The term greenhouse gas often brings to mind carbon dioxide (CO ₂ ), and rightly so, as it is a key contributor to global warming. However, a more potent pollutant and greenhouse gas A gas that is often overlooked is nitrous oxide (N ₂ O). Molecule for molecule, N ₂ O is 300 times stronger than CO ₂ and it is accumulating in the atmosphere faster than expected.

A recent study by researchers from Denmark and Spain has identified a new abiotic (non-biological) pathway for N production. ₂ O in surface waters, called photochemodenitrification, driven by sunlight. They found that this process produces N ₂ At a higher rate than biological methods such as ammonia oxidation, which was previously considered the primary source of N emissions ₂ O in surface waters.

The study includes experiments carried out in two freshwater reservoirs in southeastern Spain (Cubillas and Iznájar) and two marine coastal areas (Motril on the coast of Spain and Boknis Eck in the southwestern Baltic Sea).

Study methodology:

• Study locations: The experiments were carried out in the eutrophic reservoirs of Cubillas and Iznájar, which receive high amounts of nutrients from agricultural and urban areas. Also included were the coastal waters of Motril in the Alboran Sea, considered the most productive area of the Mediterranean, and the time series of the Boknis Eck station in the southwestern Baltic Sea, which has been monitored for a long time.
• Experiments to determine N2O production: Eight incubation experiments (experiments 1 to 8) were performed to test the effect of solar radiation on abiotic N2O production under natural oxic conditions. In experiments 7 and 9, biological N2O production was also quantified.
  • Experiments 1 to 3: Abiotic N2O production without addition of tracer (unfiltered water) in UV- and visible-transparent glass ball bottles and in dark controls with inhibition of biological activity using mercuric chloride (HgCl2). Samples were incubated for one to seven days on a floating platform.
  • Experiment 4: Abiotic N2O production without addition of tracer (filtered water through 0.7 μm filters) in quartz bottles with dark controls and inhibition of biological activity with HgCl2. The concentration of NH4+, NO2- and NO3- was measured together with N2O at different time points. The intensity of solar radiation (UVB and PAR) was also measured.
  • Experiments 5 and 6: Production of ¹⁵N₂O with addition of ¹⁵NO₂⁻ and ¹⁵NO₃⁻ as tracer isotopes in filtered water from the Cubillas and Motril coasts in quartz bottles with dark controls and HgCl2. The formation of ⁴⁵N₂O and ⁴⁶N₂O was monitored.
  • Experiment 7: Production of ¹⁵N₂O with the addition of ¹⁵NO₂⁻ and ¹⁵NO₃⁻ (for abiotic conditions with HgCl2) and ¹⁵NH₄⁺ (for biological conditions) in water from Boknis Eck in quartz bottles with dark controls.
  • Experiment 8: Effect of HgCl2 on the photochemical production of ¹⁵N₂O with the addition of ¹⁵NO₂⁻ in filtered water from Boknis Eck in quartz bottles with dark controls, comparing the presence and absence of HgCl2.
  • Experiment 9: Biological production of ¹⁵N₂O from ammonia oxidation with the addition of ¹⁵NH₄⁺ in the Cubillas and Iznájar reservoirs in the dark at in situ temperature.
• N2O analysis: The concentration of dissolved N2O was determined by gas chromatography (GC) with an electron capture detector. For the ¹⁵N labeling experiments, the concentration of ⁴⁴N₂O, ⁴⁵N₂O and ⁴⁶N₂O and isotopic ratios were measured by GC-IRMS.
• Calculation of production speeds: Photochemical N2O production rates were calculated from the increase in N2O concentration (or excess ¹⁵N-N2O) over the time of exposure to sunlight. Biological production rates were calculated from the increase in ¹⁵N-N2O over the total incubation period. Rates were converted to water volume, bottle surface area, and solar energy received (PAR and UVB).

