Heavy Metals in Tropical Forest and Agroforestry Soils: Mechanisms, Impacts, Monitoring and Restoration Strategies

Abstract

Heavy metal pollution in forest and agroforestry soils represents a persistent environmental challenge with direct implications for ecosystem functioning, food security, and human health. In tropical and subtropical regions, intense weathering, rapid organic matter turnover, and dynamic redox conditions strongly modulate metal mobility, bioavailability, and long-term soil vulnerability. This review synthesizes current knowledge on the sources, biogeochemical mechanisms, ecological impacts, monitoring approaches, and restoration strategies associated with heavy metal contamination in forest and agroforestry systems, with particular emphasis on tropical landscapes. We examine natural and anthropogenic metal inputs, highlighting how atmospheric deposition, legacy contamination, land-use practices, and soil management interact with mineralogy, organic matter, and hydrology to control metal fate. Key processes governing metal behavior include sorption and complexation, Fe–Mn redox cycling, pH-dependent solubility, microbial mediation, and rhizosphere dynamics. The ecological consequences of contamination are discussed in terms of soil health degradation, plant physiological stress, disruption of ecosystem services, and risks of metal transfer to food chains in managed systems. The review also evaluates integrated monitoring frameworks that combine field-based soil analyses, biomonitoring, and geospatial technologies, while acknowledging methodological limitations and scale-dependent uncertainties. Finally, restoration and remediation strategies—ranging from phytotechnologies and soil amendments to engineered Technosols—are assessed in relation to their effectiveness, scalability, and relevance for long-term functional recovery. By linking mechanistic understanding with management and policy considerations, this review provides a process-oriented framework to support sustainable management and restoration of contaminated forest and agroforestry soils in tropical and subtropical regions.

1. Introduction

Forestry and agroforestry systems occupy a pivotal role in addressing global environmental challenges: they sequester carbon, regulate water, preserve biodiversity and support livelihoods. However, these systems may be compromised by heavy metal pollution—originating from both natural pedogenic processes and a wide range of anthropogenic activities such as mining [1], industrial emissions [2], agrochemical inputs [3], wastewater irrigation [4], and atmospheric deposition [5]. Heavy metals entering forest and agroforestry soils threaten water quality, plant health, soil microbial function and the ecosystem services upon which human well-being depends.

Across the world, the accumulation of metals such as cadmium (Cd), lead (Pb), arsenic (As), chromium (Cr), nickel (Ni), zinc (Zn), copper (Cu) and mercury (Hg) in soils under trees and agroforestry operations is under-acknowledged in comparison with agricultural croplands. Yet tree-based systems are not exempt; for example, agroforestry established on former mining-influenced landscapes, buffer forests around industrial areas, mangrove forests in estuarine zones receiving acid mine drainage—all illustrate the vulnerability of tree-dominated soils to metal pollution.

Understanding how heavy metals enter, accumulate within, and are mobilized or retained in forest and agroforestry soils is essential for safeguarding ecosystem ser-vices. Soil chemistry governs many of the processes controlling metal speciation, mobility, and bioavailability, including sorption, complexation, redox reactions, and interactions with organic matter, while the structural complexity of forest and agroforestry soils (e.g., litter layers, root architecture, and rhizosphere processes) further amplifies these pathways. Overcoming this complexity requires integrative assessment frameworks that move beyond single-indicator approaches. Process-based interpretations that combine geochemical indicators (e.g., pH, redox potential, and Fe and Mn fractions), biological indicators (plants and microbial activity), and multiscale monitoring tools—from laboratory analyses to remote sensing—are essential for capturing the dynamic behavior of heavy metals in forest and agroforestry soils. Thus, unlike most existing reviews that primarily focus on sources, total concentrations, or ecological risks, this review adopts an integrative and process-oriented perspective. Specifically, it links redox-driven biogeochemical mechanisms, soil functional degradation and recovery, and restoration-oriented remediation strategies within a single conceptual framework. While drawing on substantial evidence from tropical and subtropical forest and agroforestry systems, where environmental change and land-use pressures are particularly intense, this review advances a broadly applicable understanding of how biogeochemical processes, ecosystem functions, and management strategies interact across forested landscapes.

This review aims to (i) identify the major sources and pathways of heavy metal pollution into forest and agroforestry soils; (ii) elucidate the biogeochemical processes that regulate heavy metal fate in these systems; (iii) assess impacts of heavy metal accumulation on soil health, vegetation, microbial communities, and ecosystem service provision; (iv) review monitoring, assessment and remediation strategies applicable to forest and agroforestry soils; and (v) highlight research gaps into heavy metal management in forest-based landscapes.

2. Literature Search Strategy and Scope of the Review

This review adopts a narrative, process-oriented, and integrative approach aimed at synthesizing current knowledge on heavy metal dynamics in forest and agroforestry soils. Rather than conducting a quantitative meta-analysis or bibliometric assessment, the manuscript focuses on the conceptual integration of biogeochemical mechanisms, ecological impacts, monitoring frameworks, and remediation strategies, with emphasis on how these components interact across soil–plant–ecosystem continua.

The literature surveyed was retrieved from major multidisciplinary scientific databases, including Scopus, Web of Science, Google Scholar, and SciELO, which collectively ensure broad international coverage across environmental sciences, soil science, ecology, and geochemistry. Literature searches were conducted using combinations of keywords, but not limited to heavy metals, potentially toxic elements (PTEs), forest soils, agroforestry, biogeochemistry, redox processes, soil contamination, Technosols, phytoremediation, soil restoration, microbial processes, remote sensing, and monitoring frameworks. Search terms were intentionally aligned with the process-based perspective adopted in this review, encompassing both geochemical and biological indicators, as well as multiscale monitoring tools ranging from laboratory analyses to proximal and remote sensing approaches.

Priority was given to peer-reviewed research articles, review papers, and well-documented case studies published primarily over the last two decades. Seminal earlier studies were also included, providing essential theoretical and methodological foundations. While the review draws on a substantial body of evidence from tropical and subtropical forest and agroforestry systems, where environmental change and land-use pressures are particularly intense [6,7,8,9,10], it also incorporates studies from temperate and boreal systems to support comparative interpretations and to advance a broadly applicable understanding of heavy metal behavior across forested landscapes.

The selected literature was critically evaluated to identify consistent patterns, dominant process controls, methodological advances, and knowledge gaps. Rather than statistically aggregating datasets, the synthesis emphasizes process-based interpretation, conceptual coherence, and cross-scale linkages among biogeochemical dynamics, soil functions, ecosystem services, and monitoring and restoration strategies.

3. Sources and Pathways of Heavy Metal Pollution in Forestry and Agroforestry Systems

Heavy metals reach forest and agroforestry soils through diverse geogenic and anthropogenic pathways. Lithogenic sources—such as parent material weathering, volcanic emissions, and natural mineralization—establish background concentrations that vary according to geological context. For instance, basaltic and ultramafic terrains may exhibit elevated Ni and Cr contents [11,12,13], while coastal and estuarine sediments commonly inherit high Fe and Mn concentrations that strongly influence the behavior of associated heavy metals [14,15]. These natural sources are particularly relevant in undisturbed forest soils, where they define a long-term geochemical baseline rather than a rapidly increasing hazardous load.

On the other hand, anthropogenic inputs tend to generate spatially heterogeneous contamination patterns. While forest soils are predominantly affected by diffuse sources such as atmospheric deposition, long-term legacy pollution, and regional geogenic enrichment [1,7,10,16], agroforestry soils are more directly influenced by management-related inputs, including mineral fertilizers, organic amendments, pesticides, irrigation practices, and periodic soil disturbance [17,18,19,20,21,22]. These contrasting input pathways not only affect total metal loads but also control their spatial distribution, chemical speciation, and short-term bioavailability, with important implications for exposure pathways and risk assessment. For instance, mining and ore processing represent the most critical source of non-point source heavy metals contamination in forest-adjacent landscapes, particularly in countries like Brazil with large mining provinces [23,24,25]. Contaminants are mobilized from waste rocks and tailings via weathering, erosion, and leaching. This process is not limited to active mines; studies in Northeastern Brazil demonstrated that waste rocks from an abandoned Cu mine remained a continuous source of emission for decades. Perlatti et al. [24] quantified this hazard, reporting waste rock Cu concentrations up to ~8000 mg kg⁻¹ and estimating a release of 7.2 tons of Cu into the adjacent stream and sediment over 30 years. This highlights a critical pathway where historical contamination sustains modern ecological risk.

Beyond these chronic sources of mobilization from abandoned sites, the potential for massive, acute contamination is realized during catastrophic failure events [26,27,28]. For instance, the Fundão dam collapse in Brazil introduced massive volumes of metal-rich tailings into riverine and coastal zones; subsequent hydrodynamic and tidal redistribution deposited this contaminated material across riparian forests, riverbanks, and estuarine soils, illustrating a vast pathway for the widespread contamination of forested receptor zones [29,30]. While mining events primarily release high concentrations of heavy metals physically across the landscape, other land management practices, particularly those associated with agroforestry systems, introduce heavy metals more diffusely through agricultural inputs [16,31,32,33]. The widespread use of phosphate fertilizers is a recognized source of Cd contamination [34,35,36] while the application of fungicidal agents, such as Cu-based sprays (e.g., Bordeaux mixture) in perennial crops like vineyards, can lead to the significant accumulation of Cu in the topsoil layers [21,22,37,38,39]. Furthermore, the application of sewage sludge as a soil amendment, while improving organic matter and fertility, also introduces a complex mix of heavy metals, the ultimate fate of which varies widely [40,41,42,43].

