A hidden chemical engine beneath our feet may be helping transform ordinary soil into a source of atmospheric pollution. In a study published in Nature Communications, Purchase, Raff, Wang and colleagues report that microbes can generate superoxide, a highly reactive oxygen species, in soil and that this activity drives the formation of biologically produced nitrogen dioxide. The finding connects two processes that are usually studied separately: microbial metabolism in the ground and the atmospheric chemistry of nitrogen oxides. It suggests that soil is not merely a passive surface that stores nutrients and carbon, but an active chemical reactor capable of producing compounds that influence air quality, climate processes and human health.
Nitrogen dioxide, or NO₂, is best known as a pollutant released when fossil fuels burn. Vehicle engines, power stations and industrial combustion generate nitrogen oxides at high temperatures, and these emissions are central to urban smog. Once in the atmosphere, NO₂ participates in reactions that produce ozone and other oxidants, while also contributing to respiratory disease and the formation of fine particles. Yet combustion is not the only pathway to nitrogen dioxide. Soils contain vast microbial communities and large reservoirs of nitrogen, creating the possibility that biological chemistry may generate nitrogen oxides even in landscapes far from roads, factories and power plants.
The new research focuses on superoxide, a molecule that carries an unpaired electron and therefore reacts rapidly with surrounding compounds. Chemically, superoxide is often written as O₂⁻, although its behavior in soil depends on acidity, moisture, mineral surfaces and the presence of other reactive molecules. Microorganisms produce superoxide as a by-product of respiration and other redox reactions, particularly when electrons are transferred incompletely to oxygen. In living cells, antioxidant systems normally control it. Outside cells or in chemically complex soil environments, however, superoxide can initiate chains of reactions involving iron, carbon compounds and nitrogen-containing molecules.
The researchers’ central conclusion is that microbial superoxide provides a direct chemical route toward biogenic nitrogen dioxide formation. In this pathway, microbial activity supplies the reactive oxygen species, while nitrogen compounds in the soil provide the chemical material needed to form nitrogen oxides. The result is a bridge between the oxygen chemistry of microbes and the nitrogen cycle that governs the movement and transformation of one of Earth’s most important elements. Rather than treating nitrogen dioxide as a product generated only by combustion or by a single specialized microbial enzyme, the study highlights how a short-lived reactive molecule can reorganize the surrounding chemistry and produce an atmospheric compound.
This mechanism is significant because superoxide does not need to accumulate to have a major effect. It may exist for only brief periods before reacting, but even transient molecules can control chemical pathways when they are produced continuously at microscopic sites. Soil is filled with such sites: thin water films around particles, microbial membranes, decaying organic matter, mineral surfaces and microscopic pores where oxygen and nitrogen compounds meet. In these confined environments, the distance between a reactive oxygen species and a potential nitrogen-containing reactant can be extremely small. Repeated microbial production may therefore create a sustained chemical flux, even when the concentration of superoxide measured across an entire soil sample appears low.
The discovery also adds complexity to the biological nitrogen cycle. Soil microbes convert nitrogen between forms through processes including nitrification, denitrification, ammonification and nitrogen fixation. These transformations determine whether nitrogen remains available to plants, is stored in organic matter, escapes as a gas or moves into waterways. Nitrogen dioxide is only one possible product, but its formation matters because it can leave the soil and enter the atmosphere. In air, nitrogen dioxide absorbs sunlight and participates in reactions that help generate hydroxyl radicals and ozone. It can also react with water and other atmospheric compounds, contributing to nitric acid and particulate pollution. A process beginning in a microbial habitat can therefore have consequences well beyond the soil profile.
The work may be especially relevant for agricultural systems, where nitrogen inputs, irrigation, tillage and organic amendments alter microbial activity and soil chemistry. Fertilizers increase the supply of nitrogen substrates, while wetting and drying cycles change oxygen availability and the distribution of reactive compounds. Crop residues and manure add carbon that can stimulate microbial metabolism, potentially changing the production of superoxide and the fate of nitrogen. These factors do not mean that every fertilized field will emit the same amount of nitrogen dioxide, but they indicate why emissions could vary sharply with soil type, temperature, moisture and management. The study points toward a need to evaluate biological nitrogen dioxide production under realistic environmental conditions rather than assuming that nitrogen oxide inventories can be based primarily on combustion sources.
The findings could also help explain why nitrogen oxide measurements sometimes differ from predictions based on conventional emission models. Atmospheric models commonly account for major industrial, transportation and soil microbial sources, but the chemical details assigned to soils may not capture every rapid reaction occurring at microbial and mineral interfaces. If superoxide-driven chemistry contributes meaningfully to nitrogen dioxide production, the strength of that source may depend on variables that are difficult to represent at large scales. Soil texture, pH, mineral composition, microbial community structure and recent rainfall could all influence the pathway. Incorporating such factors will require measurements that connect molecular reactions in soil microsites with nitrogen dioxide fluxes across fields, ecosystems and seasons.
The research does not suggest that microbial soil emissions replace fossil-fuel pollution as the dominant source of nitrogen dioxide in cities. Instead, it reveals an additional natural and potentially human-amplified mechanism. Its importance may be greatest in regions with extensive agricultural activity, high nitrogen availability or environmental conditions that stimulate microbial metabolism. Understanding the relative contribution of this pathway will require further work, including direct measurements of superoxide production, isotope tracing of nitrogen dioxide, and experiments that separate biological activity from purely chemical reactions on soil minerals. Such studies could determine when microbes are the primary drivers, how long the process continues after soil disturbance and whether land management can reduce the resulting emissions.
The broader message is that the atmosphere begins in places that may appear chemically quiet. A forest floor, pasture or cultivated field can host billions of microscopic reactions every second, many involving unstable molecules that disappear almost as soon as they form. By identifying microbial superoxide as a driver of nitrogen dioxide formation, the study gives one of those fleeting reactions a larger environmental role. It shows that the nitrogen cycle is coupled not only to microbial enzymes and greenhouse gases, but also to reactive oxygen chemistry with consequences for air pollution. As scientists work to predict how soils respond to warming, changing rainfall and rising nitrogen inputs, this newly illuminated pathway may become an important piece of the planet’s atmospheric puzzle.
Subject of Research: Microbial superoxide production and biologically driven nitrogen dioxide formation in soils
Article Title: Microbial superoxide production drives biogenic nitrogen dioxide formation in soils
Article References: Purchase, M.L., Raff, J.D., Wang, D. et al. “Microbial superoxide production drives biogenic nitrogen dioxide formation in soils.” Nature Communications (2026). https://doi.org/10.1038/s41467-026-76881-x
Image Credits: AI Generated
DOI: 10.1038/s41467-026-76881-x
Keywords: Soil microbiology, superoxide, nitrogen dioxide, nitrogen cycle, atmospheric chemistry, microbial emissions, air pollution, reactive oxygen species, biogenic nitrogen oxides
