Shifts in the Isotopic Composition of Nitrous Oxide Between El Niño and La Niña in the Eastern Tropical South Pacific.

The El Niño‐Southern Oscillation (ENSO) is a natural climate phenomenon that alters the biogeochemical and physical dynamics of the Eastern Tropical Pacific Ocean. Its two phases, El Niño and La Niña, are characterized by decreased and increased coastal upwelling, respectively, which have cascading effects on primary productivity, organic matter supply, and ocean‐atmosphere interactions. The Eastern Tropical South Pacific oxygen minimum zone is a source of nitrous oxide (N2O), a potent greenhouse gas, to the atmosphere. Here, we present the first study to directly compare N2O sources during opposing ENSO phases using N2O isotopocule analyses. Our data show that during La Niña, N2O accumulation increased six‐fold in the upper 100 m of the water column, and N2O fluxes to the atmosphere increased up to 20‐fold. N2O isotopocule data demonstrated substantial increases in δ18O up to 60.5‰ and decreases in δ15Nβ down to −10.3‰ in the oxycline, signaling a shift in N2O cycling during La Niña compared to El Niño. During El Niño, N2O production was primarily due to ammonia‐oxidizing archaea, whereas during La Niña, N2O production by incomplete denitrification supplemented that from ammonia‐oxidation, with N2O consumption likely maintaining the high site preference values (up to 26.7‰). Ultimately, our results illustrate a strong connection between upwelling intensity, biogeochemistry, and N2O flux to the atmosphere. Additionally, they highlight the combined power of N2O isotopocule analysis and repeat measurements in the same region to constrain N2O interannual variability and cycling dynamics under different climate scenarios. Plain Language Summary: Nitrous oxide is a greenhouse gas that is 300 times more potent than carbon dioxide in its ability to warm the Earth, and it can be produced and consumed through the activities of microorganisms in the ocean. The low‐oxygen waters of the tropical Pacific are a known global "hotspot" for nitrous oxide cycling as the chemical and physical conditions there provide fuel for nitrous oxide production. A natural climate cycle known as the El Niño‐Southern Oscillation, which has a warm and a cold phase, can affect the conditions that raise or lower the speed of nitrous oxide cycling. We used isotopes—small, but measurable natural differences in the masses of molecules and atoms—to understand how nitrous oxide cycling changes during each phase. We found a decrease in nitrous oxide accumulation during the warm phase, and enhanced production during the cold phase. This enhanced production during the cold phase is connected to the release of more nitrous oxide from the ocean to the atmosphere. Key Points: La Niña in the Eastern Tropical South Pacific led to a six‐fold increase in nitrous oxide concentrations compared to El NiñoIsotopocule analysis indicated that incomplete denitrification and archaeal ammonia oxidation both contributed to nitrous oxide accumulationNumerous co‐occurring production and consumption pathways likely explain the nitrous oxide distributions during La Niña [ABSTRACT FROM AUTHOR]