Organic Amendments Enhance the Remediation Potential of Economically Important Crops in Weakly Alkaline Heavy Metal-Contaminated Bauxite Residues

Abstract

Heavy metal (HM) pollution in soil has emerged as a global concern. This study introduces a novel approach to ameliorate HM-contaminated bauxite residues (BRs) characterized by weak alkalinity and low nutrient levels. By cultivating economically important crops, this method aims to enhance the remediation of heavy metal-contaminated BR while simultaneously promoting economically important crop production. Using a pot experiment, we investigated the effects of four organic amendments (peat, cow dung, bagasse, and microbial fertilizer) on the growth and BR properties of four economically important crops (castor, ramie, sugarcane, and cassava). The application of these organic amendments obviously reduced the BRs pH by 0.6–2.22%. Organic amendment applications significantly increased the soil organic matter (SOM) content and cation exchange capacity (CEC) by 14.35 to 179.94% and 6.87 to 12.14%, respectively. Additionally, the use of organic amendments enhanced BR enzyme activity, with microbial fertilizer demonstrating a substantial increase in BR invertase activity from 131.49 to 687.61%. Superoxide dismutase (SOD) activity and malondialdehyde (MDA) content remarkably increased, whereas catalase (CAT) activity did not show significant differences. HM content analysis in different plant parts revealed HMs primarily found in the plant roots. Organic amendments mitigate the transfer of HMs from roots to shoots, thereby reducing HM content in the available parts of economically important crops. The pot experiment results demonstrated the effectiveness of the four combinations in achieving both the repair and production objectives. These combinations include planting castor and ramie with cow dung, sugarcane with peat, and cassava with bagasse. These findings underscore the feasibility of cultivating economically important crops in HM-contaminated BRs, enhancing BR quality, and augmenting farmers’ incomes. This study provides a scientific basis for mine remediation and reclamation using BRs.

1. Introduction

Bauxite residue (BR) is a residual product generated from the process of bauxite washing, characterized by its fine particle size, high viscosity, extensive specific surface area, and resistance to natural settling [1]. The global accumulation of BRs has reached an estimated 5 billion tons, with an annual growth rate exceeding 0.2 billion tons [2]. Presently, the management of BR involves wet treatment, dry treatment, and semi-dry processing [3]. Research indicates that open-air storage of BR not only contaminates the atmosphere [4] but also diminishes the fertility of the adjacent soil [5], reduces microbial diversity [6], contaminates groundwater [7], and leaches soluble heavy metals, leading to increased soil alkalinity [8,9]. Implementing soil reclamation in mining areas serves a dual purpose: enhancing the ecological conditions of the mining sites and mitigating pollution, while also expanding arable land and addressing farmers’ land loss due to mining activities [10]. Considering the characteristics of fine particles, high viscoplastic index, weak acidity, low organic matter content, water swelling during reclamation, hardening upon drying, propensity for cracking, poor air permeability, and significant pulverization associated with BR, it necessitates improvement before application in mine reclamation [11,12].

Heavy metal (HM) pollution in soil is a global concern. The presence of HMs in agricultural soils has escalated owing to intensified human activity [13,14,15]. Currently, HM pollution affects 39.8% of farmlands in China. Among the investigated soil sites, the exceedance rates for Cd, Hg, As, Cu, Pb, Cr, Zn, and Ni were 7.0%, 1.6%, 2.7%, 2.1%, 1.5%, 1.1%, 0.9%, and 4.8%, respectively [16]. HMs in the soil can enter the food chain through crops, thereby exerting adverse effects on humans and animals [17]. Ullah et al. [18] discovered that farmland irrigation with HM-contaminated sewage resulted in elevated HM levels in vegetables cultivated in the soil, posing a threat to human health. Zhang [19] observed varying degrees of Cd, Pb, and Zn contamination in wheat grown near a coalmine, with Cd exhibiting the highest content. Furthermore, Huang et al. [20] found that long-term exposure to As pollution in the environment increases the risk of developing cancers, including skin, kidney, and lung cancers.

Phytoremediation is an economical and environmentally friendly technology that is widely employed to remediate HM-contaminated soils [21,22]. By planting in contaminated soil, HMs can be absorbed by the plants, and pollutants can be removed from the soil through plant harvesting, thereby achieving the objective of restoring HM-contaminated soil [23,24]. Over 700 hyperaccumulator species have been identified, and their numbers continue to grow [25]. Currently, traditional hyperaccumulators, such as Triticales sp., are frequently used for alkaline HM-contaminated soil remediation. This hyperaccumulator shows significant promise in addressing neutral or weakly alkaline soil contaminated with Cd and Zn [26]. Despite their significant advantages in remediating HM-contaminated soil, hyperaccumulators have limited economic benefits and are constrained in agricultural land applications. Introducing economically important crops provides a dual benefit; it enhances the economic income of farmers and aids in HM-contaminated soil restoration. Currently, several economically important crops such as castor, ramie, sugarcane, and cassava are effective in remediating HM-contaminated soil. Castor (Ricinus communis L.) is a long-lived herbaceous plant that belongs to family Euphorbiaceae. Its high oil yield makes it an attractive energy crop, and its oil is used to produce non-edible biofuels and biodiesel [27]. Moreover, castor shows great potential for HM-contaminated soil phytoremediation [28]. Ramie (Boehmeria nivea L.), an Urticaceae plant originating in East Asia, is one of the oldest fiber crops known to humans. Despite its development in mining areas characterized by poor soil quality or HM pollution, ramie exhibits strong growth, high biomass yield, and remarkable tolerance to and absorption of HMs, such As, Cd, Sb, Pb, and Mn [29,30,31]. Sugarcane (Saccharum officinarum) is a gramineous plant recognized as a primary sugar crop and constitutes a key source of income in many regions worldwide [32]. Its cultivation is particularly concentrated in regions with abundant sunshine such as Yunnan and Guangxi in China. Xu et al. [33] found that planting sugarcane in soils contaminated with HMs, including Pb, As, and Cd, resulted in the predominant accumulation of these metals in the roots. Importantly, the HM levels detected in the edible portions did not present any significant health risks to humans. Cassava (Manihot esculenta Crantz) is a woody, perennial shrub belonging to the family Euphorbiaceae. It is a promising bioenergy crop with a high starch content that serves as a source of food and renewable feedstock for ethanol fuel production. Notably, planting cassava in HM-contaminated soils has been found to reduce soil HM content and improve soil fertility. Furthermore, the HM (Cd, Cu, Pb, and Zn) content in the tubers, which are the edible parts of the plant, is lower than that in other parts [34,35]. The current research demonstrates the favorable tolerance of these four economically important crops to neutral and acid-contaminated soils. However, there is a paucity of research on their performance in BRs, particularly in the presence of contaminants such as As and Cd. Additionally, the mechanisms involved in remediation remain poorly understood.

