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Article

Improving Soil Health and Rice Yields with the Application of Soil Amendments in Acidic Paddy Soils

by
Jian Liu
1,
Ting Wang
1,
Lihua Lan
1,
Qingjiu Meng
2,
Jun Xu
2,
Minjun Hu
2,
Tehseen Sajid
3 and
Jun Meng
1,*
1
Laboratory of Recycling and Eco-Treatment of Waste Biomass of Zhejiang Province, School of Environment and Natural Resources, Zhejiang University of Science and Technology, Hangzhou 310023, China
2
Agricultural Technology Extension Center, Agriculture and Rural Affairs Bureau of Fuyang District, Hangzhou 311499, China
3
Institute of Biological Sciences, Khwaja Fareed University of Engineering and Information Technology, Rahim Yar Khan 64200, Punjab, Pakistan
*
Author to whom correspondence should be addressed.
Agronomy 2026, 16(1), 9; https://doi.org/10.3390/agronomy16010009
Submission received: 6 November 2025 / Revised: 6 December 2025 / Accepted: 18 December 2025 / Published: 19 December 2025
(This article belongs to the Section Soil and Plant Nutrition)
Browse Figures
Versions Notes

Abstract

The over-application of nitrogen fertilizers has expedited soil acidification, resulting in the deterioration of agricultural soil quality and a decline in rice yields. This study evaluated the performance of seven soil amendments, including lime (L), biochar (BC), composted manure (CM), and alkaline inorganic material (AM), and their combinations, such as L with BC, L with CM, and BC with AM, in regulating soil pH, nutrient levels, heavy metal bioaccumulation, and rice yields at two field sites. The results demonstrated that soil pH increased by 0.33–1.57 units after amendment application. Compared with the control, the amendments reduced the concentrations of available cadmium in soils by 7–57%, available copper by 32–91%, available nickel by 12–88%, and available zinc by 18–99%. Moreover, they induced a reduction in exchangeable H+ and Al3+ levels, improving various properties and soil health. Furthermore, these amendments caused an increase in rice yields and a decrease in Cd and Ni accumulation in rice grains by 5–30% and 11–40%, respectively. Structural equation modeling indicated that the accumulation of heavy metals in rice is mainly mediated by soil pH via its impact on exchangeable acidity. This impact subsequently modifies soil nutrient availability, thereby influencing metal bioaccumulation. Overall, the application of these amendments presents promising strategies for mitigating soil acidification and improving agricultural productivity.

1. Introduction

The rapid development of agriculture, coupled with excessive use of nitrogen fertilizers and intensive agricultural practices, has driven severe global soil acidification, with approximately 56% of the world’s arable land now affected [1]. The trend is particularly pronounced in China. From the 1980s to the 2010s, the proportion of acidic farmland (pH < 6.5) rose from 36% to 43% [2]. Current estimates indicate that about 47% of farmland has a pH below 6.5, with 15% falling below pH 5.5 [3]. A meta-analysis covering 1980 to 2024 revealed that long-term application of nitrogen fertilizer in China has led to an average soil pH decrease of 15.27% [4], with some studies reporting declines of 1.1–2.5 units [5]. If traditional practices continue, it is predicted that by 2050, approximately 13.2% of croplands could suffer from aluminum (Al) toxicity [5], and acidification may contribute to a crop yield loss of around 24% [6].
Soil acidification can cause a series of adverse effects. It directly lowers soil pH, promoting the leaching of essential base cations like calcium (Ca2+), magnesium (Mg2+), and potassium (K+), which damages soil structure and reduces fertility [7,8]. Moreover, the increase in soil acidity enhances the solubility and bioavailability of toxic heavy metals. Cadmium (Cd) is a common toxic element in Chinese farmland soils, with surveys suggesting about 27% of soil samples surpassed the risk control standard for Cd [1]. Heavy metals such as nickel (Ni), copper (Cu), and zinc (Zn), while essential micronutrients, can produce phytotoxicity at high concentrations [9,10]. Soil acidity largely determines their bioavailability; for example, reducing soil pH from 7.0 to 5.0 can increase the transfer coefficient of Cd to rice by ten times, resulting in over 70% of grain samples exceeding the safety limit of 0.2 mg kg−1 [11]. Similarly, under acidic conditions, the bioavailability of Ni increases, and its bioaccumulation inhibits plant growth and yield [12,13]. Soil acidification also disrupts the structure and activity of microbial communities, further damaging soil health and function. The consequences for food security are significant, especially in Southern China, where about 40% of the population relies on rice as their staple food [14], and soil acidification is associated with significant losses in rice yield [15]. Therefore, alleviating soil acidification is the top priority to ensure food safety.
Typical approaches to tackling soil acidification involve the utilization of soil amendments, the implementation of appropriate tillage and crop rotation, the improvement of irrigation management, and the cultivation of acid-tolerant crops [16,17]. The application of soil amendments, such as lime, organic fertilizers, and biochar, is one of the most direct and effective methods for addressing soil acidity [18]. Lime application has been demonstrated to raise soil pH by 0.85 units, reduce Cd content in rice by 48%, and boost yield by 12.9% [19]. Similarly, organic amendments like composted cattle and swine manure can raise soil pH by 0.91 and 0.64 units, respectively, and increase grain yield [20]. Biochar, known for its high porosity, specific surface area, and functional groups, is frequently employed for increasing soil pH and remediating heavy metal-contaminated soil [21,22]. Studies demonstrated that application of corn stover biochar significantly decreased soil acidity from 8.2 to 1.9 meq/100 g [23], and rice straw biochar application reduced the available Cd and Cu in the soil by 17.65% and 5.08%, respectively [24].
However, these conventional amendments have limitations for long-term and sustainable application. Biochar may exhibit toxicity to organisms due to the potential presence of hazardous substances, and long-term application may result in biochar aging and ineffectiveness [25]. Additionally, its application typically incurs high costs [26]. The long-term application of lime to agricultural soil can lead to re-acidification and an increase in physical firmness, potentially causing leaching losses of essential mineral nutrients [27]. Organic manure contains relatively high levels of heavy metals, and its long-term application to agricultural soil can lead to Cu and Zn pollution in soils, as well as changes in pH levels and organic matter [28]. In addition, most soil amendment studies are conducted under simplified and controlled laboratory conditions, resulting in insufficient evaluation of the comprehensive performance and practical trade-offs of amendments in real agricultural environments. Therefore, a comprehensive investigation under realistic field conditions is needed regarding the effects on soil acidification, improvements in soil fertility, augmentation of rice yield, and cost considerations.
The main goals of this study were to (i) investigate the mechanisms by which soil amendments reduce soil pH and exchangeable acidity, as well as their effects on soil heavy metals’ availability under actual field conditions; (ii) examine the influence of soil amendments on soil nutrients and fertility; (iii) assess the impact on crop yield and the accumulation of heavy metals in rice grains; and (iv) evaluate the costs of different soil amendment applications. This study can provide technical support for the governance of soil acidification in regional rice production and management.

