Critical Contribution of Biomass-Based Amendments in Mine Ecological Restoration: Properties, Functional Mechanisms, and Environmental Impacts

Abstract

Mining activities have caused widespread land degradation and contamination, affecting millions of hectares worldwide and posing persistent ecological risks. However, reclamation substrates are constrained by limited availability and compromised quality, which restricts their ability to fully support mine ecological restoration. Among various amendment materials, biomass-based amendments have been widely applied due to their broad availability, renewability, biodegradability, and low cost. In recent years, their role has expanded beyond simple nutrient supplementation to encompass multiple functions, including structural optimization, pollutant stabilization, and microbial regulation. This review highlights the valorisation of biomass-derived solid wastes as multifunctional amendments for mine ecological restoration. By converting agricultural and industrial wastes into green materials, these amendments improve substrate structure, stabilize heavy metals and organic pollutants, enhance nutrient cycling, and stimulate microbial activity. Potential risks, including nutrient leaching, secondary pollution, and greenhouse gas emissions, are critically assessed, with emphasis on their variability under different environmental conditions. By integrating functional benefits with ecological risks, this work underscores the critical role of biomass-based amendments as waste-to-resource strategies in advancing sustainable mine reclamation, contributing to circular economy goals, and supporting environmental engineering practices.

1. Introduction

Mining activities provide critical raw materials essential for energy production and industrial processes [1]. However, large-scale mining has caused severe environmental degradation, such as surface erosion, ecosystem destruction, and habitat loss [2]. Globally, more than 14% of nature reserves overlap with or are near metal mines, and biodiversity disturbances can extend several kilometers beyond the mining areas [3]. Mining activities cause severe land degradation, such as deforestation, open-pit excavation, compaction, and the removal of fertile surface layers, which reduces land productivity and increases the risk of natural disasters including flooding and landslides [4]. Another major concern is the release of heavy metals and other pollutants, which can migrate into soils and water systems around mining sites [5]. A global study found that heavy metal pollution from mining persists in rivers and floodplains, affecting ecosystems, human health, and about 23 million people worldwide [6]. Toxic elements such as arsenic, cadmium, lead, and mercury harm plant growth, microbial activity, and water quality, posing serious risks to ecosystems and human health [7]. These adverse impacts can persist long after mining operations have ceased, with studies reporting that ecosystem degradation and biodiversity loss may endure for decades without substantial natural recovery [8,9]. Therefore, it is necessary to adopt effective approaches to mitigate long-term environmental risks, reestablish ecosystem functionality, and improve the resilience of degraded mining landscapes [10].

Ecological restoration, defined as the process of assisting the recovery of degraded or destroyed ecosystems, seeks to enhance ecological functions while restoring natural landscapes to a state approximating their pre-disturbance condition [11]. The mine ecological restoration is commonly implemented through integrated measures such as terrain reshaping, substrate reconstruction, slope stabilization, contamination control, revegetation, and hydrological restoration [12]. The success of mine reclamation largely hinges on the appropriate selection of functional substrates and technologies, coupled with sustained post-restoration management [13]. Through restoration efforts, former mining areas can be transformed into productive landscapes that contribute to local economies, biodiversity, and environmental resilience. However, mining activities often result in the depletion or severe degradation of native topsoil, which consequently amplifies the demand for reclamation substrates [14,15]. Both exogenous topsoil and alternative substrates typically exhibit substantial deviations from original surface substrates in terms of physicochemical properties, microbial community composition, structural integrity, and long-term stability [16]. Accordingly, the use of targeted restoration amendments is critical for improving the functionality and long-term performance of reclamation substrates in mining areas. Agricultural amendments are applied repeatedly to sustain fertility and nutrient supply [17]. In contrast, mine reclamation amendments are usually applied only once, yet they are expected to exert long-lasting effects on substrate reconstruction, ecosystem restoration, and pollution control [9]. Achieving these persistent effects typically requires the combined application of diverse amendment types in large quantities. Inorganic mineral-based amendments, such as bentonite and lime, are commonly employed due to their low cost, structural stability, and ability to immobilize heavy metals, but their contributions to improving fertility, stimulating microbial activity, and fostering ecosystem recovery are relatively limited [18]. Synthetic or chemical amendments, such as polyacrylamide and EDTA, provide rapid and targeted effects but pose risks of environmental persistence and microbial disturbance [19].

Biomass is defined as organic material derived from living or recently living organisms, serving as a renewable resource [20]. Based on this concept, biomass-based amendments typically include plant residues, animal-derived products, organic wastes, and their processed by-products [21]. Biomass-based amendments are widely adopted in ecological restoration due to their wide availability, renewability, and cost-effectiveness, thereby enhancing fertility, stimulating microbial activity, and improving ecosystem stability [22]. However, limitations such as variable composition, rapid decomposition, secondary pollutant release, and uncertain long-term stability remain significant challenges [23]. Most existing reviews remain largely confined to the development of emerging categories such as biochar or microbial inoculants [24,25], or focus on microbial and fungal dynamics during ecosystem reconstruction following ecological restoration [26,27], while providing limited synthesis and comparison of the multifunctionality of biomass-based amendments in mine reclamation. In addition, evaluations of their potential environmental risks and sustainability challenges remain insufficiently systematic. This review critically examines the key issues associated with reclamation substrates in mining areas, synthesizes the characteristics and multifunctional mechanisms of various biomass-based amendments, and systematically evaluates their implications for nutrient leaching, greenhouse gas (GHG) emissions, and long-term ecological impacts. Through this synthesis, this review develops a systematic perspective to deepen understanding and advance sustainable restoration practices in mining regions.

2. Challenges Associated with Reclamation Substrates in Mine Ecological Restoration

Large-scale mining operations, particularly open-pit excavation and ore dressing, inevitably strip away and degrade the original nutrient-rich surface substrates [26]. Topsoil, as the most functionally vital component of the original surface substrates in the pre-mining ecosystem, typically exhibits high porosity and structural stability, which enhance water infiltration, aeration, and root penetration, thereby creating favorable conditions for seed germination and plant establishment [28]. It offers beneficial chemical properties, serving as a reservoir of essential nutrients and maintaining a balanced pH and cation exchange capacity (CEC) that support early plant growth and microbial activity [29]. Furthermore, it harbors a rich assemblage of native seeds and diverse microbial communities, including bacteria, fungi, and other microorganisms that are critical for nutrient cycling, symbiotic associations, and the reestablishment of a functional ecosystem [30].

In contrast, the substrates newly exposed after mining are generally composed of waste rock or tailings, exhibiting profoundly degraded physical properties with coarse texture, poor water retention, and scarce aggregation. They are markedly deficient in essential nutrients and organic matter, resulting in extremely low fertility. Moreover, sulfide oxidation and the generation of acid mine drainage induce severe acidification and mobilization of toxic metals, which further exacerbate toxicity and suppress plant colonization [15]. The synergistic effects of physical compaction, chemical contamination, and biological impoverishment create an exceptionally hostile environment where natural recovery proceeds at an exceedingly slow pace, often requiring decades or longer, and becomes nearly unattainable without human intervention [8]. As a result, the availability and quality of topsoil are severely degraded, resulting in a pronounced shortage in the supply of reclamation substrates. Under such circumstances, supplementation with topsoil or replacement with alternative substrates is considered indispensable for mine ecological restoration [31]. Accordingly, the primary sources and characteristics of reclamation substrates for mine ecological restoration are summarized in

Table 1. The primary sources and characteristics of reclamation substrates for mine ecological restoration.

