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 δ¹⁵N^β 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.