2. Materials and Methods

2.1. Site Description

Fuyang District serves as a major grain-producing area in Zhejiang Province, China. Based on the results of the third soil survey in Fuyang District, 80.16% of the cultivated land exhibits a pH value below 6.5, and approximately 34.31% has a pH value below 5.5. Two field experiments were conducted in paddy soils located in Yushan (YS, 120.09° E, 30.06° N) and Shuxi (SX, 120.10° E, 30.07° N), Fuyang District, Zhejiang Province, China. The local climate was subtropical humid monsoon, with a mean annual temperature of 16.27 °C and precipitation of 1452.5 mm. The soil pH, exchangeable H+ (Ex. H), exchangeable Al3+ (Ex. Al), cation exchange capacity (CEC), total nitrogen (TN), soil organic matter (SOM), hydrolyzable nitrogen (HAN), exchangeable Ca2+ (Ex. Ca), exchangeable Mg2+ (Ex. Mg), available phosphorus (A. P), available potassium (A. K), and total concentrations of Cd, Cu, Ni, and Zn are listed in Table S1.

2.2. Experimental Design

The field experiment encompassed eight treatments: a non-amended control (CK), lime applied at 1.5 t ha−1 (L), composted swine manure applied at 4.5 t ha−1 (CM), rice straw-derived biochar applied at 4.5 t ha−1 (BC), alkaline inorganic material applied at 2.25 t ha−1 (AM), a combination of biochar (BC) at 2.25 t ha−1 + lime (L) at 1.5 ha−1 (B + L), a combination of biochar (BC) at 2.25 t ha−1 + composted swine manure (CM) at 4.5 ha−1 (B + C), and a combination of biochar (BC) at 2.25 t ha−1 + alkaline inorganic material (AM) at 2.25 t ha−1 (B + A). There were 24 plots in total, with each plot having an area of 20 m2 (5 m length, 4 m width). Ridges, 0.2 m wide and covered with black plastic film, were established above the soil surface between adjacent plots to preclude mutual influence. The amendments for each treatment were applied only once. Subsequently, the surface soils were ploughed to a depth of 15 cm. Based on local agronomic recommendations, basal N fertilizer (urea), P fertilizer (superphosphate), and K fertilizer (potassium chloride) were applied at a rate of 72 kg N ha−1, 40 kg P ha−1, and 105 kg K ha−1, respectively. After 7 days of application of the amendments, thirty-day-old seedlings of rice were transplanted into the field plots with an inter- and intra-row spacing of 25 × 25 cm. All crop management practices adhered to local methods.
Both lime (L) and alkaline inorganic material (AM) (mainly composed of CaO, Al2O3, MgO, and SiO2) were obtained from the Wanli Shennong Company of Zhejiang (Hangzhou, China). The composted swine manure (CM) was sourced from the Ruijue Biotechnology Company of Zhejiang (Hangzhou, China), and the rice straw-derived biochar (BC) was procured from the Qinfeng Straw Technology Company of Jiangsu (Yangzhou, China). The physicochemical properties of L, CM, BC, and AM were detailed in Table S2.

