The research area is situated in the western region of Chenzhou City, Hunan provinvce, China, characterized by a sub-tropical humid monsoon climate. The geographic coordinates of the area range from 112°13’26” to 112°55’46” east longitude and 25°27’15” to 26°13’30” north latitude. The Taojia River, stretching approximately 17.59 kilometers, is a crucial water source for daily activities and production in the surrounding coastal areas. However, the upper reaches of the Taojia River have experienced sporadic and unregulated logging activities related to non-ferrous metal mining. As a result, a substantial amount of mineral waste stones and gravel have accumulated. During periods of flooding, these minerals are carried downstream, leading to the contamination of numerous cultivated soils situated along the river. Consequently, the ecological environment in the surrounding area has suffered significant damage ( Figure 1 ).

This study was divided into two segments. The first segment investigated how composting with SM and CM affected the dynamics of Pb in vegetable fields within the Taojia River Basin. In the second segment, we applied the most effective organic passivation method identified in the first part to examine the dynamics of Pb, Cd, Zn, and Cu in soil, paddy, and straw within rice fields. The selection of Pb, Zn, Cu, and Cd as HMs of environmental concern in the Taojia River Basin was based on our previous research findings (Yuan et al., 2022; He et al., 2023; Liu et al., 2024; Zeng et al., 2024). These metals were found to be of particular concern due to their higher concentrations in the affected soils, compared to other heavy metals. This made them a focus for assessing contamination and remediation efforts in the region.

This study employed a dual approach, utilizing both laboratory and field experiments. Vegetable soil, suitable for controlled laboratory settings, was chosen to investigate the mechanistic effects of compost amendments on HM immobilization under aboratory conditions. Complementing this, a field study was conducted using paddy soil, a prevalent soil type in the Taojia River Basin and an area susceptible to HM accumulation, which poses risks to agricultural productivity and food safety. This strategic selection aimed to bridge the gap between controlled experimental findings and real-world agricultural practices. By examining compost efficacy in both idealized laboratory conditions (vegetable soil) and a representative agricultural setting (paddy soil), the research sought a comprehensive understanding of its potential for HM remediation.

The SM and CM composts used in Segment One were sourced from the Shijiazhuang Biofertilizer Engineering and Technology Research Center products. The composting process involved three main stages: compost fermentation, manure storage tank fermentation, and biogas fermentation. We utilized fermented compost obtained from separately composting CM and SM with hay. During the fermentation process, the compost was regularly turned to ensure complete fermentation, and the temperature was maintained at 45°C for 15–30 days.

In the second phase of the study, two distinct treatments were implemented. In the first treatment, 10% CM compost, selected as the most effective passivator from the first segment, was incorporated into the soil—the other treatment served as a control, with no compost added. Two 20 x 20 square meter plots were prepared for this phase, and rice was cultivated in both and taken care of as per guidelines.

The concentration of HM in the digested solution was determined by inductively coupled plasma mass spectrometry (ICP-MS) (HJ 803—2016). To assess the distribution of HMs among different chemical fractions, we employed a sequential extraction procedure based on the method established by Tessier et al. (1979). This method involves several steps: the exchangeable fraction was extracted using 1 M magnesium chloride (MgCl₂), which targeted easily soluble metals; the carbonate-bound fraction was obtained with 1 M sodium acetate (NaOAc) at pH 5, allowing the identification of metals associated with carbonates; the iron and manganese oxides-bound fraction was extracted using 0.04 M hydrochloric acid (HCl), which released metals associated with these oxides; the organic matter-bound fraction was extracted using hydrogen peroxide (H₂O₂) followed by ammonium acetate (NH₄OAc) at pH 4.5, which helps to determine metals bound to organic matter; and finally, the residual fraction was assessed by mineralizing the remaining residue with a mixture of nitric acid (HNO₃) and hydrochloric acid (HCl). This comprehensive sequential extraction approach provides valuable insights into the mobility and bioavailability of heavy metals in the soil matrix (Tessier et al., 1979).

A one-way analysis of variance (ANOVA) was conducted at a 5% significance level to assess differences in descriptive statistics across compost treatments and their application rates (Hicks et al., 2022). Data were expressed as mean ± standard deviation (SD), and significant differences were evaluated using Tukey’s HSD post hoc test. Relationships between parameters were analyzed via Pearson correlation and visualized using hi-plots. Statistical analyses were performed with IBM SPSS Statistics (Version 21.0; IBM Corp., Armonk, NY, USA), while figures were generated using OriginPro software. In some cases, negative values for metal concentrations appeared in the results. These did not represent actual negative concentrations but instead reflected measurements below the detection limit of the analytical instruments, likely attributable to instrumental noise. Consequently, such values were interpreted as non-detectable concentrations (below the method’s detection threshold). While these values did not influence the overall trends or conclusions of the study, they were retained to ensure full transparency in data reporting—a standard practice in environmental research where trace metal levels may fall below reliable quantification limits.