This diffuse contamination leads to a direct pathway of risk to human health within agroforestry systems. In areas impacted by mining tailings, such as the Rio Doce estuary in Brazil, the traditional protective mechanism of metal association with Fe oxyhydroxides proved inefficient, allowing heavy metals like Cd, Cr, Cu, Ni, and Pb to bioaccumulate in edible crops such as banana fruits, cocoa beans, and cassava rhizomes above safety thresholds. This bioaccumulation posed a probable non-carcinogenic risk to vulnerable populations, particularly children consuming bananas [44]. The widespread contamination from both mining and agricultural sources is further dispersed via atmospheric transport, which, much like agricultural inputs, acts as a more diffuse source. Industrial and traffic emissions release fine particulate matter and aerosols containing heavy metals, which are transported over long distances [45]. The forest canopy intercepts airborne contaminants and acts as a dynamic biogeochemical filter rather than a passive conduit. Canopy surfaces temporarily retain particulate and gaseous metal inputs, concentrating contaminants within the phyllosphere [46]. During canopy residence, metals may undergo dissolution, organic complexation, and surface-mediated transformations that modify their speciation and solubility [46,47,48]. Dissolved organic carbon (DOC) mobilized on leaf surfaces acts as a key ligand, enhancing the proportion of soluble, complexed metal forms in throughfall and stemflow [48]. Consequently, metals reaching the forest soil originate from a mixture of atmospheric deposition, internal foliar leaching, and resuspended canopy particles, with rates and pathways varying among elements [48,49].

Additionally, estuarine and coastal forests, such as mangroves, act as important sinks for contaminants discharged from continental uplands [49,50,51]. However, the fate and bioavailability of heavy metals in these systems are governed by complex biogeochemical pathways influenced by local climate and geochemistry [51,52,53]. For instance, in semi-arid mangroves of Northeastern Brazil, a key pathway for metal immobilization—the formation of acid-volatile sulfides (AVS)—was found to be severely limited [54,55]. Queiroz et al. [54] reported that oxic conditions (Eh > +350 mV), favored by low rainfall, restricted AVS formation. Consequently, AVS acted as a minor sink, suggesting that other solid phases, particularly reactive Fe oxyhydroxides and organic matter, play the more dominant role in controlling trace metal retention and bioavailability in these specific tropical environments. Regardless of the localized biogeochemical controls that dictate metal bioavailability in soil, be they AVS limitation in mangroves or Fe oxide inefficiency near mine tailings, a critical ultimate pathway for heavy metal contamination is their transfer through the terrestrial food chain. Studies in Minas Gerais, Southeastern Brazil, revealed the bioaccumulation of toxic metals (As, Cd, Pb, and Cr) in the liver and kidneys of free-ranging rattlesnakes (Crotalus durissus) [56]. This accumulation at the top of the food web provides strong ecotoxicological evidence that heavy metals from environmental contamination sources are efficiently moving through the terrestrial trophic structure, posing a substantial and widespread risk to wildlife health [44,56,57].

4. Biogeochemical Processes Governing Heavy Metal Fate in Forest and Agroforestry Soils

The environmental fate of heavy metals in forest and agroforestry soils is governed by a dynamic interplay of biogeochemical processes that ultimately determine their mobility, speciation, and bioavailability (Figure 1). A comprehensive understanding of these processes is essential for assessing ecotoxicological risk and developing effective remediation strategies. To predict the environmental fate of heavy metals in forest and agroforestry soils, it is essential to understand the dynamic interface between anthropogenic pollution sources and the soil’s internal regulating mechanisms (Figure 1). This conceptual framework emphasizes that the transition from inert to bioavailable contaminant forms is driven by specific biogeochemical processes, including sorption, redox cycling, and rhizosphere interactions, that collectively determine the ecological risk profile of the landscape.

4.1. Sorption, Complexation and Organic Matter Interactions

Soil organic matter (SOM) exerts a dominant control over trace metal speciation in forest soils, acting as a critical non-mineral sink (Figure 1). Organic ligands—including carboxyl, phenolic, and other functional groups—strongly bind metal cations, forming stable complexes that generally reduce free ion activity and bioavailability. For metals such as Cu and Pb, strong associations with humic and fulvic acid fractions mediate retention, particularly in the surface organic horizon. However, the stabilizing effect of SOM is conditional; competition for binding sites, complexation with dissolved organic matter, and shifts in pH can rapidly modulate metal mobility (Figure 1). This is evident in agroforestry systems, where a case study in Brazil showed that Cu accumulation in banana orchard soils irrigated with mine water was largely controlled by the metal’s binding to the soil’s organic phase and Fe oxyhydroxides, demonstrating this critical mixed organic-inorganic retention mechanism [58]. The close relationship between organic and mineral phases forms a necessary foundation for stability, but it is one that is particularly susceptible to changes in the redox environment, which often occur in waterlogged or highly disturbed systems.

In the Pantanal, the world’s largest tropical wetland, where such variable redox conditions prevail, studies indicate a strong correlation among As, Fe oxyhydroxides, and organic matter [59]. The transformation kinetics of As are controlled by competitive adsorption with other species that, when mobilized, increase dissolved As concentrations [60]. Moreover, sorption mechanisms for this toxic metal are intensified in acidic environments, where adsorption affinity on mineral surfaces increases. The geochemical environment that governs As speciation—through redox-driven transformations from As(III) to As(V)—is strongly influenced by dissolved organic carbon (DOC) [61]. As(V) is favored under elevated pH; however, under conditions of high DOC concentration and subsequent medium acidification, the more toxic As(III) state is predominant [59].

In Brazilian Oxisols under oxidative conditions, Cr mobility varies along vertical gradients and is strongly influenced by speciation within the organic fraction [62]. The greater mobility of Cr(VI) in these soils occurs because, under acidic pH, Oxisols often exhibit a predominantly negative surface charge, which repels the anionic Cr(VI) species [63]. Consequently, the main removal mechanism for Cr involves reduction rather than adsorption, driven by redox-active functional groups in the SOM that convert Cr(VI), the highly mobile and toxic form, to Cr(III), less toxic and relatively immobile [62]. This process is further influenced by a potential reduction in adsorption rates due to adsorption-site saturation, which may raise soil pH and alter geochemical mechanisms. Thus, as pH increases and the amount of reactive organic matter decreases, Cr ions are less effectively removed, which raises the risk of both vertical and surface contamination [64]. These distinct but interconnected speciation-driven mechanisms reinforce the critical importance of SOM complexation and redox dynamics in controlling the behavior of these highly toxic heavy metals in tropical environments subject to hydrological fluctuations.

4.2. Microbial Mediation of Heavy Metal Fate in Forest and Agroforestry Soils

Microorganisms play a central yet often underappreciated role in governing the fate of heavy metals in forest and agroforestry soils by directly driving redox transformations [65,66], organic matter turnover [67,68], and mineral dissolution–precipitation reactions [69,70,71]. Under oxygen-limited conditions common in waterlogged forests, riparian zones, wetlands, and mangroves, microbial respiration sequentially utilizes alternative electron acceptors, including nitrate, Mn(IV), Fe(III), and sulfate [50,72,73]. The microbial reduction in Fe(III) and Mn(IV) oxyhydroxides leads to their reductive dissolution, releasing associated heavy metals into soil porewaters and transforming previously stable mineral-bound pools into mobile and potentially bioavailable forms [15,74,75]. These microbially mediated redox processes, therefore, represent a critical switch between metal immobilization and mobilization in forest soils subjected to hydrological fluctuations.

Microbial decomposition of soil organic matter further influences metal behavior by regulating the production of dissolved organic carbon (DOC) and low-molecular-weight organic compounds [76,77]. These compounds can complex metals such as Cu, Zn, and Cd, enhancing their solubility and vertical transport [77,78,79]. Studies in forest soils and column/batch experiments demonstrate enhanced dissolved metal concentrations and horizon-specific mobilization [80]. Conversely, microbial-driven humification processes contribute to the formation of stable organo-metal complexes that favor long-term immobilization in surface horizons [81,82].

In strongly reducing environments, sulfate-reducing bacteria exert a decisive control on metal fate by generating sulfide, which reacts with Fe and other metals to form insoluble sulfide minerals, including Fe monosulfides and pyrite (FeS₂) [49,54,83]. The incorporation or co-precipitation of heavy metals within these sulfide phases represents one of the most effective long-term immobilization pathways in anoxic forested and coastal systems. However, the stability of these microbially formed sinks is highly sensitive to redox re-oxidation, which can rapidly remobilize metals during drainage, vegetation loss, or extreme climatic events [84,85].

Microbial activity in the rhizosphere further amplifies these processes through close coupling with plant roots. Root exudates stimulate microbial metabolism, alter local redox conditions, and enhance ligand-mediated metal mobilization or stabilization [86,87,88,89]. This tight plant–microbe–soil coupling highlights that microbial processes cannot be treated as isolated mechanisms but must be understood as integral components of biogeochemical networks controlling heavy metal dynamics in forest and agroforestry soils.

