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Article

The Effect of Combined Application of Rhodochrosite Slag and Biochar on Cadmium Uptake in Rice

by
Jing He
1,
Zhixi Ye
1,
Fugui Chen
1,
Wentao Zhou
1,
Xin Ma
2,* and
Zhiqiang Fu
1,*
1
College of Agronomy, Hunan Agricultural University, Changsha 410128, China
2
Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(6), 1321; https://doi.org/10.3390/agronomy15061321
Submission received: 27 April 2025 / Revised: 23 May 2025 / Accepted: 26 May 2025 / Published: 28 May 2025
(This article belongs to the Section Soil and Plant Nutrition)
Browse Figures
Versions Notes

Abstract

:
Cadmium (Cd) contamination in paddy soils severely threatens rice safety and human health. Currently, the high costs and technical barriers of existing Cd remediation methods limit their development, so it’s urgent to find an economical and feasible method. Herein, the synergistic effects of rhodochrosite slag and biochar on Cd immobilization in slightly acidic Cd-contaminated paddy soils have been investigated. A field experiment with four treatments—control (CK), rhodochrosite slag (R), biochar (B), and combined rhodochrosite slag + biochar (RB)—was conducted in Hunan Province, China. Results demonstrated that RB treatment significantly increased soil pH, transferred the mobile Cd to the residual fraction, and reduced Cd availability in the soil. Cd concentrations in rice roots, stems, leaves, and brown rice decreased by 26.37%, 47.20%, 31.03%, and 51.85%, respectively, under RB treatment, achieving the lowest TF and BCF values. Furthermore, RB treatment increased rice yield by 18.73%. The synergistic interaction between biochar’s adsorption capacity and rhodochrosite slag-derived competitive ions effectively transformed Cd into stable fractions, reducing bioavailability. This study proposes a novel remediation strategy that not only enhances the Cd immobilization ability of biochar but also achieves simultaneous waste valorization and soil remediation.

1. Introduction

Cadmium (Cd) contamination in paddy soils leading to excessive Cd levels in rice has become a global issue affecting food safety and human health [1,2,3]. In recent decades, cadmium contamination in agricultural soils has become increasingly serious, with the cumulative effects of human activities, such as mining operations, metallurgical processing, wastewater irrigation, the popular application of plastic films, the widespread application of pesticides and fertilizers, as well as automobile emissions and daily life waste disposal, and a large amount of cadmium has seeped into the soil and water, posing a significant risk to the agricultural ecosystems [4,5]. Compared with other cereal crops, rice has a particularly prominent enrichment capacity for Cd [6]. The accumulation of Cd in its rice grains can be transmitted to the human body through the food chain, leading to serious problems in human organ systems such as the cardiovascular, endocrine, hematopoietic, hepatic, immune, renal, reproductive, and respiratory systems [7,8,9]. Therefore, there is an urgent need for a practicable technology to mitigate and remediate the Cd contamination of paddy soils and reduce the uptake of Cd by the rice plant and grains.
The problem of cadmium pollution in rice has received wide attention from all walks of society, and remediation techniques have emerged in an endless stream, such as chemical leaching, soil replacement method, electric restoration, thermal analysis, bioremediation restoration, agronomic regulation restoration, and so on [10]. However, these methods more or less have certain limitations, i.e., complex technology, low efficiency, poor feasibility, short duration, high economic cost, etc. Chemical immobilization (e.g., application of lime, biochar, attapulgite, sludge, etc.) is an effective remediation method to reduce the bioavailability of Cd in rice soils [11,12]. Among them, biochar is a carbon-rich substance produced through the pyrolysis of biomass, and it has attracted much attention due to its ability to sequester heavy metals through mechanisms such as surface adsorption, ion exchange, and precipitation [13,14].
However, there are several defects in the application of the biochar pyrolyzed from mono-feedstock, such as a small variety of functional groups, low porosity, and limited adsorption capacity [15]. Previous studies have shown that increasing Mn supply in hydroponic cultures reduced Cd uptake and translocation in rice [16]. Therefore, increasing the supply of competitive ions may reduce plant uptake and translocation of Cd, with Mn2+ showing the highest competitiveness with Cd2+ [17]. In pot experiments, Zhou et al. [18] found that Mn-loaded biochar reduced Cd accumulation in rice grains. In some paddy soils, low Mn content has been identified as a factor contributing to high Cd accumulation in rice [19]. Thus, combining biochar with Mn-containing compounds can create composite materials with synergistic effects, expanding the application scope and enhancing the performance of biochar. However, manganese-based materials are limited by their high cost and complex application processes. Therefore, it is urgent to develop cost-effective and easily implementable cadmium immobilization technologies.
Currently, using slag as an amendment material to restrict the migration of heavy metal contamination in soil seems to be a cost-effective method. This effectiveness stems from different operations and industries containing a variety of compounds and minerals (Fe2O3, FeO, Fe3O4, MgO, CaO, MnO, and SiO2) at varying amounts [20,21]. In a study, Liu et al. conducted research and found that the carbide slag, which contains abundant Ca, Mg, and Si, increased the soil pH value during the tillering and maturity stages of rice and reduced the available Cd content in the soil during the maturity stage [22]. Bert et al. [23] also found that Thomas basic slag application effectively immobilized lead Pb, Cd, and Zn in contaminated soils, significantly reducing their bioavailability through mechanisms such as precipitation and surface complexation. Rhodochrosite slag contains rich metal elements such as Mn, Fe, Ca, Mg, K, and Al, as well as non-metallic elements such as C, O, Si, and Fe [24]. These elements give rhodochrosite potential application value in environmental remediation, particularly in heavy metal-contaminated soils.
Therefore, this study proposes a novel and straightforward method for preparing a mixed soil remediation material comprising rhodochrosite slag and biochar. The remediation efficacy of this material on cadmium-contaminated paddy fields is investigated through four treatments: (1) control, (2) single application of biochar, (3) single application of rhodochrosite slag, and (4) combined application of rhodochrosite slag and biochar. This research examines the mechanisms of cadmium transfer and accumulation in rice-soil systems mediated by rhodochrosite slag and biochar, as well as their influence on soil cadmium retention and rice yield. This approach not only innovatively facilitates the resourceful utilization of industrial solid waste but also offers an efficient and practical technical solution for addressing cadmium pollution in paddy fields.