Phytoremediation often faces the challenge of inhibiting plant growth owing to HM stress. Therefore, the incorporation of soil amendments serves a dual purpose: enhancing remediation efficiency and alleviating the adverse effects of HM stress on plant biomass [36,37]. Soil amendments play a crucial role in promoting plant growth by enhancing the soil environment. Soil amendments are classified into two categories: organic and inorganic. However, inorganic amendments present challenges, such as high costs, limited availability, and potential environmental pollution. In contrast, organic amendments not only comprise higher nutrient content but also facilitate plant growth while simultaneously enhancing the soil environment [38]. Organic amendment applications have been shown to improve enzymatic activity and soil biogeochemical cycles, leading to changes in soil microbial communities [39,40]. In a study by Hao et al. [41], manure and other organic amendments were effective in enhancing rice soil microbial biomass and organic matter content, with manure being more effective than straw. Lei et al. [42] demonstrated that soil amendments can enhance soil fertility and porosity, with organic amendments being more beneficial than inorganic amendments for improving corn yield and water use efficiency and enhancing the cultivated land’s soil environment. Additionally, Zhao et al. [43] found that the use of organic fertilizers as soil conditioners can increase the yield of Triticum aestivum L. and Zea mays L., as well as the organic carbon concentration and large soil aggregate formation. Bagasse is a widely available byproduct of the sugar industry, and its constituents, cellulose and hemicellulose, can be converted into monosaccharides, which can be fermented into fuel ethanol [44]. The primary focus of existing research revolves around transforming bagasse into biochar for soil amendment, whereas limited attention has been paid to the direct utilization of bagasse as a soil amendment. Additionally, bagasse can improve soil’s physical and chemical properties and enhance crop productivity and quality traits [45]. Studies have demonstrated that the lignocellulose in sugarcane bagasse can serve as a carbon source to stimulate soil microbial diversity and abundance [46]. Microbial fertilizers are environmentally friendly and cost-effective amendments that are rich in beneficial microorganisms that can decompose organic matter to supply nutrients to plants, thereby promoting their growth and development [47]. Moreover, organic amendments can effectively immobilize HMs in the soil, reducing their bioavailability and toxicity to plants [48,49]. For example, peat has been found to reduce Cd and As bioavailability in rice planting soils, modify the morphological distribution of these elements, and reduce their toxicity to plants [50]. Liu et al. [51] reported that bagasse can also reduce Cd accumulation in Brassica chinensis L. and has the potential to passivate Cd in the soil. Organic amendments play a crucial role in enhancing soil properties and fertility. At present, studies have shown that the application of organic amendments in the BRs can improve BR structure and soil formation process [52]. A review of the relevant literature revealed that these four types of organic amendments (peat, cow dung, bagasse, and microbial fertilizer) show significant potential in remediating heavy metal-contaminated soils. However, their application in the remediation of BR has received limited attention.

Previous research has predominantly examined the synergistic effects of organic amendments on farmland soils contaminated with heavy metals, with minimal attention given to BRs. To bridge this research gap, we conducted a study involving the application of four types of organic amendments (peat, cow dung, bagasse, and microbial fertilizer) to BRs while simultaneously cultivating four economically significant crops (castor, ramie, sugarcane, and cassava). Specifically, this study sought to investigate (i) the impact of different organic amendments on the physical and chemical properties of HM-contaminated BRs, as well as the HM content in BRs; (ii) the influence of different organic amendments on plant growth and physiology; (iii) the effects of different organic amendments on HM accumulation in economically important crops; and (iv) the identification of suitable combinations of economically important crops and organic amendments for cultivation in HM-contaminated BRs. This study provides a scientific basis for mine remediation and reclamation using BRs.

2. Material and Methods

2.1. Materials

BR: The BR was obtained from Pingguo County, Baise, Guangxi (23°38′91″ N, 107°51′02″ E). After drying in a cool location, the BR was sieved through a 1 cm nylon mesh. The physical and chemical properties of the tested BR were determined, and the results are listed in

Table 1. Physical and chemical properties of the test BR and materials.

		BR	Peat	Cow Dung	Bagasse	Microbial Fertilizer
Physicochemical properties	pH	7.68 ± 0.03	5.93 ± 0.13	7.72 ± 0.10	7.24 ± 0.11	6.72 ± 0.08
	SOM (%)	0.54 ± 0.08	12.66 ± 0.42	11.32 ± 0.16	11.47 ± 0.10	14.11 ± 0.57
	CEC (cmol+·kg−1)	3.20 ± 0.14	/	/	/	/
	Available P (mg·kg−1)	3.59 ± 0.92	136.28 ± 11.01	242.16 ± 14.74	5.35 ± 0.70	2.32 ± 1.47
	Available K (mg·kg−1)	17.59 ± 0.35	732.70 ± 44.13	3310.10 ± 263.07	260.23 ± 0.62	93.27 ± 8.88
	Hydrolytic N (mg·kg−1)	91.01 ± 29.73	406.25 ± 79.77	637.03 ± 55.58	84.01 ± 19.80	657.93 ± 19.70
Heavy metal content (mg·kg−1)	Total As	109.74 ± 3.45	0.48 ± 0.02	2.06 ± 0.24	0.79 ± 0.53	0.87 ± 0.05
	Total Cd	1.10 ± 0.28	0.06 ± 0.02	0.54 ± 0.02	0.14 ± 0.04	0.47 ± 0.02
	Total Cr	185.44 ± 9.14	1.23 ± 0.13	24.57 ± 1.28	3.35 ± 0.63	13.31 ± 1.06
	Total Ni	76.96 ± 6.93	0.55 ± 0.001	14.22 ± 1.07	1.42 ± 1.11	26.30 ± 0.75
	Total Cu	44.53 ± 1.43	21.71 ± 2.04	26.51 ± 2.01	4.30 ± 2.38	31.04 ± 0.34
	Total Zn	233.64 ± 13.44	13.22 ± 1.68	45.26 ± 1.53	8.91 ± 0.55	40.92 ± 0.50
	Total Pb	54.87 ± 1.81	4.93 ± 0.37	10.65 ± 0.69	4.25 ± 3.17	27.55 ± 3.67
	Available As	0.15 ± 0.01	/	/	/	/
	Available Cd	0.02 ± 0.001	/	/	/	/
	Available Cr	0.007 ± 0.001	/	/	/	/
	Available Ni	0.003 ± 0.001	/	/	/	/
	Available Cu	0.06 ± 0.003	/	/	/	/
	Available Zn	0.27 ± 0.004	/	/	/	/
	Available Pb	1.94 ± 0.03	/	/	/	/

Note: Values represent mean ± standard error (n = 3). “/” indicates undetected projects.

. The As, Cd, Cr, Ni, Cu, Zn, and Pb contents in the tested BR were found to be 3.66, 3.68, 0.93, 0.77, 0.45, 0.93, and 0.46 times higher, respectively, than the agricultural land risk management and control standard (GB 15618-2018, China) [53].

