Remediation and environmental safety of paddy soil temporarily polluted by heavy metals from acid mine drainage

ABSTRACT

This study examines the remediation of paddy soils briefly contaminated by acid mine drainage (AMD) using lime (LM) and steel slag (SS) amendments. The treatment not only boosted rice yields but also significantly lowered Cd and Cu levels in the rice. Additionally, it decreased net acid generation and increased acid neutralization capacity, reducing soil acidification risk, though reacidification potential remains high. Soil amendments improved pH, raising it to neutral, and enhanced cation exchange capacity from 4.35 to 9.23 cmol/kg. Enzymatic activities were also affected: dehydrogenase activity increased, urease decreased after LM and SS application, and sucrose activity varied with the type of amendment. The findings suggest that AMD-polluted paddy soils can be effectively remediated through soil amendments and rice cultivation, but continuous monitoring and further interventions are needed for sustained soil health.

KEYWORDS:

• Acid mine drainage (AMD)
• paddy soil
• Cd/Cu contamination
• Cd/Cu pollution
• plants remediation

Introduction

In the mining and industrial areas, sulphide minerals such as pyrite oxidize in the presence of air and water, forming sulphuric acid and releasing heavy metal ions, thereby generating Acid Mine Drainage (AMD) [1,2]. The primary characteristics of AMD waters include extremely low pH and high concentrations of dissolved heavy metals. These heavy metals migrate and accumulate in surface waters, groundwater, and soil through processes such as dissolution, adsorption, and precipitation, so transitioning from solid ore to the aqueous environment. The attributes of AMD have far-reaching impacts on ecosystems: acidity and heavy metal pollution pose direct threats to aquatic life and, through the food chain, affect broader ecosystems. Concurrently, the contamination of soil and groundwater impacts plant growth and the diversity of soil microorganisms, thereby disrupting the entire ecological balance [3]. The processes of mining and transportation contribute significantly to this phenomenon, discharging copious amounts of heavy metals into the environment and rendering the soil highly prone to pollution, thereby becoming a primary vector for soil heavy metal pollution. The effects of long-term AMD pollution on the surrounding soil can be divided into three categories: chemical (such as extremely low pH and increased heavy metal content in the soil), physical (such as high bulk density, low soil water retention, and poor aggregation) and change in microbial habitat conditions (such as low soil organic matter and poor nutrient turnover). Additionally, the impact of short-term AMD pollution on soils, despite being less explored, warrants attention. Long-term pollution usually results in more severe environmental disruption [4]. Over time, the concentrations of heavy metals in the soil can increase significantly as they continue to accumulate and precipitate [5]. This prolonged pollution can lead to drastic changes in soil microbial communities, potentially causing a loss of biodiversity and the breakdown of essential soil functions [6]. Furthermore, prolonged pollution can significantly alter the soil structure and reduce fertility, impacting plant growth and soil quality an health. Moreover, soils that have been polluted over the long term are often more challenging to remediate due to the entrenched nature of the changes and the higher concentration of pollutants [7]. However, short-term impacts of pollution allow for early intervention, which can prevent the severe effects of such long-term pollution. Prompt implementation of preventive measures and remediation strategies reduces the overall cost and effort for soil recovery [8,9]. Short-term pollution can also serve as an indicator of potential long-term impacts, guiding future land use and management decisions [10]. By addressing short-term contamination, we can protect and preserve nearby ecosystems. Cadmium (Cd) and copper (Cu) represent prevalent form of heavy metal pollution in AMD [11,12]. The Bulletin of the Chinese National Soil Pollution Survey highlights a concerning trend – the incidence rates of Cd and Cu pollution exceeding the standard thresholds, reporting values of 7.0% and 2.1%, respectively. Given the present state of heavy metal pollution globally and its resulting impacts, the necessity for developing effective soil remediation techniques for areas afflicted by heavy metal pollution is paramount. This development is crucial not only for mitigating the immediate adverse effects of such pollutants but also for ensuring the long-term sustainability and health of ecosystems. Numerous remediation techniques have been developed for soil polluted with AMD. Over the years, a variety of in situ and ex situ remediation technologies have been proposed [13–15]. Among these, in situ remediation is often considered a cost-effective, environmentally friendly sustainable method for treating AMD polluted soil. In situ remediation methods include bioremediation, phytoremediation, chemical oxidation, electrokinetic remediation, soil vapor extraction (SVE), in situ thermal remediation, solidification and stabilization, and natural attenuation. Solidification and stabilization methods commonly involve the application of plants and amendments together, which are widely recognized as practical and eco-friendly strategies for enhancing soil pH levels, thereby effectively enhancing soil acidity from a low level to acceptable levels [16]. Extensive research has demonstrated that in-situ soil treatment with various amendments can reduce the bioavailability of heavy metals. The accumulation of Cd and Cu absorbed by rice from soil depends on the content of available Cd and Cu in the paddy soil. Inhibition of soil mobility of Cd and Cu is therefore crucial for reducing the accumulation of Cd and Cu in rice crops. In previous studies, it has been demonstrated that soil amendments fix soil Cd and Cu through mechanism such as complexation, adsorption, and precipitation [17]. Lime, steel slag, biochar, and other inorganic and organic amendments have been widely used to improve heavy metal polluted soils. Alkaline materials such as lime and steel slag are particularly effective in reducing the availability of soil Cd and Cu by increasing the soil pH [18]. Lime, through neutralizing both active and latent soil acids, triggers the precipitation of metal hydroxides, thereby boosting the pH and reducing the exchangeability of heavy metals. It also improves soil Ca²⁺and CEC, aiding in the immobilization of Cd and competing with Cd²⁺ at the root interface [19,20]. Steel slag is an industrial solid waste that is produced during the process of making steel; it contains nutrients such as calcium(Ca), silicon(Si), phosphorus (PO₄), and others elements beneficial for plant growth, allowing its use as fertilizer [21]. Its high porosity and large specific surface area, post-crushing, make it an effective adsorbent for metal polluted farmland leachates, trapping cations including Cu²⁺, Cd²⁺, Pb²⁺ and Zn²⁺. Furthermore, the high alkalinity of steel slag can raise the soil pH and stabilize heavy metals when applied to a polluted soil [22].