Material	Sources	Characteristics	Limitations
Topsoil	• Salvaged from the mining site • From off-site undisturbed areas	• High porosity and stable structure • Favorable chemical properties and essential nutrients • Rich in seeds and diverse microbial communities	• Risk of contaminant accumulation and nutrient loss during mining operations • Limited availability and inconsistent quality
Coal gangue	• By-product of coal mining	• Abundant and easily accessible in coal mining • Contains mineral components (Si, Al, Fe, Ca) • Exhibits good physical stability	• Low organic matter and nutrient content • Potential release of heavy metals and acid mine drainage
Fly ash	• Residue from coal combustion	• Fine texture with high specific surface area • Rich in mineral nutrients (Ca, Si, Fe, K) • Enhances aeration, porosity, and structural stability of substrates	• High salinity and alkalinity • Potential heavy metal contamination • Limited water-holding capacity
Tailings	• By-product of mineral processing	• Large available volume near mine sites • Variable texture depending on ore type • Provide structural support and limited mineral nutrients	• Poor fertility and microbial activity • Potential toxicity due to residual metals or flotation reagents
Dredged sediment	• By-product of river and lake dredging	• Fine texture and high water-holding capacity • Contains appreciable amounts of organic matter and nutrients (N, P)	• Potential contamination with heavy metals or organic pollutants • Unstable sources with heterogeneous fertility and contamination

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Given the ecological importance of topsoil in facilitating vegetation recovery and soil functionality, is considered the most ideal reclamation substrates [16]. Considerable research has focused on salvage, stockpile, and transfer techniques. Direct topsoil transfer can rapidly improve physicochemical and microbial properties in the short term. However, in the long term, such transfer may fail to fully restore biological activity, profile integrity, and organic matter content to levels comparable with undisturbed reference systems [32]. Although effective, direct transfer is often impractical in large-scale mining operations due to spatial and temporal mismatches between salvage and site restoration [33]. Consequently, topsoil is frequently stored in stockpiles for extended periods. Excessive stockpile depth can lead to compaction, which limits aeration, reduces nutrient availability, and creates unfavorable conditions in the deeper layers [34]. However, the physicochemical properties of stockpiled topsoil tend to remain relatively stable, exhibiting only minor deviations from those of reference topsoil [33]. Stockpiling primarily compromises the biological integrity of topsoil, with bacterial diversity shown to decline significantly as stockpile depth increases [34]. Prolonged storage further induces persistent shifts in microbial community composition. Initially, these conditions cause a sharp decline in microbial diversity and function [34]. Partial recovery may occur over time, ultimately resulting in a deviation from the native microbial community structure. The re-establishment of arbuscular mycorrhizal fungi (AMF) typically occurs approximately five to ten years after storage [35]. A decline in ectomycorrhizal fungi and a corresponding increase in saprotrophic fungi are observed relative to undisturbed reference surface substrates [36].

Alternative substrates can generally be categorized into two major groups based on their sources: industrial solid wastes and dredged sediments. Tailings, a major type of industrial solid waste generated during mineral-processing operations, are available in large quantities near mine sites. Their particle size and mineral composition vary greatly with ore type, leading to differences in texture [37]. Coal gangue, which is rich in minerals such as silicon, aluminum and iron [38], contains higher levels of organic matter compared with tailings [18], contributes to the enhancement of substrate stability [39]. Fly ash, a residue from coal combustion, contains essential mineral elements required for plant growth [40]. It has long been employed as a substitute for lime, as the calcium and other cations supplied by fly ash can increase pH and improve structure under acidic conditions [41]. However, the high salinity and alkalinity of fly ash may inhibit seed germination and plant establishment if applied in excess. Furthermore, these industrial solid waste substrates usually exhibit relatively low water-holding capacity, limited nutrient availability, and potential risks of heavy metal contamination. Dredged sediments obtained from rivers or lakes are characterized by fine texture and high water-holding capacity and usually contain appreciable amounts of organic matter and nutrients (N, P) [42,43]. However, sediments dredged from rivers and lakes show strong spatiotemporal variability in their sources, which gives rise to considerable heterogeneity in physical and chemical properties. In addition, dredged sediments commonly exhibit potential contamination with heavy metals and organic pollutants, while the pollutant patterns differ markedly across regions, complicating the assessment of their suitability and associated environmental risks [44].

Overall, reclamation substrates, including both topsoil and alternative substrates face challenges of limited availability, compromised quality, and the risk of secondary pollution, making them insufficient to meet the comprehensive requirements of mine ecological restoration. To overcome these limitations, amendments are required to enhance substrate performance by supplementing organic matter and essential nutrients, improving water-holding capacity and structural stability, stabilizing or immobilizing heavy metals and organic contaminants, and restoring biological activity and microbial diversity, thereby creating more favorable conditions for vegetation establishment and long-term ecosystem recovery. Among various amendment materials, biomass-based materials have been widely applied and continuously attracted attention due to their broad availability, renewability, and cost-effectiveness [20,25].

3. Classification of Biomass-Based Amendments

In mine ecological restoration, reclamation substrates are often constrained by nutrient deficiency, structural degradation, and pollutant loads, which have driven the continuous development and application of biomass-based amendments. Amendments derived from plant and animal sources, providing organic matter and essential nutrients, constitute the earliest and most widely adopted. With the growing emphasis on waste valorization in environmental management, industrial organic by-products have increasingly been introduced as alternative restoration materials. However, these conventional amendments frequently exhibit limited stability and provide insufficient direct support for the restoration of the ecosystem. To address these limitations, traditional substrates have been converted using thermochemical processes such as pyrolysis and hydrothermal carbonization, yielding amendments with enhanced stability and improved pollutant immobilization capacity [45]. In parallel, microbial inoculants have emerged as a functional category, employing selected microorganisms to directly regulate nutrient cycling and promote ecosystem restoration [46]. Collectively, the diversification of amendment categories reflects adaptive responses to restoration demands and highlights the evolutionary transition of biomass-based amendments from simple resource supplementation toward multifunctional optimization (Figure 1).

3.1. Plant-Derived Amendments

Plant-derived amendments are mainly derived from crop residues (e.g., straw, rice husk), forestry by-products (e.g., sawdust, bark) and peat materials (

Table 2. Summary of plant-derived amendments applied in mine ecological restoration.