4.3. Iron, Manganese, and Redox-Driven Transformations

Fe and Mn are among the most important geochemical regulators of trace-metal mobility in forested and coastal soils (Figure 1). Their behavior is governed by redox potential, which controls the transformation between oxidized (Fe³⁺/Mn⁴⁺) and reduced (Fe²⁺/Mn²⁺) species. Under oxidizing conditions, Fe(III)- and Mn(IV)-oxides precipitate as amorphous or crystalline oxyhydroxides, forming some of the most reactive sorbents in soils and sediments. These phases exhibit high surface area and strong affinity for trace metals, immobilizing contaminants via adsorption, co-precipitation, and incorporation into mineral structures.

Conversely, under reducing conditions common in hydromorphic forest settings—such as wetlands, riparian corridors, floodplain forests, and mangrove systems—Fe and Mn undergo reductive dissolution (Figure 1). When microbial respiration consumes oxygen and progressively shifts toward Fe(III) and Mn(IV) reduction, these oxyhydroxides dissolve, releasing previously sorbed metals into porewaters. Previous studies highlight how mangrove soils, with their strong tidal pumping and high organic matter inputs, naturally oscillate between oxidizing and reducing phases [85,90]. These oscillations amplify Fe–Mn cycling, making these environments both highly efficient sinks under oxic phases and potentially large secondary sources when reduction is dominant.

The importance of Fe–Mn redox control on contaminant mobility is well illustrated by sediment archives from the Santos–Cubatão Estuarine System, one of Latin America’s most industrialized regions. Luiz-Silva et al. reported that [91] deep sediment cores (up to 260 cm) can differentiate natural lithogenic metal inputs from anthropogenic contamination. In these profiles, Fe emerged as a geochemical proxy for steelmaking activities, strongly correlating with Cr, Mn, Ni, Zn, and Pb (Group 2), while phosphorus (P) served as a tracer for fertilizer industries, associating with Cd, REEs, Th, and U (Group 3). Because 210Pb dating was compromised by sediment disturbance, the Fe concentration profile itself served as a reliable chronological marker, reflecting increases in sedimentation rates coinciding with the expansion of the regional steel industry and subsequent environmental regulation. This case underscores how Fe-rich layers not only archive industrial history but also exert strong geochemical control on the retention or release of associated metals. Under oxic phases, these Fe–Mn oxyhydroxides stabilize contaminants; however, their reduction under changing redox conditions can remobilize these metals, threatening downstream ecosystems.

In non-industrial forest soils as well as coastal wetlands, the same fundamental mechanisms apply. The oxyhydroxides of Fe and Mn are principal sorbents for a wide array of trace metals under oxidizing conditions, where they effectively immobilize contaminants through adsorption and co-precipitation. Conversely, reductive dissolution under low Eh releases sorbed metals back into the soil porewaters, turning an effective sink into a dangerous source. The kinetics of redox-driven heavy metal mobilization are critical for environmental monitoring and risk assessment. Experimental and field studies indicate that metal release following redox shifts occurs over multiple time scales [91,92,93]. Rapid responses are typically associated with the reductive dissolution of Mn oxides, which can release surface-bound metals within hours to one day after the onset of anoxic conditions [15]. In contrast, the reductive dissolution of Fe oxyhydroxides generally requires more sustained reducing conditions, with porewater concentrations of associated metals often peaking days to weeks after flooding or saturation events [94,95]. Under prolonged anoxia, secondary immobilization processes—such as sulfide precipitation or re-adsorption onto newly formed mineral phases—may partially offset metal mobilization [50,90]. Field observations in Fe-rich estuarine tailings further demonstrate that repeated or chronic redox oscillations can sustain metal release over periods of weeks to months, emphasizing the importance of temporal resolution in monitoring strategies [94].

Hydrological fluctuations—whether driven by rainfall seasonality, tidal pumping, extreme storms, or groundwater variation—govern the timing and intensity of these redox transitions. The mining disaster in the Rio Doce estuary, Brazil (Figure 2), starkly emphasized this mechanism: the dissolution of Fe oxyhydroxides following reducing conditions liberated associated Cu and, notably, high concentrations of Mn [15]. The release of Mn revealed it as an often overlooked contaminant following large-scale tailings disasters because Mn becomes highly mobile under reducing conditions, allowing rapid downstream dispersal. The photographs below illustrate the environmental catastrophe resulting from the Fundão dam failure from a distinctively pedological perspective, capturing the massive influx of Fe-rich tailings that reshaped the estuarine soil profile (Figure 2). This visual record acts as a primary example of how such inputs redefine the estuarine matrix, bridging the gap between chemical theory and environmental impact by documenting the physical deposition processes—from acute water turbidity to the formation of new soil layers—that set the stage for the redox-driven contaminant release described in this section.

Furthermore, research on mangrove soils demonstrates that degradation, vegetation loss, and extreme climatic events destabilize the natural Fe–Mn–S cycling. In such conditions, mangroves shift from metal sinks to secondary sources, exporting trace elements into estuarine waters due to altered hydrology and enhanced reductive dissolution [86,96]. These ecosystem-level disruptions highlight how land-use changes and climate extremes exacerbate redox-driven contaminant release.

In strongly reducing, sulfidic environments—common in saturated coastal soil and sediments—trace metals may also be immobilized through sulfide precipitation. Sulfate-reducing bacteria generate sulfide that reacts with Fe²⁺ to form Fe monosulfides and, ultimately, pyrite. Trace metals may coprecipitate or become incorporated into these sulfide minerals. The degree of pyritization depends on the availability of organic carbon, sulfate, and reactive Fe.

This process can be harnessed for remediation: the addition of gypsum (CaSO₄·2H₂O) to Fe-rich mine tailings in the Rio Doce estuary induced rapid pyritization, forming highly stable pyrite phases that immobilized heavy metals [97]. However, the efficiency of pyritization varies geographically due to differences in climate, geomorphology, geology, and tidal amplitude—all factors that influence Fe availability, sulfate supply, and microbial activity.

4.4. pH, Clay Minerals, and Cation Exchange

Soil pH fundamentally governs the solubility, speciation, and sorption behavior of heavy metals [98,99,100]. In forest and agroforestry soils, acidification is a common process triggered by litter decomposition, acid rain, or the release of organic acids from roots. A decline in pH increases the solubility of many heavy metals, including Cd, Zn, and Pb, while simultaneously reducing the capacity of these cations to adsorb onto oxide, hydroxide, and organic phases [101,102]. In tropical soils, total cation exchange capacity and the concentrations of Fe and Al oxyhydroxides extracted with dithionite—both crystalline and amorphous forms—are the principal attributes controlling Cd adsorption, especially in subsurface horizons [103]. In these cases, adsorption correlates positively with dithionite-extractable Fe and Al and negatively with the SiO₂/Fe₂O₃ ratio, indicating the strong influence of sesquioxide content and degree of weathering [103].

Clay minerals and Fe and Al oxyhydroxides provide essential exchange and sorption sites [104,105,106]. In less weathered soils, minerals such as illite and smectite effectively retain Ni and Cr, while kaolinite, with its lower cation-exchange capacity, is generally less efficient. Beyond mineralogy, landscape structure and land-use regimes modulate metal retention capacity [107,108,109,110,111]. In the western Dabie Mountain region of China, a recent study has reported that the potential ecological risk of heavy metals declined in the order agricultural lands > urban areas > forests, and overall risk was higher in clay-rich and loamy soils than in sandy soils [112]. This pattern underscores the influence of fine-textured mineralogy and clay-organic associations on pollutant retention and immobilization [112].

The activity and stability of Fe oxyhydroxides are particularly sensitive to shifts in the geochemical environment. In hydrologically dynamic systems such as estuaries impacted by Fe-rich mine tailings, oscillating redox conditions trigger reductive dissolution and rapid mineralogical transformation [94,113]. In the Rio Doce estuary in Brazil, Fe-rich tailings underwent a pronounced decline in crystallinity and a substantial reduction in average mineral crystallite size over a four-year period [113]. This structural transformation sharply increased the reactivity and susceptibility of Fe oxyhydroxides to dissolution, leading to the release of previously immobilized contaminants—including trace metals—into soil porewaters and estuarine waters [113].

Monitoring soil pH and the mineralogical forms of Fe and Al is therefore not merely a measure of sorption capacity but a means of predicting the geochemical stability of metal pools in forest and agroforestry soils. These parameters integrate mineralogical, hydrological, and biological processes that collectively determine whether metals remain immobilized or become mobilized under changing environmental conditions.

4.5. Rhizosphere Processes and Plant–Soil Interactions

The rhizosphere represents the fundamental biogeochemical interface where dynamic interactions among plant roots, microorganisms, and soil govern the mobility and fate of trace elements and heavy metals [114]. In forest and agroforestry systems, plants exert both direct and indirect control over metal speciation and bioavailability [115,116,117,118]. Direct control involves root uptake and accumulation in plant tissues (leaves, stems), with some volatile elements such as mercury potentially being re-emitted to the atmosphere [115,119,120].