When concentrations of soil heavy metals exceed regulatory thresholds, varying degrees of toxicity can be manifested in crops. The bioavailability of heavy metals can effectively indicate the mobility of heavy metals and their likely ecological impacts. By utilizing soil amendments to decrease bioavailability of can enhance soil quality and facilitate crop growth. Among the various indicators of soil health, soil enzymes are highly sensitive to metal toxicity and soil pH, and can characterize the ecotoxicological effects of heavy metals in a soil [2,23]. Therefore, soil enzyme activity is an important biological indicator for assessing soil health [24]. Soil pH is also a pivotal factor in remediating soil polluted with heavy metals. Both cation exchange capacity (CEC) and soil organic matter (SOM) play a significant role in affecting bioavailability [25]. Furthermore, the potential for acid production in a soil polluted with heavy metals can be assessed through metrics such as net acid generation (ANG) and acid neutralization capacity (ANC) [26–28]. The elements silicon (Si), phosphorus (PO₄), and nitrogen (N) are essential for plant growth and development, and thus serve as fundamental indicators of soil fertility [29–31]. The impact of different soil amendments on the quality of brown rice can also be indicative of soil health, providing a practical measure for assessing the impact of remediation efforts.

In the present study, various soil amendments were formulated through the integration of lime, steel slag, compound fertilizer, and organic fertilizer. The aim of the research was to investigate the effects of these amendments on the absorption of Cd and Cu by rice within a single growing season, as well as their impact on plant growth in contaminated soils. The study examined the influence of the soil amendments on soil enzymes, soil nutrients, and trace elements, assessing the potential impacts of these amendments on the soil environment during short-term remediation of polluted sites. The findings of this research could assist in the reduction of AMD impacts in soil-rice systems over a short period.