2.3. Sample Collection and Treatment

Prior to the application of amendments, initial soil samples were collected from each plot. Samples were obtained from three points arranged in an S-shaped pattern and thoroughly homogenized to form a single composite sample. Similarly, at the rice mature stage, soil samples were collected using the identical methodology. At the maturity stage, three plant samples were randomly gathered from each plot. Both soil and plant samples were placed in plastic bags, sealed, and transported to the laboratory. The collected soil samples were positioned in a cool and dry environment for natural air-drying. Subsequently, the samples were sieved to remove stones, debris, and plant roots, followed by grinding to pass through 2 mm and 0.145 mm sieves. The soil samples were stored separately for subsequent analysis: the 2 mm samples were used for the determination of soil pH, CEC, and other properties, while 0.145 mm samples were employed for the analysis of heavy metals, available K, effective P, and other element contents. Rice samples were rinsed with deionized water, oven-dried at 105 °C for 30 min, and then further dried at 60 °C until a constant weight was achieved. Rice yields were measured by harvesting an area of 1 m2 from the middle of each plot upon the paddy reaching physiological maturity. Rice samples were ground (<0.3 mm) and stored in plastic bags until analysis.

2.4. Sample Analysis

The soil pH was measured in distilled water utilizing a pH meter (S470, Mettler Toledo, Zurich, Switzerland) at a soil-solution ratio of 1:2.5 (w/v) after shaking. To ensure measurement accuracy, the meter was calibrated using standard buffer solutions before use. The concentrations of soil bioavailable Cd, Cu, Ni, and Zn were determined through extraction with 0.1 M CaCl2 at 220 rpm, 25 °C for 60 min at a soil/water ratio of 1:10 (w/v) [29]. Total Cd, Cu, Ni, and Zn contents were measured after microwave digestion with a mixed acid system of HNO3-HF-H2O2 (4:2:2, v/v/v). The filtrates were used to quantify the concentrations of soil bioavailable and total Cd, Cu, Ni, and Zn by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES, PerkinElmer 8300, Waltham, MA, USA) [30]. The soil CEC was ascertained using the standard hydrochloric acid titration method [31]. Total nitrogen was measured using the automatic Kjeldahl apparatus (KjelFlex K360, BUCHI Labortechnik AG, Flawil, Switzerland). SOM was measured by the K2Cr2O7-H2SO4 oxidation method [32]. Exchangeable Ca2+ and Mg2+ were extracted using an unbuffered solution of 1 M NH4OAc (pH = 7), and the cation concentrations were measured using an atomic absorption spectrophotometer (AAS, iCE 3500Z, Thermo Scientific, Waltham, MA, USA) [4]. Soil available phosphorus (A. P) was analyzed using the molybdenum antimony colorimetric method [33]. The available potassium (A. K) in the soil was extracted with a 1 mol/L neutral ammonium acetate solution, filtered, and subsequently determined using AAS.
To ensure the reliability of the data, standard materials (GSS-5 for soil and GSB-23 for rice), replicates, and reagent blanks were included when determining the metal concentration of each batch of samples. The recovery rates of Cd, Cu, Ni, and Zn detection ranged from 94.5% to 108.4% for the standard materials, and the relative standard deviations of the duplicate samples were all below 5%, confirming method accuracy and precision. Before ICP-OES measurement, a calibration curve was established using six standard solutions with known concentrations for external calibration. The detection limits were ≤0.01 mg kg−1 for Cd and ≤0.05 mg kg−1 for Cu, Ni, and Zn.

2.5. Statistical Analysis

Experimental data were analyzed using Microsoft Excel 2021 and SPSS 22. Pearson correlation analysis was employed to analyze the correlations, and the results were visualized with Origin 2021. One-way ANOVA was executed with IBM SPSS Statistics 22, and significance was assessed using Duncan’s multiple range test, where distinct letters denote significant differences at p < 0.05. The data in the figures and tables are presented as the mean ± standard deviation. The structural equation model (SEM) was performed by R 4.4.1.

3. Results

3.1. Effects of Amendments on Soil pH, Exchangeable Acidity, H+, and Al3+

The pH values of soils treated with L, CM, BC, AM, B + L, B + C, and B + A are depicted in Figure 1a. The initial soil pH in Yushan was 5.50 ± 0.16. The amendments L, AM, B + L, and B + A led to a significant increase in soil pH (p < 0.05) relative to the CK, with respective increments of 0.33, 1.15, 0.46, and 0.53 units (Figure 1a). Regarding Ex. H, it significantly decreased by 0.29, 0.16, 0.07, 0.30, 0.25, and 0.30 cmol kg−1 under the treatments of L, CM, BC, B + L, B + C, and B + A (Figure 1c), compared with the CK. Meanwhile, Ex. Al significantly decreased by 0.47, 0.25, 0.20, 0.45, 0.09, and 0.46 cmol kg−1 with L, CM, BC, B + L, B + C, and B + A treatments (Figure 1d). In the case of AM treatment, the Ex. H and Ex. Al in soils from YS were not detectable because they fell below the detection limits (<0.01 cmol kg−1).
The initial soil pH in Shuxi was 5.65 ± 0.09. In comparison with the CK, the soil pH under the treatments of L, AM, B + L, and B + A significantly increased by 0.79, 0.77, 1.57, and 0.68 units, respectively (Figure 1a). Regarding Ex. H, it significantly decreased by 0.12 cmol kg−1 with B + A treatment (Figure 1c) compared with the CK and was not detected under the L, AM, and B + L treatments. As for Ex. Al, it significantly decreased by 0.34 and 0.37 c mol kg−1 under AM and B + A treatments, respectively, compared to the CK (Figure 1d), and was not detected under L and B + L treatments.

