Wildfire-driven geochemical transformations from soils to pyrogenic nanoparticles

Wildfires can substantially alter soil geochemistry in fire-prone environments, yet their influence on inorganic nanoparticles (NPs) remains poorly constrained. This study investigated nanoparticle dynamics following a wildfire in the Cantillana Reserve (central Chile) by combining field sampling, controlled combustion experiments, bulk geochemical analyses, and single-particle ICP-TOFMS. Fire exposure caused a marked increase in nanoparticle number concentration, reaching up to 1.4 × 10 9 particles per gram in ash-enriched soils, together with a shift from predominantly monometallic to multimetallic particles (e.g., Fe–Mn–Ti and Fe–Zn). Comparable patterns were observed in laboratory heating experiments, supporting temperature as a key control on elemental redistribution, reduction, and volatilization. Fire-affected samples also contained smaller nanoparticles enriched in Pb and Zn, suggesting enhanced post-fire metal mobility and potential environmental exposure. In addition, bimetallic nanoparticle ratios such as Fe/Mn and Fe/Zn may serve as geochemical indicators of wildfire severity. Overall, the results show that wildfire can strongly modify nanoparticle abundance and composition, and that nanoscale characterization provides useful insight into post-fire geochemical transformations.

Nanoparticles (NPs) have emerged as a new class of pollutants, raising environmental concerns. They originate from natural processes such as volcanic eruptions and dust storms (Hochella et al., 2019; Gelin et al., 2025; Panta et al., 2025) as well as from human activities (Lespes et al., 2020). While natural sources have long contributed to NP loads in the atmosphere and soils, anthropogenic events like wildfires can introduce unfamiliar hazardous nanomaterials into ecosystems (Hochella et al., 2019). However, the influence of wildfires on the production and behaviour of inorganic NPs in soils remains largely unknown. Understanding NP production, behavior, fate, and impacts under wildfire conditions is therefore critical.

Wildfires have increased globally in frequency and intensity due to climate change and land-use practices (Ciocca et al., 2023; Duarte et al., 2024; Farid et al., 2024). They pose a major threat to ecosystems by altering biogeochemical cycles, mobilizing contaminants, and affecting biodiversity (Hochella et al., 2019).

Wildfires produce NPs in soils through the thermal breakdown of inorganic materials and volatilization processes. Fire-induced mineral alterations release elements and change metal speciation and nutrient availability post-fire (Roshan and Biswas, 2023). The resulting NPs can contain elements such as lead (Pb), iron (Fe), manganese (Mn), titanium (Ti), aluminum (Al), cerium (Ce), and zinc (Zn) (Casanova et al., 2013; Fontboté et al., 2017). These particles can be mobilized into surrounding ecosystems, posing contamination risks to environmental and human health (Abraham et al., 2017; Rao and Parsai, 2025).

These nanoscale particles are highly reactive and easily dispersed via air and water (Hochella et al., 2019; Alam et al., 2023; Coaguila et al., 2025). For example, wildfire ash from California contained a variety of metal-bearing and carbonaceous NPs (Ti, Fe, Zn, Cu, Cr) derived from burned vegetation, structures, and vehicles (Alshehri et al., 2023), demonstrating widespread formation of incidental NPs during fires. Wildfires can also convert metals like arsenic and Fe into more mobile and toxic forms (Alam et al., 2023). Notably, Fe can transform into magnetite—a NP linked to neurodegenerative disease—and wildfires in the wildland-urban interface have been shown to generate 10 6 –10 8 NPs per Gram containing metals such as Fe, Mn, Al, Ba, Cr, Cu, Ti, Pb, and Zn, with higher concentrations in ash from man-made materials than from vegetation (Alam et al., 2025).

