Synergistic Effects of Compost and Biochar on Soil Health and Heavy Metal Stabilization in Contaminated Mine Soils

Abstract

Phytoremediation strategies present promising approaches for mitigating metal contamination in soils. This study examines the effectiveness of compost and biochar amendments, applied separately or in combination, in altering the properties of sandy mining waste soils (Sw) and affecting levels of metallic trace elements (MTEs). The research evaluates changes in soil physicochemical parameters, metal concentrations in soil pore water (SPW), and metal accumulation in Phaseolus vulgaris. Compost and biochar addition significantly affected SPW pH, which remained alkaline, while increasing SPW electrical conductivity (EC). A treatment combining 20% compost and 2% biochar (SwC20B2) enhanced soil enzymatic activities, with the highest values observed for FDA and ALP activities. Metal availability in the SPW appeared higher on D(0) compared to D(12), with notable reductions in Pb and Zn concentrations observed in the SwC20B2 treatment. Despite this decline, metal accumulation in plant shoots did not significantly differ from that in plants grown in unamended Sw, although all plants exhibited substantial growth. The minor decrease in SPW pH, likely due to compost, may have enhanced metal mobility at D(0). Notably, SPW Pb and Zn concentrations increased with higher compost rates, with SwC20B2 registering the highest Pb and Zn. Although these amendments did not directly alleviate metal mobility, they show potential for use in phytostabilization strategies by using suitable plant species.

1. Introduction

The global demand for non-ferrous metals has surged in recent decades driven by technological advancements. However, this increased demand has led to extensive soil contamination, primarily due to mining activities. Mining operations in an area not only degrade the environment and disrupt ecosystems [1] but also pose significant health risks [2] and create socio-economic challenges, particularly after mining closure [3,4]. A bibliometric analysis of soil pollution and remediation research topics from 2001 to 2020 identified metallic trace element (MTE) contamination as the central focus [5]. Furthermore, meta-analyses have shown that lead–zinc mining generates tailings contaminated with hazardous MTEs such as lead (Pb), zinc (Zn), arsenic (As), cadmium (Cd), copper (Cu), mercury (Hg), and nickel (Ni), contributing to environmental pollution and human health risks [6]. In addition to MTE contamination, mine tailings often exhibit unfavorable physicochemical conditions for living organisms. They are commonly characterized by a fine-grained texture prone to erosion, extreme pH levels, low organic matter content, and poor cation exchange capacity. These factors are critical, as they directly affect the ecological balance, particularly plant growth and microbial diversity in these environments [7,8,9]. Consequently, research efforts have intensified to develop sustainable solutions and establish effective regulatory standards for long-term environmental protection [10]. For instance, lands impacted by these hazardous contaminants could be either recycled for industrial uses [11], or decontaminated using physical and chemical methods [12]. However, these approaches are often costly. As a result, researchers advise mitigating MTEs on site using a cost-effective and environmentally friendly approach such as biological remediation [13,14,15].

Phytoremediation, a subset of biological remediation utilizes plants to mitigate organic and inorganic soil and water contamination, encompasses various processes including phytovolatilization, phytoextraction, phytofiltration, and phytostabilization [16]. Phytovolatilization converts absorbed MTEs from the soil through roots into volatile compounds through aboveground plant tissues [17]. Phytoextraction relies on hyperaccumulator plants to store significant MTEs in their aboveground tissues [18]. Phytofiltration removes contaminants from water systems [19]. Interestingly, phytostabilization stands out as the most prominent and efficient technique for immobilizing and stabilizing MTEs in the soil and rhizosphere, contrasting with the uncertain outcomes of other processes, especially phytoextraction and phytovolatilization [20]. Moreover, the selected plant cover in the phytostabilization technique should be suitable in terms of their ability to (i) tolerate metal contamination, (ii) stabilize metals in the rhizosphere, (iii) adapt to climate conditions, and (iv) produce high biomass [21,22,23]. However, the successful establishment of the phytostabilization technique requires a comprehensive understanding of the mineralogy and geochemistry of mine tailings concerning their toxicity and heterogeneity [8,22], to determine whether the site is suitable for direct revegetation or requires the addition of amendments.

Soil amendments play a key role in phytostabilization by: (i) ensuring soil enrichment with nutrients to promote plant growth, and (ii) immobilizing metals through surface interactions, dependent on the amendment’s properties [24,25]. Combining appropriate plants with effective amendments, known as aided or assisted phytostabilization, has demonstrated success in immobilizing metals [26,27,28,29,30,31]. One promising amendment is biochar, a carbonaceous material resulting from the pyrolysis of natural biomass [32]. Due to its unique physicochemical characteristics, biochar has been increasingly applied to mitigate metal contamination in soils affected by mining activities [33,34,35]. It is particularly recognized for its liming effect, which helps to neutralize soils in mining areas with extremely acidic pH levels [36,37,38]. Furthermore, biochar’s high specific surface area and abundance of functional groups facilitate the sorption of MTEs, and in some cases, enable the release of nutrients depending on the amount and composition of its ash content [39,40]. However, these properties are largely influenced by feedstock type, and pyrolysis conditions [40], which ultimately affect their ability to modify soil characteristics. Recent research suggests enhancing biochar efficiency by combining it with other inorganic (e.g., natural minerals, metal oxides, and hydroxides) or organic materials (e.g., compost, garden soil, microorganisms), to achieve optimum efficiency [41,42,43,44]. Several studies have recommended the combined application of biochar and compost to enhance the efficacy of soil amendments in remediation programs [45,46,47]. Compost enriches the soil with nutrients and organic matter, which promotes plant growth [48], though it decomposes relatively quickly. Biochar, with its porous structure and high cation exchange capacity, not only retains nutrients released from compost, reducing leaching losses, but also adsorbs heavy metals [49,50], thereby decreasing their mobility and bioavailability in contaminated soils. This synergistic use of biochar and compost improves soil fertility and structure while mitigating metal toxicity, making it a promising strategy for remediating contaminated mining soils [26,51]. Yet, their application must align with sustainability and circular economy principles, favoring local production from organic or residual biomass and low-emission processing systems, notably for biochar [52,53]. Transport over long distances and unsustainable production methods could undermine the environmental benefits of such amendments. Indeed, biochar should ideally be produced from locally available biomass using advanced pyrolysis technologies equipped with emission control systems, in order to minimize the release of greenhouse gases such as methane and other syngas components, and to ensure that these emissions do not compromise its environmental benefits [54].

