Substrate‐Specific Potential N 2 O Production and Hybrid Isotopic Signatures in the Epipelagic Eastern Indian Ocean

Nitrous oxide (N 2 O) production in epipelagic oligotrophic oceans remains poorly constrained because multiple microbial pathways can co‐occur under predominantly oxic conditions and may respond differently to oxygen and substrate availability. Here, we combined multi‐substrate 15 N tracer incubations ( 15 NH 4 + , 15 N‐urea, 15 NO 2 − , and 15 NO 3 − ), isotopocule‐based partitioning of labeled N 2 O, and microbial community and genome analyses to investigate pathway‐specific potential N 2 O production in the epipelagic eastern Indian Ocean. Substrate‐specific potential N 2 O production exhibited distinct vertical distributions and oxygen responses. Ammonium‐, urea‐, and nitrite‐driven N 2 O production generally increased from well‐oxygenated waters (>100 μmol L −1 O 2 ) toward oxycline waters (<100 μmol L −1 O 2 ), whereas nitrate‐driven production showed a non‐monotonic response with a maximum at intermediate low‐O 2 concentrations of ∼20–50 μmol L −1 . Isotopic labeling further revealed widespread hybrid N 2 O production, indicating that labeled substrates were coupled with nitrogen intermediates during N 2 O production in epipelagic waters. Microbial community patterns and regional genomic potential were consistent with ammonia‐oxidizing archaea as candidate contributors to hybrid N 2 O production in oxic waters, denitrification‐associated taxa near the oxycline, and nosZ ‐bearing lineages as candidate groups with potential for N 2 O reduction. Depth‐integrated potential N 2 O production was positively related to primary productivity, suggesting that carbon fixation, particle export, and nitrogen remineralization may jointly enhance epipelagic N 2 O production. Together, these results provide process‐level constraints on oxygen‐dependent pathway partitioning of epipelagic N 2 O production and highlight a potential carbon‐nitrogen linkage whose climatic effect depends on sea‐air exchange, water‐column transport, and biological N 2 O consumption.