Influence of Iron Mineralogy on Reactive Nitrogen Gas Emissions from Soil

Iron exists abundantly in soil in multiple oxidation states and mineral forms, and is the dominant redox-active metal. Recent evidence suggests iron chemistry plays a central role in producing volatile reactive nitrogen oxides (NO_y = NO + NO₂ + HONO + other N-oxides), which are key air pollutants contributing to tropospheric ozone formation, acid deposition, and respiratory health risks. Current atmospheric models do not accurately represent soil NO_y fluxes, particularly HONO and NO₂, because biogenic production mechanisms remain poorly characterised. In moist soils, HONO is primarily produced by bacterial and archaeal ammonia oxidisers. However, as soils dry, nitrite and nitrate can accumulate and pH often decreases, potentially favouring abiotic nitrate reduction as a critical HONO source.

To investigate how soil iron mineralogy, concentration, and speciation influence NO_y emissions during drying, HONO and NO₂ fluxes are measured over 24 hours using an ICAD-HONO/NO₂-210L system (Airyx GmbH) from soil microcosms amended with three iron oxides (ferrihydrite, goethite, magnetite) at two concentrations (3.5% and 4.5% total iron). Microbial community responses will also be assessed via quantitative PCR targeting nitrogen cycling genes. We hypothesise that: 1) soil drying will increase nitrate accumulation and lower pH, leading to HONO fluxes that peak at intermediate moisture when thin water films enable reactions but allow gas diffusion; 2) ferrihydrite-amended soils will exhibit the highest NO_y emissions due to its Fe³⁺ reduction potential, transition metal adsorption, and reactive oxygen species generation; and 3) NO_y emissions will increase with iron oxide concentration, although high concentrations may suppress microbial activity via metal toxicity.

Preliminary results show substantial HONO emissions from all iron amendments in neutral soils (~pH 7), followed by a steep decline as soils dry. A late NO₂ peak was observed, possibly due to physical release during drying or shifts in microbial pathways. Increasing goethite concentrations correlated with higher HONO emissions, whereas ferrihydrite showed a negative correlation. Future work will examine the pH modulation of the relationship between iron and NO_y fluxes by repeating flux measurements in acidic soils (~pH 5.5), where we expect enhanced nitrite protonation, and altered iron solubility and redox activity.