2. Materials and Methods

2.1. Experimental Site

The experiment was conducted in 2022 in Rongheqiao Community, Beishan Town, Changsha County, Hunan Province (113°4′0″ E, 28°26′23″ N) (Figure 1). The site is located in the East Asian monsoon region, characterized by a subtropical monsoon climate with spring temperatures of 15~25 °C, summer temperatures of 18~36 °C, and winter temperatures of 5~15 °C. Annual precipitation ranges from 1000 to 2000 mm. The basic soil properties were as follows: organic matter content 19.70 g/kg, total nitrogen 0.68 g/kg, total phosphorus 0.74 g/kg, total potassium 6.45 g/kg, available nitrogen 77.30 mg/kg, available phosphorus 15.77 mg/kg, available potassium 134.95 mg/kg, pH 5.45, and Cd content 0.89 mg/kg.

2.2. Materials

The rice variety used in the experiment was Yuzhenxiang (a medium-late maturing indica rice variety with a growth period of approximately 114 days, provided by Hunan Golden Agricultural Seed Industry Co., Ltd., Changsha, China).
Biochar Preparation: Rice straw and rice husk (provided by Henan Lize Environmental Protection Technology Co., Ltd., Zhengzhou, China) were pyrolyzed at 500 °C for 2 h under oxygen-free (vacuum) conditions. The basic properties of the biochar are shown in Table 1.
Rhodochrosite Slag: Provided by the College of Chemistry and Chemical Engineering, Central South University, Changsha, China. The elemental composition of rhodochrosite slag was determined by Changsha Research Institute of Mining and Metallurgy Co., Ltd. (Changsha, China) using ICP-OES, as shown in Table 2.

2.3. Experimental Design

Based on previous studies [25], in this research, the application rate of biochar was set at 15,000 kg/ha, and the rhodochrosite slag application rate was 2% of the biochar. The experiment was designed as a randomized block design with four treatments: CK (no application of rhodochrosite slag or biochar), B (single application of 15,000 kg/ha biochar), R (single application of 300 kg/ha rhodochrosite slag), and RB (combined application of rhodochrosite 15,000 kg/ha slag and 300 kg/ha biochar). Each treatment was replicated three times, totaling 12 plots, each with an area of 30 m2. All plots were irrigated individually, and the ridges were wrapped with plastic film. Local practices for pest, disease, and weed control, as well as water management, were followed. Rice was sown on 19 May 2022, and harvested on 20 September 2022. The transplanting density was 230,000 hills per hectare, with three seedlings per hill. Fertilizer application rates were as follows: N 150 kg/ha, P2O5 90 kg/ha, and K2O 120 kg/ha. Nitrogen and potassium fertilizers were applied in a 5:3:2 ratio for basal, tillering, and panicle fertilization, while phosphorus fertilizer was applied as a basal dose.