Material	Substrate	Region	Application Rate	Co-Applied Materials	Observed Effects
Wheat straw	Iron ore tailings, mixed with river sand (1:1) [47]	Pilbara iron ore mine tailings, Western Australia	0%, 1%, 2%, 5%, and 10%	/	Increased EC, total C, N, and CEC; promoted microbial biomass carbon and respiration; enhanced plant shoot biomass; increased uptake of Co, Cu, Fe, Mn, Zn, Cr, and Ni in shoots
Rice husk	Co-contaminated soils (PAHs + Zn, Cr) [48]	Liaoning, China	2%	/	Enhanced rhizosphere microbial activity and PAH degradation; decreased Zn and Cr bioavailability
Pine bark	Acid Cu-polluted mine soil [49]	Touro abandoned Cu mine tailing area, Galicia, NW Spain	6, 12, 24, 48, and 96 g kg−1	Alone/with crushed mussel shell (1:1)	Stimulated bacterial and fungal growth; improved microbial function recovery; increased DOM
Coniferous bark compost	Acidic mine tailings (Cu, Ni, S) [50]	Outokumpu Cu–Ni mine tailings, Finland	0%, 5%, 10%, and 20%	/	Increased pH, CEC, nutrient availability, microbial diversity
Peat	Coarse-textured reclaimed soils combined with lean oil sand or tailing sand [51]	Athabasca Oil Sands Region, Alberta, Canada	50 kg N·ha−1	/	Reduced N leaching; enhanced water-holding capacity, increased nutrient availability for vegetation growth
	Borrow pit soil with insufficient salvaged topsoil [52]	Cold Lake, Alberta, Canada	20 kg·m−2	Alone/with aspen woodchips biochar (1.5 kg·m−2)	Improved soil fertility and moisture retention under limited topsoil conditions

). These materials are rich in organic matter and supply essential nutrients, thereby improving fertility and enhancing substrate structure. In addition, they promote aggregate formation and stimulate microbial activity, which are critical for early vegetation establishment in mine ecological restoration. Wheat straw significantly improved the physicochemical properties of iron ore tailings and promoted the growth of perennial ryegrass, particularly by enhancing microbial activity [47]. Rice husk can also modify the rhizosphere environment and stimulate the growth of microbial communities, thereby improving rhizosphere environment and facilitating the restoration process [48]. Pine bark amendment significantly stimulated microbial activity in Cu-polluted acidic mine site, producing transient increases in bacterial growth and sustained enhancements in fungal growth over two years [49]. They can also be composted to further increase nutrient availability [50]. In addition, peat, formed from partially decomposed plant residues under waterlogged and anoxic conditions, has a high organic-carbon content, cation-exchange capacity, and excellent water-holding properties, which enable it to effectively support vegetation establishment in degraded mine soils [51]. Field experiments conducted in western Canada further demonstrated that peat, either applied alone or in combination with biochar, significantly increased soil organic-carbon and total-nitrogen concentrations, thereby improving the fertility of disturbed boreal forest soils with insufficient topsoil [52]. However, their rapid decomposition often leads to unstable nutrient release and may cause nitrogen immobilization and competition with plants [53]. Thus, although plant-derived amendments are the most accessible and widely applied, their short-term effects necessitate integration with other restoration strategies to sustain long-term ecosystem recovery.

3.2. Animal-Derived Amendments

Animal-derived amendments mainly include livestock manure and by-products such as bone meal. Owing to their high concentrations of nitrogen and phosphorus, these materials can rapidly improve fertility and stimulate microbial activity. However, potential safety issues and nutrient losses restrict their large-scale application in mine reclamation, emphasizing the need for proper stabilization and management measures. To ensure both safety and effectiveness, animal-derived amendments are generally recommended to undergo stabilization processes such as composting or anaerobic digestion prior to application [54]. Among animal-derived materials, co-composting rice straw with poultry manure effectively degrades high-silica residues, producing a nutrient-balanced and well-matured organic amendment [55]. Serving as a nitrogen-rich source, poultry manure complements rice straw, thereby mitigating straw residue burning and improving its physicochemical and biological properties. In contrast, bone meal, a low-cost and readily available by-product from slaughtering and bone processing, has emerged as a practical phosphate source alternative to synthetic hydroxyapatite [56]. By ameliorating acidic soils and promoting the in situ precipitation of stable metal–phosphate minerals, it effectively reduces heavy metal bioavailability and ecological risks, highlighting its potential as an economical and sustainable amendment for mine tailings and metal-contaminated sites [57]. These materials and their application effects are summarized in

Table 3. Summary of animal-derived amendments applied in mine ecological restoration.

Material	Substrate	Region	Application Rate	Co-Applied Materials	Observed Effects
Poultry manure compost	Coal surface mine spoil [58]	Pennsylvania, USA	112 Mg ha−1 (dry weight)	With paper-mill sludge	Enhanced fertility and vegetation growth; increased microbial activity; composting reduced nutrient loss and stabilized organic matter
Chicken and sheep manure compost	Reclaimed coal-mine soil [59]	Huaibei, Anhui Province, China	200 mg N·kg−1	Alone/with wheat-straw biochar (0%, 1%, 3%)	Chicken manure showed higher N mineralization potential (~36% above sheep manure); faster initial N release rate
Bone meal	Metal-contaminated soils from historic mining sites [56]	Parys Mountain, Leadhills, Wanlockhead, United Kingdom	2%	/	Increased soil and leachate pH; reduced Zn, Pb, Cd, and Cu concentrations in leachate; decreased metal bioavailability
	Acidic Ultisols [57]	Anhui, Hunan, Guangdong, Zhejiang, Southern China	1–2 g/kg	Alone/with biomass ash and alkaline slag	Decreased exchangeable acidity; reduced exchangeable Al3+; promoted metal–phosphate formation

.

3.3. Industrial Organic By-Products

Industrial organic by-products include urban sewage sludge, food waste, and agro-industrial residues. They are generated in large quantities, making them readily available and cost-effective sources of organic matter and nutrients. Food waste application can improve water retention, increase organic carbon content, and partially compensate for the scarcity of original surface substrates [60]. In long-term tailings reclamation studies, food waste-derived amendments have been shown to improve substrate bulk density, porosity, aggregate stability, and nutrient availability, thereby sustaining vegetation establishment and enhancing substrate quality over multiple years [61]. Urban sewage sludge contains more nitrogen and phosphorus and hosts abundant microbial populations. However, the potential presence of organic pollutants and pathogens may constrain its direct use in mine reclamation. To ensure environmental safety and improve material stability, sewage sludge generally requires pretreatment or stabilization, typically through composting or anaerobic digestion [62,63]. The fermentation of sewage sludge to generate organic acids effectively reduced the pH of bauxite residue to near neutral, concomitantly weakening the alkaline mineral peaks of alumino-ferric oxides and albite. The treated residue further demonstrated its suitability as a growth substrate by supporting the germination and growth of Zoysia japonica in pot experiments [64]. By-products from the papermaking process, such as primary paper mill sludge and short paper fiber, contain abundant fibrous materials and calcium-based fillers that can improve soil aggregation, enhance water retention, and neutralize acidity in mine reclamation substrates [58,65]. Bagasse, a major by-product of the sugar industry, is rich in cellulose, hemicellulose, and lignin, with high organic carbon content and a porous structure that enable it to serve as both a structural improver and an organic carbon source in mine land reclamation [66]. Their characteristics and application outcomes are summarized in

Table 4. Summary of industrial organic by-products applied in mine ecological restoration.