In this context, the present study investigates the combined effects of biochar and compost mixed in different proportions on the physicochemical and biological properties of a sandy mining waste (Sw) contaminated mainly with Pb, Zn, and Cu, originating from Touissit mine tailings (Morocco). Additionally, Phaseolus vulgaris was used as an indicator species to assess plant responses to metal stress to understand plant behavior accurately in polluted soil amended with compost and/or biochar. Ultimately, the findings contribute to identifying the most suitable amendment mixture for an effective assisted phytostabilization strategy for Touissit mine tailings.

2. Materials and Methods

2.1. Mine Tailings Samples

The soil samples used in this study were collected from a site designated for the reprocessing of mine tailings in the Touissit-Boubker mining district located in the Oriental region of Morocco. These tailings, originating from past lead and zinc extraction activities [55], exhibit multi-metal contamination, primarily with Pb, Zn, and Cu [56]. Specifically, the investigated soil, classified as "sandy waste" based on its apparent texture, represents the final residue remaining after the reprocessing of old mine tailings. To ensure representative sampling, a systematic collection approach was employed along a straight line through the center of the sandy waste (Sw) deposits, based on the observed homogeneity of the soil at depths ranging from 0 to 30 cm. A composite sample was obtained by mixing five equally sized subsamples.

The soil sampling took place in May 2023, and the subsequent experiment was conducted in January 2024.

2.2. Origin of Amendments

The biochar used in this study was provided by La Carbonerie (Crissey, France) and produced from a mixture of hardwood feedstocks, including oak, beech, and hornbeam wafers and chips. The biomass tissues were mechanically separated and dried at 105 °C, then heated to 150 °C over a period of 2 h. Subsequently, the temperature was increased from 150 °C to 500 °C at a constant rate of 2.5 °C per minute, corresponding to a heating duration of approximately 140 min. Once 500 °C was reached, the biomass was maintained at this temperature for an additional 3 h to complete the pyrolysis process [57]. After this treatment, the biochar was crushed and sieved to achieve a particle size ranging from 0.2 to 0.4 mm. This specific biochar was selected for its proven effectiveness in retaining metals, especially Pb, as reported in previous studies [58,59,60]. The compost used was a commercial product recommended for plants growing in modular trays, manufactured by Klasmann-Deilmann (Bourgoin-Jallieu, France). It consisted of a blend of blond and black frozen Sphagnum moss, with the following properties: texture size of 0–5 mm, 35% dry weight, 90% organic matter, a pH of 6, electrical conductivity of 35 mS/m, 80% water-holding capacity, and an NPK ratio of 14:10:18 in kg·m⁻¹.

2.3. Phytotoxicity Bioassay Using Phaseolus vulgaris

2.3.1. Soil Preparation and Treatments

The collected soils from the Touissit area were air-dried at room temperature and sieved to 2 mm. Additionally, the compost was mixed with perlite at a 4:1 (v/v) ratio. Subsequently, the sandy waste (Sw) was amended with compost at increasing proportions of 0%, 5%, 10%, and 20% (w/w). In parallel, biochar was incorporated at two different doses: 0% and 2% (w/w). At each compost dose, biochar was either included or omitted, allowing for an evaluation of both individual and combined amendment effects. The amendment rates and mixture types were chosen to be consistent with those used in our previous experiments, which employed similar percentages in related remediation contexts [15,61]. In total, seven treatments were compared to the unamended Sw (100% of Sw). For each treatment, five pots were prepared, corresponding to five plants, to ensure biological replication.

Table 1. Abbreviations and descriptions of treatment mixtures for sandy waste (Sw) amended with varying proportions of biochar (B) and/or compost (C) (w/w) (n = 5).

Abbreviation	Sw (%)	Biochar (%)	Compost (%)
Sw	100	0	0
SwB2	98	2	0
SwC5	95	0	5
SwC5B2	93	2	5
SwC10	90	0	10
SwC10B2	88	2	10
SwC20	80	0	20
SwC20B2	78	2	20

summarizes the different soil mixtures, treatment abbreviations, and the respective proportions of biochar and/or compost (w/w).

To assess the water-holding capacity (WHC) of each treatment, pots filled with dried mixtures were abundantly watered. Excess water was allowed to drain until the mixture reached optimal saturation. The WHC was then calculated as a percentage difference between the fresh weight and dry weight of each mixture.

2.3.2. Experimental Design

Following soil preparation, five 400 mL pots were filled with each mixture and watered with tap water for 10 days to ensure uniform moisture distribution and stability. Three days before planting, Phaseolus vulgaris seeds were pre-germinated on moistened filter paper using deionized water and incubated in darkness at a constant temperature of 22 °C ± 2 with a humidity level of 65%. After the pre-germination period, seedlings were sown, marking day 0 (D(0)) as both planting day and the completion of mixture stabilization. The plants were cultivated for twelve days (D(12)) under controlled conditions in a growth chamber, where they received adequate watering to maintain the WHC of each treatment using tap water. The growth chamber was set to a photoperiod of 16 h light and 8 h dark, with a temperature of 20 °C ± 2. At the end of the 12-day growth period, the plants were harvested and separated into leaves, stems, and roots. The roots were thoroughly and meticulously washed with tap water to remove all soil particles while preserving the entire root biomass. Finally, all plant parts were oven-dried for 72 h, and the dry weights of each were subsequently recorded (Figure 1).

