Soil Nitric Oxide Emissions and Air Quality Impact

Soil nitric oxide (NO) emissions arise chiefly from microbial nitrification and denitrification processes in natural and managed ecosystems. These emissions contribute substantially to the global nitrogen oxide (NOx) budget—estimated at around 10–15 % of total NOx—and exert a pronounced influence on tropospheric chemistry. Once released, NO reacts with atmospheric oxidants to form nitrogen dioxide (NO2), ozone (O3) and secondary particulate matter, all of which have deleterious effects on human health, vegetation and climate forcing. Emission rates are highly responsive to soil temperature, moisture and nitrogen availability, leading to episodic “pulsing” events after rainfall or irrigation. Agricultural fertiliser regimes, land-use change and climate warming all modulate soil NO fluxes, complicating efforts to predict trends in background NOx and ozone formation. Improved mechanistic parameterisations and satellite retrievals have advanced our understanding of spatial and temporal variability, yet substantial uncertainties remain in regional budgets and emission controls. Practical applications include refining air quality models, informing fertiliser management to mitigate NO release and integrating soil emissions into comprehensive ozone abatement strategies.

Research from Nature Portfolio

Recent studies employing mechanistic parameterisations in a major agricultural plain have demonstrated that soil NOx emissions can substantially undermine ozone reduction targets. In this region, soil-derived NOx comprises a significant fraction of total emissions during peak growing seasons, reducing the efficacy of anthropogenic control measures and necessitating revised mitigation approaches that account for biogenic sources. Another investigation in a high-temperature agricultural basin revealed some of the highest soil NO fluxes observed to date. These fluxes exhibit a non-linear response to fertiliser application, soil moisture and temperature, and were under-represented in conventional air chemistry models. Adjusting model inputs to match field observations elevated simulated tropospheric ozone, underscoring the potential for targeted management practices—such as optimised fertiliser timing and moisture control—to improve regional air quality.

Soil Nitric Oxide Emissions and Air Quality Impact publication trend

The graph below shows the total number of articles in soil nitric oxide emissions and air quality impact across all publications each year (not limited to Nature Index journals).

Technical terms

Soil nitric oxide (NO): A reactive nitrogen gas produced by microbial nitrification and denitrification in soils.

Nitrogen oxides (NOx): The sum of nitric oxide and nitrogen dioxide, key precursors to ozone and secondary particulate matter.

Ozone (O3): A toxic oxidant formed in the lower atmosphere by photochemical reactions involving NOx and volatile organic compounds.

Nitrification: Microbial oxidation of ammonium (NH4⁺) to nitrate (NO3⁻), releasing NO as a by-product.

Denitrification: Microbial reduction of nitrate to gaseous nitrogen species, including NO and nitrous oxide (N2O).

Parameterisation: A simplified mathematical representation of complex biogeochemical processes within models.

Pulsing: Short-lived spikes in emissions triggered by rapid changes in soil moisture or temperature.

Tropospheric column density: The total amount of a trace gas present in a vertical column of the troposphere.

Soil moisture index: A normalised measure of soil water content relative to its minimum and maximum values.

References

1. Mechanistic representation of soil nitrogen emissions in the Community Multiscale Air Quality (CMAQ) model v 5.1. Geoscientific Model Development (2019).
2. Daily Cropland Soil NOx Emissions Identified by TROPOMI and SMAP. Geophysical Research Letters (2020).
3. The underappreciated role of agricultural soil nitrogen oxide emissions in ozone pollution regulation in North China. Nature Communications (2021).
4. Unusually high soil nitrogen oxide emissions influence air quality in a high-temperature agricultural region. Nature Communications (2015).
5. Satellite NO2 trends reveal pervasive impacts of wildfire and soil emissions across California landscapes. Environmental Research Letters (2023).
6. Insights into soil NO emissions and the contribution to surface ozone formation in China. Atmospheric Chemistry and Physics (2023).
7. Sensitivity of Modeled Soil NOx Emissions to Soil Moisture. Journal of Geophysical Research: Atmospheres (2023).