The addition of SM compost increased soil pH, with the magnitude of the rise demonstrating a direct proportionality to the quantity applied. However, no significant difference in pH elevation was observed between the 2% and 5% SM treatments. CM compost also enhanced soil pH substantially, though the extent of improvement did not correlate proportionally with the application rate ( Table 1 ). Overall, composted manure amendments resulted in a modest pH increase, maintaining the soil under slightly alkaline conditions.

SOM increased more markedly with CM compost than with SM compost. Additions of 2%, 5%, and 10% CM compost yielded SOM values of 0.660 g/kg, 0.606 g/kg, and 0.623 g/kg, respectively, whereas SM compost applications produced lower SOM levels of 0.495 g/kg, 0.421 g/kg, and 0.414 g/kg at equivalent rates ( Table 1 ). TN content rose significantly to 0.172 g/kg in soil treated with 2% SM compost. However, increasing the SM compost rate to 5% did not induce further TN changes; a notable increase to 0.210 g/kg occurred only at the 10% application rate. In contrast, TN levels in CM compost-amended soils exhibited a dose-dependent proportionality ( Table 1 ). TP content showed a substantial and linear increase in soils amended with SM compost, corresponding to the application rate. Conversely, CM compost additions did not significantly elevate soil TP content ( Table 1 ).

At a 2% SM compost application rate, the soil’s DTPA-extractable Pb content reached 14.345 mg/kg. However, increasing the SM compost rate to 5% reduced the DTPA-extractable Pb content to 11.877 mg/kg. A further increase to 10% SM compost elevated the soil’s DTPA-extractable Pb content to 14.006 mg/kg. In contrast, the addition of 2% CM compost resulted in DTPA-extractable Pb content of 19.235 mg/kg. Beyond this rate, increasing the CM compost quantity produced a proportional decline in DTPA-extractable Pb levels. The highest passivation efficiency was achieved at a 10% CM compost rate, yielding an DTPA-extractable Pb content of 11.561 mg/kg ( Figure 2a ).

The relationship between soil pH levels and DTPA-extractable Pb demonstrated a weak correlation, suggesting that pH was not the primary driver of Pb activity. However, the introduction of SM compost revealed significant positive correlations between compost application and SOM (p < 0.05) and TP content (p < 0.05) ( Figure 2b ). Similarly, CM compost addition showed a significant positive correlation with SOM (p < 0.05). Additionally, a dose-dependent relationship emerged between the soil’s available Pb content and the compost application rate (p < 0.05) ( Figure 2c ).

Figure 3 demonstrated that the soil pH (7.10 ± 0.05) in the 10% CM compost treatment was significantly higher than that in the control (6.71 ± 0.07). No significant differences were observed in SOM between the two treatments. Soil TN content was marginally elevated under the 10% CM application, while soil TP content increased significantly in the 10% CM compost treatment compared to the control.

The comprehensive analysis of all HMs revealed that their levels were significantly elevated (p < 0.05) in soil treated with 10% CM compost compared to control group ( Figure 3 ). In the paddy, a similar trend was observed for Zn and Cu, as their concentrations increased in parallel with their levels in the soil. In contrast, Pb and Cd concentrations in the paddy showed a more pronounced decrease under 10% CM compost treatment.

In the straw, Zn and Cu concentrations were notably higher following 10% CM compost application, whereas Pb and Cd levels exhibited a significant decline compared to the control group. Regarding the different forms of HMs in soil, paddy, and straw, Figure 4 presents their respective distribution percentages across various fractions, including the exchangeable, carbonate-bound, Fe-Mn oxides-bound, organic matter-bound, and residual states. The total contents of these HM fractions after 10% CM compost treatment and in untreated conditions are shown in Figures 5 and 6 , respectively.

Figure 7a showed the correlation among different states of HMs across different HMs and treatments. Except for the exchangeable state (p > 0.05), all other states of HMs exhibited statistically significant positive correlations with each other. Soil, paddy, and straw correlations were examined for a more in-depth analysis, as shown in Figure 7a . In soil, different states of HMs displayed positive correlations. However, in the case of straw, no clear correlation pattern was observed. Conversely, in paddy, a significant relationship was noted between various states of HMs, except for the exchangeable state and the residue state, which did not exhibit a significant correlation ( Figure 7b ).

In Figure 8a , we can observe the correlations among the total content of HMs in soil, paddy, and straw and their relationships with various soil properties for the 10% CM and CK treatments. Regarding soil properties, the soil pH showed significant correlations with Zn, Cu, and Cd content, while soil TP was associated with Cu and Cd. Notably, Zn exhibited a strong association with both Cu and Cd. Additionally, a robust relationship was found between Cu and Cd.