2.3.3. Soil Pore Water

On days D(0) (planting) and D(12) (harvesting), 10 mL of soil pore water (SPW) was collected from each pot for every treatment/mixture using a soil moisture sampler (Rhizon^®, Model MOM, Rhizosphere Research Products, Wageningen, The Netherlands). The pH and electrical conductivity (EC) of the SPW from each treatment were measured using a multi-parameter device (Seven Excellence, Mettler-Toledo AG, Greifensee, Switzerland) after calibration according to AFNOR norm NF T01-013.

2.3.4. Metals Concentration Analysis

Metal analysis was conducted using inductively coupled plasma atomic emission spectroscopy (ICP-AES), with a ULTIMA 2 instrument (HORIBA, Labcompare, San Francisco, CA, USA), on two sample types: (1) soil pore water (SPW) and (2) various parts of Phaseolus vulgaris plants. For SPW analysis, 5 mL from each was acidified with 83 μL of 69% HNO₃ before ICP-AES measurement. After oven-drying at 60 °C for 72 h, the dry weights of the individual plant parts were recorded for each plant. The dried samples were then finely ground separately, and 200 mg of each powdered sample was digested in aqua regia (1:2 v/v) using a microwave digestion system (Multiwave 3000; Anton Paar GmbH, Graz, Germany) at 180 °C for 45 min.

2.3.5. Biogeochemical Index

The mobility index (MI) was calculated to represent the metal uptake from soil to root and its translocation from roots to aboveground parts of the plant. Specifically, the MI was determined for three different levels: MI1 refers to the metal uptake from soil to root, MI2 denotes the metal translocation from root to stem, and MI3 represents the translocation from stem to leaves [62,63]. These key indicators were designed to elucidate the relationship between the source and the receiving level, highlighting metal mobility and its capacity to transfer to other plant compartments. The calculations for these indices are outlined in Equations (1)–(3) as follows:

$${\mathrm{MI}}_1={\displaystyle \frac{\left[\mathrm{Mean} \mathrm{metals} \mathrm{in} \mathrm{roots}\right] \mathrm{mg}·{\mathrm{kg}}^{-1}}{\left[\mathrm{Mean} \mathrm{metals} \mathrm{in} \mathrm{soil}\right] \mathrm{mg}·{\mathrm{kg}}^{-1}}}$$

(1)

$${\mathrm{MI}}_2={\displaystyle \frac{\left[\mathrm{Mean} \mathrm{metals} \mathrm{in} \mathrm{stems}\right] \mathrm{mg}·{\mathrm{kg}}^{-1}}{\left[\mathrm{Mean} \mathrm{metals} \mathrm{in} \mathrm{roots}\right] \mathrm{mg}·{\mathrm{kg}}^{-1}}}$$

(2)

$${\mathrm{MI}}_3={\displaystyle \frac{\left[\mathrm{Mean} \mathrm{metals} \mathrm{in} \mathrm{Leaves}\right] \mathrm{mg}·{\mathrm{kg}}^{-1}}{\left[\mathrm{Mean} \mathrm{metals} \mathrm{in} \mathrm{stems}\right] \mathrm{mg}·{\mathrm{kg}}^{-1}}}$$

(3)

Soil and plant metal concentrations were determined using the aqua regia digestion method followed by heating, and were finally measured by ICP-AES, as described in the previous section.

2.3.6. Soil Enzymatic Activities

Three enzymatic activities were assessed in the collected soils using three replicates: fluorescein diacetate (FDA) hydrolysis, acid phosphatase, and alkaline phosphatase. FDA hydrolysis, an indicator of overall microbial activity, was measured by mixing 0.1 g of air-dried soil with 5 mL of 60 mM potassium phosphate buffer (pH 7.6) and 50 μL of a 50 mM FDA solution. The mixture was then incubated at 37 °C with shaking (150 rpm) for 3 h. Following incubation, samples were centrifuged, and the absorbance of the supernatant was measured at 490 nm using a spectrophotometer (SPECTROstar Nano, BMG LABTECH GmbH, Ortenberg, Germany) with MARS data analysis software (version V5.02 R3). FDA hydrolysis activity was calculated using the coefficient (ε = 8000 L·mol⁻¹·cm⁻¹) and expressed in nmol of fluorescein released per minute per gram of soil. Phosphatase activities, which reflect soil fertility through the hydrolysis of organic phosphorus compounds, were measured using p-nitrophenyl phosphate (PNPP) as a substrate. Acid phosphatase activity was measured by mixing 2 g of air-dried soil with 2 mL of 0.1 M sodium acetate buffer (pH 5.0). For alkaline phosphatase activity, 2 g of soil was mixed with 2 mL of 0.1 M Tris-HCl buffer (pH 8.0). These mixtures were shaken overnight at 150 rpm at room temperature. After centrifugation, 100 μL of the resulting supernatant was combined with 100 μL of 5 mM PNPP solution in a microplate. The reactions were incubated at 25 °C for 1 h and stopped by adding 100 μL of 0.1 M NaOH. Absorbance was measured at 410 nm using a spectrophotometer (SPECTROstar Nano, BMG LABTECH GmbH, Ortenberg, Germany) with MARS data analysis software (version V5.02 R3). Phosphatase activities were calculated using the coefficient (ε = 19,500 L·mol⁻¹·cm⁻¹) and expressed as nmol of p-nitrophenol produced per minute per gram of soil.