Rhizosphere activity critically modulates contaminant availability through mechanisms such as the exudation of organic acids and siderophores, pH regulation, and root-driven redox alterations. In mining-impacted environments, selecting species with phytostabilization traits is essential. Studies in tropical soils have shown that native species enhance the association of Cu with organic matter and amorphous Fe oxyhydroxides in the rhizosphere, reflected by low translocation factors to aboveground tissues [121]. For instance, low-molecular-weight organic acids exuded by roots and microorganisms play a central role in regulating metal mobility in the rhizosphere through mechanisms such as chelation, complexation, acidification, and precipitation (

Table 1. Summary of low-molecular-weight organic acids, their target heavy metals, and associated biogeochemical mechanisms of action in the rhizosphere.

Organic Acid	Target Metals	Primary Mechanism of Action
Oxalic Acid	Pb, Cd, Zn, Al	Precipitation: Forms highly stable, insoluble metal-oxalate crystals (e.g., lead oxalate), effectively immobilizing metals in the rhizosphere and reducing toxicity [122].
Citric Acid	Cd, Pb, Cu, Ni	Complexation/Solubilization: A powerful multidentate ligand that forms soluble metal-citrate complexes, increasing metal mobility and facilitating plant uptake (phytoextraction) [123].
Malic Acid	Cd, Al, Zn	Chelation & Detoxification: Often secreted as a stress response to Al or Cd; it chelates ions in the rhizosphere to prevent root entry or sequesters them in vacuoles once inside the plant [122,124,125].
Acetic & Succinic Acids	Cd, Pb	Acidification: Reduces rhizospheric pH, which promotes the dissolution of metal-bearing carbonates and oxides, thereby increasing metal bioavailability [124,126,127].

).

In environments subject to hydromorphic regimes and redox fluctuations, such as estuaries and areas affected by mine tailings, plant–Fe interactions become particularly complex. The living roots of species such as Typha domingensis and Hibiscus tiliaceus actively participate in Fe cycling [128]. Radial oxygen loss (ROL) from roots can create localized oxidation zones, promoting the formation of Fe oxide plaques that sequester metals at the root surface. In mine tailings exposed to redox-active conditions, even pre-existing Fe oxyhydroxides may rapidly be altered, including decreased crystallinity, which increases their susceptibility to dissolution and the release of associated contaminants [113].

In addition, in anoxic soils such as mangroves, root necromass plays a critical role in long-term immobilization of trace elements [129]. Fe and other elements including S, Zn, and Cu are preferentially stored in fine dead roots, which act as microenvironments for pyrite formation under anoxic conditions [50,90]. The coprecipitation of trace metals with pyrite associated with this root necromass represents an efficient mechanism for locking contaminants within the solid phase [130], with important implications for restoring soil functions in replanted mangroves [96].

Subsurface biomass, therefore, acts as a dual-phase filter: while living roots promote immediate sorption through oxide formation, the extensive pool of root necromass enhances sulfide-based retention, resulting in net accumulation of elements that exceeds biological uptake rates [130]. Integrated management systems, such as crop intercropping, also show promise. Agroforestry designs pairing Phyllostachys praecox with the hyperaccumulator Sedum plumbizincicola have demonstrated higher phytoremediation efficiency for Cu and Cd compared to pure forest stands [131]. Understanding these rhizosphere processes is therefore indispensable for developing effective assisted phytoremediation strategies and ecological restoration approaches.

4.6. Climate Change as a Driver of Metal Mobility and Soil Function

Climate change represents an emerging and cross-cutting driver of heavy metal behavior in forest and agroforestry soils, primarily through its effects on soil redox potential, pH, temperature, and hydrological regimes [132]. Alterations in precipitation patterns, including increased rainfall intensity, prolonged droughts, and more frequent flooding events, can strongly modify soil aeration and moisture dynamics, thereby accelerating redox oscillations that control metal mobilization and immobilization processes [132,133]. Under such conditions, heavy metals previously stabilized in organic matter complexes, Fe–Mn oxyhydroxides, or sulfide phases may be remobilized, increasing their bioavailability and ecological risk [86,133,134,135,136].

Rising soil temperatures and shifts in vegetation cover associated with climate change further influence organic matter turnover, microbial activity, and rhizosphere processes, indirectly affecting heavy metal speciation and/or transport [133,134,136]. In addition, extreme events such as floods and erosion pulses can redistribute contaminated soil particles across landscapes, altering exposure pathways for plants, animals, and humans [135]. Although these processes are increasingly recognized as critical, their long-term implications for heavy metal dynamics and soil functional resilience remain insufficiently constrained, particularly in forest and agroforestry systems undergoing rapid land-use change [137]. Incorporating climate-driven variability into monitoring frameworks and restoration planning is therefore essential for anticipating future risks and ensuring the durability of remediation and restoration interventions.

5. Impacts and Monitoring of Heavy Metal Contamination

Heavy metal contamination poses multifaceted challenges to forest and agroforestry systems, affecting soil functioning, vegetation health, and the safety of edible products derived from contaminated landscapes (Figure 3). Understanding these impacts requires integrating soil geochemistry, plant physiology, and ecosystem service frameworks, since metal behavior in soils is governed by processes such as sorption, redox transformations, organic complexation, and biotic interactions [44,138,139]. The manifestation of these impacts differs between land-use systems. In agroforestry settings, heavy metal contamination poses a higher risk of transfer to the food chain due to repeated soil–plant interactions, crop uptake, and management practices that enhance metal bioavailability [140,141,142]. In contrast, forest soils often store metals for longer periods, resulting in lower immediate exposure but potentially greater long-term ecological legacy effects [143,144,145]. Because these mechanisms directly influence soil fertility, plant performance, and pollutant mobility, contamination assessment cannot rely on total heavy metal concentrations alone; instead, it requires an evaluation of functional consequences for ecosystems and human well-being [146].

This section synthesizes current knowledge on how heavy metals alter soil processes and ecosystem services, how vegetation responds to metal exposure—including phytotoxicity, physiological stress, and risks to food safety in forest and agroforestry systems—and the tools available to monitor these impacts through geochemical, biological, and soil-health-based assessment protocols (Figure 3). Together, these subsections provide a mechanistic yet application-oriented overview of contamination pathways, emphasizing the links between soil chemistry, plant function, and ecological resilience [44,147].

Case studies from Brazilian forested environments further illustrate how contamination outcomes are shaped by interactions among source strength, mineralogical context, hydrology, and vegetation. Research in mining-impacted estuaries has shown that Fe dynamics and sulfide stability regulate both sequestration and re-mobilization of heavy metals, demonstrating that disturbances to vegetation or redox conditions can rapidly shift soils from metal sinks to sources [113]. Other studies highlight species-specific accumulation patterns in wetland macrophytes and the potential for native vegetation to immobilize metals or, alternatively, facilitate their transfer into edible products [44,139]. These insights reinforce the need for monitoring frameworks that couple contaminant measurements with indicators of soil function, plant health, and ecological services (Figure 3), enabling site-specific risk evaluation and informed management actions [24,148].

5.1. Impacts on Soil Function and Ecosystem Services

Heavy metals exert pervasive effects on soil functioning by disrupting chemical, physical, and biological processes that underpin ecosystem services (Figure 3). At the chemical level, elevated metal concentrations disturb nutrient equilibria, reduce cation exchange capacity, and interfere with redox buffering systems essential for nitrogen, sulfur, and Fe cycling. These disruptions alter the availability of key nutrients and accelerate the mobilization of toxic species, particularly in systems undergoing hydrological or redox fluctuations. In mangrove and wetland soils, where biogeochemical stability is strongly controlled by redox oscillations, heavy metals interact with Fe and sulfur pools to form transient complexes whose stability depends on vegetation-driven anoxia [86]. When metal loads exceed the soil’s buffering capacity, these reactions can compromise both nutrient cycling and contaminant retention. In abandoned mining districts, diffuse contamination from altered mine tailings can introduce exceptionally high concentrations of Pb, Zn, and Tl into soils and these metals subsequently leach into streams and reservoirs used for irrigation and human consumption, directly undermining water-quality regulation services [149].

Physical soil structure is similarly impaired. Metal toxicity reduces root growth and modifies rhizodeposition patterns, which in turn affects aggregation processes. Reduced fine-root biomass diminishes the soil’s capacity to maintain aggregate stability and retain fine particles—components that act as primary vectors for trace metals. Field studies in mangrove restoration sites demonstrate that vegetation structure strongly influences soil texture and stabilization, with restored plots showing higher silt content and organic matter relative to unvegetated controls [150]. In degraded mangroves, however, vegetation loss triggers a collapse of physical stability: erosion removes clay- and silt-rich fractions that previously sequestered metals, increasing their mobility and downstream export [86]. Similar processes have been documented in Mediterranean mining landscapes, where wind and water erosion of tailings mobilize metal(loid)-contaminated particles, indicating a breakdown of soil physical stability and a reduced capacity to retain pollutants [149].

Biological processes are among the most sensitive to metal stress. Toxicity decreases microbial biomass, alters community composition, and suppresses enzyme activities such as β-glucosidase and acid phosphatase—key drivers of carbon turnover and phosphorus mineralization. Restoration trajectories reported for mangrove wetlands indicate that soil enzyme activity increases markedly with vegetation recovery, reflecting improved nutrient cycling and enhanced microbial functioning [150,151,152]. Conversely, in contaminated or vegetation-degraded settings, enzymatic inhibition can slow decomposition, impair nitrogen transformations, and diminish the soil’s capacity to support primary production [153,154]. These biological impairments extend beyond microbial processes: in contaminated hunting reserves in Spain, Pb derived from both mining and ammunition residues readily enters the food web, posing risks to wildlife and humans and degrading habitat-support functions [149].