At elevated concentrations, these metal-bearing NPs can induce oxidative stress in biological systems (Titma et al., 2016). Metal oxide NPs (e.g., oxides of Fe, Mn, Zn) generate reactive oxygen species (ROS) and disrupt cellular processes when inhaled or ingested. ZnO NPs show high cytotoxicity via Zn dissolution and ROS generation, while MnO 2 NPs can cause delayed neurotoxic and inflammatory effects (Zaitseva et al., 2015; Titma et al., 2016). Even relatively less bio-reactive NPs such as Fe and Mn oxides (e.g., in welding fumes) provoke oxidative stress and lung inflammation (Xia et al., 2023). Additionally, wildfire-derived TiO 2 NPs can produce ROS that damage cells (Bogdan et al., 2015), and CeO 2 NPs can cause oxidative stress, inflammation, and genotoxicity due to their redox activity (Dahle and Arai, 2015; Park et al., 2018; Santos et al., 2025).

The mobilization of toxic metals and NPs by fire heightens the risk of soil and water contamination (Paul et al., 2022; Farid et al., 2024). A major concern is the transfer of contaminants via runoff into surface waters (Gleasman et al., 2025), where they can compromise water treatment systems and threaten water quality (Alam et al., 2023; Coaguila et al., 2025).

Mediterranean-climate regions have seen increasing wildfire activity (Turco et al., 2017; Ruffault et al., 2020). Central Chile, in particular, has experienced record-breaking wildfire seasons exacerbated by a megadrought and recurrent heatwaves (González et al., 2018; Cordero et al., 2024). The Cantillana Nature Reserve near Santiago has been repeatedly affected by severe wildfires (González et al., 2020). Such events leave persistent contaminants, including heavy metals (Pb, Mn, Zn, Fe) and organic compounds that degrade soil and water quality (Draper et al., 2022; Sánchez-García et al., 2023). Elements like Fe, Ti, and Mn—abundant in local mineral deposits—are especially susceptible to thermal mobilization during intense fires (Guerin et al., 2024).

In this context, a central question is whether wildfires alter the formation, composition, and environmental behaviour of inorganic NPs, particularly heavy-metal-bearing multimetallic species. The size and mass distribution of these particles is critical because it governs their transport, bioavailability, and toxic effects. High-temperature combustion in wildfires could generate multimetal or mixed-phase NPs (potentially within fire plumes), but their formation mechanisms and post-fire evolution remain poorly understood.

To address these issues, we employed a hierarchical approach. First, bulk soil analysis by inductively coupled plasma optical emission spectrometry (ICP-OES) was performed to assess large-scale elemental redistribution in fire-affected soils. Next, controlled laboratory burns of soil at 400 °C and 550 °C were conducted to replicate wildfire conditions and examine NP formation under different thermal regimes. Finally, single-particle inductively coupled plasma time-of-flight mass spectrometry (spICP-TOFMS) was used to quantify nanoparticle concentrations and characterize NP size, mass, and elemental composition (Tharaud et al., 2022; Meng et al., 2023), providing insight into temperature-driven transformations.

Study area and sample collection

The Cantillana Nature Reserve, located ∼60 km southwest of Santiago in Chile’s Coastal Range, represents a critical site for post-wildfire studies. The area is characterized by a Mediterranean climate with cool, wet winters and hot, dry summers that promote fire propagation, a tendency intensified by the recent megadrought (González et al., 2020). Vegetation includes sclerophyllous forests dominated by native species such as Lithraea caustica and Quillaja Saponaria (Gajardo Michell, 1994), together with exotic Pinus radiata and Eucalyptus globulus plantations that contribute to fire severity in urban–rural interfaces (altosdecantillana.org). Soils derive from Mesozoic volcanic–sedimentary formations, rich in silicates (quartz, feldspars, micas) and Fe oxides such as hematite, magnetite, and goethite (Oliveros et al., 2010; Casanova et al., 2013). Hydrothermal mineralization linked to the nearby Alhué district provides sulfides including pyrite, chalcopyrite, sphalerite, and galena, which may oxidize during fire and release Pb, Zn, and other trace metals (Fontboté et al., 2017). Accessory rare earth element (REE)-bearing minerals (e.g., monazite, allanite) contribute Ce inputs upon weathering (Ramos et al., 2016; Collao et al., 2019). These mineralogical characteristics, combined with recurrent drought and wildfire events, make Cantillana particularly prone to the generation of ultrafine and reactive nanoparticles during combustion. Field sampling was conducted in July 2024, 7 months after a wildfire (10 December 2023). Two zones 0.7 km apart were selected: an unburned zone (3 plots) and a burned zone (6 plots, including ash-rich sites near charred trees). Surface soil (0–5 cm) was collected from each plot (∼0.5 kg per sample) and stored in plastic bags. All samples were air-dried and sieved (<2 mm) prior to analysis. Extended geological, climatic, and vegetation descriptions are provided in Supplementary Information and Supplementary Figures S1, S2.

