Wildfires forge hidden nanoparticles that reshape soil chemistry

When a wildfire tears through a forest, the destruction is visible for kilometers: charred trunks, gray ash, blackened soil. But according to a new study from central Chile, the most consequential changes may be invisible to the naked eye. Fire, it turns out, is a nanoparticle factory, and the tiny particles it produces could reshape how scientists understand post-fire contamination, soil recovery, and even human health risks.

The research, led by Cristian Suarez-Oubiña of Université Paris Cité and colleagues, examined soil from the Cantillana Nature Reserve, a fire-prone Mediterranean ecosystem about 60 kilometers southwest of Santiago. The team combined field sampling seven months after a December 2023 wildfire with controlled laboratory combustion experiments and a cutting-edge analytical technique called single-particle inductively coupled plasma time-of-flight mass spectrometry, or spICP-TOFMS. Their findings, published in Frontiers in Environmental Chemistry, reveal that fire exposure can increase nanoparticle concentrations in soil by more than an order of magnitude, reaching up to 1.4 billion particles per gram in ash-enriched soils.

Nanoparticles are defined as particles between roughly 1 and 100 nanometers in diameter, small enough that a single gram of them presents an enormous surface area for chemical reactions. They occur naturally in volcanic ash, dust storms, and weathering minerals, but wildfires introduce a particularly hazardous variety. As vegetation and soil minerals burn, elements like iron, manganese, titanium, zinc, lead, and cerium are thermally fragmented, volatilized, and re-condensed into nanoscale particles. These particles are small enough to be carried by wind and water far from the fire itself, and their high surface-to-volume ratio makes them unusually reactive in biological systems.

The health implications are significant. Metal oxide nanoparticles such as zinc oxide and manganese dioxide are known to generate reactive oxygen species, molecules that damage cells and trigger inflammation. Iron oxide nanoparticles, including magnetite, have been linked to neurodegenerative disease in previous research. Cerium oxide nanoparticles can cause oxidative stress, genotoxicity, and inflammation due to their redox activity. Even relatively benign iron and manganese oxides, such as those found in welding fumes, provoke lung inflammation at elevated concentrations. Wildfire smoke rich in nanoparticles has been shown to trigger stronger inflammatory and toxic responses than equal doses of larger particles.

In the Chilean field samples, unburned soil contained between 10 million and 100 million nanoparticles per gram, over 95 percent of which were single-element particles, mostly titanium. After the fire, the picture changed dramatically. Iron-bearing nanoparticles increased from 13 million per gram in unburned soil to 43 million per gram in ash-rich soil. Manganese-bearing nanoparticles rose tenfold, from 21 million to 220 million per gram. Lead and zinc nanoparticles, negligible before the fire, appeared in substantial quantities afterward, with lead peaking at 27 million per gram in burned soils. The proportion of multimetallic particles, those containing two or more elements, increased from less than 5 percent in unburned soil to 18 percent in ash.

Laboratory experiments confirmed these patterns and identified temperature as the key driver. When the team heated unburned Cantillana soil at 400 degrees Celsius, mimicking low-intensity surface fires, and at 550 degrees Celsius, mimicking intense crown fires, nanoparticle production increased sharply with temperature. At 550 degrees Celsius, iron and manganese nanoparticles reached 256 million and 213 million per gram respectively, while lead, zinc, and cerium nanoparticles also peaked. The hottest conditions favored complex multimetallic particles such as iron-manganese-titanium and iron-zinc combinations, suggesting that high temperatures provide the activation energy needed to volatilize metals as vapors that then nucleate into new particles upon cooling.

One counterintuitive finding was that nanoparticles became smaller, not larger, after fire exposure. Mean iron nanoparticle diameters decreased from 240 nanometers in unburned soil to 160 nanometers in ash-rich soil, while zinc nanoparticles shrank from 119 to 90 nanometers. Lead nanoparticles averaged just 50 to 60 nanometers. The researchers attribute this to combustion-induced fragmentation and rapid quenching of the post-fire aerosol, which preserved particles at small sizes before they could coarsen through sintering. Volatilization of labile components also left behind smaller, refractory particle cores. These ultrafine particles are more chemically reactive and more easily transported by wind or water than their larger counterparts.

The study also uncovered a novel geochemical fingerprint of fire severity. The ratio of iron to manganese in bimetallic nanoparticles dropped from approximately 18 in unburned soil to approximately 4 in ash-rich soils, reflecting manganese enrichment under higher burn intensity. Similar shifts appeared in iron-titanium and iron-zinc ratios. Because nanoparticles persist in soils long after the flames are gone, these ratios could serve as durable indicators of past fire intensity, complementing traditional burn severity assessments based on vegetation and soil observations. Such nanoscale indicators might eventually help guide post-fire remediation and policy decisions.

Environmental risks extend beyond inhalation. The proliferation of fine nanoparticles after fire may weaken soil aggregation and increase erosion, while also providing greater surface area for pollutant adsorption. Ultrafine lead and zinc nanoparticles may be highly mobile and bioavailable, readily absorbed by organisms. Ash-driven alkalinity can slow metal nanoparticle dissolution, extending their persistence in the environment, while interactions with pyrogenic organic matter can alter metal speciation and enhance toxicity. Runoff from burned watersheds can transfer these particles into surface waters, potentially compromising water treatment systems and threatening downstream water quality.

The authors caution that their study is based on a single wildfire-affected site in central Chile with limited replication, so the patterns should not be directly generalized to all fire-prone environments. Still, the work demonstrates that coupling bulk geochemistry with nanoscale characterization provides a more complete understanding of post-fire soil transformations. As climate change and land-use practices drive wildfires to increase in frequency and intensity globally, from the Mediterranean Basin to California to Chile, understanding the nanoscale consequences of fire becomes increasingly urgent. The invisible particles left behind may be among the most enduring legacies of the flames.

Subject of Research: Wildfire-driven geochemical transformations from soils to pyrogenic nanoparticles

Article Title: Wildfire-driven geochemical transformations from soils to pyrogenic nanoparticles

Article References: Suarez-Oubiña, C., Tharaud, M., Taraconat, N., Michot, V., Rojas, C., & Benedetti, M. F. (2026). Wildfire-driven geochemical transformations from soils to pyrogenic nanoparticles. Frontiers in Environmental Chemistry, 7, Article 1797359. https://doi.org/10.3389/fenvc.2026.1797359

Image Credits: AI Generated

DOI: 10.3389/fenvc.2026.1797359

Keywords: Wildfire-driven, geochemical, transformations, soils, pyrogenic, nanoparticles, scientific research