Collectively, these chemical, physical, and biological perturbations translate into cascading impacts on ecosystem services. Provisioning services decline as reduced soil fertility and metal toxicity impair biomass production, timber quality, and non-timber resources. Regulating services such as carbon sequestration, sediment retention, and water filtration are compromised when microbial decomposition slows, when aggregates destabilize, or when contaminant immobilization fails. Supporting services—including soil formation, biogeochemical cycling, and habitat provisioning—are diminished through the loss of microbial diversity and the disruption of root–soil feedback, which maintains wetland functioning. Urban environments exhibit similar vulnerabilities: long-term monitoring of green spaces in Finland shows that park soils accumulate large quantities of Cr, Cu, Fe, Mn, Ni, and Zn from traffic emissions, with vegetation type and park age determining both the magnitude of accumulation and the potential for contaminant leaching—highlighting the role of urban soils as key providers of purification and water-regulation services [155].

Nevertheless, the capacity of these soils to immobilize contaminants is finite; heavy-metal accumulation is recognized as a major pressure in urban soil-quality frameworks such as the Urban Soil Quality Index (uSQI), which identifies pollutant immobilization as one of the first ecosystem services to degrade under elevated PTE concentrations [156].

The resilience of metal-immobilization mechanisms is particularly threatened in mangrove ecosystems subjected to extreme weather events or climate-driven mortality. The loss of vegetation reduces belowground oxygen consumption and rapidly increases redox potential, shifting soils from anoxic to suboxic or oxic conditions [86]. This transition destabilizes Fe sulfides and other reduced mineral phases, releasing previously immobilized metals into porewater and tidal channels. Observations from southeastern Brazil reveal that the combined effects of sulfide oxidation and fine-particle erosion can result in the loss of over 90% of a mangrove’s metal inventory—transforming the ecosystem from a long-term contaminant sink into an active source to adjacent aquatic systems [86,157].

Conversely, successful restoration can reverse many of these impacts. Evidence from multi-year mangrove rehabilitation projects shows that vegetation reestablishment improves soil organic matter content, enhances nutrient stocks, and steadily reduces heavy-metal concentrations as plant uptake, rhizosphere sorption, and organic matter accumulation strengthen contaminant retention [150]. Mixed-species and appropriately spaced plantings further enhance these benefits, stabilizing soil texture, promoting microbial recovery, and restoring biogeochemical functions critical for ecosystem service provision. Similar principles apply to agroforestry systems, where multifunctionality and biodiversity underpin key services such as nutrient cycling, carbon sequestration, and soil conservation; heavy-metal contamination diminishes these functions by constraining microbial activity, nutrient retention, and productivity [158,159,160].

Overall, evidence across forested, wetland, agricultural, and urban systems reveals that heavy metals impose strong, multi-layered constraints on soil functions that support ecosystem services. Yet the combined influences of vegetation cover, organic-matter accumulation, and redox regulation create opportunities for recovery, especially when restoration strategies explicitly target soil-based ecosystem functions [156]. Understanding these interactions is essential for designing rehabilitation strategies that sustain long-term soil health and the ecosystem services upon which human well-being depends.

5.2. Vegetation Responses, Phytotoxicity and Food Safety

Heavy metal contamination in forest and agroforestry soils represents a major ecological and human health concern due to its capacity to induce phytotoxicity and promote transfer into the food chain [138]. The severity of plant damage depends primarily on metal bioavailability rather than total soil concentrations, and bioavailability is tightly regulated by soil physicochemical parameters such as pH, redox potential, and the abundance of organic matter [138,139]. Consequently, assessing ecological risk requires understanding the dynamic interactions between soil chemistry and plant physiological responses [139].

Tree seedlings and understory vegetation in contaminated landscapes often display visible symptoms of metal stress, including reduced biomass, chlorosis, and diminished reproductive output. These macroscopic impairments reflect profound cellular and metabolic disturbances [138]. At the cellular level, toxicity commonly manifests as oxidative stress, driven by the excessive production of reactive oxygen species (ROS) that disrupt lipids, proteins, and nucleic acids [138,146]. Metals can also interfere directly with central metabolic pathways, including the photosynthetic apparatus, thereby compromising carbon assimilation and energy balance [146].

Plants have evolved multiple defense strategies to mitigate heavy metal stress [128,139,161]. These include exclusion mechanisms that limit uptake, and internal detoxification mechanisms such as chelation by phytochelatins, sequestration into vacuoles, and compartmentalization within root tissues [162,163,164,165]. The extent to which metals are translocated from roots to shoots is a critical determinant of toxicity and varies markedly among elements. Manganese serves as a clear example: the divalent Mn²⁺ ion becomes highly toxic when present at elevated concentrations and is readily translocated under conditions of low pH and low redox potential [146].

However, previous studies demonstrated that phytotoxicity is not solely dictated by the presence of metals, but also by the chemical environment that modulates their activity [75,113,166]. In an urban technosol contaminated with metals and PAHs, phytotoxicity to Phaseolus vulgaris and Medicago sativa was unexpectedly low despite high total metal concentrations [147]. The alkaline pH (7.8–8.6) reduced soluble metal species in the porewater, substantially limiting bioavailability. This case illustrates how soil alkalinity can function as a powerful natural buffer against metal-induced phytotoxicity, highlighting the importance of local soil chemistry in shaping vegetation response.

In dynamic hydromorphic settings such as estuaries and wetlands, plant-driven processes can strongly influence metal mobility [127,167]. Species-specific differences in rhizosphere chemistry, including oxygen release, organic acid exudation, and redox modification, were shown to exert contrasting controls on heavy metal speciation and accumulation in estuarine soils [139]. These species-dependent effects have direct implications for phytoremediation success, underscoring the need to select plant species that not only tolerate contamination but also influence metal dynamics in desirable ways.

The most critical consequence of heavy metal contamination in forest and agroforestry systems is the transfer of metals into edible crops [44,54]. This represents an immediate human health concern, particularly in subsistence-based forested regions. A striking example comes from the Rio Doce estuary, where Fe-rich mine tailings contaminated agricultural soils. Concentrations of Cd, Cr, Cu, Ni, and Pb in cocoa beans, cassava tubers, and banana fruits exceeded regulatory thresholds [44]. Hazard Index calculations indicated a significant non-carcinogenic risk for children consuming bananas, highlighting the heightened vulnerability of specific demographic groups and the need for continuous monitoring and public health intervention [44]. This case provides unequivocal evidence that even low-input agroforestry systems are not immune to food-chain contamination when embedded within polluted landscapes.

These findings demonstrate that vegetation responses to heavy metals are governed by a complex interplay of soil chemistry, plant physiology, and environmental dynamics. Metal uptake and toxicity are neither uniform nor predictable from total concentrations alone; instead, they vary across soil types, species, and hydrological conditions. This complexity implies that risk assessments and management strategies must integrate bioavailability metrics, species-specific traits, and local soil conditions to accurately predict phytotoxicity and safeguard food safety in contaminated forest and agroforestry systems. Accordingly, from a regulatory and food safety perspective, the interpretation of heavy metal concentrations in soils and edible plant tissues cannot rely on universal threshold values. Soil quality standards and food safety limits for metals vary substantially across local, regional, national, and international frameworks, reflecting differences in land-use categories, exposure pathways, dietary habits, and environmental backgrounds. Consequently, risk assessment in forest and agroforestry systems requires site-specific evaluation aligned with the relevant regulatory context rather than comparison against fixed global benchmarks.

5.3. Monitoring Methods and Assessment Protocols

Effective monitoring of heavy metal contamination in forest and agroforestry soils requires a multi-scale approach that integrates geospatial technologies, soil geochemistry, and biological indicators to capture both spatial patterns and functional impacts. Remote sensing and geotechnologies—ranging from satellite-based sensors to Unmanned Aerial Vehicles (UAVs)—provide essential large-scale datasets for detecting land-use change, vegetation stress, and potential contamination hotspots. Optical and multispectral imagery enable the continuous mapping of forest cover and the identification of anthropogenic disturbances [168,169,170,171], while vegetation indices such as NDVI and EVI serve as proxies for canopy vigor and early stress detection [172,173]. Active sensors, including Light Detection and Ranging (LiDAR) and Synthetic Aperture Radar (SAR), further refine assessments by quantifying aboveground biomass and vertical structure, allowing the detection of degradation processes that may be linked to contamination inputs or soil instability [174,175]. Hyperspectral imaging extends these capabilities by detecting subtle biochemical alterations in plant tissues, enabling the inversion of soil heavy metal concentrations through machine-learning algorithms such as Random Forest, as demonstrated in mining-reclaimed agricultural landscapes in China [171,176].