3.2. Effects of Amendments on Soil Nutrients

The effects of different soil amendments (L, CM, BC, AM, B + L, B + C, and B + A) on soil properties are presented in Figure 2 and Figure 3. In Yushan, the increase in CEC concentration after the seven treatments compared to the CK treatment ranged from 0.44 to 3.72 cmol kg−1. Specifically, the AM treatment significantly increased CEC content by 54% compared to the CK (Figure 2a). The CM, BC, AM, and B + C treatments significantly elevated the SOM concentration relative to the CK. However, the L treatment led to a reduction in the SOM concentration (Figure 2b). The TN concentrations in YS following treatments were considerably increased compared to the CK treatment, with the exception of the CM and AM treatments (Figure 2c). All treatments decreased the HAN concentration by 19% to 32% (Figure 2d). Except for the B + A group, all treatments in YS increased the concentration of Ex. Ca. The L, BC, AM, and B + C treatments increased it by 24%, 15%, 20%, and 34%, respectively, and the CM treatment showed a remarkable increase of 74% (Figure 3a). Additionally, all treatments led to an increase in the available P concentration ranging from 10% to 123%, with the AM treatment showing a 123% increase. Regarding the available K, it increased by 16%, 23%, and 28% under the L, BC, and B + A treatments, respectively, while the B + C treatment caused a 15% reduction.
In Shuxi, the L and BL treatments increased the concentration of CEC by 45% and 49%, respectively. The concentrations of TN did not show significant changes, and the L treatment had a significantly negative effect in comparison to the CK. None of the treatments increased SOM concentrations when compared to the CK; even the L treatment notably decreased SOM levels (Figure 2c). The L and BL treatments significantly decreased the HAN concentrations by 27% and 55%, respectively. All seven treatments significantly elevated the concentrations of Ex. Ca and Ex. Mg at the mature stage. Specifically, the L, B + L, and B + A treatments increased Ex. Ca concentrations by 140%, 118%, and 116%, while the BC, AM, and B + A treatments increased Ex. Mg concentrations by 40%, 44%, and 41%. The A. P concentration increased by 125%, 108%, and 255% with the CM, B + L, and B + C treatments in Shuxi, respectively, as compared with the CK. The A. K concentration increased by 20% to 72% with all treatments compared to the CK.

3.3. Effects of Amendments on Soil Bioavailable Cd, Cu, Ni, and Zn

The concentrations of bioavailable Cd, Cu, Ni, and Zn (A. Cd, A. Cu, A. Ni, and A. Zn) in soil treated with L, CM, BC, AM, B + L, B + C, and B + A amendments are presented in Figure 4. In Yushan, the concentration of A. Cd decreased by 25%, 7%, 40%, and 26% with the L, BC, AM, and B + A treatments, respectively, compared to the CK (Figure 4a). The concentration of A. Cu decreased by 75%, 42%, 65%, 71%, and 32% with the L, BC, AM, B + L, and B + C treatments, respectively, compared to the CK (Figure 4b). The concentrations of A. Ni decreased by 45%, 74%, 36%, and 56% after L, AM, B + L, and B + A treatments, respectively (Figure 4c). Compared to the CK, the L, AM, B + L, and B + A treatments reduced A. Zn levels by 71%, 87%, 41%, and 71%, respectively (Figure 4d).
In Shuxi, the A. Cd concentration decreased by 45%, 15%, 60%, 20%, and 16% in the application of L, AM, B + L, B + C, and B + A, respectively, as compared with the CK (Figure 4a). Regarding A. Cu, it reduced by 91%, 85%, 80%, and 79% under the treatments of L, AM, B + L, and B + A in YS, respectively, compared to the CK. Under the treatments of L, AM, B + L, and B + A, the A. Ni concentrations decreased by 82%, 49%, 88%, and 37%, respectively (Figure 2c). Additionally, the L, AM, B + L, and B + A treatments resulted in decreases in A. Zn levels by 95%, 69%, 99%, and 62% in SX, respectively (Figure 2d).
The amendments (L, AM, B + L, and B + A) in this study typically decreased the bioavailability of Cd, Cu, Ni, and Zn in both regions (Figure 4). Nevertheless, the efficacy varied between the two locations and among different metals. The CM treatments did not significantly affect the A. Cd, A. Ni, and A. Zn metals in Shuxi. However, it exerted opposite effects on A. Cu in two paddy fields, decreasing by 42% in Yushan and increasing by 31% in Shuxi. The BC treatment significantly decreased A. Cu in Yushan but had no significant effect in Shuxi. However, the combination of BC with lime, swine manure, and alkaline inorganic materials diminished its effectiveness in treating A. Cd in Yushan compared to Shuxi.