Bulk soil analysis

Major element concentrations in unburned, burned, and ash-rich soils were measured using ICP-OES (Agilent iCap Pro). Samples were digested by lithium metaborate (LiBO 2) fusion followed by acid dissolution (Service d’Analyse des Roches et des Minéraux, SARM). Results (in g/kg) are provided in Supplementary Table S1. In addition, total inorganic carbon (as CO 2), H 2 O content, and Loss of Ignition (LOI) were measured to assess soil carbon distribution (inorganic organic, Supplementary Table S2).

Controlled combustion experiments

Controlled combustion experiments were carried out at the Laboratoire de Combustion et de Détonique (PPrime Institute, Poitiers, France) to tentatively reproduce wildfire conditions and evaluate nanoparticle (NP) formation under contrasting thermal regimes (Brudo et al., 2024). Two furnace temperatures were selected: 400 °C, representing low-intensity surface fires mainly affecting organic layers, and 550 °C, mimicking high-intensity crown fires capable of stronger organic matter volatilization and mineral alteration (Araya et al., 2017). Although simplified, these controlled conditions allow reproducible assessment of NP generation mechanisms, but they do not capture the full complexity of natural wildfire events, including fluctuating temperatures, variable oxygen availability, heterogeneous fuel loads, turbulence, and longer heating durations. Therefore, the observed mineral and nanoparticle transformations should be interpreted as mechanistic trends under constrained thermal conditions rather than exact replicates of field wildfire processes.

Unburned Cantillana soils (20 g aliquots) were placed on aluminum foil within quartz trays, ensuring chemical inertness and preventing external contamination while facilitating recovery. The tubular furnace was pre-heated to the target temperature before sample insertion, with soils exposed for 15 min. After cooling, samples were weighed to calculate mass loss (grams and percentage). Experimental setup details are shown in Supplementary Figure S3.

Nanoparticle extraction

Prior to weighing and burning, all soil samples were sieved through a 1 mm mesh to remove coarse debris (stones, roots) and ensure uniform particle size distribution. For extraction, 0.5 g of each sample (unburned soil, burned soil, or ash) was placed in 50 mL Falcon tubes with 40 g of ultrapure water, allowing NPs and soluble elements to be released from the bulk soil matrix (Tharaud et al., 2022). Suspensions were agitated overnight on a roller shaker at 60 rpm to ensure homogenization. This protocol was not intended to preserve in situ aggregation, but to simulate post-fire leaching and rainfall-driven NP mobilization. After agitation, the suspensions were filtered through 1.2 μm cellulose acetate filters (Sartorius Minisart) (Wang et al., 2024). Filtrates were transferred into pre-cleaned containers and stored at 4 °C until analysis. Dilutions (5×–50×) were applied to reduce coincidence and optimize NP detection. Dilution further minimized background signals that could obscure small NPs and avoided coincidence effects, enabling accurate single-particle detection (Laborda et al., 2014).