At intermediate scales, UAV-based multispectral and hyperspectral systems enhance spatial precision, making it possible to delineate fine-grained patterns of vegetation stress and to guide field sampling efforts. Emerging applications in mangrove environments show that NIR spectroscopy, combined with chemometric models such as Partial Least Squares regression, can predict heavy metal concentrations indirectly through their covariance with spectrally active soil components like organic matter and clay, as shown for the metal-rich Botafogo estuary in Brazil [177]. These tools allow remote detection of contamination gradients and support rapid, cost-effective monitoring across complex forested terrain.

Despite the growing potential of remote sensing and UAV-based approaches for monitoring heavy metal contamination, important methodological limitations must be acknowledged. In forest and agroforestry systems, dense canopies hinder the direct inversion of soil heavy metal concentrations, often requiring reliance on indirect vegetation-based indicators [178]. A recent study carried out by Zhou et al. [179] addressed the challenges of monitoring soil arsenic in tropical agricultural regions with dense canopies by developing a synergistic approach that uses UAV hyperspectral data, satellite multispectral imagery, and using time-series Sentinel-2 multispectral and Sentinel-1 synthetic aperture radar data to infer contamination levels through vegetation proxy variables. These spectral responses, however, are frequently non-specific and may reflect multiple abiotic stressors—such as drought or nutrient deficiency—rather than metal toxicity alone, reducing diagnostic certainty. Case studies using satellite vegetation indices have shown that reliable discrimination of metal-induced stress typically requires advanced statistical or probabilistic frameworks and strong field calibration, particularly in heterogeneous landscapes. In addition, the mixed-pixel effect remains a major constraint for orbital sensors, as individual pixels integrate signals from foliage, litter, bare soil, and shadow, masking subtle geochemical anomalies [180,181,182]. Consequently, remote sensing products are most effective when used as complementary tools within integrated monitoring frameworks that combine field sampling, laboratory analyses, and process-based interpretation.

Ground-based soil assessments remain essential for quantifying contamination levels and determining environmental risk. Standardized sampling designs typically include horizon-resolved collections spanning the litter layer, root zone, and mineral subsoil to capture vertical gradients in metal inputs, retention, and mobility. Total digestion methods (e.g., triacid digestion followed by ICP-MS) provide inventories of metal stocks useful for tracking long-term trends in atmospheric deposition, as illustrated by the German National Forest Soil Inventory, where shifts in organic layer metal loads recorded declines in anthropogenic inputs between survey cycles [183]. However, because total concentrations do not reflect bioavailability, sequential extraction procedures are indispensable for operationally defining metal pools such as exchangeable, carbonate-bound, reducible (e.g., Fe oxyhydroxides), oxidizable (organic-bound), and residual fractions—information crucial for predicting mobility under fluctuating pH and redox conditions. These protocols have been widely applied in Brazilian mangrove and forest soils to assess the stability of metal associations in systems subject to hydrological variability.

In situ measurements further support this evaluation. Portable X-ray fluorescence instruments facilitate rapid field screening of contaminated sites, although calibration is essential for organic-rich forest soils [184]. Measurements of Eh, pH, and electrical conductivity enable real-time interpretation of geochemical constraints controlling metal solubility and phase transformations [86,96,185,186,187]. Integrating these geochemical indicators with soil health metrics—such as soil organic carbon, bulk density, aggregate stability, enzymatic activities, and microbial biomass—supports a functional understanding of how contamination alters ecosystem processes and services.

Biomonitoring complements soil analyses by integrating plant physiological responses into contamination assessments. Tree leaves, bark, and fine roots accumulate airborne and soil-derived metals, making them effective indicators of environmental exposure. Studies in urban forests have shown that tissues of species like Rhus typhina and Broussonetia papyrifera can accumulate substantial concentrations of Pb, Cr, Mn, Cu, and Zn, with bio-concentration factors (BCF) quantifying translocation efficiency and revealing species-dependent uptake patterns [188]. Furthermore, leaf age is a critical factor when interpreting plant tissues as biomonitors of metal contamination [189]. Older leaves tend to accumulate metals with low phloem mobility, such as Pb, Cd, and Cr, due to prolonged exposure and xylem-driven transport, often acting as terminal sinks where metals are sequestered in cell walls or vacuoles [190,191,192]. In contrast, younger leaves generally exhibit lower concentrations of these non-essential metals, reflecting exclusion mechanisms that protect actively photosynthesizing tissues. Essential micronutrients such as Zn and Cu, however, may be remobilized from senescing leaves to support new growth, resulting in distinct age-dependent concentration patterns [193,194,195]. These metrics are essential not only for detecting pollution but also for evaluating the risks posed to edible forest products, agroforestry crops, and wildlife.

To translate monitoring data into actionable environmental guidance, assessment protocols employ threshold comparisons, contamination indices, and risk-based frameworks. Differentiating geogenic from anthropogenic sources is a critical step, as metals like Ni and Cr often reflect parent material composition, while Pb typically shows pronounced surface enrichment indicative of atmospheric deposition [183]. Failure to distinguish these sources may result in misdirected remediation efforts. In dynamic systems such as mangroves, integrated indices like the soil health index have proven effective for synthesizing contamination data, combining metrics such as the Geoaccumulation Index (Igeo) and the Contamination Factor (Cf) to support management and policy decisions [116,156,196,197]. For agroforestry settings and regions where subsistence farming coexists with contamination risks, monitoring protocols must explicitly assess metal accumulation in edible plant tissues and evaluate Hazard Quotients or Hazard Indices to prevent food chain contamination and protect vulnerable populations.

Therefore, robust monitoring of heavy metal contamination in forest and agroforestry soils demands a workflow that spans remote sensing, targeted field sampling, chemical speciation, and biomonitoring, supported by risk-oriented evaluation tools. By integrating macro- to micro-scale indicators, these frameworks enable the accurate diagnosis of contamination pathways, the prediction of ecological and human health risks, and the development of informed strategies for restoration and sustainable land management.

6. Remediation and Restoration Strategies: Challenges and Opportunities

6.1. Phytoremediation and Phytostabilization in Forest Contexts

Phytoremediation has emerged as a central strategy in the restoration of heavy-metal-contaminated forest and agroforestry landscapes due to its ecological compatibility, cost-effectiveness, and capacity to reestablish vegetation while reducing contaminant mobility [198]. Because heavy metals are non-biodegradable and persist in soils where they pose risks of leaching, trophic transfer, and biomagnification, remediation strategies that harness plant–soil interactions can simultaneously address contamination and promote ecosystem recovery [199]. Within forest contexts—particularly those impacted by mining, industrial emissions, or legacy pollution—phytoremediation supports soil stabilization, nutrient cycling, microclimate regulation, and long-term carbon sequestration, thereby linking contamination control to broader ecological functions.

Phytostabilization represents a key phytotechnology for forested systems, relying on metal-tolerant species capable of immobilizing contaminants in the rhizosphere through mechanisms such as root sorption, rhizoaccumulation, the formation of organo-metal complexes, and changes in soil pH and redox conditions [198]. Native tropical species are especially advantageous because their traits have co-evolved under local climatic and edaphic constraints, enhancing establishment success and long-term persistence. Multiple studies in Brazilian mining-impacted landscapes have demonstrated that species with low translocation factors (TF ≪ 1) preferentially sequester metals in roots rather than transferring them to aboveground tissues, thereby minimizing food-web contamination risk. For example, native trees and shrubs from the Caatinga biome—including Pilosocereus gounellei, Cenostigma pyramidale, Sideroxylon obtusifolium, Myracrodruon urundeuva, and Pityrocarpa moniliformis—exhibited low bioaccumulation factors and low TF values when grown in Cu-contaminated soils, reflecting a strong rhizoaccumulation mechanism that reduces the mobile and exchangeable Cu fractions in the rhizosphere [121]. Similarly, experiments with constructed Technosols amended with bentonite and compost showed that M. urundeuva and Cedrela fissilis accumulate high Cu concentrations in roots while the Technosol matrix immobilizes 60%–80% of added Cu, highlighting the potential of soil amendments to enhance phytostabilization outcomes [148]. In redox-active wetland forests, species such as Hibiscus tiliaceus further stabilize metals by promoting Fe plaque formation—a process driven by radial oxygen loss—which traps Fe and associated trace metals on root surfaces and prevents their entry into root tissues [128].

While phytostabilization aims to immobilize metals, phytoextraction seeks to remove them from contaminated soils through the harvest of aboveground biomass [198,200]. The use of trees for phytoextraction in forest systems is constrained by slow growth rates, long rotation times, and limited biomass turnover. Therefore, tree-based phytoextraction is most effective when integrated with short-rotation coppice systems, fast-growing shrubs, or wetland macrophytes capable of producing large aboveground biomass within short timeframes. Assisted phytoremediation approaches—including the use of plant-growth-promoting rhizobacteria (PGPR), engineered microbial strains, and chelating agents such as EDTA or organic acids—can further increase metal uptake by modifying metal speciation and enhancing root absorbance capacity [198,201].