Amendments: Peat and cow dung were obtained from a flower market in Guilin City, China. The bagasse was obtained from a sugar factory in Nanning, Guangxi, China. Microbial fertilizer (approval registration No. 2436 (2018)) was purchased from Beijing Century Arms Biotechnology Co., Ltd., Beijing, China. Parameters related to the physical and chemical properties of the amendments added to the BR were measured, and the corresponding results are listed in Table 1. The amendments underwent functional group analysis using Fourier transform infrared spectroscopy (FTIR), and the results are illustrated in Figure 1.

2.2. Methods

2.2.1. Experimental Design

The pot experiments were conducted at the HM pollution phytoremediation test base at the Guilin University of Technology (24°3′43″ N, 110°18′5″ E, Guilin, China). The treated BR was loaded into plastic pots (upper diameter: 20 cm; bottom diameter: 15 cm; height: 20 cm) with a weight of 2.5 kg per pot, as determined using an electronic balance. Five treatments, including no addition, peat, cow dung, bagasse, and microbial fertilizer addition, were each added at a rate of 0.5% w·w⁻¹ (the mass ratio of amendment to BR) [45,51]. These treatments were applied to four types of economically important crops (castor, ramie, sugarcane, and cassava). These treatments were designated as CK, peat, cow dung, bagasse, and microbial fertilizer. Each treatment was replicated three times, resulting in 60 pots, with one plant grown per pot. After a stable 7-day pot-loading period, daily watering was carried out to maintain the normal growth of the plants. Plants were harvested 80 days after sowing, and the relevant indicators were determined and analyzed.

2.2.2. BR Sampling and Analysis

Following plant root removal, the BR from each pot was mixed and separated into two parts. One part of the BR sample was air-dried for BR chemistry analysis, while the other was maintained at 4 °C for biochemical analysis.

In this study, the pH, soil organic matter (SOM), available potassium, available phosphorus, and hydrolytic nitrogen in BR were assessed using established methods from the. literature [54]. The pH of the BR was measured using the ultrapure water leaching-potentiometric method. SOM was determined via potassium dichromate oxidation-spectrophotometry. Available potassium was measured using the ammonium acetate extraction method, while available phosphorus was determined through sodium bicarbonate extraction followed by molybdenum-antimony anti-spectrophotometry. Hydrolytic nitrogen was analyzed using the alkali diffusion method. The cation exchange capacity (CEC) was determined using hexaamminecobalt trichloride, and the absorbance was measured at a 475 nm wavelength after leaching [53]. Urease activity was assessed using the sodium phenol–sodium hypochlorite colorimetric method by incubating urea with BR at 37 °C for 24 h and measuring the content of ammonia produced by colorimetric assay at 578 nm [55]. Invertase activity was determined using the 3,5-dinitrosalicylic acid colorimetric method by incubating an 8% sucrose solution as the substrate, and the reducing sugar produced was quantified by measuring the absorbance at 508 nm [56]. Catalase (CAT) activity was determined using ultraviolet spectrophotometry to measure the difference in hydrogen peroxide content before and after incubation with the BR. The remaining hydrogen peroxide concentration indicated the content consumed by the enzyme-catalyzed reaction, thus reflecting CAT activity [57].

2.2.3. Plant Sampling and Analysis

At the end of the experiment, plant height was measured, and the shoots were separated from the roots using scissors. The plant parts were then washed three times with tap and deionized water. Subsequently, the samples were decolorized in an oven at 120 °C for 30 min and then to a constant weight at 60 °C. The weight of each plant was measured using an electronic balance. To determine malondialdehyde (MDA) activity in the leaves, we used the thiobarbituric acid colorimetric method [58]. This method involves the reaction of MDA with trichloroacetic acid under high-temperature and acidic conditions, resulting in a reddish-brown product. The product wavelength was measured using an ultraviolet spectrophotometer. Plant leaf superoxide dismutase (SOD) activity was measured using the nitrogen blue tetrazolium method [59]. SOD activity was determined by measuring the colorimetric changes at 550 nm. CAT activity was measured using the ammonium molybdate–ascorbic acid colorimetric method [60]. The CAT H₂O₂ decomposition reaction was terminated by adding ammonium molybdate, and the yellow complex formed at 405 nm was measured to calculate CAT activity.

2.2.4. Analysis of Heavy Metal Content in Plants and BRs

Accurate and reliable methods are essential in determining HM content in BR. This study employed the DTPA-leaching inductively coupled plasma emission spectrometer (ICP-OES, PE-Optima 7000DV, Waltham, MA, USA) to measure the effective state content of HMs in BR, and the aqua regia extraction inductively coupled plasma emission spectrometer method to determine the total content of HMs in BR. Specifically, 0.1 g of air-dried BR sample was weighed into a 50 mL conical flask, to which 10 mL of aqua regia solution (3+1 hydrochloric acid–nitric acid solution) was added and placed on an electric heating plate. Additional aqua regia solution was added until the BR sample was fully digested. After completion of digestion, the curved neck funnel and conical flask were washed with deionized water at least thrice. The transferred digestion solution and washing solution were then transferred to a 25 mL colorimetric tube, with a blank control set simultaneously. The quality of BR samples was evaluated using the national standard substance (GBW07407), and the recovery rates of As, Cd, Cr, Ni, Cu, Zn, and Pb in BR ranged from 81.03 to 102.19%.

The HNO₃-H₂O₂ digestion method, recommended by the U.S. Environmental Protection Agency (US EPA 3050b) [61], was used to determine the content of HMs in plants. The method involved adding 0.5 g of plant samples into a 50 mL conical flask, followed by the addition of 10 mL of HNO₃. The mixture was then heated on a hot plate at 80 °C overnight. After 12 h, the temperature was gradually increased to 180 °C, and 5 mL of 30% H₂O₂ was added twice when the digestion solution evaporated to about 3 mL. When the remaining digestion liquid reached approximately 1 mL, the conical flask was removed from the hot plate and allowed to cool. The curved neck funnel and conical flask were rinsed with deionized water at least three times, and the digestion and washing liquids were transferred to a 25 mL colorimetric tube, and a blank control was set at the same time. The quality of plant samples was controlled by the national BR reference material (GBW07603), and the recoveries of As, Cd, Cr, Ni, Cu, Zn, and Pb in plants ranged from 80.64 to 111.51%.

2.2.5. Statistical Analyses

The study conducted by Zhang et al. [62] employed bioaccumulation (BCF) and translocation factors (TF) as assessment metrics to investigate the capacity of economically important crops to accumulate and transfer HMs. BCF values were determined by comparing the HM content in various plant parts with that in the BR. TF values were calculated by dividing the HM content in the upper portion of the plant by that in the roots. The cumulative HM content of the plants was determined by multiplying the biomass of specific plant parts with the corresponding HM content.