3.4. Effects of Amendments on Rice Yield and Heavy Metal Accumulation in Rice Grains

As presented in Figure 5, rice yield exhibited a significant improvement in all treatments at both sites. In Yushan, the AM treatment exerted the most pronounced effect, while in Shuxi, the L treatment had the greatest impact. The BC and AM treatments significantly increased the rice yields in Yushan but had no significant effect in Shuxi. The rice yield under the CK treatment differed by 0.9 t ha−1 between Yushan and Shuxi.
Under the L, CM, BC, AM, B + L, B + C, and B + A amendments, the changes in R. Cd, R. Cu, R. Ni, and R. Zn contents are illustrated in Figure 6. In Yushan, the concentrations of R. Cd, R. Cu, R. Ni, and R. Zn under the CK treatment were 0.311 ± 0.005, 3.511 ± 0.063, 1.411 ± 0.029, and 32.973 ± 0.116 mg kg−1, respectively. The concentration of R. Cd decreased by 26%, 30%, 27%, 16%, 20%, and 26% in the application of L, CM, AM, B + L, B + C, and B + A in YS, respectively, compared to the CK. For R. Cu, the L and B + C treatments showed decreases of 4% and 13%, respectively, compared to the CK. Additionally, the R. Ni content decreased by 36%, 25%, 11%, 34%, 37%, 33%, and 40% with the L, CM, BC, AM, B + L, B + C, and B + A treatments, compared to the CK. Regarding R. Zn, only the L treatment resulted in a significant reduction compared to the CK.
In Shuxi, the concentrations of R. Cd, R. Cu, R. Ni, and R. Zn under the CK treatment were 0.771 ± 0.005, 4.338 ± 0.092, 1.413 ± 0.022, and 26.081 ± 0.670 mg kg−1, respectively. Under the L, CM, and B + C treatments, the R. Cd content in Shuxi decreased, showing a reduction of 5–68% when compared to the CK. However, the BC treatment had a different result, leading to a 53% increase in the R. Cd concentration compared to the CK. In Shuxi, the reduction in R. Cu concentration was less pronounced under the AM and B + C treatments. Under the CM, AM, B + L, and B + C treatments, the R. Ni concentrations decreased by 32%, 26%, 34%, and 51%, and the BC and B + A treatments did not show a significant effect. Notably, the L treatment resulted in an increase.

3.5. Cost Analysis

Cost exerts a substantial influence on the feasibility of implementing remedial measures in large-scale contaminated soils. The prices, denominated in US dollars, of the amendments employed in the study are as follows: lime (L), $97.493/t; composted swine manure (CM), $83.565/t; rice straw-derived biochar (BC), $278.552/t; and alkaline inorganic materials (AM), $208.914/t, respectively. The price of rice is $396.66 t−1. Table 1 shows that in Yushan, the CK treatment yielded the highest net income, followed by the L treatment. Although the BC treatment entailed a higher cost, it provided relatively lower returns, all of which were lower than those of the CK treatment. In Shuxi, the order of net income was L treatment > CK treatment > AM treatment. Though the CM amendment yielded a higher net income than the AM amendment, the AM treatment demonstrated the highest effectiveness. While the net income of the L treatment was marginally lower than the CK treatment in Yushan, it substantially exceeded that of the CK treatment in Shuxi. Collectively, these results indicate that the L treatment holds practical potential for large-scale implementation.

3.6. Soil Grade Analysis

The soil quality grade derived from multiple soil properties can intuitively reflect the effect of soil amendment application on soil quality grade. It was calculated based on the technical standard for evaluating cultivated land quality grade in China (GB/T 33469-2016) [34], as detailed in Text S1. This study found that the application of most amendments improved soil quality by optimizing most soil indicators in comparison with the CK treatment. As shown in Table 2, the soil grade improved by 1–2 grades in Yushan and increased by 1–3 grades in Shuxi when compared to the CK treatment.

3.7. Structural Equation Modeling Analysis

The SEM demonstrated the relationships among soil pH, exchangeable acidity, soil nutrients, soil-available metals, and rice-accumulated metals (Figure 7a). Exchangeable acidity was characterized by exchangeable hydrogen and aluminum, soil nutrients by CEC, SOM, HAN, exchangeable Ca2+, Mg2+, and A. P, and metal availability was denoted by A. Cd, A. Ni, and A. Zn. The R2 values indicated the extent to which the indicators accounted for the variance of each latent variable. Soil pH exerted a strong negative effect on exchangeable acidity (R2 = 0.600, path coefficient = −0.7747) and a weak positive effect on soil nutrients (R2 = 0.022, path coefficient = 0.1480). Additionally, analysis of other pathways showed that exchangeable acidity had a moderate negative effect on soil nutrients (path coefficient = −0.4177), and soil nutrients exhibited a strong direct effect on heavy metal accumulation (path coefficient = −0.6696). The findings revealed that the bioaccumulation of heavy metals in rice is primarily mediated by soil pH through its influence on exchangeable acidity, which subsequently alters soil nutrient availability and thereby affects the accumulation of heavy metals in rice grains. Overall, soil pH plays a central role in regulating soil acidity and metal availability, making it a key factor influencing soil chemical properties.