Material	Substrate	Region	Application Rate	Co-Applied Materials	Observed Effects
Spent Mushroom Compost	Subsoil cover on Pb/Zn tailings [61]	University of Limerick Pb/Zn TSF site, Ireland	33%	Alone/co-applied	Improved porosity and aggregate stability; increased organic carbon and total N; soil fertility and vegetation establishment
Compost-like Output
Urban sewage sludge	Bauxite residue–soil mixture [64]	Shenyang, China	30%	Direct/inoculated fermentation	Reduced pH; increased TOC and decreased Na and Al; enhanced Z. japonica growth and microbial diversity
Sewage sludge compost	Rare earth mining wasteland soil [66]	Meizhou, Guangdong, China	40%	Alone/co-applied	SSC improved soil fertility but increased Cu, Zn, Cd, Ni accumulation and inhibited growth; bagasse improved soil structure, and SOC; co-application enhanced root growth, nutrient uptake, and biomass while reducing heavy metal bioavailability
Bagasse
Paper mill sludge	Surface soil of mined land [58]	Pennsylvania, USA	20 or 40 Mg·ha−1 (dry weight)	With manure	Increased soil pH, total N and C, and reduced nutrient leaching; enhances N stabilization and soil fertility
Short paper fiber	Abandoned coal refuse pile in Appalachia [65]	West Virginia, USA	20% or 40%	/	Reduced infiltration; improved water retention and structural stability

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3.4. Biochar and Hydrochar

Biochar and hydrochar, produced through pyrolysis and hydrothermal carbonization respectively, represent a more intensively processed category of biomass-based materials. These products exhibit high chemical stability, strong carbon sequestration potential, and considerable capacity for immobilizing heavy metals and organic pollutants [48]. Hydrochars effectively adsorb heavy metals, owing to their porous structure and abundant oxygen-containing functional groups [67]. The addition of biochar significantly reduced the mineralization rates of C and N, exhibiting a pronounced negative priming effect that contributes to long-term carbon sequestration [68]. In addition, they can improve substrate structure, increase CEC, and provide a habitat for beneficial microorganisms. Hydrochar application has been shown to enhance CEC and enrich organic carbon content, creating more binding sites for metal cations. These improvements collectively decrease the mobility and bioavailability of heavy metals, thereby contributing to the long-term stabilization and enhance overall environmental quality [69]. Research has revealed that biochar derived from date palm waste, particularly low-temperature biochar, can effectively reduce the bioavailability of Cd, Cu, Pb, and Zn in mine tailings, while simultaneously enhancing microbial activity and organic carbon content [70]. However, thermochemical products often contain limited nutrient levels compared with raw biomass and may carry toxic by-products such as polycyclic aromatic hydrocarbons (PAHs). Furthermore, production costs can be relatively high, which restricts their large-scale deployment in restoration projects [71]. Despite these challenges, biochar and hydrochar are increasingly regarded as promising long-term solutions for stabilizing degraded environments in mining areas. The key types of biochar and hydrochar and their application effects are summarized in

Table 5. Summary of biochar and hydrochar applied in mine ecological restoration.

Material	Substrate	Region	Application Rate	Temperature	Co-Applied Materials	Observed Effects
Rice husk biochar	Co-contaminated soils (PAHs + Zn, Cr) [48]	Liaoning, China	2%	500 °C	/	Biochar performed better than rice husk; lower bioavailability of heavy metals; higher PAH removal efficiency.
Date palm waste biochar	Mine tailings (Cd, Cu, Pb, Zn) [70]	Saudi Arabia	1–5%	Low (300–400 °C) High (500–600 °C)	/	Low: decreased Cd, Cu, Pb, Zn bioavailability; high: reduced CO2 emission and maintained metal immobilization
Peanut shell biochar	Acidic Cu-mine tailings [72]	Jiujiang, Jiangxi, China	5–10%	500 °C	/	Increased pH and reduced Cu leachability; improved plant growth; stabilized Cu through precipitation and surface complexation
Modified hydrochar	Pb–Cd contaminated soils [73]	Beijing, China	1–5%	220 °C	/	Increased immobilization efficiency; decreased acid-soluble and reducible fractions; increased pH and electronegativity
Iron-modified rice husk hydrochar	Pb–Sb contaminated soil [74]	Hunan, China	1–5%	180 °C	/	Reduced bioavailable Pb and Sb by 25% and 40%; immobilization via cation exchange, precipitation, surface complexation, and formation of Fe–Sb stable minerals

.

3.5. Microbial Inoculants

Microbial inoculants constitute a functional category of amendments that rely on beneficial microorganisms to directly regulate biological processes. Although microbial inoculants differ from conventional organic residues in that they consist of living organisms rather than decomposed biomass, they can be conceptually regarded as biologically derived amendments because they are biological resources that directly regulate ecosystem processes. This group includes plant growth-promoting rhizobacteria (PGPR), nitrogen-fixing bacteria, phosphate-solubilizing bacteria, and fungi such as AMF. These organisms enhance nutrient cycling, increase plant stress tolerance, and contribute to the reestablishment of functional ecosystems [75]. Mycorrhizal inoculation has been reported to accelerate seedling establishment and biomass production while facilitating the recovery of species diversity in reclaimed mine sites [76]. AMF establish symbiotic associations with legumes and early- to mid-successional plants, thereby improving fertility and promoting vegetation growth. Their introduction into degraded environments enhances nutrient acquisition, particularly nitrogen uptake in coal mining subsidence areas, ultimately mitigating erosion and desertification [26]. In mine reclamation, microbial inoculants are particularly valuable for reintroducing biological activity into sterile substrates and fostering plant-microbe symbioses [77]. A four-year field study at an abandoned carbonate mine demonstrated that microbial inoculants can substantially alter root-associated bacterial and fungal communities and reshape soil–plant microbiome networks [78]. Nevertheless, their effectiveness is highly dependent on environmental conditions, and their persistence in the field is often limited. As a result, microbial inoculants are frequently applied in combination with amendments such as compost or biochar to improve their survival and performance [79]. These microbial inoculants and their application effects are summarized in

Table 6. Summary of microbial inoculants applied in mine ecological restoration.