SPSS (version 26.0) and Origin (2021) were used for data analysis and graphical representation of the experimental findings. To examine the statistically significant differences among the various treatments applied to the same plant, a one-way analysis of variance was conducted (p < 0.05). Additionally, dimension reduction calculation and principal component analysis were performed using the origin of the BR physical and chemical properties, BR and plant enzyme activity, BR HM content, plant growth, and physiological parameters.

3. Results

3.1. BR pH, SOM, CEC, and Enzyme Activity

Alterations in BR pH, SOM, and CEC resulting from the application of different amendments are shown in Figure 2a–c. The use of amendments had a significant impact on BR pH, as evidenced by a decrease compared with the CK (Figure 2a). In the case of castor, all four amendments caused a reduction in BR pH, ranging from 0.65 to 1.68%, compared to that of the CK. Regarding ramie, both cow dung and microbial manure application led to a decrease in BR pH by 0.91 and 1.04%, respectively, compared to that of the CK. Regarding sugarcane, the three amendments, excluding microbial fertilizer, resulted in a BR pH reduction ranging from 1.17 to 2.22% compared to that of the CK. Similarly, for cassava, the three amendments, excluding peat, caused a BR pH reduction ranging from 1.71 to 2.11% compared to that of the CK. Amendment inclusion resulted in a noteworthy enhancement in SOM content (Figure 2b). Compared with the CK, peat application increased SOM content by 74.39–113.48%. Cow dung application increased SOM content by 14.35–146.96%, whereas sugarcane bagasse application increased SOM content by 20.09–68.70%. Furthermore, microbial fertilizer application demonstrated a significant increase ranging from 84.55 to 179.94%. Amendment application to the BR where the castor was planted had no significant impact on the CEC (Figure 2c). However, compared with the CK treatment, peat application resulted in a significant increase in CEC for sugarcane- and cassava-planted BRs by 8.38 and 11.55%, respectively. Furthermore, cow dung application increased ramie-, sugarcane-, and cassava-planted BR CEC by 12.14, 10.03, and 8.45%, respectively.

The effects of various amendments on enzyme activity in BRs are illustrated in Figure 2d–f. Specifically, peat application to castor-planted BR resulted in a significant increase (61.48%) in CAT activity relative to that of the CK treatment (Figure 2d). Similarly, bagasse application resulted in a significant increase (47.49%) in CAT activity in ramie-planted BR but a significant decrease (70.01%) in cassava-planted BR. Additionally, the use of microbial fertilizer led to a significant reduction in CAT activity in sugarcane (47.10%) and cassava (63.46%) BRs. Peat application enhanced urease activity in sugarcane-planted BR by 82.61% compared to that of the CK (Figure 2e). However, cow dung application had a profound effect on the improvement of urease activity in BR. It increased the activity from 45.59 to 178.26% compared to that of the CK. Bagasse application did not have a significant effect on urease activity in castor BR but increased activity in the BR of the other three economically important crops by 35.19–282.61% compared to that of the CK. Microbial fertilizer application increased urease activity in sugarcane-planted BR by 117.39% compared to that of the CK treatment. The effects of the different treatments on BR invertase activity in the four economically important crops are shown in Figure 2f. However, compared with the CK, peat application increased invertase activity in the castor-planted BR by 93.44%. However, it had no significant effect on invertase activity in the BRs of the other three economically important crops. Furthermore, cow dung application significantly increased invertase activity in castor- and cassava-planted BRs by 104.71 and 90.62%, respectively, compared to that of the CK. Notably, microbial fertilizer application greatly improved BR invertase activity, increasing it by 131.49–687.61% compared to that of the CK treatment.

3.2. HM Content in BR

The results presented in Figure 3 depict the contents of HMs in BR after applying various amendments. Overall, the use of amendments demonstrated a tendency to decrease the contents of HMs in BR. However, the use of cow dung showed insignificant effects on the HMs content. On the other hand, the application of peat reduced the content of Cd (35.07%) in BR of castor planting, while the use of bagasse reduced Ni (8.54%) content. Moreover, the application of microbial fertilizer demonstrated a significant reduction in Cd (35.16%) and Pb (12.77%) in castor. In contrast, bagasse increased the content of Cu (5.16%) and Zn (5.96%) in sugarcane BR compared to the CK treatment. In the case of cassava, the use of bagasse reduced the content of As (11.84%), while microbial fertilizer demonstrated a reduction in As (13.61%) and Pb (9.54%), compared to the CK treatment.

3.3. HM Effective State Content in BRs

The effective state of heavy metals refers to the content of heavy metals in the BR that can be absorbed by plant roots during the plant growth period. The effects of different treatments on HM effective state contents in BRs are shown in Figure 4. Generally, amendments can increase the effective state content of each metal in BR. With the exception of Cr, nearly all treatments demonstrated an inclination to increased HM availability in the BR compared to that of the CK. Regarding castor, peat increased Cd (16.08%) availability, whereas cow dung increased Cd (28.94%), Cu (29.45%), Zn (64.26%), and Pb (11.75%) availability. Bagasse notably increased Cd (22.51%) and Pb (6.49%) availability, and microbial fertilizer application greatly increased Ni (300%) availability. Regarding ramie, peat significantly increased the Cd (15.57%) and Cu (14.30%) availability, whereas cow dung increased Cd (15.57%), Cu (21.45%), Zn (53.14%), and Pb (7.88%) availability. Bagasse notably increased Cd (12.46%) availability, and microbial fertilizer application increased Ni (99.98%) availability. Regarding sugarcane, the effective As content increased from 37.77 to 47.99% compared to that of the CK. However, the amendment had a minimal effect on improving the content of other HMs in the sugarcane-planted BR. Regarding cassava, peat and cow dung significantly increased Cd (20.53%), Cu (24.12–30.16%), and Pb (5.91–8.81%) availability. Bagasse and microbial fertilizer application increased Cd (14.67%) availability.

3.4. Plant Growth

The effects of the different treatments on plant growth are listed in

Table 2. Plant growth under different treatment conditions.