Materials and methods

Study area and soil material

The soil utilized in a pot experiment was obtained from a paddy field located at 109°11’E, 25°05’N in Liuzhou, Guangxi (Figure 1). The roadbed stones (sulphide ores) located near the paddy fields contain heavy metals, primarily Cu and Cd. After rainfall, acidic mine wastewater leaches out, and via runoff flows into the paddy fields. One month elapsed from the initial observation of acidic water leaching post-rainfall to the clearance of the roadbed stones (sulphide ores), approximately one month elapsed. This led to the pollution of the soil within a radius of approximately 1 km from the adjacent road. To ensure representative soil sampling, random fixed points were established in the contaminated paddy fields, and the ‘S’ shaped soil sampling method was employed to select six different points within the farmland. At each of these points, soil samples were collected from a depth of 15 cm below the surface. These samples were mixed and air-dried naturally. After air-drying, a portion of the dried soil was then ground in the laboratory using a mortar and pestle. Subsequently, the soil is passed through a 2 mm plastic sieve to ensure uniformity, and stored for future use in experiments. The characteristics of soil before the experiment are shown in Table 1. In the samples of paddy soil polluted with metals, the concentrations of Cd and Cu were approximately 0.4 mg/kg and 60 mg/kg, respectively. These levels exceed the Grade I standards of the National Standard (GB 15,618–2018) and likely pose a severe threat to food safety.

Figure 1. Location of pollution.

Table 1. Initial cadmium and copper contents, pH and SOM of the soil.

	pH	Cd (mg/kg)	Cu (mg/kg)	SOM (mg/kg)
CS	2.8	60	0.4	30.5
CK	5.2	31	0.08	63.7

Soil amendments and plants

To investigate the remedial effects of different proportions of lime (LM), purchased from a local market, steel slag (SS), obtained from Liuzhou Steel Plant, and base fertilizers-compound fertilizer (CF) and a mix of compound and organic fertilizers (CF+OF), the CF and OF was purchased from Liuzhou Flower Market. We designed combinations on polluted soil as shown in Table 2. These mixtures were incorporated into the polluted soil (CS) at varying ratios and then placed in plastic pots. The rice variety planted, Liu Feng Xiang Zhan, was selected for the pot tral. The cultivation and management techniques implemented were consistent with line established local agricultural practices. The cultivation was conducted under natural conditions with regular irrigation. The pot experiment was carried out at the Liuzhou Agricultural Science Research Institute. For the preparation of the potting soil, the soil mixture was initially saturated with water, ensuring a level 5 cm above the soil surface. This saturation was maintained for a period of 2 days to ensure adequate soil moisture in the substratum. Following this, rice seedlings were transplanted into pots, with each pot containing two plants. Three independent replicates were prepared for each treatment combination.

Table 2. Combination treatments and dosage.

Treatment	CS	SS	LM	CF	OF
	(g)	(g)	(g)	(g)	(g)
CS+SS①+CF	3000	60	–	1.5
CS+SS②+CF	3000	105	–	1.5
CS+SS①+CF+OF	3000	60	–	1.5	5
CS+SS②+CF+OF	3000	105	–	1.5	5
CS+LM①+CF	3000	–	45	1.5
CS+LM②+CF	3000	–	75	1.5
CS+LM①+CF+OF	3000	–	45	1.5	5
CS+LM②+CF+OF	3000	–	75	1.5	5
CS	3000	–	–	–
CS+CF	3000	–	–	1.5
CS+CF+OF	3000	–	–	1.5	5

Collection and preparation of samples

After a quarter’s cultivation and upon the maturation of the rice, the rice was harvested and subjected to further processing. The rice grains were collected, and key parameters such as the total grain count and the height of the rice plant were measured. Subsequently, the glasshouse-grown rice plants were removed from the pots, and the soil adhering to the roots was thoroughly washed off using tap water. This was followed by a secondary rinse with deionized water to ensure complete removal of any residual soil particles. The cleaned plants were then air-dried and the separated into their constituent parts: roots, stems, leaves, and husked grains(brown rice), Each component was finely ground using a pulverizer to passed through a 0.15-mm plastic sieve. The finely ground, sieved samples were carefully stored in airtight bags to prevent contamination and degradation. The material remaining after sieving was preserved separately for analysis.

