In this study, Elizabeth Leon-Palmero and colleagues describe an abiotic process that generates significant amounts of nitrous oxide (N₂O). N₂O is both a strong greenhouse gas, and a potent destroyer of stratospheric ozone.  The rapid increase of N₂O concentration in the atmosphere over the last decade underscores the need to better identify the sources of N₂O to reduce uncertainty in climate assessments and optimize mitigation strategies.

The researchers conducted experiments in two freshwater reservoirs in Southeast Spain (in Granada and Córdoba), off the coast of the Mediterranean Sea (Motril, Granada), and in the Baltic Sea (Boknis Eck station, Germany). They detected a consistent and substantial increase in N₂O concentrations, which was correlated with the dose of sunlight received during the experiments. Using isotope tracers, the authors demonstrated that sunlight promotes the abiotic conversion of nitrite to N₂O. They also showed that nitrate contributed to the formation of nitrous oxide after being photoreduced to nitrite. This reaction was named “photochemodenitrification”. In addition, the production rates due to photochemodenitrification were larger than those attributed to microbial ammonia oxidation, the main biological process that produces N₂O in surface waters.

The production of N₂O by photochemodenitrification is greater at the water surface and decreases with depth, because sunlight attenuates in the water column. This suggests that photochemodenitrification could have a disproportionate impact on N₂O fluxes to the atmosphere. This is because N₂O newly formed at the air/water interface can diffuse more rapidly into the atmosphere compared to N₂O produced and stored in the deeper layers of the water column (as typically occurs with biological production).

This new process could significantly contribute to global N₂O emissions in both freshwater and marine surface waters, especially in regions with higher availability of inorganic nitrogen and greater solar radiation, such as eutrophic freshwaters and coastal areas, and tropical regions. This discovery represents an advance in the study of global N₂O sources, highlighting uncertainties in our current understanding of global N₂O inventories and fluxes.

The article, Sunlight drives the abiotic formation of nitrous oxide in fresh and marine waters, first appeared online March 13 in Science.

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