Plant	Treatment	Plant Height /cm	Root Biomass /g·plant−1	Shoot Biomass /g·plant−1	Stem Biomass /g·plant−1	Total Biomass /g·plant−1
Castor	CK	20.00 ± 1.00 b	0.62 ± 0.09 b	0.70 ± 0.07 a	/	1.32 ± 0.15 b
	Peat	19.83 ± 1.42 b	0.86 ± 0.12 ab	0.82 ± 0.24 a	/	1.68 ± 0.36 ab
	Cow dung	28.00 ± 2.65 a	1.11 ± 0.25 a	1.31 ± 0.16 a	/	2.42 ± 0.34 a
	Bagasse	20.17 ± 1.01 b	0.92 ± 0.08 ab	1.20 ± 0.19 a	/	2.12 ± 0.11 ab
	Microbial fertilizer	25.67 ± 2.09 ab	0.92 ± 0.09 ab	1.27 ± 0.25 a	/	2.20 ± 0.30 ab
Ramie	CK	19.00 ± 6.00 bc	0.07 ± 0.02 b	0.23 ± 0.07 b	/	0.30 ± 0.09 b
	Peat	28.67 ± 1.45 b	0.09 ± 0.02 ab	0.29 ± 0.03 b	/	0.38 ± 0.06 b
	Cow dung	48.83 ± 5.29 a	0.15 ± 0.02 a	0.63 ± 0.06 a	/	0.78 ± 0.08 a
	Bagasse	13.33 ± 1.86 c	0.08 ± 0.012 b	0.19 ± 0.03 b	/	0.27 ± 0.04 b
	Microbial fertilizer	11.67 ± 0.33 c	0.06 ± 0.004 b	0.14 ± 0.02 b	/	0.20 ± 0.02 b
Sugarcane	CK	73.00 ± 6.11 a	1.08 ± 0.27 a	1.45 ± 0.12 a	0.38 ± 0.04 a	2.91 ± 0.32 a
	Peat	80.33 ± 5.04 a	1.74 ± 0.59 a	2.52 ± 0.64 a	0.66 ± 0.14 a	4.92 ± 1.32 a
	Cow dung	75.67 ± 6.12 a	1.43 ± 0.36 a	2.06 ± 0.49 a	0.65 ± 0.12 a	4.14 ± 0.96 a
	Bagasse	77.67 ± 3.48 a	1.38 ± 0.19 a	1.90 ± 0.25 a	0.50 ± 0.02 a	3.78 ± 0.46 a
	Microbial fertilizer	82.83 ± 6.42 a	1.04 ± 0.13 a	1.91 ± 0.15 a	0.60 ± 0.03 a	3.55 ± 0.22 a
Cassava	CK	24.67 ± 2.05 a	0.57 ± 0.27 a	1.63 ± 0.60 a	/	2.03 ± 0.96 a
	Peat	29.00 ± 3.22 a	0.59 ± 0.21 a	1.69 ± 0.66 a	/	2.27 ± 0.87 a
	Cow dung	30.33 ± 1.76 a	0.63 ± 0.17 a	2.08 ± 0.81 a	/	2.70 ± 0.96 a
	Bagasse	31.67 ± 1.45 a	0.97 ± 0.16 a	3.29 ± 0.61 a	/	4.25 ± 0.73 a
	Microbial fertilizer	29.33 ± 1.33 a	0.50 ± 0.14 a	1.22 ± 0.30 a	/	1.72 ± 0.43 a

Note: Values represent mean ± standard error (n = 3). Different letters indicate significant differences between treatments for the same plant (p < 0.05). “/” indicates undetected projects.

. Although the amendments did not affect sugarcane and cassava growth, cow dung addition notably increased castor and ramie growth. Specifically, compared with the CK treatment, cow dung application resulted in a significant increase in plant height, root biomass, and total castor biomass by 40, 80, and 83.65%, respectively. Similarly, for ramie, cow dung application increased plant height, root, shoot, and total biomass by 157.02, 114.71, 175.65, and 161.74%, respectively, compared to those of the CK. Compared with the CK, peat application had a pronounced effect on sugarcane growth. Specifically, plant height, shoot, root, stem, and total biomass increased by 10.05, 61.63, 73.69, 73.35, and 69.13%, respectively. Moreover, compared with the CK, bagasse application resulted in significant increases in cassava plant height, shoot, root, and total biomass by 28.38, 70.80, 101.72% and 109.72%, respectively.

3.5. Plant Physiological Parameters

The effects of different treatments on the physiological parameters are shown in Figure 5. The results indicated that CAT content in the leaves of the four plants was not significantly affected (Figure 5b). Moreover, MDA (Figure 5a), and superoxide dismutase (SOD) content (Figure 5c) in castor and cassava leaves were not different after treatment with the amendments. However, for ramie, the bagasse application increased MDA and SOD content in the leaves by 99.70 and 49.58%, respectively, compared to that of the CK. Furthermore, microbial fertilizer application increased leaf SOD content by 59.72%. Compared to the CK, peat application increased leaf SOD content by 43.68%. Additionally, microbial fertilizer application significantly increased leaf MDA and SOD content by 73.85 and 31.03%, respectively.

3.6. Plant HM Content

Plant shoot HM content is shown in Figure 6. In castor, compared to the CK, As content in the plant shoots decreased by 26.87–30.00% when three of the four amendments were used, excluding the peat amendment. Furthermore, microbial fertilizer application decreased Cu content (47.27%) in castor shoots, whereas cow dung application significantly increased Cu content (151.14%) in the same region. For ramie, compared with the CK treatment, peat application increased Cu content (202.54%) in the plant shoots, cow dung application increased Zn content (72.87%) in the plant shoots, and microbial fertilizer application significantly decreased Ni content (49.92%) and Zn content (60.46%) in the plant shoots, with minimal effect on the content of other HMs. Additionally, bagasse application decreased Ni content (50.31%) in ramie shoots but increased Cu content (117.21%) in the same region. Regarding sugarcane, compared with the CK treatment, all four amendments led to a significant increase in Ni content in the shoots (73.43–230.61%) but a significant decrease in Cr content (85.76–91.18%) and Ni content (84.09–97.05%) in the stems. Peat and cow dung application had little effect on the HM content in sugarcane shoots, whereas microbial fertilizer application reduced As content (27.53%) in plant shoots. Moreover, bagasse application decreased plant shoot As content (28.98%) and Cu content (46.93%) but significantly increased plant shoot Cr content (116.91%). Regarding cassava, compared with the CK treatment, all four amendments decreased plant shoot Cr content (by 37.89% to 69.88%) and Pb content (by 35.94% to 52.74%). Except for the bagasse application, the other three amendments decreased Zn content in cassava shoots by 24.32% to 52.60%. Additionally, bagasse application decreased cassava shoot Cd content (84.69%) but increased shoot Cu content (62.47%).

Plant root HM content under different treatments is shown in Figure 7. Compared with the CK treatment, peat application increased the content of Ni (133.66%) in castor roots, Cu (253.01%) in ramie roots, and Pb (37.45%) in cassava roots. Cow dung application increased Cr (153.10%), Ni (154.74%), Pb (174.60%), and Zn (26.88%) content in ramie roots. Bagasse application increased Cu (133.98%) and Pb (38.58%) content in ramie roots compared to that of the CK treatment but resulted in a significant decrease in Zn (36.98%) content. Microbial fertilizer application significantly decreased Zn in castor and ramie roots and Cd in cassava roots by 37.44, 44.06, and 31.93%, respectively.