Material	Substrate	Region	Application Rate	Co-Applied Materials	Observed Effects
Funneliformis mosseae (AMF)	Simulated coal-mining subsidence soils [80]	Beijing, China	50 g inoculum per hole	/	Enhanced N, P, K, Ca, and Mg uptake; increased shoot and root biomass; improved hormone regulation; promoted root recovery and plant tolerance
	open-pit coal mine dump [81]	Inner Mongolia, China	100 g AMF inoculum per plant	/	Accelerated leaf decomposition; increased macroaggregate formation and GRSP accumulation; enhanced mineral-associated organic carbon; promoted carbon stabilization and aggregate development
multiple AMF (Rhizophagus, Claroideoglomus, Funneliformis)	Degraded copper tailings/mine-impacted soils [82]	Jiangxi Province, China		/	Enhanced plant biomass and chlorophyll; restored AMF community composition; improved soil nutrient availability (P, N); reduced Cu uptake and metal toxicity;
P-solubilizing bacteria	Cd/Pb/Zn-contaminated nonferrous metal mine tailings [83,84]	Dabao Mountain mining area, Guangdong, China	5 mL bacterial suspension injected into rhizosphere every 10 days × 4 times (total 45 days)	With Straw compost (1%)	Increased available P and chlorophyll; reduced Cd/Pb/Zn and increased plant extraction efficiency; improved soil enzyme activities and microbial metabolic heat
Si-solubilizing bacteria					Increased available Si and improved ryegrass root length; enhanced soil redox potential; increased antioxidant enzyme activity; promoted transformation of metals to exchangeable forms and enhanced phytoextraction
K-solubilizing bacteria					Strongest stimulation of soil urease and phosphatase among single strains; increased available K; enhanced microbial calorimetric parameters; promoted ryegrass biomass accumulation and metal uptake.
Combined strains					Most effective treatment; increases in ryegrass growth and antioxidant enzymes; highest Cd/Pb/Zn extraction; largest decrease in soil metal concentrations; strongest enhancement of metal bioavailability.
Mixed microbial inoculant	Abandoned carbonate mine restoration soil [78]	Nanjing, Jiangsu, China	1 L/m2 inoculum		Reshaped root, fine-root, rhizosphere and bulk soil bacterial/fungal communities; enhanced plant–soil nutrient coupling; increased complexity and stability of soil microbiome network; increased functional traits in root and nodule microbiomes

.

4. Functional Mechanisms of Biomass-Based Amendments in Mine Ecological Restoration

Biomass-based amendments from different origins can, to a certain extent, reflect inherent differences in their properties, thereby elucidating their respective advantages and limitations in mine site ecological restoration. However, their effectiveness is not determined solely by source or composition, but more critically by the interactions they initiate upon incorporation into reclamation substrates. Through synergistic interactions, these mechanisms alleviate mining-induced disturbances such as structural degradation, nutrient depletion, pH imbalance, metal contamination, and disruption of microbial functionality [85]. Comprehensively understanding these processes is crucial for elucidating the differential remediation effects of various biomass-based amendments and serves as a prerequisite for the design of rational, rapid, and sustainable reclamation substrates in mine ecological restoration. Accordingly, four key aspects are considered: physical improvement and structural optimization, chemical processes and contaminant immobilization, nutrient cycling and fertility enhancement, and biological mechanisms with microbial activation, which together elucidate the functional roles of biomass-based amendments in mine ecological restoration, as illustrated in Figure 2.

4.1. Physical Improvement and Structural Optimization

Degraded mining environments are typically characterized by a lack of stable aggregates, low porosity, and high susceptibility to surface cracking, all of which severely constrain water infiltration, root establishment, and microbial activity [86]. Biomass-based amendments can alleviate these limitations through multiple physical pathways, mainly by promoting aggregate formation and stabilization, improving porosity and water retention, and enhancing crack resistance and surface stability. In practical applications, composite amendments often exert these functions simultaneously. Although each component contributes more prominently to specific functions, their combined action optimizes substrate structure from micro-scale aggregation to macro-scale stability, thereby creating a favorable physical environment for nutrient conservation, microbial activation, and vegetation recovery.

(1) Promotion of aggregate formation and stabilization

Aggregates, as the fundamental units of substrate structure, represent heterogeneous assemblages of organic matter and mineral particles, and their formation is crucial for ecosystem development [87]. Plant residues, animal manures, and compost decompose to release polysaccharides, humic acids, and other binding agents, which interact with mineral particles and strengthen the aggregation process, thereby improving substrate structure [88]. Sugarcane mulch, applied as an exogenous organic amendment to Fe ore tailings, facilitated aggregate formation and ecological rehabilitation by providing carboxyl, aromatic, and carbonyl groups that were sequestered by Fe-rich minerals through ligand exchange and hydrophobic interactions [89]. In backfilled topsoil used for mine reclamation, chemical fertilizer application significantly reduced the proportion of macro-aggregates (>2 mm), whereas pig manure and cow manure increased their proportion, with chicken manure exhibiting the most pronounced effect [90]. Additionally, extracellular polymeric substances secreted by bacteria serve as biogenic binding agents that promote the aggregation and stabilization of microaggregates [91]. In parallel, fungi contribute to aggregate stability through hyphal enmeshment and binding, as well as the structural reinforcement provided by hyphal networks [92]. This implies that AMF microbial inoculants can likewise promote the formation and stabilization of aggregates. Biochar, enriched with carboxyl and hydroxyl groups, enhances microaggregate cementation through interactions with mineral particles and organic matter [25]. Meanwhile, its microporous structure protects organic matter from decomposition and promotes its stabilization within aggregates as microaggregate-associated carbon [93].

(2) Improvement of porosity and water retention

Compost and plant residues, with their low density and porous texture, enrich organic carbon and enhance CEC during decomposition, which improves water retention and complements aggregate-mediated regulation [94]. Biochar, with its well-developed microporous structure, stores water and reduces gravitational loss, while its high surface area and porosity reinforce water retention [24]. A meta-analysis of published data demonstrated that biochar amendments significantly improved all tested substrate physical properties, with bulk density markedly reduced, porosity substantially increased, and notable enhancements observed in available water-holding capacity and saturated hydraulic conductivity [95]. In addition, the formation of aggregates further optimizes pore size distribution, whereby macropores facilitate infiltration and micropores retain water, thereby improving overall water-holding capacity and redistribution [96].

(3) Enhancement of crack resistance and surface stability

The crack resistance of substrate surfaces largely depends on structural stability [97]. Biomass-based amendments strengthen substrate resilience by combining aggregation benefits with enhanced plasticity, which together reduce fissuring under alternating wet–dry cycles. Studies has shown that the incorporation of 10% fine-particle biochar most effectively suppressed reclamation substrate cracking, with the surface crack ratio, crack segment number, total crack length, and average crack width reduced by 31.29%, 30.78%, 14.18%, and 20.45%, respectively [98]. Meanwhile, long-fiber materials such as straw and wood chips act as reinforcement, effectively reducing the shrink and swell ratio of clayey substrates and inhibiting crack propagation [99]. For example, two primary mechanisms have been identified for straw-reinforced substrates: improved ductility, which minimizes local tensile stress and cracking and thereby enhances slope stability, and the spatial constraints imposed by the straw–substrate network, which further strengthen the reinforcing effect [100]. Organic by-products such as paper sludge and bagasse, which contain abundant fibrous components, further enhance mechanical stability and provide additional support to the surface layer [89]. Short paper fiber, a solid by-product of the papermaking process enriched in fibrous components, has been demonstrated to perform effectively as an amendment, with the 20% application rate yielding the best results [65]. Biomass-based amendments improve crack resistance and surface stability through the combined effects of plasticity enhancement and fibrous reinforcement.

