Nitrous oxide ($\text{N}_{2}\text{O}$) is a naturally occurring compound that acts as both a potent greenhouse gas and a substance that depletes the ozone layer. Though its atmospheric concentration is low, $\text{N}_{2}\text{O}$ is roughly 300 times more effective at trapping heat than carbon dioxide, making it the third most significant long-lived greenhouse gas. The gas has an atmospheric lifetime of approximately 114 to 120 years, allowing it to migrate to the stratosphere where it breaks down the protective ozone layer. Natural production of $\text{N}_{2}\text{O}$ originates almost entirely from the global cycling of nitrogen, a biological process driven by microorganisms in terrestrial and aquatic environments.
Microbial Factories in the Soil
Natural soils are the single largest source of $\text{N}_{2}\text{O}$ emissions, with undisturbed terrestrial ecosystems contributing approximately 60% of the total natural flux. The gas is released as an unintended byproduct of microbial metabolism driving the nitrogen cycle, specifically through denitrification and nitrification. These reactions are carried out by various bacteria and archaea that use nitrogen compounds for energy.
Denitrification is the dominant pathway for $\text{N}_{2}\text{O}$ production in soils. It occurs when facultative anaerobic bacteria reduce nitrate ($\text{NO}_{3}^{-}$) to dinitrogen gas ($\text{N}_{2}$) in a stepwise process. $\text{N}_{2}\text{O}$ is an intermediate in this chain, and its release is favored when the final enzyme, $\text{N}_{2}\text{O}$ reductase, is inhibited or inactive. Denitrification is promoted by low oxygen levels, which often occur in water-saturated soil microsites.
Nitrification, conversely, is an aerobic process where ammonia-oxidizing bacteria and archaea convert ammonium ($\text{NH}_{4}^{+}$) to nitrate ($\text{NO}_{3}^{-}$). $\text{N}_{2}\text{O}$ is released as a side-product when intermediate compounds, such as hydroxylamine, are unstable during the oxidation steps. Although nitrification is aerobic, the $\text{N}_{2}\text{O}$ yield increases under slightly oxygen-limited or fluctuating moisture conditions. The interplay between these two processes, known as coupled nitrification-denitrification, is a major factor in $\text{N}_{2}\text{O}$ emission from natural soils.
Production in Oceans and Waterways
Oceans and associated waterways are the second most significant natural source of nitrous oxide, contributing roughly 35% of the natural flux. $\text{N}_{2}\text{O}$ production in aquatic systems is driven by microbial processes, including nitrification in the water column and denitrification in oxygen-depleted zones and sediments. The majority of the $\text{N}_{2}\text{O}$ produced in the open ocean results from nitrification, where ammonia is oxidized in the subsurface layers.
A substantial portion of marine $\text{N}_{2}\text{O}$ flux originates from Oxygen Minimum Zones (OMZs), which are layers where dissolved oxygen levels plummet to near zero. In these hypoxic or anoxic environments, the microbial community switches to using other compounds as electron acceptors, favoring the denitrification pathway. This process involves the reduction of nitrate, producing $\text{N}_{2}\text{O}$ as an intermediate before it is fully reduced to $\text{N}_{2}$ gas.
$\text{N}_{2}\text{O}$ often accumulates in OMZs because low oxygen levels enable its production, but conditions are not always sufficiently anoxic to complete the reduction to $\text{N}_{2}$ gas. When these waters mix or upwell to the surface, the dissolved $\text{N}_{2}\text{O}$ is released into the atmosphere. Freshwater systems, estuaries, and coastal vegetation also contribute minor amounts to the overall aquatic budget through similar microbial mechanisms.
Minor Natural Contributors
Beyond the large-scale microbial processes in soil and water, a few smaller, non-biological processes naturally contribute to the atmospheric $\text{N}_{2}\text{O}$ budget. Natural biomass burning, such as wildfires sparked by lightning, releases $\text{N}_{2}\text{O}$ through the incomplete combustion of organic matter. Nitrogen compounds contained within the vegetation and soil are partially converted to $\text{N}_{2}\text{O}$ during the burning process.
Atmospheric chemical reactions also serve as a minor source, contributing a small fraction of total natural emissions. For example, lightning discharges provide the high energy needed to initiate the chemical fixation of atmospheric nitrogen and oxygen, leading to the formation of nitrogen oxides that eventually result in $\text{N}_{2}\text{O}$ production. The total contribution from these smaller sources, including natural inland waters and atmospheric production, accounts for about 4% of the natural emissions.
The Natural Nitrous Oxide Budget
The total natural flux of $\text{N}_{2}\text{O}$ from all sources—soil, ocean, and minor contributors—was historically in balance with natural removal processes, maintaining a stable pre-industrial atmospheric concentration. This natural flux is estimated to fluctuate around 11.7 to 12.1 teragrams of nitrogen per year ($\text{Tg N yr}^{-1}$). The primary natural sink for atmospheric $\text{N}_{2}\text{O}$ is its destruction in the stratosphere through photolysis, where high-energy ultraviolet radiation breaks the molecule apart.
For millennia, the rate of natural production was matched by stratospheric destruction, keeping the atmospheric concentration stable at approximately 270 parts per billion (ppb). However, current emissions from human activities have fundamentally disrupted this long-term balance. Total annual anthropogenic emissions have risen significantly, now exceeding the atmosphere’s natural ability to remove the compound. This has resulted in a nearly 25% increase in atmospheric $\text{N}_{2}\text{O}$ concentration since the pre-industrial era.