4. Discussion

The effectiveness of soil amendments may be related to their main components. Multiple studies have shown that L predominantly consists of calcium oxide (CaO), which elevates soil pH by dissolving and releasing OH- into the soil solution [35,36]. In this study, the alkaline materials L and AM were composed of CaO (Table S2), notably improving soil pH. The intensification of soil acidification leads to an increase in H+ and Al3+ concentrations and a reduction in calcium and magnesium levels, which significantly affects the availability of soil nutrients [4]. In Shuxi, the content of Ex. H was roughly two-fold that of the CK treatment when CM and BC were applied, and the content of Ex. Al remained approximately twice that of the CK treatment when the B + C treatment was implemented. This might be due to the elevated levels of nitrogen and organic acids in CM, which react with alkaline salts to release more H+.
The CEC, as an indicator of soil fertility and health, enables soil to retain and exchange positive ions such as Ca2+ and Mg2+, thereby enhancing soil fertility [37]. In the two paddy fields, most amendment treatments significantly increased the soil CEC content, implying an improvement in soil fertility. The increase in soil CEC might be attributed to the enhanced surface area of soil particles (e.g., effects of L and AM) and the provision of hydroxyl and other oxidized functional groups (e.g., effects of BC and CM). In addition, higher SOM and CEC imply a strong pH buffering capacity, mitigating the toxicity of elements like Al to plants [38]. However, for the SOM in the two regions we studied, all treatments either significantly increased or remained unchanged compared to the CK treatment, except for the L treatment. TN and HAN are essential for plant growth, supporting plant nutritional needs and improving overall soil health. Ex. Ca and Ex. Mg are adsorbed by soil particles, facilitating aggregate formation, improving soil structure, neutralizing soil acidity, and regulating soil pH, which in turn affects nutrient availability. Soil aggregates are the basic units constituting soil structure, and their stability directly affects the pore structure, water movement, and pollutant fixation in soils [39]. Alkaline materials and CM contain natural Ca and Mg, which contribute to the improvement of soil structure and CEC [40]. BC possesses a high specific surface area and negative charge, enabling it to enhance soil stability, water retention, and nutrient cycling in paddy soils [41]. In the two paddy fields, the A. P concentration increased under all seven treatments when compared to the control. Lime treatment could increase soil pH, convert soil phosphates into more soluble forms, and enhance the A. P. Additionally, it could also reduce the fixation of P in the soil, thereby facilitating P uptake by plants. Likewise, the A. K content showed a significant increase in the Shuxi field. However, in the Yushan field, it exhibited a decline influenced by various factors, including climate.
Changes in soil physical and chemical properties induced by amendment application further affect the bioavailability of heavy metals in soils. This study revealed that soil pH and CEC were the most important factors influencing available Cd concentration in soils. The elevation of soil pH by amendments such as lime and AM directly promotes the immobilization of heavy metals through multiple mechanisms. Increased pH drives the precipitation of metals like Cd and Cu as insoluble hydroxides or carbonates. More importantly, a higher pH increases the net negative surface charge on soil colloidal particles (e.g., clay minerals, Fe/Al oxides), thereby enhancing non-specific CEC [42]. In this study, L and AM treatments enhanced the CEC content (Figure 2a), facilitating the adsorption of Cd, Cu, Ni, and Zn on the surfaces of soil particles. Concurrently, the rise in soil pH can promote specific adsorption of these metals onto oxide surfaces (inner-sphere complexation), a process strongly dependent on their hydrolysis constants and affinity for ligand sites [43].
In this study, the significant negative correlation between available Cd, Ni, and Zn concentrations and soil pH (Figure S1) underscores the pH-dependent immobilization. This result aligns with prior research findings [44,45]. Specifically, the acidic CM application (Table S2) led to a decrease in soil pH (Figure 1a) but increased the concentration of bioavailable metals. Besides pH effects, the organic components within composite amendments played a crucial role. They contribute functional groups (e.g., carboxyl, phenolic) that chelate metals directly and promote the formation of complexes on clay and oxide surfaces through mineral-organic interactions [46]. Overall, soil amendments improve soil characteristics by leveraging their unique properties to chelate and immobilize heavy metals.
Generally, Cd accumulation in rice is affected by soil texture, metal accumulation, and soil pH [47]. A lower pH can promote Cd release and solubility, resulting in enhanced absorption by rice plants and elevated metal accumulation in the grains [48]. In this study, the application of L and AM significantly increased soil pH (Figure 2a) and remarkably reduced available metal in soils (Figure 4) as well as metal uptake by rice (Figure 6). A previous study demonstrated that CM amendment could immobilize Cd in soils, resulting in lower Cd levels in rice grains [49]. This study revealed that L, CM, AM, B + L, B + C, and B + A treatments had significant inhibitory impacts on the uptake of Cd and Ni in two paddy fields. The Ni concentration in all rice samples is below the recommended limit of 2.0 mg kg–1 set by the European Commission [50]. Among all the amendments, B + C treatment had a notably superior immobilization effect on Cd in the Shuxi field. However, the Cd accumulation in the rice grains still slightly exceeded the limit of 0.2 mg kg–1 set by Chinese safety guidelines (GB2762-2022). This indicates that efforts are needed to enhance the pollution control of Cd in rice in the study area. The SEM analysis revealed that soil pH regulated the bioaccumulation of metals by gradually influencing the exchangeable acidity and soil nutrients.
The variation in the performance of identical amendments between the YS and SX fields may stem from their distinct initial soil physicochemical characteristics. The soil at YS possessed much greater native SOM content and total metal load than the soil at SX, which had a higher initial CEC but lower levels of exchangeable Ca2+ and Mg2+. The abundant SOM in YS might supply a larger reservoir for metal binding, enabling organic-based amendments to interact more effectively with native soil matrices. In contrast, the pure inorganic amendments, such as lime, might be more sensitive to the initial base saturation and CEC status, as observed at the SX site. This may be due to the higher CEC yet lower saturation in the SX region, which may alter the competitive adsorption kinetics between the cations from amendment sources (e.g., Ca2+ from lime) and the target metals from exchange sites. Therefore, site-specific remediation strategies should prioritize a detailed assessment of soil parameters to tailor the selection and application of soil amendments.