2.4. Sampling and Analysis

2.4.1. Sample Collection

At rice maturity, soil samples were collected from the 0~20 cm layer in each plot using a three-point sampling method. The samples were air-dried, ground, and sieved through a 100-mesh nylon sieve for further analysis. Representative rice plants were collected from each plot, washed with tap water and deionized water, and divided into roots, stems, leaves, and panicles. The samples were oven-dried at 105 °C for 30 min and then at 80 °C to constant weight. The samples were ground and sieved through a 0.15 mm nylon sieve for further analysis. Brown rice was separated using a husker and ground for analysis.

2.4.2. Measurements

Measurements: Soil pH was measured using a pH meter with a soil-to-water ratio of 1:2.5 [26]; the available nitrogen content was measured via the alkali hydrolysis diffusion method, the available phosphorus content was measured via the sodium bicarbonate extraction molybdenum blue colorimetry method, and the available potassium content was measured via the ammonium acetate extraction flame photometry method [27]; the concentrations of total N, total P, total K in soil were determined according to the descriptions by Bao [28]; soil organic carbon (SOC) was analyzed via the oxidation of potassium dichromate method. Soil total Cd content was determined using HNO3-HClO4-HF [29]; soil Cd fractions were analyzed using the BCR sequential extraction method [30]; Cd content in rice roots, stems, leaves, and brown rice was determined using HNO3-HClO4 digestion [31,32] and measured by atomic absorption spectrophotometry (PinAAcle 900T, Perkin Elmer, Waltham, MA, USA). The translocation factor (TF) and bioaccumulation factor (BCF) were calculated using the following formulas [26,33]:
TFx−y = Cy/Cx
BCF = Ct/Cs
where x and y represent different parts of the rice plant, TFx−y represents the translocation factor of Cd from part x to part y, Cx and Cy represent Cd content in parts x and y, respectively; BCF represents the bioaccumulation factor of Cd in a specific part of the rice plant, Ct represents Cd content in the plant part, and Cs represents soil total Cd content.

2.5. Data Analysis

Data were processed using Excel 2021 (Microsoft Corporation, Redmond, WA, USA) and SPSS 26 (International Business Machines Corporation, New York, NY, USA) for statistical analysis, and graphs were generated using Origin 2025 (Origin Lab Corporation, Northampton, MA, USA). Significant difference between different treatments was analyzed using one-way analysis of variance (Duncan’s test) by SPSS 26 at p < 0.05 level.

3. Results

3.1. Effects of Combined Application of Rhodochrosite Slag and Biochar on Cd Content in Different Parts of Rice

Compared with CK treatment, the B, R, and BR treatments were all effective in reducing the cadmium content in various parts of rice plants, and the order of Cd content in rice plants was as follows: root > stem > leaf > brown rice. Compared with the CK treatment, the RB treatment was the most effective in reducing the Cd content in rice roots, stems, leaves, and brown rice, which reached significant differences (p < 0.05) with the CK treatment, with reductions of 26.37%, 47.20%, 31.03%, and 51.85%, respectively (Table 3). In addition, the brown rice Cd content of RB treatment reached 0.135 mg/kg, which was significantly reduced by 31.82% and 29.32% compared with B and R treatments, respectively, and was lower than the national rice food safety standard (0.2 mg/kg). These results indicate that the RB treatment was the most effective in reducing the Cd content in rice roots, stems, leaves, and brown rice, and inhibited rice Cd uptake more effectively than the other treatments.

3.2. Effects of Combined Application of Rhodochrosite Slag and Biochar on Soil Total Cd Content and pH

The results revealed that the application of rhodochrosite slag and biochar significantly increased the total cadmium content in the soil (Figure 2a). Compared with the CK, the total Cd content in all treatments increased by 8.41% to 26.09%. Except for the R treatment, the other treatments were significantly different from the CK treatment (p < 0.05). Among all the treatments, RB treatment was the most effective in increasing the total cadmium in the soil. The pH increased by 5.76%, 2.42% and 6.88% with the application of biochar and rhodochrosite slag in the B, R and RB treatments, respectively, compared to the CK treatment (Figure 2b). Similarly, except for the R treatment, the other treatments were significantly different from the CK treatment (p < 0.05), among which the RB treatment had the most significant effect on increasing soil pH. The research results show that the mixed application of rhodochrosite slag and biochar in cadmium-contaminated paddy fields can effectively alleviate soil acidification and enhance the soil’s ability to fix cadmium.