2.4. Data Analysis

Data analysis was performed using Excel (Microsoft Office 365, 2016) for data management and IBM-SPSS Statistics version 25.0 for statistical tests. The normality of the data was checked with the Shapiro–Wilk test, and Levene’s test was used to assess equal variances. For normally distributed data, an ANOVA was used for comparing means, followed by Tukey’s test for identifying significant differences (p < 0.05). For data that were not normally distributed, the Kruskal–Wallis test was applied. Additionally, a paired Student’s t-test was used to evaluate statistical differences between parameters measured from D(0) to D(12).

3. Results and Discussion

3.1. Soil Basic Physicochemical Characteristics

3.1.1. Substrates Water Holding Capacity (WHC)

Results from the WHC measurements indicated that the addition of biochar and compost, either individually or in combination, significantly enhanced WHC compared to the control soil (Sw). Interestingly, the co-application of biochar and compost further increased WHC compared to the levels observed in Sw (16.86%), following the trend SwB2 (123.01%) < SwC5 (144.72%) < SwC5B2 (152.96%) < SwC10 (164.94%) < SwC10B2 (171.17%) < SwC20 (193.06%) < SwC20B2 (206.52%) (Figure 2).

Specifically, applying 2% biochar alone increased the WHC by 3.88% over the unamended Sw, whereas 5% compost led to a 7.54% increase in WHC, reflecting the impact of differing application rates. According to Alghamdi et al. (2020), the addition of finely particulated biochar to sandy loam soils enhanced water retention due to its large pore volume and specific surface area [64]. Furthermore, a meta-analysis by Razzaghi et al. (2020) found that adding biochar to coarse-textured soils, such as sandy and loamy sand, decreased soil bulk density by 11%, and improved soil–water interactions by increasing soil water content at field capacity by 51%, permanent wilting point by 47%, and plant-available water by 45% [65]. The high organic matter content (90%) of compost contributed to reducing soil bulk density [48], thereby increasing WHC. This is consistent with findings by Rivier et al. (2022), who observed increased water retention in sandy soils with compost application [66]. Additionally, Głąb et al. (2020) highlighted the synergistic effect of biochar and compost, showing that biochar contributed to maintaining long-term WHC improvements that might otherwise diminish over time with compost alone [49].

3.1.2. Soil Pore Water (SPW) pH and Electrical Conductivity (EC)

SPW pH and EC exhibited variation across different treatments, as summarized in

Table 2. Basic physicochemical characteristics of soil pore water (A), including pH, electrical conductivity, Pb, Zn, and Cu of unamended and amended Sandy waste (Sw) with different mixtures of compost and biochar. Different letters (a, b, c, d) indicate significant differences between biochars (n = 5 ± SE; p < 0.05). The different asterisks in part (B) reveal significant differences within the two sampling days, namely the planting and harvesting days, with * (p < 0.05), ** (p < 0.01), and *** (p < 0.001).

A	SPW pH				SPW EC (µs·cm−1)				SPW Pb (mg·L−1)				SPW Zn (mg·L−1)				SPW Cu (mg·L−1)
	D(0)		D(12)		D(0)		D(12)		D(0)		D(12)		D(0)		D(12)		D(0)		D(12)
Sw	8.42 ± 0.06	a	8.24 ± 0.04	ab	1276 ± 94.60	ab	1302 ± 37.27	ab	0.12 ± 0.02	d	0.12 ± 0.01	ab	0.17 ± 0.02	c	0.99 ± 0.10	ab	0.09 ± 0.03	ab	0.24 ± 0.04	ab
SwB2	8.39 ± 0.02	ab	8.17 ± 0.02	ab	1465 ± 93.51	ab	1478 ± 118.58	a	0.09 ± 0.01	d	0.16 ± 0.01	ab	0.15 ± 0.03	c	1.36 ± 0.18	a	0.06 ± 0.002	b	0.26 ± 0.02	a
SwC5	8.16 ± 0.04	ab	8.17 ± 0.03	ab	1021 ± 38.22	c	1048 ± 55.94	bc	0.21 ± 0.02	cd	0.22 ± 0.07	ab	0.29 ± 0.02	bc	0.62 ± 0.10	bc	0.09 ± 0.01	ab	0.20 ± 0.06	ab
SwC5B2	8.19 ± 0.02	ab	8.16 ± 0.04	ab	1544 ± 145.90	a	1422 ± 113.91	ab	0.71 ± 0.20	ab	0.39 ± 0.12	a	0.44 ± 0.09	ab	0.70 ± 0.06	bc	0.16 ± 0.03	a	0.20 ± 0.03	ab
SwC10	8.22 ± 0.04	ab	8.13 ± 0.04	b	1068 ± 29.88	bc	951 ± 37.10	c	0.20 ± 0.03	cd	0.12 ± 0.01	ab	0.23 ± 0.03	bc	0.52 ± 0.04	cd	0.07 ± 0.003	b	0.11 ± 0.004	bc
SwC10B2	8.15 ± 0.03	b	8.25 ± 0.03	ab	1330 ± 51.68	ab	1149 ± 51.99	bc	0.62 ± 0.08	bc	0.20 ± 0.02	ab	0.42 ± 0.05	ab	0.43 ± 0.03	cd	0.12 ± 0.02	ab	0.11 ±0.01	bc
SwC20	8.15 ± 0.09	b	8.23 ± 0.03	ab	1275 ± 168.11	ab	1091 ± 76.98	bc	0.81 ± 0.14	ab	0.20 ± 0.09	ab	0.51 ± 0.06	a	0.37 ± 0.06	cd	0.09 ± 0.01	ab	0.11 ± 0.02	bc
SwC20B2	8.26 ± 0.08	ab	8.32 ± 0.06	a	1303 ± 39.15	ab	922 ± 29.53	c	1.10 ± 0.10	a	0.11 ± 0.02	b	0.55 ± 0.05	a	0.20 ± 0.01	d	0.10 ± 0.01	ab	0.10 ± 0.01	c
B	SPW pH				SPW EC (µs·cm−1)				SPW Pb (mg·L−1)				SPW Zn (mg·L−1)				SPW Cu (mg·L−1)
	D(0)/D(12)				D(0)/D(12)				D(0)/D(12)				D(0)/D(12)				D(0)/D(12)
Sw	—				—				—				**				**
SwB2	**				—				***				**				**
SwC5	—				—				—				*				—
SwC5B2	—				—				—				*				*
SwC10	—				*				*				**				**
SwC10B2	**				—				*				—				—
SwC20	—				—				*				—				—
SwC20B2	—				**				**				**				—