Analyticals methods

Cadmium in paddy samples was determined according to the National Food Safety Standard for the Determination of Cadmium in Foods (GB5009.15–2014). After samples digestion(Model DTD-40 Digester, Changzhou Pusen Electronic Instrument Factory), Cd was analyzed by Graphite Furnace Atomic Absorption Spectrophotometry [32]. Determination of copper was by Flame Atomic Absorption Spectrometry (AAS ZEEnit700 model, Germany), following the methodology of Teodoro [33]. A Flame Atomic Absorption Spectrophotometry method (HJ 491–2019) and the Graphite Furnace Atomic Absorption Spectrophotometry method (GB/T 17,141–1997) were employed to determine the concentrations of Cu and Cd in soil, respectively. A Toxicity Characteristic Leaching Procedure (TCLP) [34] was use to determine the potential for water-soluble heavy metals within the waste to leach out and contaminate groundwater.

Soil urease and sucrase activities were determined by the indophenol blue colorimetric method and the 3,5-dinitrosalicylic acid colorimetric method, respectively. The measurement of soil dehydrogenase activity was based on the determination of triphenyl formazan formed from the reduction of 2,3,5-triphenyl tetrazolium chloride (TTC) in the soil according to a modified Tabatabai method [35]. The instrument used for above measurement is Ultraviolet-visible Spectrophotometer(UV-2000, Unicosh (Shanghai) Instruments Co., Ltd)

Soil pH(pHS-25cw, Shanghai Selon Scientific Instrument Co.,Ltd.) was determined with a soil to water ratio of 1: 2.5 [32]. Soil organic matter (SOM) was determined by the potassium dichromate oxidation-external heating method [36], and cation exchange capacity (CEC) was measured using ammonium acetate method [37].

Soil available phosphorus (AP) was determined using the molybdenum blue method [38]. Soil available Si was determined by silicon molybdenum blue spectrophotometry (UV-2000, Unicosh (Shanghai) Instruments Co. Ltd) (NY/T 1121.15–2006) [39].

Soil acidification potential was characterized by Net Acid Generation (NAG) and Acid Neutralization Capacity (ANC). During the NAG determination, an excess of H2O2 (4.5 mol/L) was added to the soil sample, and the pH of the resulting suspension recorded as the NAG-pH. Subsequently, the suspension was titrated with NaOH, and the NAG of the sample calculated based on the volume of NaOH required for neutralization. For ANC determination, soil samples are immersed in an excess of HCl and heated to near-boiling conditions. The reaction suspension was then titrated with NaOH, and the ANC of the soil is calculated based on the volumes of HCl and NaOH used in the titration process.

Statistical analysis

The data analysis was conducted using SPSS 21.0. Single-factor analysis of variance (ANOVA) and Pearson correlation analysis were performed on the target variables.

Results

Effects of amendments on rice growth

Treatment CS+SS①+CF produced the highest average plant height (77.83 cm) and a substantial yield (1107 grains per plant). Despite a relatively high error margin, it is marked with a statistical significance, indicating its effectiveness. Conversely, CS+SS②+CF showed a slightly lower plant height (74 cm) but achieved the highest yield (1251 grains per plant). Treatments with added organic fertilizer (i.e. CS+SS①+CF+OF, CS+LM①+CF+OF) generally showed a decrease in yield compared to their counterparts without organic fertilizer. This might be due to the different nutrient release rates or interactions between organic and inorganic components in the soil. The control group CS (polluted soil), as expected, showed the least effectiveness in both growth and yield, underscoring the positive impact of soil amendments.

Figure 2. Plant height and average yield of rice grains per plant of rice. Different letters indicated that there were significant differences in the application of amendments (p < 0.05) (single factor analysis of variance (ANOVA)).