3.3. Effects of Combined Application of Rhodochrosite Slag and Biochar on Soil Cd Fractions

Across all treatments, Cd was predominantly present as acid-extractable and reducible fractions, with the sequential order of Cd fractions being acid-extractable > reducible > residual > oxidizable. Compared to the CK treatment, treatments with R or RB increased the acid-extractable Cd content, while B treatment significantly reduced acid-extractable Cd by 12% (p < 0.05) (Table 4). Notably, no significant difference in acid-extractable Cd was observed between RB and CK. However, both RB and R treatments significantly enhanced reducible, oxidizable, and residual Cd fractions compared to CK (p < 0.05), with RB showing the most pronounced effects. Specifically, reducible Cd increased by 54.55% (RB), 43.08% (B), and 15.81% (R); oxidizable Cd rose by 40.54% (RB), 24.32% (B), and 10.81% (R); residual Cd increased by 45.28% (RB), 16.98% (B), and 5.66% (R), respectively.
Notably, the R treatment showed no significant changes in Cd fractions relative to CK. In contrast, the percentage of the most unstable acid-extracted Cd content decreased from 50.7% (CK) to 39.7% and 40.5% under the B and RB treatments, respectively, with the lowest percentage of acid-extracted Cd under the B treatment (Figure 3). Concurrently, reducible Cd increased significantly by 46.6% (B) and 44.9% (RB) compared to CK treatment. The percentage of Cd in the residual form, which is the least bioavailable form of Cd, increased from 7.7% (CK) to 7.9% and 8.7%, with a peak in the RB treatment, indicating that the RB treatment significantly promoted the conversion of reactive Cd to a more stable residual form. These results indicate that although RS alone failed to reduce acid-extractable Cd, its combination with BC synergistically promoted Cd transformation into stable fractions, thereby significantly immobilizing Cd in contaminated soil and decreasing Cd bioavailability.

3.4. Effects of Combined Application of Rhodochrosite Slag and Biochar on Cadmium Translocation and Bioaccumulation Factors in Rice

The bioaccumulation factor (BCF) is commonly used to characterize the ease of element migration in the soil-plant system [34]. The translocation factor (TF) reflects the ability of heavy metals to translocate within plants, with higher TF values indicating greater ease of metal movement [33]. Compared to the control (CK), the application of passivators significantly reduced both the TF and BCF of Cd in different parts of rice (Table 5). Under the same treatment, the reduction in TF from roots to stems and from roots to rice ranged from 10.50% to 28.28% and from 9.33% to 33.33%, respectively. The RB treatment exhibited the lowest TF values for root-to-stem and root-to-brown rice translocation, with significant reductions of 28.28% and 33.33%, respectively, compared to CK. This indicates that the RB treatment effectively inhibited Cd translocation from roots to stems and from roots to brown rice. For stem-to-brown rice translocation, the TF increased in the B treatment but decreased significantly in the other two treatments, with the R treatment showing the lowest value (0.087 mg/kg), a 20.91% reduction compared to CK, the RB treatment followed, with no significant difference between B and RB treatments.
The BCF values for Cd in different rice tissues followed the order roots > stems > leaves > brown rice. Across all treatments, the RB treatment consistently resulted in the lowest BCF values for roots, stems, leaves, and brown rice, significantly lower than other treatments. Compared to CK, the RB treatment reduced BCF by 41.89% in roots, 58.40% in stems, 46.51% in leaves, and 62.50% in brown rice. These findings demonstrate that the RB treatment effectively suppressed Cd uptake and translocation in rice, highlighting its potential for mitigating Cd contamination in paddy systems.

3.5. Effects of Combined Application of Rhodochrosite Slag and Biochar on Rice Yield

Compared with the CK treatments, B, R, and RB treatments all effectively increased the rice yield, with the increased range being 11.82, 8.40%, and 18.73% (Figure 4). Among all treatments, RB treatment had the highest yield, reaching 6.89 t/ha, followed by the single B treatment, with a yield of 6.49 t/ha, which was significantly increased by 18.73% and 11.82%, respectively, compared with CK (p < 0.05). Although the single application of rhodochrosite slag (R) had a certain yield-increasing effect, there was no significant difference compared with CK.