In wetland and riparian forests, where hydrological dynamics amplify metal bioavailability, certain macrophytes have shown exceptional potential for phytoextraction. Eleocharis acutangula, for instance, exhibits rapid and efficient Ba uptake in flooded sediments, achieving maximum translocation within 105 days and enabling the removal of up to 58 kg ha⁻¹ of Ba in its aerial tissues [202]. Likewise, Typha domingensis has demonstrated extraordinary efficacy for Mn remediation in mining-impacted estuaries, displaying shoot Mn concentrations of ~6858 mg kg⁻¹, high bioconcentration and translocation factors, and the ability to phytoextract 147 tons of Mn over areas of ~19,000 m² [203]. The mechanism underpinning this performance is rhizospheric acidification, which mobilizes Mn from Fe plaques and short-range-ordered Mn oxides, thereby enhancing root uptake [203]. In parallel, T. domingensis also exhibits a high potential for Fe phytoextraction, with shoot concentrations exceeding 3500 mg kg⁻¹ in Fe-rich estuarine soils [128].

Collectively, these studies demonstrate that successful phytoremediation in forest contexts depends on matching plant functional traits to local contamination profiles, hydrological regimes, and restoration goals. Phytostabilization is particularly suited to forested uplands and semi-arid woodlands where erosion control, root-based metal sequestration, and long-term soil recovery are priorities, whereas phytoextraction becomes more feasible in wetland forests and riparian zones dominated by fast-growing macrophytes. Because many forest landscapes serve as subsistence agricultural systems or buffer zones around food-production areas, the selection of species must also consider food safety, non-edibility of harvested biomass, and the risks of metal transfer into the food chain. Ultimately, integrating native vegetation, soil amendments, and assisted phytotechnologies offers a viable pathway for restoring ecosystem function while mitigating contamination hazards across forested environments [121,148,198,203].

Thus, evidence confirms that phytoremediation and phytostabilization can substantially reduce ecological risks in forested systems when plant traits and soil geochemistry are properly aligned; however, the long growth cycle of woody plants remains a significant limiting factor for immediate site recovery and economic feasibility. In highly contaminated or structurally degraded soils where metal mobility is high, plant-based approaches alone may be insufficient unless integrated with strategies that balance slow remediation returns with the immediate economic needs of land users [204,205]. In this context, agroforestry intercropping systems—particularly alley cropping—offer a viable solution by establishing fast-growing woody species in wide rows while utilizing inter-row spaces for short-cycle, high-value non-food crops [206,207,208]. This arrangement not only allows for annual income generation during the extended remediation period but also improves land-use efficiency, stimulates rhizospheric microbial activity to accelerate contaminant breakdown, and provides a sustainable bridge between short-term economic viability and long-term ecological restoration.

These opportunities and constraints directly motivate the next section, which examines the development and application of Technosols and engineered substrates as complementary tools for restoring heavy-metal-impacted forest and agroforestry landscapes.

6.2. Technosols and Engineered Substrates

Technosols, as defined by the World Reference Base for Soil Resources, are soils containing ≥ 20% artifacts within the upper 100 cm, intentionally or unintentionally formed by human activity [209]. These anthropogenic soils occur across a wide range of disturbed landscapes and increasingly serve as a central component of remediation and restoration strategies in tropical environments [210]. Their potential lies in their capacity to immobilize contaminants, rebuild soil structure, promote vegetation establishment, and reestablish key ecosystem functions under conditions where natural soil formation is severely constrained.

Unintentional Technosols may emerge rapidly following catastrophic environmental disturbances, such as mining dam collapses. The Fundão Dam disaster, which released more than 60 million m³ of Fe-rich tailings into the Rio Doce estuary [29,30], offers a striking example (Figure 2). The deposition of fine sediments led to major shifts in the solubility, speciation, and sorption mechanisms of heavy metals, as the reduced crystallinity of Fe oxyhydroxides under fluctuating redox conditions facilitated the mobilization of Cu, Cr, Mn, and Ni [94,113]. Within four years, however, vegetation colonization triggered rapid pedogenic transformation of the tailings, promoting cumulization, melanization, and the formation of low-crystallinity Fe minerals that stabilized organic matter inputs, ultimately generating the first documented estuarine Technosol profile in a tropical hydromorphic environment [211]. These findings highlight that even severely altered substrates can undergo surprisingly fast pedogenesis when biological and geochemical processes interact under tropical conditions.

Beyond unintentional formation, Technosols can be deliberately engineered to restore contaminated or structurally degraded soils. A growing body of research has explored the use of mining wastes, construction and demolition waste (C&DW), and mineral amendments as parent materials for constructing functional soils. Technosols composed of Fe mining tailings (IMT) and C&DW, for example, have shown promising results for supporting vegetation growth while stabilizing large volumes of industrial waste [212]. When mixed in varying proportions, these materials provided a physically stable matrix capable of sustaining Urochloa brizantha, primarily through improved structural aggregation and nutrient supply derived from the blended waste materials [212]. This approach presents a sustainable alternative for solid waste management in Brazil, where IMT dam instability and C&DW disposal remain critical challenges.

Technosols engineered with sorptive mineral amendments and organic materials have shown strong potential for immobilizing potentially toxic metals. Asensio et al. [148] demonstrated that Technosols amended with bentonite and compost provide high Cu sorption capacity, reducing metal mobility and enhancing soil conditions for tropical native trees such as Myracrodruon urundeuva and Cedrela fissilis [148]. In this system, sorption processes at bentonite surfaces limit Cu availability, while root activity contributes to phytostabilization and accumulation within plant tissues, forming a combined geochemical and biological barrier to contamination [148]. Similar plant-driven immobilization has been noted in other tropical systems, where rhizosphere acidification promotes Mn hyperaccumulation in wetland macrophytes, reinforcing the importance of plant–soil interactions in Technosol-based phytoremediation [203].

The long-term formation and maturation of Technosols can also lead to the recovery of key soil functions, including carbon sequestration. Studies of limestone mine Technosols chronosequences showed rapid pedogenesis marked by pedoplasmation, rubification, and melanization, driven by root inputs and organo-mineral interactions—particularly those mediated by Ca in dolomitic substrates [213]. After 20 years, these Technosols not only developed well-defined horizons but also exhibited organic carbon stocks 2.7-fold higher than adjacent natural soils, illustrating their capacity to restore soil functioning under tropical conditions [214]. Soil quality assessments using the Soil Management Assessment Framework (SMAF) confirmed equivalent or superior soil functioning relative to native soils, emphasizing the value of these systems for long-term land reclamation [214].

More recent work shows that Technosols produced entirely from C&DW can support native Brazilian tree species and restore soil health with surprisingly high efficiency. Technosols with added compost increased cation exchange capacity (CEC), total organic carbon, and nutrient concentrations in plant tissues, resulting in higher soil health index scores than natural Ferralsols [215]. These improvements in fertility and biological activity allowed Guazuma ulmifolia and Piptadenia gonoacantha to develop biomass comparable to or greater than that observed in reference soils, demonstrating the feasibility of using recycled waste materials as nature-based solutions for land reclamation [215].

Together, these studies reveal that Technosols and engineered substrates can act as powerful tools for remediating contaminated and degraded tropical landscapes. Their effectiveness depends on the careful selection and mixing of parent materials, the integration of sorptive and nutrient-rich amendments, and the promotion of vegetation capable of driving pedogenic and biogeochemical processes. Whether unintentionally formed after extreme events or deliberately constructed for restoration purposes, Technosols offer a flexible and scalable approach to stabilizing contaminants, improving soil structure, enhancing carbon sequestration, and accelerating the recovery of essential ecosystem services in tropical regions.

Despite their demonstrated effectiveness, the long-term evolution of Technosols remains unclear for risk management. Field evidence indicates that metal immobilization in Technosols can remain durable over decadal timescales when stabilization mechanisms are supported by sustained neutral to alkaline pH, the formation of secondary mineral phases, and limited redox oscillations [216,217,218,219,220,221]. Long-term trials using carbonate-rich and oxide-amended Technosols have shown persistent partitioning of metals into oxide-associated, organic, and residual fractions after more than a decade, indicating low remobilization potential under stable conditions [219,220]. However, progressive weathering, organic matter decomposition, shifts in redox regimes, root activity, and physical disturbance may alter fixation pathways and promote metal redistribution among operational fractions [213,217]. These processes are metal-specific and amendment-dependent, underscoring the need for long-term monitoring that integrates chemical speciation, bioaccessibility assays, leaching tests, and vegetation responses to ensure that Technosols remain effective sinks rather than delayed sources of contamination.

6.3. Integrated Approaches for Sustainable Remediation and Restoration of Metal-Impacted Forest Soils

The long-term recovery of metal-contaminated forest and agroforestry soils requires integrated strategies (Figure 4) that align ecological processes with technological innovation and socio-economic incentives [222]. Beyond traditional engineering-based remediation, emerging approaches now emphasize biological mechanisms, circular economy principles, and landscape-scale monitoring frameworks [1,223]. An effective remediation and restoration plan must be site-specific (Figure 4), taking into account factors like the type of metal contamination and the challenging physical and chemical properties of the soil [1,54]. Furthermore, implementing these strategies often necessitates an interdisciplinary approach that incorporates ecological and social factors to successfully regenerate contaminated sites, sometimes leading to the creation of novel ecosystems [222]. This schematic workflow (Figure 4) emphasizes that sustainable restoration is not a single event but a cumulative process, advancing from initial risk assessment and the construction of Technosols or phytoremediation strategies toward the final establishment of productive agroforestry systems capable of reinstating vital ecosystem services (Figure 4).