5. Conclusions

This study comprehensively evaluated the improvement effects of various amendments and their combinations on two typical acidified paddy fields. The results indicated that the application of L, AM, B + L, and B + A significantly reduced the levels of exchangeable H+ and Al3+ in both fields. The four treatments markedly increased soil pH by 0.33–1.57 units and synchronously decreased the available Cd, Cu, Ni, and Zn concentrations in soils by 7–99%. After remediation, the soil quality grade was improved by at least one grade, and the accumulation of Cd and Ni in rice grains was decreased by 5–30% and 11–40%, respectively. The cost-effectiveness analysis of the amendment application demonstrated that L treatment had a positive impact as compared with the CK. Additionally, the SEM analysis revealed that soil pH regulated the bioaccumulation of metals by influencing the exchangeable acidity and subsequently soil nutrients. Overall, this study compared the effects of various amendments on soil acidity, nutrients, rice yield, and heavy metal bioaccumulation, providing an important reference for soil quality improvement. Future research should explore mineral-organic interface dynamics and microstructural changes to more fully understand the mechanism of heavy metal immobilization and soil nutrient regulation under soil amendment application.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy16010009/s1, Text S1: Calculation of soil quality grade; Figure S1: Relationships between soil pH and concentrations of available Cd (a), Cu (b), Ni (c), and Zn (d) in soils; Table S1: Basic physicochemical properties of initial soils collected from Yushan and Shuxi.; Table S2: Physicochemical properties of lime (L), composted swine manure (CM), rice straw-derived biochar (BC), and alkaline inorganic material (AM).