4.2. Chemical Mechanisms and Contaminant Immobilization

Degraded mining environments often experience extreme pH conditions. Acid mine drainage leads to acidification, while strong evaporation in arid and semi-arid regions causes alkalization or salinization. Elevated heavy metal concentrations and complex redox conditions further increase ecological risks Biomass-based amendments not only improve substrate structure through physical processes but also immobilize contaminants via multiple chemical pathways [101]. Depending on their type, these amendments may function primarily by buffering pH, passivating heavy metals, or regulating redox reactions. In practice, they often act synergistically to decrease heavy metal mobility and bioavailability, thus creating a more stable chemical environment that facilitates plant establishment and ecological restoration.

(1) pH buffering and neutralization

Mine soils and tailings are frequently subjected to prolonged leaching by acid mine drainage, which results in severe acidification. Acidification deteriorates substrate physicochemical properties, decreases nutrient availability, and markedly increases the solubility and mobility of heavy metals [102]. Under such conditions, alkaline biochar and composts rich in carbonate or ash can effectively neutralize acidity, elevate pH, and thereby mitigate acid-induced degradation of substrate functionality while reducing the bioavailability of heavy metals [103]. The ash fraction of alkaline biochar, rich in carbonates and (hydr)oxides, dissolves to neutralize acidity and restore pH balance [104]. For instance, peanut shell biochar (pH 10.35) effectively neutralized copper mine tailings (pH 3.85), alleviating acidification stress [72]. Arid and semi-arid mining regions and waste dumps with strong evaporation are often affected by salt accumulation and alkalization [105]. Under such conditions, organic-rich biomass-based amendments effectively buffer pH fluctuations and neutralize excessive alkalinity. They release acids that mobilize Ca²⁺ from CaCO₃ to displace Na⁺ and reduce exchangeable sodium percentage, while humic substances enhance buffering capacity, thereby alleviating alkalinity stress [106]. In parallel, functional microbial inoculants such as salt-tolerant PGPR and AMF offer a biological pathway for saline soil remediation by improving nutrient status and pH. They also enhance ionic homeostasis through increased K⁺/Na⁺ ratios and activate plant stress-response mechanisms that collectively strengthen salinity tolerance [107].

(2) Stabilization of heavy metals

Beyond pH regulation, biomass-based amendments contribute to the stabilization of heavy metals through direct interactions with functional groups and organic complexes. Surface functional groups such as carboxyl, hydroxyl, and phosphate provide abundant binding sites that immobilize metals including Cd, Pb, Zn, and As via adsorption and ion exchange [101]. In particular, humic substances rich in oxygen-containing functional groups chelate with or complex metal cations, forming stable organo–metal associations that reduce their solubility and bioavailability [82]. The porous structure and large specific surface area of biochar enhance sorption capacity, while its surface charge properties facilitate electrostatic interactions with heavy metal ions [108]. Moreover, certain biomass amendments can modify redox potential, driving valence transformations such as the reduction of Cr(VI) to the less toxic Cr(III) and the stabilization of As predominantly in the form of As(V), which further decreases their environmental risk [109,110]. These processes reduce metal leaching and limit uptake by plants and microorganisms, thereby mitigating ecological and human health risks [111].

(3) Immobilization and degradation of organic pollutants

Mine sites are often subjected to organic contaminants, including PAHs, petroleum hydrocarbons, and flotation reagents derived from mining activities. Biomass-based amendments have shown considerable potential in mitigating such contamination. The hydrophobic surfaces and aromatic structures of compost and biochar facilitate the adsorption and immobilization of hydrophobic organic pollutants through π–π interactions, partitioning, and hydrophobic effects, thereby reducing their mobility and bioavailability [68]. Compost and crop residues can stimulate microorganisms to secrete key enzymes, thereby accelerating the cometabolic degradation of recalcitrant organics such as PAHs [59]. In addition, humic substances and certain biochars possess electron-shuttling properties, acting as mediators that facilitate electron transfer between microbes and pollutants, which promote the cleavage and structural transformation of organic contaminants [104]. A 126-day experiment demonstrated that the application of sewage sludge and sludge compost in alfalfa-planted soils increased PAHs removal to 60%, representing a 20%–25% improvement compared with the unamended control [112]. Another 150-day pot trial showed that rice husk biochar amendment in alfalfa-planted co-contaminated soils enhanced PAHs removal to 65%, with an additional 20% increase over the control, while Zn and Cr were simultaneously immobilized through adsorption and stimulation of rhizosphere microbial activity [48]. Overall, biomass-based amendments achieve simultaneous immobilization of heavy metals and degradation of organic pollutants through combined mechanisms of physical sorption, microbial cometabolism, and redox regulation, thereby providing an efficient pathway for the integrated remediation of co-contaminated mine soils.

4.3. Nutrient Cycling and Fertility Enhancement

Mining-impacted areas are typically nutrient-depleted and exhibit poor fertility. In the short term, deficiencies in organic carbon and readily available nutrients constrain plant establishment and early growth, whereas in the long term, nutrient leaching and trace element deficiencies impede the maintenance of sustained fertility and ecosystem functions [113]. Such limitations not only hinder vegetation recovery but also weaken microbial activity and functionality. Biomass-based amendments improve nutrient cycling and enhance fertility through multiple pathways, including continuous carbon input that promotes humus formation, mineralization–buffered nutrient release that combines rapid availability with long-term retention, and the supplementation of macro- and micronutrients [114]. Collectively, these processes act synergistically to restore nutrient content and provide a stable fertility foundation for mine ecological restoration.

(1) Exogenous carbon input and humification

Biomass-based amendments provide exogenous carbon inputs that not only supply energy and substrates for microbial recovery but also facilitate the formation and transformation of complex organic molecules such as polysaccharides and aromatics, thereby accelerating humus accumulation [115]. Compost products in particular deliver partially humified intermediates enriched in aromatic backbones and oxygen-containing functional groups (–COOH, –OH), which act as precursors and catalysts that initiate humification processes and shorten their timescale from decades to seasons [116]. These intermediates undergo condensation, π–π stacking, hydrogen bonding, and covalent coupling, driving the stabilization of organic matter into humic substances [117]. The resulting relatively stable carbon pool provides long-term storage of carbon and nutrients and improves physicochemical properties, thereby laying a foundation for ecosystem restoration.