3.7. The Ability of Plants to Accumulate and Transport Heavy Metals

Table S1 presents the impact of various amendments on the BCF value of plant roots (BCF_root). Compared to the CK treatment, the application of peat increased the BCF_root value of castor for Ni (142.22%), the BCF_root value of ramie for Cu (247.66%), and the BCF_root value of cassava for Pb (50.00%). However, it notably decreased the BCF_root value of ramie for Zn (30.15%). Similarly, the application of cow dung increased the BCF_root value of ramie for Ni (112.90%) compared to the CK treatment. Moreover, the application of bagasse increased the BCF_root value of ramie for Cu (125.11%) and cassava for Pb (48.33%) but decreased the BCF_root value of ramie for Zn (40.44%) compared to the CK treatment. Furthermore, the application of microbial fertilizer significantly reduced the BCF_root values of Zn in castor and ramie by 33.82 and 46.69%, respectively, and decreased the BCF_root value of Cd (45.80%) in cassava.

Table S2 presents the impact of different amendments on the BCF value of plant shoots (BCF_shoot). The BCF value of the sugarcane stems (BCF_stem) is shown in Table S4. Compared to the CK treatment, the four amendments increased the BCF_shoot values of Cr and Ni in sugarcane by 133.33–200.00% and 150.00–275.00%, respectively. However, they reduced the BCF_shoot values of cassava by 57.41–68.52%, 86.14–91.03%, and 89.19–97.30%, respectively. Furthermore, compared to the CK treatment, the BCF_shoot values of Cd and Cu in the ramie exhibited significant increases of 49.02 and 199.12%, respectively. Notably, the application of cow dung increased the BCF_shoot values of Pb in the castor and Zn in the ramie by 54.55 and 77.78%, respectively. Moreover, the application of bagasse elevated the BCF_shoot value of Cu (174.56%) in the ramie, while it considerably decreased the BCF_shoot value of Cd (82.02%) in the cassava.

The impact of various amendments on the TF value of the four economic plants is presented in Table S3. Generally, the addition of amendments tends to decrease the TF value. The TF value of the sugarcane stems (TF_stem) is shown in Table S4. Compared to the CK treatment, the four amendments reduced the TF values of castor shoots to Ni, cassava shoots to Cr and Pb, and sugarcane stems to Cr and Ni by 53.01–67.47%, 74.69–87.45%, 69.57–78.97%, 85.37–91.89%, and 90.68–94.47%, respectively. In the sugarcane, apart from the microbial fertilizer, the other three amendments decreased the TF value of As in shoots by 34.34–40.87%. Additionally, the microbial fertilizer led to a substantial reduction of 96.85% in the TF value of sugarcane stems to Cd, compared to the CK treatment. For castor, except for cow dung, the other three amendments significantly lowered the TF value in plant shoots of Cr by 53.47–64.36%. Furthermore, the application of peat and bagasse reduced the TF value in plant shoots of As by 47.48 and 51.30%, respectively. Regarding ramie, the application of cow dung and microbial fertilizer reduced the TF value in plant shoots of Ni by 58.87 and 60.48%, respectively, compared to the CK treatment.

3.8. Plant HM Accumulation

HM accumulation in plants under different treatments is shown in Figure 8. In castor, application of the four amendments led to a significant increase in root Ni accumulation ranging from 194.04 to 259.65% compared to that of the CK treatment. Peat application significantly enhanced Cr accumulation (197.40%) in castor roots. Similarly, cow dung application increased Cd accumulation (115.94%) in castor roots and Zn (109.73%) and Pb (160.62%) accumulation in shoots. Bagasse application increased Cd (101.02%) and Cu (103.29%) accumulation in roots, as well as increasing Zn accumulation (89.39%) in the shoots. Moreover, microbial fertilizer application notably increased Zn accumulation (119.97%) in the shoots. Regarding ramie, peat application increased Cu accumulation in both roots and shoots by 337.50 and 276.06%, respectively, compared to that of the CK treatment. Cow dung application had a substantial impact on HM accumulation in ramie roots and shoots, increasing Cr (526.10%), Cd (79.80%), Ni (552.59%), and Pb (568.11%) accumulation in the roots and significantly increasing the accumulation of seven HMs in the shoots by 59.30–601.26%. Compared with the CK, bagasse application increased Cu accumulation (118.76%) in ramie shoots, whereas bagasse and microbial fertilizer application led to a significant decrease in Pb accumulation in ramie shoots of 59.14 and 64.63%, respectively. Regarding sugarcane, the four amendments significantly reduced Cr and Ni accumulation in the stems by 77.41–87.45% and 80.75–95.90%, respectively, compared to that of the CK treatment. Compared with the CK treatment, peat application notably increased Zn (69.23%) accumulation in the leaves and Pb (223.36%) accumulation in the stems. Cow dung and bagasse application increased Pb (189.36%) and Cr (110.59%) accumulation in the roots, respectively. Additionally, microbial fertilizer application increased Cd (163.63%) accumulation in the stems. Regarding cassava, the four amendments significantly reduced shoot Cr and Ni accumulation by 72.97–91.46% and 75.20–88.52%, respectively, compared to that of the CK treatment. Furthermore, compared with the CK, bagasse application increased Pb accumulation (140.40%) in the roots, whereas microbial fertilizer application significantly decreased Pb accumulation (76.30%) in the shoots.

3.9. The Principal Component Analysis

With the addition of various amendments, the physical and chemical properties, as well as the bioavailable HMs content, of the four planting BRs were analyzed using PCA. The results of the analysis are presented in Figure 9. The first principal component (PC1) accounted for 33.7, 32.2, 34.5, and 37.7% of the variance contributions for castor, ramie, sugarcane, and cassava, respectively. Similarly, the second principal component (PC2) contributed 20.2, 23.0, 15.6, and 22.5% to the variance for the respective crops. The cumulative variance contributions of the two principal components were 53.9, 55.2, 50.1, and 60.2% for castor, ramie, sugarcane, and cassava, respectively. For castor, the first principal component was characterized by pH, SOM, CEC, invertase, urease, and the availability of seven HMs (As, Cd, Cr, Ni, Cu, Zn, and Pb). The second principal component was characterized by SOM, invertase, urease, catalase, and the availability of three HMs (As, Cr, and Ni). In the case of ramie, the first principal component included CEC, urease, catalase, and the availability of five HMs (Cd, Cr, Cu, Zn, and Pb). The second principal component encompassed SOM, CEC, invertase, and the availability of seven HMs (As, Cd, Cr, Ni, Cu, Zn, and Pb). Sugarcane exhibited SOM, CEC, sucrase, urease, and the availability of six HMs (As, Cd, Ni, Cu, Zn, and Pb) as the characteristic indicators for the first principal component. The second principal component was characterized by SOM, CEC, catalase, and the availability of five HMs (As, Cd, Ni, and Cu). Regarding cassava, the first principal component was characterized by SOM, CEC, urease, invertase, catalase, and the availability of seven HMs (As, Cd, Cr, Ni, Cu, Zn, and Pb). The second principal component included pH, CEC, catalase, and the availability of four HMs (As, Cr, Ni, and Zn).

