Photochemodenitrification: A Novel Photochemical Pathway for Nitrous Oxide Production in Aquatic Systems



By Elizabeth León-Palmero and collaborators

@leon-palmeroNitrous oxide (N2O) is both a strong greenhouse gas and a potent destroyer of stratospheric ozone, which protects us from harmful ultraviolet (UV) radiation from the sun. Since preindustrial times, atmospheric N₂O levels have increased by 23%, mainly due to more reactive nitrogen in the environment, largely from industrial fertilizer production. A significant amount of this nitrogen ends up in freshwaters and coastal waters, where it boosts N₂O production. The rapid increase in N2O concentration in the atmosphere over the last decade, even faster than predicted by the Intergovernmental Panel on Climate Change (IPCC), emphasizes the need for better identification of the sources of N2O. 

Ammonia oxidizers and denitrifiers are microbial groups that are supposed to produce N2O in aquatic systems. However, we have recently described the discovery of a new photochemical process that generates the potent greenhouse gas N2O in fresh and marine waters. Without the mediation of the usual microorganisms that are known to produce N2O, this abiotic mechanism, that we termed "photochemodenitrification", occurs in both fresh and marine surface waters and is driven by sunlight in the presence of inorganic nitrogen.

We conducted experiments in two freshwater reservoirs in Southeast Iberian Peninsula (Granada and Córdoba), off the coast of the Mediterranean Sea (Motril, Granada), and in the Baltic Sea (Boknis Eck station, Germany). In these experiments we detected a consistent and substantial increase in N2O concentrations, which was correlated with the dose of sunlight received during the experiments. Using isotope tracers (i.e., 15N-nitrite and 15N-nitrate to track the formation of N2O), we demonstrated that sunlight promotes the abiotic conversion of nitrite to N2O. We also showed that nitrate contributed to the formation of nitrous oxide after being photoreduced to nitrite by sunlight. This reaction was named “photochemodenitrification”. 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 with depth. 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 a major advance in the study of global N2O sources, highlighting uncertainties in our current understanding of global N2O inventories and fluxes.

This research work was partly funded by the Spanish Association of Terrestrial Ecology and the Iberian Society of Ecology (AEET-SIBECOL) through a grant for Young Researchers to Dr. Leon-Palmero (PHOTON, 2021).