Changes in soil fertility

For the remediation of the heavy metal-polluted soil, the application of compound fertilizer alone or in combination with organic fertilizer does not demonstrate superiority over other treatment groups. This suggests that the use of steel slag and lime does indeed improve the soil environment. Following the application of steel slag and lime, soil pH increased to neutral. The treatments of CS+LM②+CF+OF and CS+SS②+CF+OF resulted in the most significant increase in soil organic matter and showed a significant difference (p < 0.05) compared to CS alone. Available Si content was generally higher with steel slag treatment than lime and was significantly different from CS (p < 0.05). Overall, soil organic matter and available Si content were more improved under steel slag treatment compared to lime treatment.

Effects of amendments on rice

Figures 3 and 4 indicate a clear downward trend in the Cd and Cu contents of rice roots. It is evident that a sole application of compound fertilizer, or its combined use with organic fertilizer, can reduce the accumulation of heavy metals in the roots even without soil amendments. However, the application of lime and steel slag offers a more pronounced advantage in reducing their accumulation, with their effectiveness increasing as the amounts of lime and steel slag increase. Most importantly, the content of Cd and Cu in rice grains is significantly controlled by lime and steel slag by reducing the accumulation of Cd and Cu in the rice roots. The contents of Cd and Cu in brown rice decreased by 4.65% to 60% and 31.5% to 57.2% respectively, compared to that from the control soil (p < 0.05). This underscores the efficacy of lime and steel slag in mitigating the accumulation of potentially toxic heavy metals in rice, particularly in the consumable parts.

Figure 3. The content of Cd in rice roots, stems and rice after the application of amendments. Different letters indicated that there were significant differences in the application of amendments (p < 0.05) (single factor analysis of variance (ANOVA)).

Figure 4. The content of Cu in rice roots, stems and rice after the application of amendments. Different letters indicated that there were significant differences in the application of amendments (p < 0.05) (single factor analysis of variance (ANOVA)).

Leaching of cadmium and copper by amendments

It can be seen from Figure 5 that application of compound fertilizer can reduce the leaching of Cd and Cu. However, when applied in conjunction with steel slag and lime, there is no decrease in the leaching of Cd and Cu. On the contrary, compared to the compound fertilizer alone, the steel slag treatment can even increase the leaching of Cd and Cu. This inconsistency with previous studies leads us to consider several possibilities [40,41]: (i) the compound fertilizer may contain organic matter or other complexing agents capable of forming more stable complexes with heavy metal ions in the soil, thereby reducing their leaching, (ii) the nutritional elements provided by the compound fertilizer might compete with heavy metal ions on the soil particle surface for adsorption sites or participate in ion exchange. For instance, added K and PO₄ might displace Pb or Cd cations on soil particles, thus diminishing the leaching of these heavy metals, (iii) the supplied nutrients could enhance the activity and diversity of soil microorganisms. Certain microbes have the ability to transform or immobilize heavy metals in the soil, reducing their bioavailability and mobility.

Figure 5. Leaching amounts of Cd and Cu after amendment application. Different letters indicated that there were significant differences in the application of amendments (p < 0.05) (single factor analysis of variance (ANOVA)).

Soil enzyme activity

In this study, the activities of urease, dehydrogenase and sucrase in the soil were assessed. When treated with CS + CF and CS + CF + OF the soil exhibited an increase in urease activity ranging from 22.7% to 37.8%. Conversely, sucrase activity decreased significantly, falling between 31.8% and 71.7%. Dehydrogenase activity remained unchanged, with a recorded activity of zero. The soil treated with CS + CF and CS + CF + OF showed no statistically significant difference in the activities of urease and dehydrogenase (p > 0.05). However, sucrase activity differed significantly (p < 0.05). Furthermore, the addition of lime amendment to the soil resulted in a decrease in urease activity by 10.3% to 45.1%, and a substantial reduction in sucrase activity, ranging from 361% to 1433%, while simultaneously promoting dehydrogenase activity. Urease activity exhibited no significant difference (p > 0.05), but marked differences were observed for dehydrogenase and sucrase (p < 0.05). Additionally, the inclusion of steel slag in the soil led to an increase in urease activity by 8.7% to 92% and a dramatic decrease in sucrase activity, which ranging from 678% to 2544%, while also enhancing dehydrogenase activity. Significant differences were observed in the activities of dehydrogenase and sucrase (p < 0.05), as well as in the urease activity for treatments CS+SS②+CF and CS+SS②+CF+OF (p < 0.05). In contrast, treatments CS+SS①+CF and CS+SS①+CF+OF showed no significant difference in urease activity (p > 0.05). This study indicates the complex interactions between soil treatments and enzyme activities, offering insights into their potential implications for soil health and productivity.