PCA was conducted to assess the growth physiological characteristics of four plant species and the physical and chemical properties of the BR. The findings of the analysis are presented in Figure 9. The PC1 accounted for 32.5, 48.6, 41.1, and 37.1% of the variance contributions for castor, ramie, sugarcane, and cassava, respectively. The PC2 contributed 24.8, 19.2, 20.7, and 19.5% of the variance for the respective plants. The cumulative variance contributions for these two components were 57.3, 67.8, 61.8, and 56.6%, respectively. Regarding castor, the key indicators of the first principal component were plant height, shoot and root biomass, SOM, SOD, and CAT. The second principal component was characterized by shoot and root biomass, SOM, CEC, and MDA. In the case of ramie, the first principal component encompassed plant height, shoot and root biomass, CEC, and pH, while the second principal component involved the same growth characteristics along with SOM, CEC, SOD, CAT, and MDA. For sugarcane, the first principal component included plant height, shoot and root biomass, stem biomass, SOM, CEC, SOD, and MDA, while the second principal component involved plant height, shoot and root biomass, stem biomass, and pH. Lastly, in the case of cassava, the first principal component consisted of plant height, shoot and root biomass, CEC, and MDA, whereas the second principal component comprised pH, CEC, SOD, CAT, and MDA.

4. Discussion

4.1. Organic Amendment Effects on Physical and Chemical Properties in BRs

The pH, SOM, and CEC play crucial roles in plant growth and in improving BR properties [63]. This study demonstrated that amendment application resulted in BR pH reduction while enhancing the SOM content and CEC (Figure 2a–c), which aligns with previous research findings. Organic amendments are rich in essential minerals (such as N, P, and K) that supply the nutrients required for plant growth [64,65]. Furthermore, these amendments can elevate organic carbon levels in the soil, thereby positively influencing favorable soil structure development [66]. Among the amendments investigated, microbial fertilizer exhibited the most increase in SOM content. This may be attributed to the high SOM content inherent to the microbial fertilizer itself, as well as its ability to enhance microbial diversity and contribute to SOM formation and decomposition within aggregates [67]. Moreover, microorganisms can decompose SOM and generate carbonic acid, ultimately resulting in a decrease in soil pH [68].

CEC is a crucial parameter used to assess soil fertility. Higher CEC enhances soil nutrient availability [69,70]. In this study, all amendments resulted in an increase in the BR CEC (Figure 2c). Among these amendments, peat and cow dung exhibited the most effect on CEC. This effect can be attributed to higher humic acid (HA) content in the compost product [71]. During the composting process, the elemental composition of HA undergoes changes, leading to an increase in the presence of functional groups such as phenolic, carboxyl, and carbonyl groups [72,73]. Ren et al. [74] discovered that nitro-HAs increased soil CEC, whereas the hydroxyl groups in HA and fulvic acid (FA) facilitated cation binding, thereby promoting mineral particle formation. Furthermore, studies have demonstrated that the application of various amendments such as poultry manure [75], compost [76], and biomass residues [77] to soil results in an increased proportion of large aggregates and stimulates microbial activity.

4.2. Organic Amendment Effects on BR and Plant Enzyme Activity

Soil enzyme activity is important for nutrient cycling, microbial metabolism, and soil mineralization [39,40]. Shaaban et al. [78] observed that manure and straw application stimulated enzymatic activity (CAT, urease, and invertase) in the soil. These enzymes are highly involved in SOM mineralization. Previous studies have established a positive correlation between CAT and aerobic microorganisms. Under poor soil permeability conditions, microorganisms compete with plant roots for oxygen and alter microbial metabolic pathways under oxygen stress, thereby notably limiting nutrient availability and root growth [79,80]. Ren et al. [81] discovered that organic fertilizers affect soil enzyme activity by enhancing microbial communities, including bacteria and fungi. In this study, amendment application stimulated soil CAT, urease, and invertase activity (Figure 2d–f). Notably, the use of microbial fertilizer resulted in the greatest increase in invertase activity. This can be attributed to the microbial fertilizer enriching the available carbon content, promoting root secretion, enhancing soil microbial biomass and activity [82], and potentially introducing a higher diversity of beneficial bacteria into the soil [83]. Furthermore, enhanced soil enzyme activity could be attributed to the improved soil structure resulting from amendments. These amendments enhance soil permeability, provide accessible carbon substrates, and facilitate the participation of soil enzymes in carbon, nitrogen, and phosphorus cycling [84]. This, in turn, promotes plant absorption, soil mineralization, and microbial degradation [85].

Changes in plant growth conditions result in corresponding alterations in plant enzyme activity [86]. Alterations in the soil environment, such as an increase in HM or salt content, can prompt plants to generate excessive reactive oxygen species (ROS), causing damage to cell membranes, cell walls, and lipid peroxidation [87]. MDA, the end product of lipid peroxidation, is an indicator of plant oxidation levels [88]. Antioxidant enzymes, including CAT, SOD, and POD play vital roles in scavenging ROS in plant leaves [89]. SOD is recognized as the primary line of defense against plant ROS [90]. In this study, plant enzyme activity was positively correlated with BR physicochemical properties (pH, SOM, and CEC) (Figure 9). Amendment application resulted in an increase in MDA content in the plants. This increase may be attributed to the amendments increasing bioavailable HMs in the soil, leading to their absorption and subsequent ROS production during the metabolic process. The increase in leaf SOD content could be a response to HM stress in plants. As shown in Table S3, heavy metal ions are translocated to the shoots of the plant, triggering activation of the leaf defense mechanism and thus mitigating oxidative damage caused by oxygen-free radicals [91].

4.3. Organic Amendment Effects on BR HM Content

Soil HM content is remarkably influenced by pH. Numerous studies have demonstrated that a decrease in pH leads to an increase in bioavailable HM leaching from soils [92,93]. Zuo et al. [94] observed that vermicompost application induced changes in HM morphological distribution in the soil. This alteration resulted in an elevated proportion of unstable HMs (weak acid extraction and exchangeable states) while reducing the proportion of stable HMs (residue state). In this study, a negative correlation was observed between the pH and available HM content (Figure 9). Organic amendment application led to alterations in the soil environment, resulting in a reduction in soil pH and an increase in HM ion mobility [95]. This finding aligns with previous research, wherein a decrease in soil pH corresponds to an increase in soil Eh. During this transition, the soil environment shifts from a reducing to an oxidizing state, leading to increased HM solubility [96]. However, this may be attributed to the amendment of HM morphological distribution in the soil [97]. Although bioavailable HM content increased, the overall quantity of HMs in the soil tended to decrease.

