Resolving Nitrous Oxide Source Ambiguity in Ammonia-Oxidizing Archaea-Dominated Nitrification: Mechanistic Modeling of the Hybrid Pathway and Coupled Denitrification

Nitrous oxide (N 2 O) emissions linked to ammonia-oxidizing archaea (AOA) lack a process-resolved kinetic framework, limiting accurate source attribution in wastewater nitrification. A mechanistic model was developed for N 2 O production in AOA-dominated systems, explicitly resolving the archaeal N-nitrosation hybrid pathway and coupling nitrite generation by AOA to heterotrophic denitrification via intracellular carbon storage. The model was calibrated using dynamic batch experiments with an AOA-enriched culture across dissolved oxygen gradients (1.47–7.35 mg L –1 ) and independently validated against temporal profiles of nitrogen species and N 2 O. The rate constant for the archaeal hybrid N 2 O production was quantified as k AOA = 0.4966 m 3 g -1 d -1, yet yielded only 0.032–0.085% of oxidized nitrogen as N 2 O. Simulations indicated that N 2 O in AOA systems originated predominantly (>96%) from heterotrophic denitrification, while the archaeal hybrid pathway remained low-yield and insensitive to dissolved oxygen. In contrast, canonical ammonia-oxidizing bacteria-mediated systems exhibited higher N 2 O yields (3.1–8.7% of oxidized nitrogen), which were strongly suppressed under elevated oxygen. By transforming the conceptual hybrid pathway into a predictive, process-resolved framework, this model provided a kinetic basis for moving N 2 O mitigation strategies beyond uniform oxygen control toward approaches that account for nitrifier identity.