3.6. Relationships Between Brown Rice Cadmium Content and Soil Cadmium Fixation

Principal component analysis (PCA) was used to study the effects of rhodochrosite slag and biochar on soil Cd bioavailability and brown rice Cd content. The results showed that the first principal component accounted for 67.00% and the second principal component accounted for 16.80% of the total variables (Figure 5). The R and B treatments were closer to the CK treatment, and the RB treatment was the furthest away from the CK treatment. Therefore, the effect of the mixed application of biochar and rhodochrosite slag in reducing the bioavailability of Cd was greater than that of the application of biochar and rhodochrosite slag alone, and the difference was significant with the other treatments. RB treatment could partially increase soil pH, whole Cd content, oxidized Cd, reducible Cd, and residual Cd content, and at the same time, significantly reduce Cd transport coefficients of rice parts, Cd enrichment coefficients of brown rice, and Cd content of brown rice.

4. Discussion

4.1. Effects of Rhodochrosite Slag and Biochar on Soil pH and Cadmium Availability

The present study confirmed that both single and combined applications of rhodochrosite slag (RS) and biochar (BC) effectively reduced soil Cd bioavailability and inhibited Cd uptake by rice, aligning with previous findings on amendment-driven metal immobilization [35,36]. This may be due to the fact that the ash produced during pyrolysis of biochar contains a large number of alkaline substances (e.g., CaCO3, K2O, etc.), and the oxygen-containing functional groups on the surface (e.g., –OH, –COOH) can adsorb H+ through ion exchange, thus increasing the saturation of salt-based ions in the soil, effectively neutralizing the acidity of the soil, and increasing the pH value of the soil [37,38]. Another reason may be that rhodochrosite slag is rich in CaO, MgO, and SiO2, which can be hydrolyzed and ionized to release OH- when applied to the soil and plays an important role in reducing the active acidity of the soil [39,40]. Some studies have shown that the use of biochar containing Mn for the remediation of heavy metal-contaminated soils can affect the basic physicochemical properties of the original soil [26]. Therefore, with the application of rhodochrosite slag, the pH of Cd-contaminated soil increased significantly. He et al. [41] showed that the application of steel slag to Cd-contaminated soil could significantly increase soil pH due to the high content of calcium hydroxide and alkaline oxides in the slag, which gave the slag the characteristic of high alkalinity and thus possessed the ability to increase pH. Tang et al. [42] demonstrated that the application of biochar could significantly change the basic soil physical and chemical properties (pH, EC, and organic carbon content), and the range of changes became more pronounced with the increase of biochar application. The above results were similar to the results of this study.
The bioavailability of Cd in soil is strongly governed by its chemical speciation. The acid-extractable fraction exhibits the highest activity, readily releasing mobile-free Cd2+ that are easily absorbed by plants, representing the primary source of soil Cd contamination [43]. In contrast, reducible and oxidizable fractions remain stable under typical soil conditions but may release Cd under redox fluctuations, while the residual fraction exhibits minimal bioavailability due to strong mineral lattice integration [44]. Our results demonstrated that BC and RS application reduced the acid-extractable Cd fraction by 12~−0.85% while enhancing reducible, oxidizable, and residual fractions by 5.81~54.55%, 10.81~40.54%, and 5.66~45.28%, respectively. Notably, although the RS–BC combination did not significantly reduce the absolute content of acid-extractable Cd, it markedly decreased its proportion relative to total Cd, as well as substantially enhancing the stable residual fraction. Compared with the CK, the residual Cd and total cadmium content in the soil increased significantly by 45.28% and 26.09%, respectively (p < 0.05), highlighting the synergistic role of RS and BC in promoting Cd stabilization. Ca2+ and Si in RS are favorable for the precipitation of Cd as Ca-Cd silicates or carbonates. Prior studies indicate that the application of silicon (Si) additives to soil can release effective silicon to induce the co-precipitation of Cd and Si in soil [41]. Liu et al. applied biochar, calcium carbide slag, and natural magnets to Cd-contaminated soils and found that all three amendments significantly inhibited the migration of As and Cd from soil, with calcium carbide slag having the optimal effect, as the active functional groups of calcium carbide slag (–OH, C–O, OH−, CO32−) immobilized Cd by precipitation, adsorption, ion exchange, and electrostatic interaction [22]. In addition, the reducing properties of biochar materials help to promote the reductive decomposition of metal oxides in slag, which facilitates the complexation reaction between those in slag and Cd in soil; biochar materials are conducive to the improvement of soil pH and the sequestration of Cd-containing metal complexes due to their large porosity, large specific surface area, and abundance of surface functional groups [45]. High pH also enhanced the sequestration of Cd in the soil, so the soil total Cd of the RB treatment was significantly higher than that of the CK treatment. These findings emphasize the complementary roles of RS and BC in reducing Cd bioavailability through physicochemical immobilization and competing ionic effects. However, the exact adsorption mechanisms of BC and RS require further elucidation via advanced characterization techniques (e.g., SEM, XRD, FTIR, etc.) to clarify structural interactions and immobilization pathways.