For instance, agromining and biomining represent innovative, bio-based strategies that couple phytoremediation with resource recovery [223,224]. These systems are typically developed in specific environments, such as ultramafic soils, which are naturally rich in metals like Ni [200,223]. Hyperaccumulator species are cultivated to extract extraordinarily high levels of metals from contaminated soils, producing biomass that can later serve as a raw material for metal recovery or bioenergy production [225,226]. For instance, species like Noccaea muralis s.l. are established hyperaccumulators that have historically been studied for their potential in nickel agromining [223]. In forest systems, short-rotation woody species with high metal uptake capacity may gradually deplete bioavailable metal pools while generating marketable biomass [200,223,226]. The practical implementation of phytoextraction in sites contaminated with metals like Pb, Cd, and Ni has been an area of growing interest for research and pilot projects [1,225,227]. However, economic viability depends on the concentration of recoverable metals, biomass yield, harvesting logistics, and the existence of legal frameworks for biomass handling and metal recovery [228,229]. Integrating these bioeconomic opportunities into forest restoration programs could help offset remediation costs and enhance stakeholder engagement.

Natural regeneration processes can be enhanced through targeted interventions that accelerate ecological succession and soil recovery, which is critical in post-mining lands [230]. Assisted natural regeneration relies on the exclusion of further disturbance, the reintroduction of native or pioneer vegetation, and soil amendments to foster microbial and vegetative recolonization [231]. Revegetation is a key aspect of this, essential for mitigating risks associated with former mining sites, particularly those contaminated with nickel [231]. Microbial inoculants—such as metal-tolerant rhizobia, endophytic bacteria, and Dark Septate Endophyte (DSE) fungi—play pivotal roles by increasing plant tolerance, improving nutrient acquisition, and modifying metal bioavailability [232,233,234]. Accordingly, DSE fungi, for example, have been isolated from metal-polluted soils and studied for their tolerance and metal accumulation in vitro [232,233,234]. The success of these interventions is context-dependent, requiring site-specific consideration of contamination type, climatic regime, and vegetation composition [1,230,235]. Furthermore, biological assays, such as phytotoxicity tests, are critical for understanding the ecological consequences of metal contamination by assessing their effects on plant growth and health.

Advances in geospatial technologies are transforming the monitoring and management of metal contamination in forested landscapes [236]. Remote sensing and GIS-based approaches allow the detection of contamination gradients, mapping of soil metal hotspots, and identification of vegetation stress indicators such as chlorosis, defoliation, or canopy thinning [1,235,236]. The integration of satellite imagery, UAV-mounted hyperspectral sensors, and LiDAR-derived canopy models with geochemical sampling enables the development of spatial risk models [236]. These tools are instrumental for tracking the process of ecological restoration monitoring over time and space [235,236]. Beyond contamination, Integrated Monitoring (IM) programs, such as those implemented in Swedish forest ecosystems, investigate the combined effects of acidification, eutrophication, and heavy metals on water, soil, and biological systems [237]. These programs generate long-term time series data that reveal trends in environmental processes, facilitating modeling and prediction [182,237]. When linked to carbon credit systems and ecosystem service valuation, these data provide quantitative metrics for guiding policy and funding decisions [236].

The technical feasibility of forest soil remediation must be supported by governance frameworks that ensure long-term implementation [1,238,239]. Effective remediation requires coordination among environmental agencies, research institutions, and local communities, as well as financial mechanisms capable of sustaining multidecadal restoration projects [32,222]. While regulation of soil contamination may be less directly addressed than water or air pollution in some national contexts, policy development is pivotal for ensuring the effective adoption and implementation of new soil remediation technologies [240,241,242]. Economic factors pose a significant challenge, as the cost of remediation can be prohibitively high, particularly for advanced or extensive clean-up methods. Public awareness and education on the importance of soil health and remediation technologies are also crucial for shaping the future landscape of environmental management [243,244,245].

Despite recent advances, several challenges constrain the large-scale remediation of metal-contaminated forest soils. The long temporal scales required for tree establishment and the instability of metal–Fe–Mn associations under fluctuating redox conditions remain major barriers [185,246,247]. Furthermore, limited baseline data across tropical forest types complicate risk assessment and monitoring, particularly in the context of metal mining sites [1,75,159,231]. Key research priorities include the long-term tracking of soil–plant–microbe interactions, comparative screening of native species for phytostabilization and phytoextraction potential, evaluation of Technosol amendment durability, and development of remote sensing tools capable of detecting early metal stress in vegetation [148,223,236]. Future research should also integrate human health risk assessment into restoration planning, promoting holistic management of contaminated forest ecosystems [1,240,243,244].

Furthermore, from a management and policy perspective, the cost-effectiveness of remediation strategies is a critical consideration, particularly when interventions are implemented at large spatial scales [205,248]. Technologies such as Technosols and engineered amendments often involve higher initial investment due to material inputs, transport, and site preparation, but they may offer rapid risk reduction and functional recovery in severely degraded or soil-less environments [249,250]. Phytoremediation and nature-based approaches typically require medium to high costs and demand longer timeframes to achieve meaningful contaminant stabilization, especially in woody systems [204,251].

Balancing short-term economic constraints with long-term environmental benefits remains a key challenge for land managers and decision-makers. Integrating remediation strategies with productive agroforestry systems, ecosystem service valuation, and land-use planning can improve economic viability while enhancing soil function and landscape resilience. Future research linking biogeochemical performance to cost–benefit analyses will be essential to support evidence-based remediation policies.

7. Limitations, Knowledge Gaps, and Research Challenges

Despite the comprehensive synthesis presented in this review, several limitations must be acknowledged. First, the available literature exhibits substantial heterogeneity in terms of analytical methods, spatial scales, soil classifications, and reporting standards, which constrains direct quantitative comparisons across studies. Variations in extraction procedures, speciation approaches, and monitoring timeframes introduce uncertainties when extrapolating metal behavior across forest and agroforestry systems. For instance, an emerging challenge concerns the role of microplastics as vectors for heavy metals in forest and agroforestry soils. Quantifying metal adsorption, transport, and bioavailability associated with microplastic particles remains analytically complex due to difficulties in sampling, separation, and speciation, posing significant challenges for monitoring and risk assessment frameworks [252,253].

Second, the geographic distribution of the literature synthesized in this review reflects both an intentional analytical focus and the uneven development of process-based studies worldwide. Rather than aiming for uniform global coverage, this review prioritizes forest and agroforestry systems in tropical and subtropical regions. While this focus strengthens the relevance of the conceptual framework for regions under acute environmental pressure, it also implies that some biogeochemical processes—particularly those operating under cold or permafrost-influenced conditions—are not addressed in detail. As such, the findings and interpretations presented here should be understood as most directly applicable to forested landscapes characterized by high weathering intensity, dynamic redox regimes, and strong human disturbance.

Third, scale mismatches remain a persistent challenge. Many biogeochemical processes controlling heavy metal fate—such as redox oscillations, microbial transformations, and rhizosphere interactions—operate at the microscale, whereas monitoring and management decisions are often made at landscape or regional scales. Bridging these scales introduces uncertainty, particularly when using remote sensing or spatial proxies to infer subsurface processes.

Ultimately, long-term uncertainties impact the assessment of remediation and restoration outcomes. The durability of immobilization mechanisms, including phytostabilization, sulfide formation, and Technosol performance, remains insufficiently constrained under prolonged weathering, land-use change, and climate-driven hydrological variability. These limitations highlight the need for long-term monitoring frameworks and standardized assessment protocols to support adaptive management in contaminated forest and agroforestry landscapes.

8. Conclusions

This review advances the understanding of heavy metal pollution in forest and agroforestry soils by adopting an integrative, process-oriented perspective that links biogeochemical mechanisms, soil functional dynamics, and monitoring and restoration-oriented management strategies. Rather than treating contamination solely as a problem of sources or concentrations, the framework presented here emphasizes how redox-driven transformations, organic matter interactions, mineral dynamics, microbial activity, and plant–soil interactions jointly regulate heavy metal mobility, bioavailability, and ecological risk.

From a management standpoint, the synthesis highlights that effective responses to heavy metal contamination require moving beyond single-indicator assessments toward multi-scale diagnostic frameworks that combine soil chemistry, biological indicators, and spatial monitoring tools. Restoration strategies must be context-specific, accounting for local hydrology, soil mineralogy, vegetation traits, and land-use history. In this regard, the integration of phytotechnologies, Technosols, and assisted natural regeneration emerges as a flexible toolbox capable of supporting both contamination control and ecosystem service recovery in forested landscapes.

The review also underscores the relevance of heavy metal biogeochemistry for policy and governance. Contaminated forest and agroforestry soils directly affect food safety, carbon sequestration, water regulation, and biodiversity conservation. Embedding soil-health indicators and long-term monitoring requirements into forest restoration programs, land-use planning, and ecosystem service valuation frameworks will be essential to align remediation efforts with broader sustainability goals.

Looking forward, progress in this field will depend on long-term, process-based studies capable of capturing the effects of climate variability, land-use change, and soil evolution on metal stability. Advances in remote sensing, trait-based plant selection, and engineered soil systems must be coupled with socio-economic considerations to ensure that remediation strategies are both environmentally effective and operationally feasible. Ultimately, safeguarding forest and agroforestry soils from heavy metal pollution is not only a technical challenge but a strategic investment in ecosystem resilience and human well-being.