Time-of-flight ICP-MS analysis

Nanoparticle measurements were performed at the PARI analytical platform, Institut de Physique du Globe de Paris (IPGP), using a Vitesse ICP-TOFMS system (Nu Instruments). Operating conditions are summarized in Supplementary Table S3, and element-specific particle detection limits in terms of mass and equivalent diameter are reported in Supplementary Table S4. In brief, mass spectra were acquired over m/z 45–260 amu with an effective dwell time of 83 μs and a 14-min acquisition per run. A multi-element dissolved calibration (Ti, Fe, Mn, Zn, Ce, Pb; 0.1–20 ppb) was applied, and certified gold nanoparticle standards (40, 60 and 80 nm; BBI Solutions, United Kingdom) were employed to calculate the transport and nebulisation efficiencies, which were validated before analysis using sonicated NP solutions (60s at 42 KHz). The transport efficiency ranged between 6% and 8% (particle frequency method) and nebulization efficiency was approximately 5%. Before each sample run, the sample introduction system was thoroughly rinsed with 1% HNO 3, followed by ultrapure water for 3 min to minimise carry-over and cross-contamination. Data were processed with Nu Codaq and NuQuant software. Particle events were identified by setting element-specific intensity thresholds at mean + 3σ of the dissolved signal background. Elemental targets and monitored isotopes (48 Ti, 54 Fe, 55 Mn, 64 Zn, 140 Ce, 208 Pb) reflect the specific geochemical fingerprint identified for the study site through the initial elemental screening of soils and ashes. Accordingly, the nanoparticle-level analysis was focused on these elements to capture the main post-fire geochemical trends observed in the Cantillana system. Two major matrix elements (Al and Si) were excluded from the NP-level dataset because they generate high background signals and may saturate the detector, implying that aluminosilicate NPs are likely underrepresented in the present analysis. Fe was monitored at 54 Fe as a compromise isotope: 56 Fe often saturates detectors and suffers severe polyatomic interferences (e.g., 40 Ar 16 O +, 40 Ca 16 O +), whereas 57 Fe signals are too weak. We acknowledge that other fire-affected environments may display different elemental signatures, including elements reported in wildfire residues such as Cr, Cu, or As. Therefore, the reported NP compositions should be interpreted as a targeted site-specific characterization rather than an exhaustive survey of all possible wildfire-derived nanoparticles. For each sample, the NP number concentration, elemental composition, and size (mass) distribution were determined. Hierarchical agglomerative clustering was applied to categorize NPs into distinct compositional families (Tharaud et al., 2022).

Bulk sample composition

Unburned soils had high Si and Al concentrations (220–285 g/kg Si; 66–83 g/kg Al), which decreased slightly in burned and ash-rich soils (Supplementary Table S1). Fe remained relatively stable (45–78 g/kg) or even increased in some ash samples. Calcium (Ca) and phosphorus (P) were enriched in ash (e.g., Ca up to 57 g/kg; P 3.7 g/kg) likely due to biomass ash inputs (Bodí et al., 2014; Roshan and Biswas, 2023). Magnesium (Mg) and sulfur (S) showed inconsistent changes, while sodium (Na) decreased markedly in fire-affected soils: Na dropped from 23 g/kg in unburned soil to <10 g/kg in some ash samples, indicating volatilization and leaching losses. Potassium (K) and Ti remained relatively unchanged. These bulk trends varied between plots, reflecting heterogeneity in fire severity and ash deposition.

To highlight elemental redistribution, impacted soils were normalized to unburned composition using Ti as a reference (Figure 1). Enrichment factors >1 indicate fire-induced enrichment, while <1 indicates depletion relative to baseline. This analysis showed Ca and P enrichment in ash plots and strong depletion of Na, Si, and Al in burned soils, although trends were not uniform across all sites. Overall, bulk composition alone provided limited insight into post-fire changes, necessitating nanoparticle-level analysis.

Complementary to the elemental data, CO 2, LOI and H 2 O contents also increased in fire-affected soils (Supplementary Table S2). CO 2 rose from 3.9% in unburned soils to 22% in ashes, reflecting oxidation of charred biomass. LOI exceeded 15% in ashes, consistent with enhanced organic matter transformation. Elevated H 2 O contents further suggest secondary hydrated mineral phases or partial rehydration during cooling.