Effect of amendments on soil acidification

As illustrated in Figure a, the application of both steel slag and lime reduced the net acid production of the soil, with steel slag demonstrating a stronger capacity. Soils treated with lime and steel slag amendments exhibited a significant reduction in net acid production ranging from 25.9% to 2013% compared to polluted soils, with a notable difference (p < 0.05). As depicted in Figure b, both steel slag and lime were found to enhance the soil neutralization capacity. Specifically, the application of 105 g of steel slag increased the soil’s neutralizing potential. Lime treatment resulted in an increase in soil neutralization capacity by 98.6%-207%, while the steel slag treatments increased neutralization capacity by 82.4% for CS+SS②+CF and by 1383% for CS+SS②+CF+OF, both showing significant differences (p < 0.05). These data highlight the potential of these amendments for improving soil quality and mitigating acidity.

Pearson correlation analysis

The accumulation of heavy metals and growth patterns in rice may be associated with changes in soil physico-chemical properties induced by soil amendments. Urease activity showed no significant correlation with plant height, grain yield, or the accumulation of Cd and Cu in rice grains. Conversely, sucrase activity had a positive correlation. Additionally, dehydrogenase activity was positively correlated with plant height but negatively correlated with the accumulation of Cd and Cu in the rice. Available PO₄, Si, pH, CEC and DEH were negatively correlated with the accumulation of Cd and Cu in rice but positively correlated with plant height. NAG showed a positive correlation with the accumulation of Cd and Cu in rice grains, and a negative correlation with plant height and grain yield. These findings underscore the complex interactions between soil amendments and both the physiological growth and heavy metal accumulation in rice.

Discussion

The findings of this study indicate that in short-term polluted paddy fields here the soil is acidic, and the levels of Cd and Cu exceed National Standards, potentially posing a severe threat to food security. Remedial measures are necessary to mitigate the hazards of these heavy metals and the accumulation in grains. We employed lime and steel slag as soil amendments and compared their effects on the remediation of heavy metal pollution and the accumulation of heavy metals in rice. Overall, both steel slag and lime amendments not only reduced the accumulation of heavy metals in rice roots but also decreased their concentration in the aerial parts of the rice plant, notably in the rice grains. This effect can be attributed to the complex process of heavy metal uptake by and translocation from rice roots to brown rice [42], the study indicates that during the transport of Cd and Cu metals might co-precipitate. This impedes the return of Cd and Cu to the ground, thereby reducing the levels of Cd and Cu in brown rice [43,44]. The application of lime and steel slag amendments significantly increased the available Si content in the soil and improved the soil pH (Table 3). Following the application of lime and steel slag, there was a decrease in the concentration of the two heavy metals in rice organs and an increase in average plant height and yield (Figure 2).

Table 3. Effects of treatments on SOM, CEC, pH, available Si and available PO_4..