Author Contributions

J.L.: data curation, writing—review and editing, conceptualization, funding acquisition. T.W.: data curation, writing—original draft, visualization. L.L.: methodology, conceptualization. Q.M.: investigation, project administration. J.X.: investigation, project administration. M.H.: investigation, project administration. T.S.: methodology, writing—review and editing. J.M.: supervision, conceptualization, funding acquisition, project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Fundamental Research Funds from Zhejiang University of Science and Technology (2025QN053), the National Natural Science Foundation of China (42307017), Zhejiang Province “Three Rural Nine Strategies” Science and Technology Cooperation Program (2024SNJF068), and the Public Welfare Technology Application Research Project of Zhejiang Province, China (LGF21D010002).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare that the research 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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Figure 1. Effects of different amendments on soil pH (a), exchangeable acidity (b), exchangeable H+ (c) and exchangeable Al3+ (d) (mean ± SD, n = 3). Different capital (lowercase) letters indicate significance at p < 0.05 for treatments in the YS (SX) paddy field.
Figure 1. Effects of different amendments on soil pH (a), exchangeable acidity (b), exchangeable H+ (c) and exchangeable Al3+ (d) (mean ± SD, n = 3). Different capital (lowercase) letters indicate significance at p < 0.05 for treatments in the YS (SX) paddy field.
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Figure 2. Effects of different amendments on soil cation exchange capacity (CEC) (a), soil organic matter (SOM) (b), total nitrogen (TN) (c), and hydrolyzable nitrogen (HAN) (d). Different capital (lowercase) letters indicate significance at p < 0.05 for treatments in the YS (SX) paddy field.
Figure 2. Effects of different amendments on soil cation exchange capacity (CEC) (a), soil organic matter (SOM) (b), total nitrogen (TN) (c), and hydrolyzable nitrogen (HAN) (d). Different capital (lowercase) letters indicate significance at p < 0.05 for treatments in the YS (SX) paddy field.
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Figure 3. Effects of different amendments on soil exchangeable Ca2+ (a), exchangeable Mg2+ (b), available P (c), and available K (d). Different capital (lowercase) letters indicate significance at p < 0.05 for treatments in the YS (SX) paddy field.
Figure 3. Effects of different amendments on soil exchangeable Ca2+ (a), exchangeable Mg2+ (b), available P (c), and available K (d). Different capital (lowercase) letters indicate significance at p < 0.05 for treatments in the YS (SX) paddy field.
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Figure 4. Effects of different amendments on the concentrations (mg kg−1) of available Cd (a), Cu (b), Ni (c), and Zn (d) (mg kg−1) in soils. Different capital (lowercase) letters indicate significance at p < 0.05 for treatments in the YS (SX) paddy field.
Figure 4. Effects of different amendments on the concentrations (mg kg−1) of available Cd (a), Cu (b), Ni (c), and Zn (d) (mg kg−1) in soils. Different capital (lowercase) letters indicate significance at p < 0.05 for treatments in the YS (SX) paddy field.
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Figure 5. Effects of different amendments on the yields of rice grains. Different capital (lowercase) letters indicate significance at p < 0.05 for treatments in the YS (SX) paddy field.
Figure 5. Effects of different amendments on the yields of rice grains. Different capital (lowercase) letters indicate significance at p < 0.05 for treatments in the YS (SX) paddy field.
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Figure 6. Effects of different amendments on the concentrations (mg kg−1) of Cd (a), Cu (b), Ni (c), and Zn (d) in rice grains. Different capital (lowercase) letters indicate significance at p < 0.05 for treatments in the YS (SX) paddy field.
Figure 6. Effects of different amendments on the concentrations (mg kg−1) of Cd (a), Cu (b), Ni (c), and Zn (d) in rice grains. Different capital (lowercase) letters indicate significance at p < 0.05 for treatments in the YS (SX) paddy field.
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Figure 7. The relationships among the five factors in improving soil acidification with structural equation model (a). The direct, indirect, and total effects of the soil pH, exchangeable acidity, soil nutrients, and soil-available metal on the metal accumulation in rice grains (b). Exchangeable acidity (Ex. H and Ex. Al), soil nutrients (CEC, SOM, HAN, Ex. Ca, and Ex. Mg), metal accumulation in rice (A. Cd, A. Ni, and A. Zn), and rice-available metals (R. Cd and R. Cu). Solid and dotted arrows indicate positive and negative effects, respectively. ** and *** indicate the significance levels at p < 0.01 and p < 0.001, respectively.
Figure 7. The relationships among the five factors in improving soil acidification with structural equation model (a). The direct, indirect, and total effects of the soil pH, exchangeable acidity, soil nutrients, and soil-available metal on the metal accumulation in rice grains (b). Exchangeable acidity (Ex. H and Ex. Al), soil nutrients (CEC, SOM, HAN, Ex. Ca, and Ex. Mg), metal accumulation in rice (A. Cd, A. Ni, and A. Zn), and rice-available metals (R. Cd and R. Cu). Solid and dotted arrows indicate positive and negative effects, respectively. ** and *** indicate the significance levels at p < 0.01 and p < 0.001, respectively.
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Table 1. The costs of lime (L), composted swine manure (CM), rice straw-derived biochar (BC), alkaline inorganic material (AM), B + L, B + C, and B + A. Total cost includes material costs, material application, land tilling costs, and land management expenses.
Table 1. The costs of lime (L), composted swine manure (CM), rice straw-derived biochar (BC), alkaline inorganic material (AM), B + L, B + C, and B + A. Total cost includes material costs, material application, land tilling costs, and land management expenses.
SiteTreatmentsTotal Cost
($ ha−1)		Yield
(t ha−1)		Total Income
($ ha−1)		Net Income
($ ha−1)
YushanCK417.837.753075.632657.80
L626.748.263278.032651.29
CM856.558.003174.852318.30
BC1733.988.503373.271639.29
AM950.568.633424.872474.31
B + L1253.488.043188.741935.25
B + C1483.298.303293.901810.62
B + A2047.358.543389.151341.79
ShuxiCK417.838.243270.022852.19
L626.748.983562.652935.91
CM856.558.653432.872576.32
BC1733.988.283284.321550.34
AM950.568.403331.472380.92
B + L1253.488.733463.942210.46
B + C1483.298.673439.011955.72
B + A2047.358.253271.761224.41
Table 2. The effects of application of lime (L), composted swine manure (CM), rice straw-derived biochar (BC), alkaline inorganic material (AM), B + L, B + C, and B + A on soil quality grade.
Table 2. The effects of application of lime (L), composted swine manure (CM), rice straw-derived biochar (BC), alkaline inorganic material (AM), B + L, B + C, and B + A on soil quality grade.
SiteTreatmentComposite IndexSoil Grade
YushanCK0.8454
L0.8643
CM0.8474
BC0.8623
AM0.9032
B + L0.8663
B + C0.8533
B + A0.8852
ShuxiCK0.8036
L0.8324
CM0.8255
BC0.8165
AM0.8414
B + L0.8623
B + C0.8384
B + A0.8444
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Liu, J.; Wang, T.; Lan, L.; Meng, Q.; Xu, J.; Hu, M.; Sajid, T.; Meng, J. Improving Soil Health and Rice Yields with the Application of Soil Amendments in Acidic Paddy Soils. Agronomy 2026, 16, 9. https://doi.org/10.3390/agronomy16010009

AMA Style

Liu J, Wang T, Lan L, Meng Q, Xu J, Hu M, Sajid T, Meng J. Improving Soil Health and Rice Yields with the Application of Soil Amendments in Acidic Paddy Soils. Agronomy. 2026; 16(1):9. https://doi.org/10.3390/agronomy16010009

Chicago/Turabian Style

Liu, Jian, Ting Wang, Lihua Lan, Qingjiu Meng, Jun Xu, Minjun Hu, Tehseen Sajid, and Jun Meng. 2026. "Improving Soil Health and Rice Yields with the Application of Soil Amendments in Acidic Paddy Soils" Agronomy 16, no. 1: 9. https://doi.org/10.3390/agronomy16010009

APA Style

Liu, J., Wang, T., Lan, L., Meng, Q., Xu, J., Hu, M., Sajid, T., & Meng, J. (2026). Improving Soil Health and Rice Yields with the Application of Soil Amendments in Acidic Paddy Soils. Agronomy, 16(1), 9. https://doi.org/10.3390/agronomy16010009

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