Controlled-laboratory combustion experiments

Controlled combustion at 400 °C and 550 °C strongly influenced NP production and composition. The absence of dispersion processes (e.g., wind, runoff) under laboratory conditions enhanced recovery of thermally generated particles, thus reflecting their maximum formation potential. NP concentrations increased with temperature (Figure 2). At 550 °C, Fe and Mn reached 2.56 × 10 8 ± 1.62 × 10 8 and 2.13 × 10 8 ± 1.73 × 10 8 g -1, respectively. Pb, Zn, and Ce also peaked under these conditions, with 1.22 × 10 7 ± 1.05 × 10 7, 1.42 × 10 7 ± 1.16 × 10 8, and 1.18 × 10 7 ± 8.5 × 10 6 g -1, respectively. Multimetallic NPs were favored, particularly Fe–Mn, Fe–Ti–Mn (Supplementary Figure S4), and Fe–Zn. For example, Fe–Ti–Mn reached 8.3 × 10 7 g -1, Fe–Mn 1.2 × 10 8 g -1, and Ti–Zn 4.6 × 10 7 g -1. NP masses were typically 30–80 fg.

Field nanoparticle concentrations and composition

Unburned soils contained on the order of 10 7 –10 8 g -1, predominantly monometallic. Over 95% were single-element particles, mostly Ti (∼60% of total count), with smaller fractions of Fe and Mn (Figure 3A). Multimetallic NPs were rare (<5%).

In burned soils, NP concentrations were several times higher than background, and remained elevated even 7 months post-fire (Figure 2). Fe-bearing NPs increased to 3.9 × 10 7 g -1 in burned soil and 4.3 × 10 7 g -1 in ash-rich soil (against 1.3 × 10 7 g -1 in unburned). Mn-bearing NPs rose dramatically, from 2.1 × 10 7 g -1 (unburned) to 2.2 × 10 8 g -1 in ash. Pb and Zn NPs, negligible in unburned soil, appeared after fire: Pb peaked at 2.7 × 10 7 g -1 in burned soils before declining in ash (6.7 × 10 6 g -1), while Zn showed variable increases. Ce NPs decreased with fire exposure, from 7.8 × 10 6 g -1 in unburned soil to 7.9 × 10 5 g -1 in ash.

The fraction of multimetallic NPs increased with fire exposure (Supplementary Figure S5), from <5% in unburned soil to 14% in burned soil and 18% in ash. NP composition also shifted: Ti-dominated NPs, dominant in unburned soil, became nearly absent in ash, while Mn-based NPs became the most abundant (>75% of particles in ash; Figures 3A,B). Pb NPs also rose in abundance, though their ultrafine size limited their mass contribution.

Fire exposure influenced NP mass (Figure 4; Supplementary Figure S6) and size distributions (Figure 5). Mean Fe NP diameters decreased from 240 nm (unburned) to 160 nm (ash-rich), Mn from 120 to 100 nm, and Zn from 119 to 90 nm. Pb NPs averaged 50–60 nm, while Ce remained 50–60 nm, with smaller particles likely below detection limits. Particle sizes were derived from NP mass fractions assuming spherical geometry and the most common oxide forms for each element (e.g., Fe 2 O 3, TiO 2, MnO 2, ZnO, CeO 2, PbO; see Supplementary Table S4 for densities, detection limits and calculation details).

Post-fire elemental redistribution

The major-element patterns reflect two key processes: volatilization losses of mobile elements and accumulation of ash-derived nutrients. Sodium exhibits the most pronounced depletion in burned soils, consistent with its volatilization at >400 °C and removal during post-fire rainfall (Certini, 2005; Pereira et al., 2014). In contrast, nutrients like Ca, P, and Mg show enrichment in ash-rich soils, indicating concentration of mineral residues from burned biomass (Bodí et al., 2014; Roshan and Biswas, 2023). Sulfur and K display moderate enrichment, while refractory elements Al and Fe remain relatively stable. The high Fe (78 g/kg) and Al (83 g/kg) levels in ash are at the upper end of global wildfire-ash observations (Bodí et al., 2014; Roshan and Biswas, 2023), implying the fire was intense enough to involve mineral soil layers. Fe and Al likely underwent solid-state transformations and vapor-phase transport before re-condensing in ash (Mukasyan and Rogachev, 2017). Intense burning also mixes Fe- and Al-rich deeper soil into surface ash, contributing to their post-fire enrichment. In fact, our ash Ca and P concentrations fall within global wildfire-ash ranges (Sánchez-García et al., 2023), and the pronounced Na loss is consistent with ash leaching observed in other fires (Balfour and Woods, 2013). The Fe and Al contents in our ash samples slightly exceed typical values, reflecting the intense combustion of mineral soil components in this fire.