4.2. Effects of Rhodochrosite Slag and Biochar on Cd Uptake and Yield in Rice

Reducing Cd accumulation in brown rice can be achieved through two primary strategies: decreasing Cd bioavailability in soil and inhibiting Cd translocation from roots to grains. The results of this study showed that both single and combined applications of RS and BC significantly reduced Cd concentrations in rice roots, stems, leaves, and brown rice, with the RB treatment showing the most pronounced reduction. This may be due to the large specific surface area and abundant functional groups of biochar, which can adsorb, complex, and co-precipitate heavy metals, thereby altering Cd fractions in soil, reducing its bioavailability, and ultimately decreasing Cd content in rice grains [32]. The Mn, Fe, Ca and Si in RS compete with Cd for root adsorption sites and transport channels, thereby reducing Cd uptake by plants [22]. It can be seen that the combined application of BC and RS can enhance the soil’s retention capacity for cadmium through complexation, adsorption, precipitation, competitive adsorption and other methods, thereby achieving the purpose of reducing the cadmium content in rice. Principal component analysis showed that cadmium content in brown rice was negatively correlated with soil pH, reducible Cd, oxidized Cd and residual Cd, suggesting that RB treatment may reduce the bioavailability of Cd in soil by increasing soil pH, thus reducing the absorption of Cd by rice.
The translocation and bioaccumulation factors reflect the plant’s ability to absorb Cd [26]. In this study, the single and combined applications of RS and BC reduced the BCF in different parts of rice. Studies have shown that the BCF of Cd in crop grains is linearly related to soil Cd content, and the BCF generally decreases with increasing soil Cd content [46]. This finding is consistent with the results of the present study. Principal component analysis confirmed that brown rice Cd content negatively correlated with soil pH and stable Cd fractions (reducible, oxidizable, residual), indicating that amendments mitigate Cd uptake by altering its speciation and bioavailability. Previous studies have made similar conclusions and suggested that the addition of amendments such as BC, SL, and FMO can decrease HM uptake in plants [47]. Additionally, the RB treatment decreased the TF of root to grain and root to stem by 33.3% and 28.28%. This phenomenon may be attributed to RS-derived SiO2 promoting Cd co-precipitation in root cell walls, thereby reducing Cd uptake and its translocation to aerial parts [48]. These findings indicate that RS can enhance the root sequestration of Cd, effectively decreasing Cd accumulation in stems, leaves, and brown rice.
Notably, the combined RS–BC treatment (RB) not only reduced the bioavailability of Cd but also promoted rice growth, resulting in a significant increase in rice yield by 18.8% compared with the control. This promotion may be due to the fact that, on the one hand, the presence of mineral elements (e.g., Si, K, and Ca) in RS provides essential nutrients for the growth of rice plants, and some studies have shown that Si has a significant promotion effect on rice growth, which can significantly increase the yield [49]; meanwhile, biochar can increase the organic carbon content in the soil, improve soil fertility, and promote crop growth [50]. On the other hand, mixed application of biochar and manganese lignite residue increased soil pH, resulting in a more neutral soil environment, reducing the bioavailability of Cd in the soil, and thus mitigating the negative effects of Cd stress on rice yield, so that the mixed application of BC and RS lignite residue can further increase the rice yield compared to its application alone [51]. Mu et al. found that the combination of biochar and phosphogypsum can effectively improve and repair Cd-contaminated soil and improve the yield and quality of cabbage [52], which is consistent with the results of this study. However, this study was only limited to one combination application plan, and a better combination application ratio remains to be further verified.

5. Conclusions

The results of this experiment showed that the combined (RB) treatment of rhodochrosite slag (RS) and biochar (BC) could significantly increase the soil pH compared with the B and R treatments. At the same time, after applying BC and RS, the soil Cd form showed a tendency to change from soluble to insoluble state, and the residual Cd content increased by 5.66~45.28% compared with that of CK treatment, and the bioavailability of soil Cd was significantly reduced, effectively passivating the Cd in soil. Among them, the RB treatment was the strongest in Cd fixation, and it significantly reduced the Cd content of brown rice, effectively alleviated the poisonous effect of Cd on rice, and increased the rice yield by 18.73%. In conclusion, the RB treatment not only improved soil acidification but also retained a higher Cd-fixing capacity, which increased rice yield by guaranteeing the safety of brown rice for consumption. These findings suggest that the combined application of biochar and rhodochrosite residue is an economically feasible strategy in Cd-contaminated rice fields. Future research should focus on the cadmium content of biochar and phosphate fertilizers, further optimize RS/BC ratios, and evaluate long-term effects to maximize scalability. In addition, the physicochemical properties of BC and RS should be characterized and analyzed by multidisciplinary methods, such as physics and chemistry, to further reveal the mechanism of Cd adsorption by the combined application of BC and RS.