Key findings:

• Solar radiation drives abiotic N2O formation in both fresh and marine waters.
• In experiments 1 to 4 it was found that significant increase in N2O concentration in irradiated samples compared to dark controls, which confirms the photochemical production of N2O.
• The production of ⁴⁵N₂O and ⁴⁶N₂O from ¹⁵NO₂⁻ and ¹⁵NO₃⁻ was significantly higher in the sun in Cubillas, Motril and (for ¹⁵NO₂⁻) in Boknis Eck.
• Longer exposure to sunlight led to greater N2O formation.
• The increase in ⁴⁵N₂O and ⁴⁶N₂O concentrations was greater when adding ¹⁵NO₂⁻ than when adding ¹⁵NO₃⁻.
• No clear differences were found between freshwater and marine systems, suggesting that the reaction may depend more on substrate availability and sunlight than on system type or salinity.
• The ratio of ⁴⁵N₂O to ⁴⁶N₂O production correlated with in situ availability of ¹⁴N-NO₂⁻ and ¹⁴N-NO₃⁻Higher availability of ¹⁴N promoted the formation of ⁴⁵N₂O.
• Nitrite was reduced to concentrations below the detection limit in irradiated samples, indicating its consumption in the photochemical production of N2O.
• HgCl2 inhibited biological activity but had no consistent effect on photochemical N2O production (Experiment 8). In some cases, HgCl2 slightly reduced the production of ⁴⁵N₂O and ⁴⁶N₂O, suggesting a possible role of microbial surfaces or HgCl2-sensitive processes in abiotic production.
• Photochemical production of N2O was observed in the range of UVB (280–320 nm) and PAR (400–700 nm) radiation.
• In the Cubillas and Iznájar reservoirs, it also contributed biological production of N2O from ammonia oxidation to total N2O concentration, although photochemical production was also significant.

Summary of conclusions:

The study showed that Solar radiation is a significant factor contributing to the abiotic formation of nitrous oxide in various aquatic ecosystems.This process depends on the availability of inorganic forms of nitrogen, particularly nitrite and nitrate, and the intensity of solar radiation. The findings suggest that photochemical production of N2O may be an important, yet under-recognized source of emissions of this important greenhouse gas from water surfaces.

Additional materials:

The document also includes supplementary texts, figures (S1 to S7) and tables (S1 to S8), which provide more detailed information on the experimental setup, results, and statistical analyses. Spring

The work is published in the magazine Science .

Glossary of key terms

• Abiotic: Non-living, relating to physical or chemical processes that are not mediated by living organisms.
• Biotic: Living, relating to processes that are mediated by living organisms.
• Nitrous oxide (N₂O): A potent greenhouse gas and ozone-depleting substance that occurs naturally in the atmosphere, but its concentration is increasing due to human activity.
• Photochemical: Pertaining to chemical reactions that are initiated or accelerated by the absorption of light.
• Isotope marker (tracer): An atom with an unusual number of neutrons, used to trace the path of chemical substances or processes in environmental or biological systems (in this case, ¹⁵N).
• Nitrite (NO₂-): A nitrogen anion that is an intermediate in the nitrogen cycle, such as in nitrification and denitrification.
• Nitrate (NO₃-): The most oxidized form of inorganic nitrogen, an important nutrient in many ecosystems.
• Ammonia (NH₄+): A form of nitrogen that is produced by the decomposition of organic matter and is oxidized in nitrification.
• Freshwater tanks: Artificial water bodies created by damming river flows, often used for water supply, irrigation, and recreation.
• Marine coastal areas: Parts of the ocean that are located near the coast and are influenced by land processes.
• Quantum yield efficiency: The number of molecules that undergo a specific reaction per photon of absorbed light.
• Headspace: The gas space above a liquid sample in a closed container, used for the analysis of dissolved gases.
• Gas chromatograph (GC): An instrument used to separate and quantify individual gaseous components in a mixture.
• Isotope Ratio Mass Spectrometer (IRMS): A type of mass spectrometer used to measure isotope ratios in chemical substances.
• UVB radiation (280 – 320 nm): The part of ultraviolet radiation from the Sun that can have harmful effects on living organisms.
• PAR (photosynthetically active radiation, 400 – 700 nm): The portion of sunlight that plants and other photosynthetic organisms use for photosynthesis.
• Eutrophic: Rich in nutrients, often leading to increased primary production and potential water quality problems.
• Oligotrophic: Poor in nutrients, with low primary production.
• Monomictic: A lake that only mixes once a year, usually during the winter.
• Stratification: The creation of layers of water of different temperatures and densities that prevent mixing.
• Epilimnion: The warmer, well-lit surface layer of a stratified lake.