	SOM	CEC	pH	available Si	available PO4
	(mg/kg)	(cmol/kg)		(mg/kg)	(mg/kg)
CS	32.84 ± 1.03c	2.74 ± 0.14g	3.42 ± 0.35d	16.67 ± 3.87g	10.45 ± 2.87h
CS+CF	31.93 ± 0.70c	2.97 ± 0.079g	3.28 ± 0.30d	16.12 ± 0.55g	9.45 ± 0.23fg
CS+CF+OF	43.93 ± 4.23b	3.06 ± 0.52g	3.03 ± 0.01d	30.81 ± 23.54g	13.96 ± 2.05g
CS+LM①+CF	32.09 ± 0.74c	8.51 ± 0.14cd	7.07 ± 0.23ab	102.85 ± 17.52f	20.09 ± 0.95e
CS+LM①+CF+OF	42.44 ± 0.28b	7.09 ± 0.21f	7.3 ± 0.14a	164.04 ± 20.82ef	24.55 ± 2.54bc
CS+LM②+CF	31.38 ± 0.97c	7.64 ± 0.48ef	7.45 ± 0.21a	138.45 ± 19.02e	19.99 ± 2.08e
CS+LM②+CF+OF	52.96 ± 3.71a	8.01 ± 0.62de	7.21 ± 0.05a	143.97 ± 10.79e	25.41 ± 2.5ab
CS+SS①+CF	33.80 ± 2.45c	11.97 ± 0.048a	6.51 ± 0.33c	437.24 ± 48.22c	17.47 ± 0.0046f
CS+SS①+CF+OF	44.89 ± 2.65b	8.51 ± 0.36cd	7.12 ± 0.46ab	518.36 ± 5.57d	22.8 ± 0.31cd
CS+SS②+CF	35.55 ± 2.83c	8.96 ± 0.28bc	6.7 ± 0.31bc	350.02 ± 25.9b	21.83 ± 0.4de
CS+SS②+CF+OF	55.53 ± 4.54a	9.33 ± 0.24b	7.03 ± 0.03ab	684.08 ± 29.17a	27.35 ± 0.31a

Different letters indicated that there were significant differences in the application of amendments (p < 0.05) (Single factor analysis of variance (ANOVA)).

Table 4. Pearson coefficients of correlation between some soil physico-chemical properties and plant traits.

	High	Grains	Cd-Root	Cd-Stem	Cd-Rice	Cu-Root	Cu-Stem	Cu-Rice
TN	0.148	−0.015	−0.427	−0.381	−0.027	−0.455	−0.462	−0.478
NH^4+-N	0.563	0.325	−0.443	−0.552	−.612*	−0.558	−0.541	−.607*
NO^3--N	0.121	−0.109	−0.089	−0.181	−0.327	−0.172	−0.118	−0.102
AP	0.716*	0.283	−0.906**	−.922**	−0.216	−0.900**	−0.912**	−.864**
SOM	0.196	0.497	−0.462	−0.328	0.195	−0.508	−0.524	−0.537
Urease	0.077	0.511	−0.144	−0.077	0.397	−0.211	−0.236	−0.129
Sucrase	0.879**	−0.499	0.841**	0.939**	0.622*	0.900**	0.862**	0.901**
Si	0.782**	0.581	−0.756**	−0.785**	−0.218	−0.721*	−0.739**	−0.686*
pH	0.805**	0.267	−0.800**	−0.915**	−0.561	−0.817**	−0.790**	−0.823**
ANC	−0.144	0.044	−0.169	−0.068	0.272	−0.265	−0.312	−0.238
NAG	−0.873**	−.666*	0.812**	0.849**	0.413	0.801**	0.817**	0.829**
CEC	0.946**	0.575	−0.682*	−0.875**	−0.683*	−0.753**	−0.777**	−0.728*
DEH	0.842**	0.583	−0.806**	−0.892**	−0.390	−0.820**	−0.812**	−0.790**

DEH: Dehydrogenase

Significance levels of correlations: *p < 0.05; **p < 0.01.

The immobilization of heavy metals is a critical factor in the risk assessment of remediation strategies for polluted soils [14]. Figure 5 indicates that the combined application of compound fertilizers, as well as the mix of compound and organic fertilizers, can reduce the leaching of heavy metals. This result is inconsistent with prior findings, yet it is evident that the application of lime and steel slag indeed decreases the leaching of heavy metals. pH, as a key parameter influencing the leaching of heavy metals [45], is significantly increased after the application of lime and steel slag [20], as shown in Table 3, leading to reduced leaching of Cd and Cu. In addition, amendments can immobilize and precipitate heavy metals through adsorption, complexation, or precipitation [46,47], further mitigating the risk associated with heavy metal pollution.