(2) Nutrient mineralization and slow release

The easily degradable fractions of biomass-based amendments, such as proteins, amino acids, and soluble sugars, undergo rapid microbial mineralization. This process releases available nitrogen, phosphorus, and potassium, which support nutrient supply for early plant establishment and initial growth [118]. At the same time, biochar and humic substances, owing to their high CEC and well-developed microporous structures, can effectively adsorb and gradually release nutrient ions, thereby reducing leaching losses and runoff risks [119]. In reclaimed soils from coal mining subsidence areas, the incorporation of 1% and 3% biochar with chicken manure reduced nitrogen leaching by 21.49% and 28.31%, respectively, compared with the non-biochar treatment [59]. The temporal complementarity between mineralization and slow release helps maintain a relatively stable nutrient supply regime, improves nutrient synchrony across plant–substrate–microbe systems, and ultimately establishes a solid foundation for long-term fertility restoration and sustainable ecosystem recovery in degraded mining areas. Biofertilizers, consisting of bacterial and fungal inocula, can further complement organic fertilizers to reduce reliance on chemical inputs and enhance nutrient use efficiency. Through processes such as nitrogen fixation and the mobilization of phosphorus, potassium, and iron, these microbial inoculants improve nutrient bioavailability and reinforce fertility restoration in degraded soils [120].

(3) Micronutrient supplementation and ionic rebalancing

Biomass-based amendments are often enriched in secondary and micronutrients such as Ca, Mg, Fe, and Zn, with the input of these elements helping to alleviate ionic imbalances in mining environments caused by prolonged leaching, acidification, or weathering [121,122]. In calcareous soils, the application of grain-derived compost has been reported to enhance micronutrient availability, with Fe²⁺, Zn²⁺, Mn²⁺, and Cu²⁺ increasing by 57.1%, 66.1%, 56.9%, and 44.9%, respectively, over the course of one year [123]. Calcium and magnesium supplementation improves colloidal stability, promotes aggregate formation, and optimizes the rhizosphere environment, thereby enhancing plant nutrient and water uptake [124]. Iron and zinc, as essential micronutrients, play critical roles in photosynthesis, enzyme activation, and metabolic regulation, and also contribute to soil redox processes that influence metal dynamics, thereby supporting plant and microbial functions [125]. Moreover, these mineral elements can bind with organic matter to form stable organo–mineral complexes, further strengthening nutrient retention and contributing to structural stability [126]. Overall, by supplementing micronutrients and correcting ionic disequilibria, biomass-based amendments contribute to restoring nutrient balance and supporting plant health in degraded mining ecosystems.

4.4. Microbial Activation and Ecosystem Restoration

In mine ecological restoration, the fundamental goal is to achieve holistic ecosystem recovery and sustain its structure and functions over the long term. The activation and functional recovery of microbial communities represent a key driving force in attaining this objective [127]. Biomass-based amendments improve physicochemical properties and nutrient conditions, thereby facilitating microhabitat improvement and creating essential prerequisites for microbial community revival [128]. They also promote the recovery of metabolic pathways and the rebuilding of symbiotic relationships, thereby supporting microbial functional reconstruction and long-term ecosystem stability [62].

(1) Microhabitat improvement and microbial community revival

Biomass-based amendments enhance porosity, water retention, and organic matter content, thereby creating more suitable microhabitats for microorganisms [24,129]. Such effects are illustrated by the porous structure of biochar, which simultaneously offers protective microhabitats and acts as a carbon source for microbial survival and functional recovery in degraded environments [91]. These changes also alleviate stresses such as acidification, salinization, and heavy metal contamination, stabilize the soil microenvironment, and promote the restoration of microbial diversity and early community revival [130]. The combined application of pig-manure biochar and phosphate-solubilizing bacteria effectively alleviates Pb/Cd stress, enhances organic acid secretion and electron transfer, and facilitates the stabilization of Cd as phosphates and the partial transformation of Pb into carbonates and phosphates, thereby improving remediation efficiency in mine soils under co-contamination conditions [131]. Such microhabitats enhance microbial diversity and functional redundancy, which are critical prerequisites for the sustained recovery and stability of ecosystems.

(2) Metabolic and enzymatic activation

Biomass-based amendments activate microbial metabolism by supplying substrates and alleviating environmental stress, which subsequently induce the synthesis and enhanced activity of key enzymes [132]. This is typically reflected in the increased activities of urease, phosphatase, and dehydrogenase, which expedite organic matter decomposition and nutrient transformation [133]. For instance, a four-year field study in abandoned mine soils demonstrated that microbial inoculants markedly elevated rhizosphere organic carbon, ammonium, and the activities of urease and phosphatase, thereby promoting nutrient availability and plant nutrient acquisition [78]. Moreover, the elevated enzymatic activity further accelerates nutrient mineralization and organic matter turnover, reinforcing microbial metabolic functions through a positive feedback loop [26]. As a result, the strengthened microbial metabolism further extends to core processes such as nitrogen fixation, nitrification–denitrification, and sulfur redox reactions, which are essential for nutrient cycling and contaminant attenuation [88]. Consistently, a recent study on the ecological restoration of acidic mine tailings demonstrated that the application of compost in combination with clay, marble waste, and narrow-leafed lupine significantly enhanced microbial biomass and the activities of nutrient-acquiring enzymes (β-glucosidase, N-acetyl-β-glucosaminidase, L-arginase, and acid phosphatase), thereby reinforcing soil carbon, nitrogen, and phosphorus cycling and improving remediation efficiency [85].

(3) Rebuilding symbiosis and ecosystem stability

The progressive recovery of physicochemical properties and microbial community functions creates essential prerequisites for vegetation establishment. Vegetation recovery subsequently enriches the rhizosphere with carbon inputs and ecological niches, fostering microbial activity and diversity, which in turn accelerates community succession and facilitates the reconstruction of microbial functions [134]. In this process, plant–microbe interactions are strengthened, exemplified by the nitrogen-fixing symbiosis between legumes and rhizobia and the mutualistic association between AMF and host plants, which effectively enhance nutrient use efficiency and increase plant stress tolerance [76,107,135]. The introduction of exogenous functional inoculants further accelerates this process. For example, at an open-pit coal mine reclamation site in Inner Mongolia, AMF inoculation was shown to markedly enhance the restoration of sea buckthorn vegetation [81]. In nonferrous metal mine tailings heavily contaminated with Cd, Pb, and Zn, inoculation with phosphorus-, silicon-, and potassium-solubilizing bacteria significantly improved physicochemical properties, increased nutrient availability, enhanced enzyme activities, and promoted rapeseed growth, while simultaneously reducing the leaching toxicity of heavy metals and facilitating their immobilization [83]. Similarly, inoculation with PGPR consortia improved soil quality and ryegrass performance, increased nutrient availability, and mitigated heavy metal stress, demonstrating their effectiveness in supporting ecosystem recovery in mine-impacted areas [84]. In addition, a four-year field experiment at an abandoned carbonate mine site applied a microbial inoculant consortium composed of actinomycetes, bacilli, and filamentous fungi. The inoculants significantly reshaped soil and root microbial communities, enhanced root and nodule functional traits, and ultimately improved soil–plant functionality during mine restoration [78].

5. Nutrient Leaching and Secondary Pollution Risks

Biomass-based amendments provide nutrients and organic matter that can substantially improve fertility and promote vegetation establishment in mine reclamation. However, their application also raises concerns regarding nutrient leaching and potential secondary pollution, which may compromise long-term ecological safety.

