Particle-associated denitrification is the primary source of N 2 O in oxic coastal waters.

The heavily human-perturbed coastal oceans are hotspots of nitrous oxide (N 2 O) emission to the atmosphere. The processes underpinning the N 2 O flux, however, remain poorly understood, leading to large uncertainties in assessing global N 2 O budgets. Using a suite of nitrogen isotope labeling experiments, we show that multiple processes contribute to N 2 O production throughout the estuarine-coastal gradient, sustaining intensive N 2 O flux to the atmosphere. Unexpectedly, denitrification, rather than ammonia oxidation as previously assumed, constitutes the major source of N 2 O in well-oxygenated coastal waters. Size-fractionated manipulation experiments with gene analysis further reveal niche partitioning of ammonia oxidizers and denitrifiers across the particle size spectrum; denitrification dominated on large particles and ammonia oxidizers on small particles. Total N 2 O production rate increases with substrate and particle concentrations, suggesting a crucial interplay between nutrients and particles in controlling N 2 O production. The controlling factors identified here may help understand climate feedback mechanisms between human activity and coastal oceans.