Concerning paddy, soil pH and TP were linked to Pb and Cu content, with Pb demonstrating a robust correlation with Cu. Likewise, there was a strong relationship between Cu and Cd. Regarding straw, soil pH and TP were associated with Zn and Cu content and soil pH was associated with Cd. Zn exhibited correlations with both Cu and Cd. It’s important to note that any relationships not mentioned were found to be statistically insignificant. Figure 8b presents the correlations among soil properties for the 10% CM and CK treatments. Except for the significant association between soil pH and soil TP, all other connections between soil properties were non-significant (p > 0.05).

Remediating soil HM pollution through livestock manure compost involves both direct and indirect mechanisms (Xiao et al., 2020). Direct mechanisms include HM passivation through complexation, adsorption, precipitation, and redox reactions within the soil. Indirectly, CM compost alters key soil properties such as pH, OM, TN, and TP content. After a 15-day greenhouse culture, the available Pb content in the untreated vegateble soil sample was 8.062 mg/kg. However, upon adding the two types of compost, the available Pb content in the soil increased. During the greenhouse culture period, microbial activity within the soil samples played a multifaceted role in influencing HMs. Microbial processes such as enzymatic catalysis (Saha et al., 2021), adsorption, dissolution, and redox reactions (Bai et al., 2020; García-Carmona et al., 2017; Jacob et al., 2018) contributed to these changes. Additionally, microorganisms produced various organic acids, including formic acid, butyric acid, citric acid, malic acid, and lactic acid, which facilitated HM dissolution and complex formation with HM elements in the soil (Manikandan et al., 2023).

Moreover, microbial enzymes enhanced HM solubility by reducing metal oxides (Ayangbenro and Babalola, 2017). These mechanisms contributed to the observed increase in available Pb content following compost application (Silva et al., 2005; Gaur et al., 2021). However, when compost addition exceeded a critical threshold, the available Pb content began to decline. Our results indicated that adding 10% CM compost reduced available Pb by 58.09% compared to adding 2% CM compost. This reduction could be attributed to multiple mechanisms: an increase in OM enhanced cation-exchange capacity, promoting greater Pb ion binding. Additionally, organic compounds in compost formed stable Pb complexes, reducing solubility and mobility. Compost also stimulated microbial activity, leading to biosorption and Pb precipitation. Furthermore, changes in soil pH due to compost addition likely facilitated Pb precipitation into less soluble forms.

These findings suggest that adjusting the dosage of SM and CM compost, while considering soil physicochemical properties, can aid in mitigating and remediating Pb contamination. The application of organic manure compost to Pb-contaminated soil influenced HM dynamics through microbial processes, including enzymatic catalysis, which enhanced HM solubility. Consequently, fixed Pb in the soil was released, increasing available Pb content. In cases where small amounts of SM and CM compost were added, residual Pb partially dissolved from the soil. However, as compost addition increased, Pb transitioned from extractable and reducible states to a more stable residual state, leading to an increase in oxidizable and available Pb content. Livestock and poultry manure compost not only enriched the soil with humus and inorganic elements but also facilitated Pb complexation and precipitation, ultimately reducing its bioavailable concentration.

The addition of SM and CM compost significantly increased SOM content compared to the control, with the extent of increase correlating with compost dosage. SOM plays a key role in enhancing organic adsorption and chelation intensity, promoting HM precipitation and reducing their bioavailability (He et al., 2022). This transformation resulted in more HM elements forming insoluble complexes (Liu et al., 2018). The SM and CM composts used in this study contained varying SOM compositions, combined with inorganic compounds capable of forming mineral precipitates and binding HMs. These interactions effectively immobilized HMs in the soil, further limiting their bioavailability. Moreover, both composts exhibited minimal effects on soil pH, and the correlation between pH and DTPA-Pb was weak. This suggests that under weak alkaline conditions, soil pH plays a limited role in controlling available Pb content and is not the primary influencing factor (Farrell and Jones, 2009).

Soil pH serves as a crucial indicator of the soil’s acidity or alkalinity. Where as SOM is a significant parameter in assessing land fertility, as it plays a pivotal role in supplying essential nutrients required for plant growth and development (Taskin et al., 2021). The introduction of a 10% CM led to a 5.7% increase in soil pH compared to the control group, demonstrating the effectiveness of this amendment. Interestingly, the addition of 10% CM compost did not result in any substantial difference in SOM compared to the control group.