A. At planting day (D(0)), the sandy waste soil (Sw) displayed an alkaline pH, reaching the highest value of 8.42 among all treatments. In contrast, the addition of biochar and compost in various proportions slightly reduced SPW pH, with the lowest recorded value (8.15) observed in the SwC10B2 and SwC20 treatments. By day 12 (D(12)), the SPW pH of Sw remained alkaline, although slightly decreased to 8.24, with no statistical differences observed among treatments except for SwC10 (8.13) and SwC20B2 (8.32), which recorded the lowest and highest values, respectively. Despite the acidic nature of the compost used in this study (pH 6), the alkaline character of the sandy waste soil remained around 8. Previous studies attributed the alkalinity of Touissit tailings to their high carbonate content, resulting in elevated pH [67,68]. Additionally, biochar may have contributed to maintaining the alkalinity of the mixture due to its liming effect resulting from the high composition of alkali salts of carbonates and hydroxides in the ash fraction [36,69]. Wu et al. (2020) found that biochar had greater potential than lime to increase acidic soil pH, which aligns with our findings that, despite the presence of 20% compost in the SwC20B2 treatment, SPW pH remained the highest [70].

Regarding SPW EC, the initial D(0) and subsequent D(12) values were respectively 1276.32 μS·cm⁻¹ and 1302.46 μS·cm⁻¹, indicating relatively high conductivity (Table 2A). Similarly, increased EC levels have been stated in other mining wastes in Northeastern Morocco, such as Kettara mine with 9500 μS·cm⁻¹ [71]. This elevated SPW EC could potentially be attributed to the presence of components such as soluble salts, electrically conductive minerals, and metal sulfides found in mining wastes [72,73]. However, adding amendments resulted in fluctuating average values of SPW EC. At D(0), compost addition to Sw slightly reduced SPW EC, whereas the combination of compost/biochar consistently increased SPW EC, with SwC5B2 reaching the highest value at 1544.92 μS·cm⁻¹. Similarly, at D(12), treatments incorporating both compost and biochar generally exhibited higher SPW EC than those with compost alone, except for SwC20B2, which showed the lowest value at 922.58 μS·cm⁻¹. Nonetheless, the highest EC values at D(12) were observed in the SwB2 (1478.8 μS·cm⁻¹) and SwC5B2 (1422.38 μS·cm⁻¹) treatments. The increase in SPW EC in treatments with compost and biochar is likely attributable to the effect of biochar. Previous studies have linked higher soil EC to biochar’s high ash content and the presence of soluble alkaline cations [74,75]. Moreover, Bartoli et al. (2022) specifically examined the influence of heating rate and feedstock nature on the electrical conductivity of biochar, concluding that polar oxygenated functional groups, influenced by pyrolysis temperature rather than the heating rate or feedstock type, contributed to higher electrical conductivity [76].

3.1.3. SPW Metal Concentrations

The SPW concentrations of Pb, Zn, and Cu exhibited notable variability among different treatments and sampling days, as shown in Table 2A. On D(0), the control treatment Sw displayed relatively low SPW concentrations of Pb (0.12 mg·L⁻¹), Zn (0.17 mg·L⁻¹), and Cu (0.09 mg·L⁻¹). In contrast, treatments with compost and compost/biochar showed higher concentrations. Specifically, SwC20 and SwC20B2 registered the highest SPW Pb concentrations at 0.81 and 1.10 mg·L⁻¹, and SPW Zn at 0.51 and 0.55 mg·L⁻¹, respectively. Similarly, the compost/biochar combination in SwC5B2 increased SPW Cu to 0.16 mg·L⁻¹, although this was not statistically different from unamended Sw. However, biochar alone slightly reduced SPW concentrations of Pb (0.09 mg·L⁻¹), Zn (0.15 mg·L⁻¹), and Cu (0.06 mg·L⁻¹), while compost alone led to increases, notably in Pb (0.21 mg·L⁻¹) and Zn (0.29 mg·L⁻¹). Although no statistical differences were observed compared to Sw, these results suggest that the increase in MTE concentrations in SPW was more strongly linked to compost addition than to biochar. Additionally, all compost-containing treatments slightly lowered Sw pH at D(0), which may have driven MTEs mobilization, as small pH changes can directly affect metal release and mobility [77]. Moreover, the release of dissolved organic carbon from compost may also contribute to metal mobilization, as organic ligands can complex with MTEs and increase their solubility [78]. Indeed, Medyńska-Juraszek et al. (2020) observed similar trends, with compost increasing the availability of Pb and Cd due to changes in MTEs speciation and the presence of organic compounds [79].

