ca–Mg–P fertilizer reduced the uptake of Cd by the crop root system and thus reduced the distribution of Cd in the plant to the grains. In addition, Mg was an essential element for plant growth and development, and the absence of Mg affects plant growth metabolism. Studies have found that Mg can increase Cd absorption and transport in rice seedlings (Kikuchi et al., 2008). Therefore, the introduction of Mg into Cd-exceeding soils inhibited the uptake and accumulation of rice Cd. The T9 treatment has a relatively high content of lime, which contains a large amount of Ca ions that improve soil pH and affect the migration of Cd transformation (Chen et al., 2024). Some studies have shown that Ca protects the integrity of cell wall and plasma membrane, blocks the entry of Cd into rice roots, and reduces the uptake of Cd in roots. Meanwhile, Ca regulates Cd translocation in rice by modulating the expression of OsNRAMP5 and OsHMA2 transporter genes, thereby regulating Cd translocation in rice (Rizwan et al., 2012). Ca ions in plant root cell walls were able to exchange with Cd ions and compete with Cd ions for binding sites on root transporters, thus reducing the concentration of neutralizing Cd ions of root (Sheng et al., 2022). In this experiment, T3 and T9 could significantly improve the soil pH, reduce the soil available Cd, and introduce a large number of P, Si, Ca, and Mg elements. Through the antagonism between different elements, the fixation and transformation effect inhibited the enrichment and transport of Cd in various rice organs (Liu et al., 2023).

4.3 Mechanisms of soil conditioner composition on rice yield

Soil pH was a direct factor affecting rice yield, and some studies have found that the acidic soil rice yield was significantly lower than that of neutral soil. Soil peracid was an important factor limiting the rice yield improvement (Chen et al., 2017). It was found that exchangeable Al and exchangeable Ca in acidic soil jointly drive the response mechanism of crop yield to soil pH, and the critical value of exchangeable Al/Ca was 0.108 (Thomas et al., 2015). The soil conditioners formulated in this study all contained Al and Ca ions, and soil conditioners were able to regulate the exchangeable Al/Ca values of soil. Soil conditioner application could improve the soil pH value, which was beneficial to the increase of rice yield. In addition, our study revealed that soil conditioner increased the effective number of grains and the fruiting rate and reduced the number of empty grains in rice to some extent compared with the control, which, in turn, increased the theoretical and actual yields. Compared with the control rice yield, the T3 actual rice yield increased by 15.2% (early rice) and 12.5% (late rice), respectively. The phosphorus in Ca–Mg–P fertilizer can increase the plant stress resistance and improve rice yield and quality. Phosphorus was also an important component of many intracellular compounds and an important element involved in plant photosynthesis, energy transportation, and metabolism (Chang and Sung, 2004). Some studies had shown that magnesium fertilizer increased the grain yield of soybean, early rice, late rice, corn, and millet (Prasad, 2009). Therefore, Ca–Mg–P fertilizer increased the soil magnesium content and then increased the rice yield. In addition, the proportion of Si in Ca–Mg–P fertilizer was approximately 25%–40%, and Si content was considered to have effectively improved the photosynthesis of the plant, the resistance to fall, and the resistance of the plant. Some studies have demonstrated that augmented silicon fertilization significantly reduces empty grain formation through silicon-mediated reinforcement of cell wall structures while enhancing filled grain development and grain filling efficiency via phytohormonal regulation during panicle differentiation, thereby contributing to improved rice yield outcomes (Duan et al., 2007). These findings exhibited congruity with the yield dynamics observed under the T3 treatment. In conclusion, the T3 treatment based on Ca–Mg–P fertilizer was effective in preventing Cd and increasing rice yield.

5 Conclusion

Among the nine treatments using a uniform experimental design, the T3 treatment had the best effect in reducing Cd content and increasing rice yield, which could significantly increase soil pH and reduce the available Cd content of the soil compared with the control treatment as well as reduce the Cd content in the grains of middle and late rice by 59.69% and 51.26%, respectively, and increase the yields of rice by 15.2% and 12.5% by decreasing the enrichment coefficients and the transport coefficients, respectively. The best formulation was optimized based on the regression model, lime/Ca–Mg–P fertilizer/sodium humate in the ratio of 1:5:0.58. Therefore, under the conditions of this experiment, the use of the optimal formulation of soil conditioner can effectively reduce the Cd uptake of various rice organs and realize the high-yield cultivation and safe utilization of rice in acidic Cd-polluted farmland.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Author contributions

JX: Writing – original draft. HH: Conceptualization, Writing – original draft. JJ: Writing – review & editing. ZL: Writing – original draft, Data curation. XJL: Writing – original draft, Methodology. YL: Writing – original draft, Methodology. XML: Writing – original draft, Investigation. XH: Writing – original draft, Investigation. LQ: Resources, Writing – original draft. QR: Writing – original draft, Formal analysis. LZ: Writing – original draft, Methodology.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This work is financially supported by Jiangxi Provincial Major Science and Technology R&D Special Project “Jie bang gua shuai” Project (20213AAF02026), National Natural Science Foundation of China (32160754, 32060725, 32160767), Basic Research and Talent Cultivation of Jiangxi Academy of Agricultural Sciences (JXSNKYJCRC202516), and Early-Career Young Scientists and Technologists Project of Jiangxi Province (409055638009).

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fagro.2025.1713644/full#supplementary-material

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