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Microorganisms encoding for the N₂O reductase (NosZ) are the only known biological sink of the potent greenhouse gas N₂O, and are central to global N₂O mitigation efforts. Yet, the ecological constraints selecting for different N₂O-reducers strains and controlling the assembly of N₂O-respiring communities remain largely unknown. Of particular biotechnological interest are clade II NosZ populations, which usually feature high N₂O affinities and often lack other denitrification genes. Two planktonic N₂O-respiring mixed cultures were enriched under limiting and excess dissolved N₂O availability to assess the impact of substrate affinity and N₂O cytotoxicity, respectively. Genome-resolved metaproteomics was used to infer the metabolism of the enriched populations. We show that clade II N₂O-reducers outcompete clade I affiliates for N₂O at sufficiently low sludge dilution rates (0.006 ^h-1), a scenario previously only theorized based on pure-cultures. Under N₂O limitation, all enriched N₂O reducers encoded and expressed only clade II NosZ, while also possessing other denitrification genes. Two Azonexus and Thauera genera affiliates dominated the culture. We explain their coexistence with the genome-inferred metabolic exchange of cobalamin intermediates. Conversely, under excess N₂O, clade I and II populations coexisted. Notably, the single dominant N₂O reducer (genus Azonexus) expressed most cobalamin biosynthesis marker genes, likely to contrast the continuous cobalamin inactivation by dissolved cytotoxic N₂O concentrations (400 μM). Ultimately, we demonstrate that the solids dilution rate controls the selection among NosZ …