Author Contributions

Conceptualization, Z.F.; methodology, W.Z., X.M., and Z.F.; validation, J.H.; formal analysis, J.H., Z.Y., and F.C.; writing—original draft preparation, J.H. and Z.Y.; writing—review and editing, F.C., W.Z., X.M., and Z.F.; funding acquisition, Z.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The datasets used or analyzed during the current study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
RSRhodochrosite slag
TFTranslocation factor
BCFBioaccumulation factor
BCBiochar

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Figure 1. Map of the experiment site.
Figure 1. Map of the experiment site.
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Figure 2. Effects of different treatments on soil total Cd content (a) and pH value (b). CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag. Error bars represent the standard error (n = 3). Different letters indicate a significant difference at p < 0.05.
Figure 2. Effects of different treatments on soil total Cd content (a) and pH value (b). CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag. Error bars represent the standard error (n = 3). Different letters indicate a significant difference at p < 0.05.
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Figure 3. The percentage distribution of soil Cd fractions under different treatments. CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; and RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag. Error bars represent the standard error (n = 3).
Figure 3. The percentage distribution of soil Cd fractions under different treatments. CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; and RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag. Error bars represent the standard error (n = 3).
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Figure 4. The effect of combined application of rhodochrosite slag and biochar on rice yield. CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; and RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag. Error bars represent the standard error (n = 3). Different letters indicate a significant difference at p < 0.05.
Figure 4. The effect of combined application of rhodochrosite slag and biochar on rice yield. CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; and RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag. Error bars represent the standard error (n = 3). Different letters indicate a significant difference at p < 0.05.
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Figure 5. Principal component analysis to study the effects of rhodochrosite slag and biochar on soil Cd bioavailability and brown rice Cd content. Note: Ac-Cd: acid-extractable Cd; Red-Cd: reducible Cd; Oxi-Cd: oxidized Cd; Res-Cd: residual Cd; Soil Cd: total Cd in soil; Brown rice represents Cd concentrations in brown rice; TF: translocation factors; and BCF: bioconcentration factor. CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; and RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag.
Figure 5. Principal component analysis to study the effects of rhodochrosite slag and biochar on soil Cd bioavailability and brown rice Cd content. Note: Ac-Cd: acid-extractable Cd; Red-Cd: reducible Cd; Oxi-Cd: oxidized Cd; Res-Cd: residual Cd; Soil Cd: total Cd in soil; Brown rice represents Cd concentrations in brown rice; TF: translocation factors; and BCF: bioconcentration factor. CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; and RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag.
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Table 1. Basic properties of biochar.
Table 1. Basic properties of biochar.
Total Nitrogen
(%)		Total Phosphorus
(mg/kg)		Total Potassium
(mg/kg)		Available Nitrogen
(mg/kg)		Rapidly Available Phosphorus
(mg/kg)		Rapidly Available Potassium
(mg/kg)
0.265.77 × 1034.72 × 104176.00242.002.2 × 104
Table 2. The component ratio of rhodochrosite slag. Others include CuO, ZnO, SrO, As2O3, Cl, Cr2O3, and V2O5.
Table 2. The component ratio of rhodochrosite slag. Others include CuO, ZnO, SrO, As2O3, Cl, Cr2O3, and V2O5.
Molecular FormulaSiO2MnOAl2O3CaOFe2O3SO3K2OMgONa2OTiO2P2O5BaOOther
Content (%)39.7218.1213.8810.925.264.423.122.230.980.560.460.130.20