Nanoparticle generation mechanisms

The sharp rise in NP production at 550 °C indicates a thermal threshold for nanoparticle formation, consistent with temperature-dependent metal volatilization and condensation kinetics (Sellmann et al., 2019). Higher temperatures provide the activation energy to overcome kinetic barriers, releasing metals as vapours that nucleate into particles. In our controlled burns (with minimal dispersion), this resulted in abundant NPs at 550 °C, highlighting how flame temperature and duration govern NP yields. Wildfires likely generate NPs through two processes: (1) thermal fragmentation of existing minerals into nanoscale debris, and (2) redox-driven nucleation of new particles from volatilized metals at high temperature (Lassenberger et al., 2017; Cotin et al., 2022). The strongly reducing conditions in burning organic matter (Huang et al., 2016) promote reduction of metals like Fe 3+ and Mn 4+ to more volatile forms, which then co-precipitate with other elements upon cooling (Doerr and Cerdà, 2005). This mechanism likely contributed to the formation of complex Fe–Mn–Ti particles observed only under the hottest conditions (Supplementary Figures S4, S7), although the multimetallic character of these particles may also have been reinforced by processes occurring during ash formation.

Nanoparticle size dynamics

It is counterintuitive that smaller NPs were found after high-temperature exposure, since heat usually coarsens particles (Granqvist and Buhrman, 1976). However, our results indicate that combustion-induced fragmentation and rapid quenching dominated over sintering. Fast cooling of the post-fire aerosol likely preserved many particles at small sizes before they could grow (Strobel and Pratsinis, 2007). Also, volatilization of labile components left behind smaller, refractory particle cores, boosting the surface-to-volume ratio of the NP population (Neary et al., 1999). These ultrafine particles are more chemically reactive and more easily transported by wind or water.

Monometallic to multimetallic shift

Wildfire-induced changes in NP composition reflect the differing thermal stability of soil minerals. TiO 2 is highly refractory and remains largely in bulk form under fire (Hu et al., 2020), whereas manganese-bearing minerals decompose and reduce at lower temperatures (Lee et al., 1978). Combustion of biomass also releases Mn, producing Mn oxide NPs (Davar et al., 2009). Consequently, fire-affected soils become dominated by Mn-rich NPs, often in multimetallic forms (Supplementary Figure S8). Manganese oxides can incorporate other metals through surface reactions, facilitating multimetal nanoparticle assembly in ash (Lee et al., 1978; Cotin et al., 2022). This process likely contributed to the predominance of Mn-based multimetallic NPs in the ash from our study, although additional processes related to ash formation and particle association may also have influenced their final composition. Meanwhile, Ti-based nanoparticles nearly vanished in the ash despite Ti’s high melting point. This disappearance suggests that Ti was incorporated into larger crystalline phases or glassy slags under prolonged heating (Hanaor and Sorrell, 2010), removing it from the nanoparticle size fraction. Likewise, Ce-bearing NPs decreased significantly in ash-rich soils, likely because Ce was oxidized to Ce(IV) and crystallized as larger CeO 2 particles (Janoš et al., 2014), which fall outside the detection range of our NP analysis.