TN and TP are vital nutrients for plant growth, and assessing their total quantities in the soil provided valuable insights into the soil’s nutrient supply capacity for fertilization (Francioli et al., 2016). The incorporation of CM compost significantly enhanced the N and P content in the soil. Compared to the control group, soil treated with 10% CM exhibited a notable 15.7% increase in TN and a substantial 30.6% boost in TP. This pattern aligned with the findings of Bożym and Siemiątkowski (2018), where the application of CM compost similarly resulted in a significant rise in soil pH. This effect was attributed to the alkaline nature of CM compost, which had a relatively high pH value and was applied in substantial quantities.

Notably, in China, the primary source of soil pollution in agricultural fields stemmed from inorganic fertilizers. While these fertilizers were essential for enhancing crop yields, they often led to the accumulation of harmful HMs and other toxic substances in the soil (Dong et al., 2021). Additionally, the unselective application of inorganic fertilizers altered soil pH and microbial activity, further exacerbating soil degradation (Saha et al., 2018). However, compost proved effective in mitigating HM contamination in soils, offering a sustainable alternative to the excessive use of inorganic fertilizers. A comparison of these results with those of other studies highlighted the gradual release of plant nutrients into the soil through the biodegradation of animal manure, suggesting that increasing the proportion of manure could enhance various soil quality attributes (Zhang et al., 2019; Iqbal et al., 2022).

High manure application rates consistently led to increased soil levels of HMs, namely Zn, Pb, Cu, and Cd. This trend aligned with the findings of previous studies (Kebrom et al., 2019). Following the addition of the 10% CM, the HM content, particularly Zn and Pb was notably higher in soil treated with CM than in the control group. The gap between Zn and Pb was particularly pronounced, indicating a significant accumulation of these HMs. Variations in the spatial distribution of HMs and dust deposition may have contributed to differences in soil HM content (Zhao et al., 2021).

HMs existed in various forms, each with distinct environmental behaviors and levels of toxicity. Among these forms, the exchangeable and carbonate-bound states exhibited high mobility and enhanced bioavailability, as highlighted by Zhang et al. (2018). Conversely, the iron-manganese oxide-bound state and the organic matter- and sulfide-bound state were relatively stable, with reduced bioavailability compared to the weak acid state, which represented a biologically potentially available form. When HMs were in the residue state, they demonstrated high stability and generally posed minimal risks to organisms and the environment, as their impact was usually insignificant.

Figures 4 , 5 , and 6 illustrated the distribution of HM forms in the soil. Notably, Cu and Cd predominantly existed in the residue state, with Cu having the highest proportion, followed by Cd, Zn, and Pb. Conversely, Zn, Cu, and Cd primarily existed in the less favorable exchangeable form. Pb had the highest proportion among these four HMs in its exchangeable form. It was imperative to take timely measures to prevent the migration of Pb to crops, as doing so could help mitigate crop pollution (Jiang et al., 2022).

In this study, the addition of CM compost resulted in increased HM levels in the straw and paddy compared to the pre-compost application period. This increase may have been due to the nutrient-rich composition of CM compost, which enhanced overall plant growth and, consequently, facilitated greater uptake of HMs by plant tissues (Zhang et al., 2019). However, the exchangeable state of Zn, Pb, and Cu in the paddy decreased following compost application. This reduction was likely due to the binding of these HMs to organic matter in the compost, leading to the formation of stable complexes that rendered them less bioavailable.

Furthermore, the total contents of Pb and Cd in rice and straw declined, while Zn and Cu increased. The decreased levels suggested effective immobilization facilitated by the organic components of CM compost, which enhanced the retention of these HMs in less soluble forms (Saha et al., 2018). In contrast, the increased levels of Zn and Cu may have indicated improved nutrient availability due to compost application or a synergistic effect that enhanced the absorption of these HMs (Zhao et al., 2021). Correlation analysis revealed significant associations (p < 0.05) between paddy soil pH, TP, and Pb and Cd levels. These findings suggested that composting primarily reduced the accumulation of Cd and Pb in rice seeds and straw by elevating soil pH and TP, which aligned with (He et al., 2022).

Additionally, the application of CM compost notably increased the exchangeable Pb fraction in the soil but had a limited effect on Pb accumulation in rice grains. Therefore, CM compost could serve as a valuable soil amendment for enhancing soil fertility and reducing Cd and Pb accumulation in paddy fields (Tang et al., 2021). However, the potential risk of metal accumulation in the soil due to the repeated application of chicken manure compost should be carefully considered. Moreover, while straw return to the field is generally recognized as beneficial for soil enrichment (Glithero et al., 2013), it could also exacerbate the accumulation of HMs. In this rice-growing region, returning straw to the field should not be done uncritically. Given the enrichment of Zn and Cu in the straw, proper treatment of the straw was essential to prevent secondary pollution.