On D(12), while the control (Sw) maintained its Pb concentration, significant increases were observed in SPW Zn and Cu, with levels 5.8 and 2.6 times higher than those recorded at D(0), respectively. Meanwhile, treatments with higher compost proportions (between 10% and 20%) exhibited lower SPW MTE concentrations. Notably, SwC20B2 showed a marked decrease compared to Sw, with SPW Zn and Cu reduced by 4.9-fold and 2.4-fold, respectively. Conversely, the addition of 2% biochar to Sw led to the highest SPW Zn and Cu concentrations among all treatments, with increases of 1.4 and 1.1 times, respectively. As for SPW Pb, the highest concentration was found in SwC5B2 (0.39 mg·L⁻¹), though not significantly different from Sw.

These findings were supported by paired sample Student’s t-tests between D(0) and D(12) (Table 2B), revealing nuanced differences. Significant variations were observed for all three metals in Sw, with p < 0.01 for SPW Zn and Cu. The SwB2 and SwC10 were the only treatments showing significant differences for all three metals across the two sampling days. Both showed increased metal concentrations at D(12) compared to D(0), except for SwC10, where SPW Pb decreased. Interestingly, SwC20B2 showed a significant decrease in SPW Pb and SPW Zn over time. This behavior highlights the importance of amendment interactions. The initial increase in metals with SwC20B2 at D(0) likely reflects the influence of compost, whereas the clear decrease at D(12) suggests that biochar may have moderated this effect over time. Campos et al. (2020) reported that biochar can effectively reduce the mobility of Pb, Zn, and Cu in acidic contaminated soils, primarily through pH elevation [80], while Paz-Ferreiro et al. (2014, 2018) emphasized that biochar’s efficacy depends on factors such as feedstock type, soil conditions, and the presence of other amendments [81,82]. In our study, although compost alone tended to increase SPW MTE concentrations, its combination with biochar, particularly at higher compost rates (20%), ultimately led to reduced concentrations, even below those observed in the control. In parallel, the presence of plants may also influence these dynamics by fluctuating the chemical environment in the rhizosphere. As part of their defense mechanisms, roots release various compounds, including low-molecular-weight organic acids, which can chelate MTEs and limit their uptake [83]. Moreover, compost and/or biochar generally enhance root development and exudation, which in turn can influence MTEs speciation and toxicity in soil [84]. However, such effects may vary depending on plant species, phenological stage, and soil biological and geochemical characteristics [85,86].

3.2. Substrates Enzymatic Activities

The enzyme activities of fluorescein diacetate (FDA), alkaline phosphatase (ALP), and acid phosphatase (ACP), exhibited distinct patterns in both unamended and amended Sw as shown in Figure 3. In unamended Sw, FDA activity was the highest (4.35 nmol·g⁻¹·min⁻¹), followed by ACP (0.08 nmol·g⁻¹·min⁻¹), and then ALP (0.01 nmol·g⁻¹·min⁻¹). Upon addition of 2% biochar to Sw, FDA activity significantly increased to 8.57 nmol·g⁻¹·min⁻¹. However, the addition of 5% compost, though higher than the unamended Sw, did not show a significant difference. Notably, combining compost (5%, 10%, or 20%) with 2% biochar consistently enhanced FDA activity compared to compost alone, with SwC20B2 recording the highest FDA activity (10.26 nmol·g⁻¹·min⁻¹). Conversely, ALP enzymatic activity presented contrasting results compared to FDA. In particular, treatments containing compost alone displayed increasing values, with SwC20 exhibiting the highest ALP activity (0.09 nmol·g⁻¹·min⁻¹), significantly different from Sw. Interestingly, biochar alone resulted in extremely low ALP values, as did SwC5B2 treatment, which registered a value of 0. However, SwC5 showed a slight increase (0.02 nmol·g⁻¹·min⁻¹) over Sw, although not statistically significant. This suggests that biochar addition negatively influenced ALP activity while compost enhanced it. Similarly, biochar addition to Sw exhibited the lowest ACP activity (0.05 nmol·g⁻¹·min⁻¹). Overall, all treatments presented lower ACP activity than Sw, except SwC20 treatment, which displayed a slightly higher ACP activity (0.09 nmol·g⁻¹·min⁻¹), though not statistically different from Sw.

Enzymatic activity levels serve as key indicators of soil microbial health, making them valuable biological markers for monitoring the impacts of pollutants and assessing soil quality [87]. Indeed, FDA enzymatic assays provide insights into the hydrolytic activity of a range of enzymes, including proteases, esterases, and lipases, which play crucial roles in catalyzing the decomposition of complex organic compounds, promoting organic matter degradation, and enhancing nutrient availability in the soil for plants [88,89]. Furthermore, phosphatase activities are widely used as a marker to assess the organic phosphorus hydrolysis process [90], acting as stress indicators owing to their high degree of sensitivity to anthropogenic activities [91]. Regarding our findings, the observed decline in such enzymatic activity in Sw could potentially be attributed to the impact of metal ions on enzyme active sites, resulting in the denaturation of enzyme proteins [92,93]. However, the recorded increase in FDA activity in the presence of biochar and compost suggests that compost provides essential nutrients, enriching the Sw and supporting its microbial function.

Moreover, the incorporation of biochar into soil can provide physical and chemical support for microbial communities due to its porous structure and high carbon content [94,95]. A similar observation was made by Mukhopadhyay et al. (2022), who found that FDA activity levels increased with higher concentrations of a combined amendment consisting of yard waste biochar and compost, with varying rates for compost (2–10%) and biochar (1–5%), when applied to mining soil [51]. Additionally, an increase in ALP activity in Sw was also noted when the soil was amended with either 20% compost or 20% compost and 2% biochar. In contrast, ACP activity in unamended Sw did not significantly differ from that in SwC20 and SwC20B2, despite being contaminated by Pb, Zn, and Cu. This suggests that either the specific microorganisms responsible for ACP activity have developed metal tolerance, or that the lower metal solubility, influenced by soil physicochemical characteristics, remained below the toxicity threshold [96].