Easily mineralizable nitrogen (NO₃⁻ and NH₄⁺) and soluble phosphorus contained in biomass-based amendments are readily released under rainfall or irrigation, leading to nutrient losses into groundwater and surface waters and substantially increasing the risk of eutrophication [136]. Soil column leaching experiments demonstrated that the application of compost and pig slurry markedly increased soluble organic carbon and dissolved N, P, and K, resulting in a downward displacement of N and K [137]. Such nutrient leaching is typically most pronounced during the initial months to years following application. Although this rapid release benefits early plant establishment, it also highlights the risk of nutrient leaching in mine reclamation settings. However, as nutrient reserves are gradually depleted and nutrient cycling becomes stabilized through plant uptake and microbial assimilation, the intensity of leaching progressively declines [138]. Compost and straw have been reported to initially increase the mobility of N, P, and K, but under long-term field conditions they can reduce nutrient leaching by enhancing aggregate stability and stimulating microbial activity [139]. In a three-year mine reclamation study in Pennsylvania, poultry manure mixed with primary paper mill sludge at a C/N ratio of 29 caused pronounced N leaching, mainly during the first two autumn seasons, compared with the C/N 20 treatment, whereas composted poultry manure showed minimal N losses (<1%) [58]. In addition, concerns have been raised in some reclamation regions such as parts of the Athabasca Oil Sands regarding the potentially excessive use of organic amendments [51]. These observations underscore the need for rational dosing and site-appropriate application strategies to avoid unintended environmental impacts.

Secondary contamination poses long-term and cumulative risks. Animal-derived amendments are associated not only with substantial nitrogen losses through leaching, but also with potential risks such as pathogen transmission and antibiotic or hormone residues [140]. Livestock manure contains various antibiotic residues and antibiotic resistance genes (ARGs), which can persist even after composting. These residual ARGs and pathogens may enter ecological restoration substrates through application and subsequently transfer to plants, thereby facilitating cross-media dissemination and accelerating the environmental spread of antibiotic resistance [141]. Industrial organic by-products, including urban sewage sludge, food waste, and agro-industrial residues, exhibit highly heterogeneous compositions that can pose significant environmental risks. In particular, they may cause the accumulation of heavy metals, organic pollutants, and other toxic compounds [112]. These contaminants can be mobilized through leaching, while fluctuations in pH or redox conditions may facilitate their transformation into bioavailable forms. As a result, their long-term stability remains uncertain, and their environmental safety varies considerably across regions. Leaching experiments indicated that the organic matter in sewage sludge compost (SSC) promoted reducing conditions in soil, which increased the leaching concentration of As by approximately one order of magnitude. In addition, the cumulative release of Cd, Cr, Cu, and Pb from SSC accounted for less than 5% of their total contents [142]. In contrast to co-application strategies, the exclusive use of SSC improved soil nutrient availability but simultaneously promoted the accumulation of Cu, Zn, Cd, and Ni, ultimately inhibiting the growth of Eucalyptus urophylla [66].

Long-term monitoring studies have shown that, in some mining sites that have undergone ecological restoration for many years, nutrient imbalance and the remobilization of heavy metals or organic contaminants may still occur, underscoring the necessity of continuous risk assessment [127,143]. Therefore, future research should not only establish long-term monitoring frameworks and identify the environmental drivers of these risks but also account for the site-specificity and uncertainties associated with amendment applications. Striking a balance between improving fertility and ensuring ecological safety remains a central issue.

6. Greenhouse Gas Emissions and Carbon Sequestration Potential

Although nutrient leaching and secondary pollution risks underscore the local ecological uncertainties associated with biomass-based amendments, their environmental impacts extend beyond site-level safety concerns. The application of biomass-based amendments also affects GHG emissions and carbon sequestration processes, thereby linking mine reclamation practices with global climate change mitigation and carbon neutrality strategies [23].

The application of biomass-based amendments in mine ecological restoration often induces short-term increases in GHG emissions. The decomposition of organic matter and the stimulation of microbial activity accelerate CO₂ release [144]. Nitrogen-rich inputs further drive nitrification and denitrification processes, leading to substantial N₂O emissions [17]. Moreover, under poor drainage or seasonal waterlogging, oxygen limitation may create anaerobic microsites that occasionally promote CH₄ production [145]. Previous studies have shown that although compost and manure substantially improve fertility, they also increase GHG fluxes during the initial application period, particularly under conditions of high nitrogen availability [146]. In contrast, biochar generally exhibits a mitigation effect, as its structural stability and capacity to modulate microbial communities can effectively reduce N₂O emissions and suppress CO₂ release [24]. In a 30-day incubation with mine soil, the addition of date palm feedstock substantially enhanced respiration activity, with CO₂–C fluxes increasing by up to 1664% [70]. On this basis, the application of low-temperature biochar further increased CO₂ emissions, albeit to a much lesser extent, whereas high-temperature biochars exhibited no significant effect and in some cases reduced cumulative CO₂ release [70]. These results suggest that the intrinsic properties of amendments, including C/N ratio, degree of stabilization, maturity, and application rate, together with environmental conditions such as soil pH, redox potential, temperature, and moisture, play a pivotal role in regulating short-term GHG emissions [54,118].

Beyond its role in emission reduction, biochar functions as a long-term carbon sink due to its highly stable aromatic carbon structures, with residence times in soil extending from decades to centuries [93]. Other biomass-based amendments also contribute to carbon sequestration by supplying labile substrates, enhancing humification, and promoting microbial transformation [23]. In addition, these amendments improve aggregation and provide physical protection for organic matter, thereby reducing carbon losses through mineralization [87]. Microbial pathways further reinforce this process, as autotrophs, methanotrophs, and acetogens can fix CO₂ via the Wood–Ljungdahl and related metabolic routes, facilitating its incorporation into more stable organic carbon content [26,81,127].

Overall, the carbon cycling effects of biomass-based amendments in mine reclamation systems are inherently dual, acting either as sources of GHG emissions or as pathways for long-term carbon storage. Future research should emphasize long-term monitoring, integrate life-cycle assessments, and establish predictive frameworks that quantify net carbon effects under diverse environmental conditions, thereby clarifying how biomass-based amendments regulate carbon cycling and GHG emissions to promote sustainable mine ecosystem restoration.

7. Conclusions

Biomass-based amendments, characterized by low cost, availability, and biodegradability, hold great promise for mine ecological restoration. They can enhance structure, improve nutrient cycling, and stimulate microbial functions, thereby accelerating vegetation recovery and ecosystem reconstruction. However, their application may also pose risks such as nutrient leaching, secondary pollution, and GHG emissions. Overall, these amendments represent vital tools for sustainable mine reclamation, but require site- and material-specific optimization. In addition, the selection of biomass-based amendments should follow a site-specific and resource-dependent principle, considering the variability in local biomass availability and field application conditions across different mining areas. Future work should also strengthen the investigation of synergistic mechanisms between biomass-based amendments and other types of amendments, as these interactions may markedly influence overall restoration performance. At the same time, it is necessary to advance long-term monitoring and risk assessment and to further explore their potential contributions to carbon neutrality, the circular economy, and ecological civilization.