In this study, FTIR analysis revealed the prevalence of specific functional groups, including carboxyl, carbon, and phenolic hydroxyl groups, within the four amendments, as shown in Figure 1. The peak observed at approximately 1600 cm⁻¹ was attributed to either C=O stretching or C=C vibrations in the carboxylic acid. Simultaneously, carboxylic acids exhibit a robust complexing capability with Cd and Pb [98], thereby effectively diminishing the BR content of these metals, as shown in Figure 3. Furthermore, discernible oxygen-containing functional groups at 1161–1034 cm⁻¹, such as aliphatic C–O–C and alcohol–OH [99], were present, whereas the peak at 779 cm⁻¹ may be associated with aromatic C–H in-plane and out-of-plane deformation [100]. Previous studies have confirmed the involvement of these functional groups in complex HM precipitation processes, which constitute the primary adsorption mechanism [40,101]. Oxygen-containing functional groups play a beneficial role in immobilizing HM ions, and an augmented cation content accelerates the fixation efficiency, thereby enhancing HM ion removal [102]. Kang et al. [103] discovered that HAs and FAs in compost products could form stable metal complexes by binding to HMs, thereby reducing their content through adsorption and precipitation.

The reduction in BR HM content observed in this study can be explained by several mechanisms, primarily the loose and porous structure of the amendment-facilitated HM adsorption [104,105]. Additionally, this may be ascribed to the presence of various functional groups that can form organic metal complexes via complexation with HM ions. Moreover, the reduction in HM content in the BR could also be linked to an increase in BR CEC, which improves the ability of BRs to absorb metal cations by increasing the number of binding sites on the surface of BR particles or the electrostatic accumulation capacity of the molecular surface [106,107].

4.4. Organic Amendment Effects on Plant Growth and HM Content

Soil amendment obviously improves the soil environment and promotes plant growth. Consistent with previous research, when plants were exposed to HM stress, amendment application decreased the stress level experienced by plants. This reduction in stress may be attributed to the ample nutrient supply provided by the plant amendments [108]. In this study, we observed a positive correlation between BR physicochemical properties and plant growth (Figure 9), suggesting that the amendments enhanced plant growth by improving BR physicochemical properties. Organic amendments served as carbon sources for the soil and facilitated root growth (Table 1). This effect can be attributed to the synergistic interaction between the roots and the amendments [109], which leads to increased microbial diversity in the soil and an improved rhizosphere environment [110,111]. In the current study, we discovered that microbial fertilizer was less effective in promoting plant growth than the other three organic amendments. This disparity could be attributed to the relatively low HA and FA concentrations in the microbial fertilizers, despite the provision of ample organic matter and additional nutrients. Conversely, the beneficial substances responsible for enhancing plant growth in the organic amendments were likely HAs and FAs. Moreover, the amendments enhanced HMs availability in the BR and facilitated their absorption by the plants. Specifically, our findings indicated that the use of amendments increased the content of most HMs in the plant roots (Figure 7, Table S1), whereas their content in the surrounding BR decreased. This suggests that the majority of the HMs were absorbed by the roots. However, we also observed an increase in the content of certain HMs in the plant shoots, such as Cr, Ni, and Cu (Figure 6, Table S2). This increase may be attributed to the high mobility of these metals in plant vascular systems. It is likely that these HMs bind to xylem cell walls, allowing them to be transported to plant shoots [112]. Furthermore, the HAs and FAs present in compost products increase the mobility of certain HMs in the soil [113]. Our study revealed that sugarcane roots accumulated high amounts of HMs (Figure 8, Table S4). This accumulation can be attributed to the extensive root biomass of sugarcane and its unique physiological structure, including numerous fine roots that facilitate HM absorption [114]. However, the HM content in the available sugarcane parts remained very low. This is likely due to the modification process, which enables HMs to enter the root vacuole and form carbonate bonds at cell-wall exchange points, effectively sequestering them within plant roots [115]. Additionally, the variation in HM accumulation across different parts of the plant generally follows the pattern of root > leaf > stem > fruit [116].

Peat application increased Ni content in sugarcane shoots and decreased As content. Moreover, there was a decrease in Cr and Ni accumulation in the sugarcane stems, whereas Pb accumulation remarkably increased within acceptable limits. When cow dung was applied, there was a substantial increase in the Cu and Zn content in the shoots of both castor and ramie plants. This increase subsequently contributed to significant Zn and Pb accumulation in the castor shoots. However, it is worth noting that organic amendment application increased the accumulation of seven HMs (As, Cd, Cr, Ni, Cu, Zn, and Pb) in ramie shoots. Additionally, bagasse application resulted in increased As, Cu, and Zn accumulation in cassava. This can be attributed to the organic amendments promoting the biomass of economically important crops and potentially enhancing HM uptake by plant shoots. Therefore, cultivating economically important crops in BRs contaminated with HMs is a viable approach. As long as the HM content in the edible portion of economically important crops remains below the threshold set by national food safety standards, this practice can serve the dual purposes of remediation and production.

5. Conclusions

In this study, we discovered that organic amendment application reduced BR pH and increased the SOM and CEC content. Additionally, organic amendment application has demonstrated the potential to enhance BR enzyme activity. This finding suggests that the amendments contributed to an improvement in BR quality. Compared with the CK, we observed a notable increase in SOD activity and MDA content. However, CAT activity did not show any differences. These results indicate that amendments alter the BR environment and activate plant defense mechanisms. These findings demonstrate the crucial role of SOD in ROS production. Moreover, amendment application led to enhancements in plant height, root, shoot, and total biomass. Amendment application resulted in a decrease in the overall amount of HMs in BR but an increase in effective HM content, which promotes HMs absorption by plants. Notably, HM accumulation in plants occurred primarily in the roots. Amendment application facilitated HM absorption by roots while inhibiting their uptake by plant shoots. Compared with the CK, peat and cow dung exhibited more effects in increasing HM accumulation in plant roots. Additionally, all four organic amendments reduced the ability of plants to transport HMs. Furthermore, the four amendments led to a decrease in Cr and Ni content in the edible portion of sugarcane. Notably, the HM content of the economically important crops remained below the national food safety standard threshold when amendments were added. Based on the pot experiments, we demonstrated that four combinations exhibited good results: castor and ramie planting in BR with the addition of cow dung, sugarcane planting in BR with the addition of peat, and cassava planting in BR with the addition of sugarcane. These specific combinations were observed to enhance plant growth while also resulting in increased HM accumulation in plant shoots. Importantly, the HM content in the available parts of economically important crops remained below the national control threshold, ensuring the feasibility of simultaneous remediation and production. Although microbial fertilizer application proved to be effective in improving BR properties, it was found to be less effective in remediating HM-contaminated BRs than the other three organic amendments. Our findings present a novel approach for remediating HM-contaminated BRs. This study demonstrates that the concurrent planting of economically important crops and organic amendment application can effectively restore the BR and facilitate crop production. This approach not only enhances BR quality but also mitigates the economic losses associated with BR remediation. This study provides a scientific basis for mine remediation and reclamation using BRs.