3.3. Plant Growth Characteristics

3.3.1. Dry Weight (DW) and Metal Accumulation in Phaseolus vulgaris

The dry weights of Phaseolus vulgaris stems and roots on Sw soil did not significantly differ from those of plants grown on amended Sw (Figure 4). In contrast, a significant difference in leaf biomass production was observed. Specifically, SwC10 exhibited the highest leaf dry weight (0.25 g), while SwC5B2 had the lowest (0.09 g) compared to the control Sw (0.16 g). Although not statistically significant, a similar trend was observed in root dry weights, with SwC10 recording the highest (0.55 g) and SwC5B2 the lowest (0.25 g) compared to the control (0.43 g). Furthermore, at each compost level (5%, 10%, or 20%) mixed with 2% of biochar, a slight decrease in dry weights was observed compared to compost alone. These variations in plant organ dry weights were inversely correlated with metal accumulation levels, as shown in Figure 5. For instance, SwC5B2, which had the lowest dry weight, exhibited the highest leaf and stem Pb concentrations at 94.59 and 117.30 mg·kg⁻¹, respectively. In contrast, SwC10, which had the highest shoot dry weight, showed the lowest Pb concentration in stems (17.55 mg·kg⁻¹). Similar trends were observed for Zn and Cu, except in SwB2, where Zn and Cu accumulated in leaves, reaching 289.51 and 65.07 mg·kg⁻¹, respectively.

In terms of root metal accumulation, shown in Figure 4, the highest Pb levels were once again recorded in SwC5B2 (1162.63 mg·kg⁻¹), while SwC10 presented the greatest Zn levels (1058.99 mg·kg⁻¹) and Sw recorded the highest Cu at 199.29 mg·kg⁻¹. Additionally, the combined application of compost and biochar led to increased metal accumulation in shoots compared to compost alone, which was negatively correlated with dry weights. Specifically, plants grown with both compost and biochar had lower shoot and root dry weights than those grown with compost alone. This suggests, at first sight, that metal accumulation was more closely linked to plant development rather than the composition of the amendment itself. Consequently, a dilution effect was observed [97], where low organ dry weights resulted in higher metal content. Similarly, Lebrun et al. (2019) observed a decrease in Pb and As from mining soil when applying biochar alone or associated with compost and iron. This reduction was partially attributed to increased plant dry weight in amended treatments which diluted metal concentrations in plant tissues [26].

Similarly, all treatments involving amendments showed the same trend with increased Zn accumulation in shoots, followed by Pb and then Cu. This pattern likely reflects the availability of these metals in the soil. In fact, metal concentrations measured in soil pore water (SPW), as shown in Table 2, followed the same accumulation order observed in plants. This suggests that the labile metal fractions in SPW were phytoavailable, thereby facilitating their uptake by Phaseolus vulgaris roots and subsequent translocation to aboveground parts. In fact, Zn easily forms strong bonds with organic compounds and iron/manganese oxides, making it the most mobile metal in the soil [98]. This Zn mobility can be attributed to the fact that it is an essential nutrient for plants at low concentrations [99], and hence, they do not prevent its uptake from the soil. Additionally, various factors influence metal speciation and mobility in soil, including acidic pH, high organic matter content, and lighter soil texture [100,101]. Given that Sw is alkaline, metals tend to stabilize under such pH conditions, and lower uptake of these metals will occur [102]. However, He et al. (2024) suggested that despite their alkalinity, sandy soils may contain more available forms of metals compared to clay-rich soils, which tend to immobilize metals within the soil matrix [103]. Therefore, Pb, Zn, and Cu availability could be influenced not only by soil composition and texture but also by plant-induced changes in the rhizosphere.

Overall, although some treatments resulted in lower Pb, Zn, and Cu accumulation, these differences were not statistically significant compared to Sw. However, metal accumulation in shoots was significantly higher in the presence of amendments, especially in SwC5B2 for Zn and Cu. This is in agreement with the study by Karer et al. (2018) on the immobilization of metals in contaminated soil with biochar and compost, suggesting that Cu may complex with the dissolved organic fraction of compost rather than the biochar surface, resulting in increased Cu availability in amended soils [46]. However, this observation contrasts with several previous findings. For instance, Cheng et al. (2023) found that the co-application of biochar and compost to an acidic Pb-Zn mining soil led to a reduction in Pb, Zn, Cu, and Cd availability, while improving soil physicochemical and biological properties, leading to increased ryegrass biomass [104]. Similarly, Novak et al. (2019) showed that the application of 5% beef cattle manure biochar in combination with 5% compost to an acidic mining soil mainly contaminated with Cu, Zn, and Cd resulted in a reduction in Zn and Cd accumulation in shoots and roots of switchgrass [105]. Indeed, the ability of biochar with its negative surface functional groups and compost, rich in its humic substances, to form stable complexes with metal ions in mining soils can reduce metal bioavailability [47]. However, the effects of these amendments can vary depending on factors previously mentioned, such as amendment feedstock type and production methods, as well as the soil’s overall physicochemical characteristics, which may explain the contrasting results observed in this study. Moreover, the application rates of both biochar and compost are key factors. The doses used in this study were appropriate for laboratory-scale testing to ensure observable effects, but their applicability at the field scale may vary depending on environmental and soil-specific conditions. From this practical perspective, the field applicability of this amendment strategy in Morocco deserves to be studied.