Table 3. Effect of different treatments on Cd content in roots, stems, leaves, and brown rice of rice plants. CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag. Error bars represent the standard error (n = 3). Different letters indicate a significant difference at p < 0.05.
Table 3. Effect of different treatments on Cd content in roots, stems, leaves, and brown rice of rice plants. CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag. Error bars represent the standard error (n = 3). Different letters indicate a significant difference at p < 0.05.
TreatmentRoot
(mg/kg)		Stem
(mg/kg)		Leaf
(mg/kg)		Brown Rice
(mg/kg)
CK3.641 ± 0.046 a2.497 ± 0.076 a0.293 ± 0.003 a0.274 ± 0.007 a
B2.899 ± 0.030 b1.722 ± 0.062 c0.215 ± 0.005 b0.198 ± 0.012 b
R3.608 ± 0.291 a2.197 ± 0.052 b0.221 ± 0.009 b0.191 ± 0.006 b
RB2.678 ± 0.056 b1.315 ± 0.055 d0.196 ± 0.011 b0.135 ± 0.014 c
Table 4. Content of cadmium in acid-extractable (Ac-Cd), reducible (Red-Cd), oxidized (Oxi-Cd), and residual fractions (Res-Cd) in soil under different treatments. CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; and RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag. Error bars represent the standard error (n = 3). Different letters indicate a significant difference at p < 0.05.
Table 4. Content of cadmium in acid-extractable (Ac-Cd), reducible (Red-Cd), oxidized (Oxi-Cd), and residual fractions (Res-Cd) in soil under different treatments. CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; and RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag. Error bars represent the standard error (n = 3). Different letters indicate a significant difference at p < 0.05.
TreatmentAc-Cd (mg/kg)Red-Cd (mg/kg)Oxi-Cd (mg/kg)Res-Cd (mg/kg)
CK0.350 ± 0.015 ab0.253 ± 0.021 b0.037 ± 0.002 c0.053 ± 0.003 c
B0.308 ± 0.012 b0.362 ± 0.018 a0.046 ± 0.001 b0.062 ± 0.003 b
R0.360 ± 0.017 a0.293 ± 0.005 b0.041 ± 0.001 c0.056 ± 0.001 bc
RB0.353 ± 0.015 ab0.391 ± 0.007 a0.052 ± 0.002 a0.077 ± 0.002 a
Table 5. Effects of different treatments on transport factor (TF) and bioconcentration factor (BCF) of cadmium in rice tissues. CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; and RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag. Error bars represent the standard error (n = 3). Different letters indicate a significant difference at p < 0.05.
Table 5. Effects of different treatments on transport factor (TF) and bioconcentration factor (BCF) of cadmium in rice tissues. CK: No application of rhodochrosite slag or biochar; B: 15,000 kg/ha of biochar; R: 300 kg/ha of rhodochrosite slag; and RB: 15,000 kg/ha of biochar and 300 kg/ha of rhodochrosite slag. Error bars represent the standard error (n = 3). Different letters indicate a significant difference at p < 0.05.
TreatmentTFBCF
TFroot-stemTFroot-riceTFstem-riceBCFrootBCFstemBCFleafBCFrice
CK0.686 ± 0.022 a0.075 ± 0.003 a0.110 ± 0.004 ab5.300 ± 0.247 a3.626 ± 0.050 a0.427 ± 0.021 a0.399 ± 0.019 a
B0.594 ± 0.016 b0.068 ± 0.004 a0.115 ± 0.003 a3.736 ± 0.087 b2.220 ± 0.106 c0.277 ± 0.005 b0.255 ± 0.019 b
R0.614 ± 0.033 ab0.054 ± 0.006 b0.087 ± 0.005 b4.814 ± 0.320 a2.936 ± 0.029 b0.295 ± 0.014 b0.255 ± 0.013 b
RB0.492 ± 0.028 c0.050 ± 0.004 b0.103 ± 0.013 ab3.079 ± 0.053 b1.515 ± 0.089 d0.226 ± 0.016 c0.154 ± 0.015 c
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He, J.; Ye, Z.; Chen, F.; Zhou, W.; Ma, X.; Fu, Z. The Effect of Combined Application of Rhodochrosite Slag and Biochar on Cadmium Uptake in Rice. Agronomy 2025, 15, 1321. https://doi.org/10.3390/agronomy15061321

AMA Style

He J, Ye Z, Chen F, Zhou W, Ma X, Fu Z. The Effect of Combined Application of Rhodochrosite Slag and Biochar on Cadmium Uptake in Rice. Agronomy. 2025; 15(6):1321. https://doi.org/10.3390/agronomy15061321

Chicago/Turabian Style

He, Jing, Zhixi Ye, Fugui Chen, Wentao Zhou, Xin Ma, and Zhiqiang Fu. 2025. "The Effect of Combined Application of Rhodochrosite Slag and Biochar on Cadmium Uptake in Rice" Agronomy 15, no. 6: 1321. https://doi.org/10.3390/agronomy15061321

APA Style

He, J., Ye, Z., Chen, F., Zhou, W., Ma, X., & Fu, Z. (2025). The Effect of Combined Application of Rhodochrosite Slag and Biochar on Cadmium Uptake in Rice. Agronomy, 15(6), 1321. https://doi.org/10.3390/agronomy15061321

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