Nitrous oxide (N2O) is a naturally occurring gas found in Earth’s atmosphere. It plays a role in the natural biogeochemical cycles, particularly the nitrogen cycle. While human activities have increased its atmospheric concentration, natural processes consistently release N2O into the atmosphere.
The Microbial Engines of Natural Nitrous Oxide
The primary biological processes responsible for natural N2O production are nitrification and denitrification, carried out by diverse microorganisms. Nitrification involves the oxidation of ammonium (NH4+) to nitrite (NO2-) and then to nitrate (NO3-), primarily by ammonia-oxidizing bacteria (AOB) and archaea (AOA). During this process, N2O can be released as a byproduct, particularly under conditions of fluctuating oxygen levels or when ammonia oxidation rates are high.
Denitrification is a sequential process where nitrate (NO3-) is reduced to dinitrogen gas (N2) through a series of intermediates, including N2O. This process occurs in anaerobic or oxygen-limited environments where microbes use nitrate as an electron acceptor. When denitrification is incomplete, N2O can escape into the atmosphere instead of being fully converted to harmless N2. Nitrifier denitrification, a specific pathway within nitrification, also contributes to N2O production as ammonia-oxidizing bacteria reduce nitrite to N2O and N2 under anoxic conditions. The relative contribution of nitrification and denitrification to N2O production varies, with nitrification dominating under lower moisture conditions and denitrification under higher moisture conditions.
Nitrous Oxide from Terrestrial Environments
Land-based ecosystems are a major natural source of N2O, with soils under natural vegetation contributing a substantial portion of these emissions. The microbial processes of nitrification and denitrification occur widely in soils, leading to N2O release. Uncultivated soils cover a large part of the Earth’s land surface, and the N2O produced by the microbes within them contributes to natural emissions.
Factors such as soil moisture, temperature, and nutrient availability significantly influence N2O production in these terrestrial environments. N2O emissions increase with higher soil temperatures, reflecting increased microbial activity, up to around 104°F (40°C). Soil moisture also plays a substantial role; emissions can be elevated when soils are saturated for extended periods, creating anaerobic conditions favorable for denitrification. Soils with higher organic matter content exhibit greater N2O emissions due to more available carbon for microbial activity and increased nitrogen mineralization.
Nitrous Oxide from Aquatic Environments
Water bodies also contribute to natural N2O emissions, with similar microbial processes driving its production. Oceans are a significant natural source of N2O, where microbes in both surface waters and deeper oxygen-minimum zones engage in nitrification and denitrification. Oceanic N2O emissions are estimated to contribute between 10% and 53% of the global total from aquatic sources.
Freshwater systems, including lakes, rivers, and estuaries, also release N2O through microbial activity. Similar to terrestrial environments, the levels of oxygen and nutrient loads in aquatic systems influence N2O emissions. Low oxygen conditions can promote denitrification, leading to N2O release, while elevated nutrient inputs can stimulate microbial activity and increase N2O production. Some aquatic animals can emit N2O, attributed to denitrification by ingested bacteria within their anoxic guts.