3.3.2. Mobility Index of Metals

The mobility index (MI) results provide valuable insights into the bioavailability and translocation of Pb, Zn, and Cu within the substrates used for cultivating Phaseolus vulgaris. This study specifically examined how these metals are taken up from soil and transported within the plant, focusing on three levels: MI1 (soil to roots), MI2 (roots to stems), and MI3 (stems to leaves), as shown in Figure 6. The results indicate that Pb uptake by the root system, quantified by MI1, reached its highest value (0.25) in soil amended with 5% compost and 2% biochar (SwC5B2) (Figure 6A). This observation implies that the synergistic effect of compost and biochar at specific ratios enhances the bioavailability and subsequent uptake of Pb by the root system. Furthermore, Pb translocation from roots to stems, represented by MI2, reached its maximum value (0.21) in soil amended with 20% compost and 2% biochar (SwC20B2), indicating a moderate transfer of Pb from the root system to the aerial parts of the plant. Notably, MI3, which quantifies Pb translocation from stems to leaves, was significantly higher (3.12) in soil amended with 10% compost and 2% biochar (SwC10B2), suggesting an efficient mobilization of Pb from the stems to the foliar tissues. Regarding Zn, the highest MI1 value (0.30) was observed in soil amended with 10% compost (SwC10) (Figure 6B), indicating significant uptake of Zn from the soil matrix to the root system in the absence of biochar.

The translocation of Zn from roots to stems (MI2) reached its highest value (0.21) in the SwC5B2 substrate, while the MI3 value peaked (2.65) in soil amended with 2% biochar (SwB2), implying an efficient mobilization of Zn from stems to leaves in the presence of biochar. Cu followed a different pattern, with the highest MI1 value (0.29) observed in the unamended Sw (Figure 6C). The translocation of Cu from roots to stems (MI2) was more pronounced (0.39) in the SwC20B2 substrate, suggesting that the combination of compost and biochar facilitates Cu movement within the plant. Furthermore, the highest MI3 value (2.81) was recorded in SwB2, implying a substantial translocation of Cu from stems to leaves in the presence of biochar.

Overall, these findings suggest that biochar and compost amendments influenced the mobility and translocation of Pb, Zn, and Cu in ways that did not fully align with the initial hypothesis. Specifically, it was anticipated that biochar and compost would immobilize metals in the soil, thereby reducing their bioavailability and uptake by plants. However, the results revealed moderate to high mobility and uptake of the metals from the soil to the roots (MI1), with significant translocation from roots to stems (MI2) and considerable transfer from stems to leaves (MI3). One key similarity between these metals is the trend of high translocation from stems to leaves (MI3), suggesting that once these metals are taken up by the roots, they are efficiently transported to the aerial parts of the plant. This pattern was particularly pronounced for Pb and Zn in specific combinations (e.g., SwC10B2 for Pb and SwB2 for Zn), and for Cu, which showed high translocation in the SwB2 substrate. To better understand this phenomenon, it is essential to consider all the possible interactions between soil properties, soil amendments, and plant physiology. Notably, the Sw soil is naturally alkaline, which typically promotes metal precipitation as insoluble hydroxides or compounds like carbonates or phosphates [98,106]. However, SPW pH measurements indicated that the application of compost and biochar reduced this alkalinity, especially at D(0). The expected liming effect of biochar, which promotes metal precipitation, was not prominent when applied to the Sw soil owing to its alkaline nature [107,108]. Additionally, Li et al. (2021) explained that less-decomposed compost contains non-humified organic compounds that may form soluble metal complexes, increasing metal mobility. In contrast, well-decomposed compost forms stable organometallic complexes through humic substances, reducing metal availability [109]. Furthermore, the choice of Phaseolus vulgaris as the test plant likely played a key role in the mobility of MTEs in the soil matrix. Indeed, this species is known to take up and translocate high concentrations of metals, including Pb, Zn, and Cu, from roots to leaves as reported in the literature [110,111,112]. Silva-Gigante et al. (2023) reported that under metal stress, Phaseolus vulgaris generates reactive oxygen species (ROS), which are toxic and disrupt plant metabolism [113]. However, Gutiérrez-Martínez et al. (2020) stated that common bean possesses antioxidant defense mechanisms involving enzymes such as superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase that convert these toxic oxygen species into harmless molecules, including H₂O and O₂ [114]. This inherent defense mechanism may explain the high mobility indices (MI3) observed for Pb, Zn, and Cu, especially the high translocation from stems to leaves. The ability of Phaseolus vulgaris to counteract metal-induced stress may allow it to tolerate high metal concentrations, possibly contributing to enhanced metal transport within the plant.

4. Conclusions

The co-application of compost and biochar moderately influenced the properties of sandy waste soil, with slight changes in pH and EC observed over the 12-day Phaseolus vulgaris growth period. These shifts, although limited, affected the bioavailability of MTEs in SPW, particularly at the time of planting. Notably, SwC20B2 increased SPW Pb and Zn concentrations at D(0), but later showed a significant reduction at D(12), suggesting a delayed stabilizing effect. Although this mitigation of MTEs in SPW did not consistently lead to reduced metal accumulation in plant shoots, plant growth was maintained along with soil quality. The increased MTE accumulation observed in some treatments was linked to the strong ability of Phaseolus vulgaris to uptake MTEs, supporting its role as a bioindicator. These results suggest that plant exudation interfered with the amendment effects, rendering the stabilizing capacity of compost and biochar limited in this alkaline sandy soil. Nevertheless, compost-biochar combinations at 20% and 2%, respectively, still show promise in improving soil conditions while limiting MTEs mobility in Sw. This effect would likely be more effective if the amendments were applied with phytostabilizer species exhibiting an excluder response. Further testing with varying biochar concentrations and alternative biochar types could provide deeper insights into its behavior under alkaline conditions. Overall, this study provides preliminary insights into the combined effects of compost and biochar on alkaline sandy mining wastes and highlights a potential strategy for integrating phytostabilization into the remediation of Touissit mining soils. Lastly, for practical applications, these amendments should be sourced locally and produced under environmentally friendly conditions to align with the principles of the circular economy. This is crucial for ensuring the long-term sustainability of these remediation strategies.