Nanoparticles as fire severity proxies

Changes in NP metrics across burned versus unburned soils hint at novel indicators of fire severity. We observed systematic shifts in elemental ratios of bimetallic NPs with increasing burn intensity (Figure 6; Supplementary Figure S9). For example, the Fe/Mn ratio in Fe–Mn particles dropped from ∼18 (unburned) to ∼4 (ash-rich soils), reflecting Mn enrichment under higher severity (Figure 6A). Similarly, Fe/Ti and Fe/Zn ratios distinguished lower vs. higher intensity conditions (with Fe/Ti ratios of 6–20 in field samples versus 2–4 in controlled burns; Supplementary Figure S9). The proportion of multimetallic particles also rose from <5% to 18% as fire severity increased. These nanoparticle signatures could complement traditional burn severity indices based on vegetation and soil observations (Keeley, 2009). Because nanoparticles persist in soils long after the fire, they provide a durable geochemical fingerprint (illustrated in Figure 7) of fire intensity that could aid in post-fire assessment and restoration efforts. Because traditional metrics often struggle to relate fire behavior to ecological impact (Keeley, 2009), these nanoscale indicators could provide a more direct measure of fire severity to inform post-fire remediation and policy.

Environmental implications

The post-fire proliferation of fine NPs may influence soil stability and contaminant dynamics. Smaller particles contribute less to soil aggregation, potentially weakening soil structure and increasing erosion (Shakesby and Doerr, 2006). On the other hand, more numerous fine particles provide greater surface area for adsorption of pollutants, which can either immobilize or disperse contaminants. Wildfire-generated NPs span a range of elements (Fe, Mn, Ti, Zn, Ce, Pb; Supplementary Figure S10) and include hazardous species. For instance, Cr(VI)-bearing nanoparticles have been identified in wildfire ash (Lopez et al., 2023). The ultrafine Pb and Zn NPs observed in our soils may be highly mobile and bioavailable; such particles can be readily inhaled or absorbed by organisms, with evidence for lung inflammation and systemic uptake (Moreno-Ríos et al., 2022; Smutná et al., 2022). Ash-driven alkalinity can slow metal NP dissolution, extending their persistence, while complexes with organic matter may stabilize NPs but modulate their bioavailability (Tipping, 2002). Moreover, interactions with pyrogenic organic matter can alter metal speciation and enhance toxicity (Bodí et al., 2014; Sánchez-García et al., 2023). Inhalation of wildfire smoke rich in NPs has been shown to trigger stronger inflammatory and toxic responses than equal doses of larger particles (Rager et al., 2021). These findings highlight nanoscale fire residues as a potentially important component of post-fire environmental risk. However, because this study is based on a single wildfire-affected site in central Chile, uses a field sampling design with limited replication, and does not include comparisons across other fire-prone ecosystems, the patterns reported here should not be directly generalized to all fire-prone environments. Instead, they are better interpreted as a site-specific but mechanistically informative case study. Nevertheless, our results support incorporating nanoparticle monitoring into post-fire soil and water assessments to better identify contamination hotspots and guide remediation efforts.

Wildfire substantially modified soil geochemistry in the Cantillana system at both bulk and nanoparticle scales. At the bulk level, burned and ash-affected soils showed depletion of volatile elements and enrichment of less volatile components, indicating major post-fire elemental redistribution linked to ash inputs, mineral soil incorporation, and high-temperature alteration. Evidence of carbonate decomposition and subsequent mineral rehydration further supports the strong impact of fire on soil mineral transformations and post-fire weathering processes.

At the nanoparticle scale, both field observations and controlled combustion experiments showed that increasing temperature promoted a marked rise in NP abundance together with a shift from predominantly monometallic to multimetallic particles. In particular, Fe-, Mn-, and Ti-bearing nanoparticles became more compositionally complex after burning, while the reduction in NP size and mass suggests that volatilization and fragmentation were important controls on post-fire nanoparticle formation. These results support temperature as a key driver of nanoparticle redistribution and compositional change after wildfire.

The post-fire enrichment of fine Pb- and Zn-bearing nanoparticles also suggests an increased potential for metal mobility and environmental exposure in burned soils. In addition, bimetallic nanoparticle ratios such as Fe/Mn and Fe/Zn may provide useful geochemical indicators of wildfire severity. Although this study is based on a single wildfire-affected site, it shows that coupling bulk geochemistry with nanoscale characterization can provide a more complete understanding of post-fire soil transformations and may improve the assessment of wildfire impacts on soil and water systems.