Adequate nutrition is crucial for plant survival in polluted soils. Studies have shown that amendment mixtures can enhance soil fertility, immobilize heavy metals, promote plant growth and yield, increase microbial activity, and reduce ecological risks [48,49]. Table 3 demonstrates that both steel slag and lime can improve soil nutrients status, as SOM, CEC, available Si, available PO₄, and pH levels are restored or elevated to fertility levels from a depleted state. Typically, the restoration of nutrients also involves soil enzymes, which also play a vital role in the soil nutrient cycle and can serve as effective indicators for assessing soil fertility [50]. We selected soil enzyme sensitive to soil heavy metals toxicity as activity indicators such as urease, dehydrogenase, and sucrase. Based on previous long-term remediation efforts, the potential for heavy metal-polluted soils to recover to their initial state has been demonstrated by other workers [7,51]. The soil in this study was subjected to short-term pollution and underwent only one season of remediation. After the application of lime amendments, a decrease in the activity of urease and sucrase was observed, whereas dehydrogenase activity increased. Conversely, after the application of steel slag amendments, an increase in the activities of dehydrogenase and urease was noted, but sucrase activity decreased (Figure 6). This indicates that the restoration effect on soil enzyme activities during short-term remediation did not follow a consistently positive and evident trend [52,53].

Figure 6. Soil urease, dehydrogenase and sucrase activity after amendment application. Different letters indicated that there were significant differences in the application of amendments (p < 0.05) (single factor analysis of variance (ANOVA)).

A significant increase in soil pH was observed after the application of amendments. Nevertheless, given the short-term nature of the remediation, it is necessary to assess the risk of re-acidification of the polluted soil [54,55]. An analysis of ANC and NAG changes (Figure 7) indicates that while the amendments enhanced soil ANC, they concurrently reduced soil NAG. Numerically, the values for both ANC and NAG for the two amendments were positive. An ANC > 0 signifies a capacity to neutralize soil acidity. A NAG value > 0 suggests that a soil still possesses the potential for acid production, suggesting a high risk of re-acidification [56]. Regular monitoring and timely replenishment of amendments are therefore crucial for mitigating the risks of soil acidification.

Figure 7. Effects of amendment application on soil net acid production capacity and soil acid neutralization capacity. Different letters indicated that there were significant differences in the application of amendments (p < 0.05) (single factor analysis of variance (ANOVA)).

These findings indicate that short-term soil remediation cannot be compared with those of long-term remediation and soil amendments exhibit complex interactions with rice physiological growth and the accumulation of heavy metals (Table 4), yet lime and steel slag as amendments hold substantial potential for the remediation of polluted soils and the safe production of crop plants such rice.

Conclusion

Pot experiments were conducted to investigate the effects of short-term AMD on a polluted farmland. The study applied various combinations of lime and steel slag as amendments to rice crops, which led to a reduction in both available Cd and available Cu in the soil. There was also a notable accumulation of Cd in the rhizosphere of rice, while the accumulation of Cd and Cu in the brown rice reduced. Soils polluted with AMD, low pH and poor CEC were improved after remediation. However, soil enzyme activity did not show a significantly improvement, and soil acidification was greatly inhibited, this necessitates long-term ecological monitoring. The amendments used promoted a small increase in the accumulation of soil nutrients. The results of this study can provide a theoretical basis for the use of amendments to improve soils polluted by AMD in the short term, and can hopefully be extended to other polluted soils.

Credit author statement

Ting Huang: Conceptualization, Formal analysis, Investigation, Writing – original draft. Youxuan Zhou: Resources, Data curation, Writing – review. Siyu Yin: Methodology, Writing -review and editing. Dongmei Deng: Writing – Reviewing and Editing, Supervision, Project administration, Funding acquisition

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was funded by National Natural Science Foundation of China [Grant Numbers: 31570506, 31700470] and the Guangxi Natural Science Foundation [Grant Numbers: 2017GXNFBA198099].