The heavily human-perturbed coastal oceans are hotspots of nitrous oxide (N₂O) emission to the atmosphere. The processes underpinning the N₂O flux, however, remain poorly understood, leading to large uncertainties in assessing global N₂O budgets. Using a suite of nitrogen isotope labeling experiments, we show that multiple processes contribute to N₂O production throughout the estuarine-coastal gradient, sustaining intensive N₂O flux to the atmosphere. Unexpectedly, denitrification, rather than ammonia oxidation as previously assumed, constitutes the major source of N₂O 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 N₂O production rate increases with substrate and particle concentrations, suggesting a crucial interplay between nutrients and particles in controlling N₂O production. The controlling factors identified here may help understand climate feedback mechanisms between human activity and coastal oceans.

Lateral input of dissolved organics may play a significant role to support productivity in oligotrophic ocean although associated biogeochemical evidences are lacking in the field. Ammonia oxidation (AO), the first step of nitrification that bridges organic remineralization and nitrate, is potentially an immediate responder. By using ¹⁵N‐NH₄⁺, the spatial distribution of AO was investigated in the northern South China Sea, where Kuroshio Current intrudes frequently. AO ranged widely (0.001 to 134 nmol · L⁻¹ · day⁻¹) in space and the depth‐integrated (200 m) AO peaked where the Kuroshio influence is moderate suggesting that enhanced AO had occurred due to lateral mixing. Since oligotrophic Kuroshio is characterized by high dissolved organic nitrogen (DON), such lateral mixing not only introduces external DON into the northern South China Sea but also enhances NH₄⁺ regeneration and subsequent oxidation to complicate the conventional new production in the boundary zone with DON gradient.

Nitrogen (N), as a critical element for microbial metabolisms, recycles rapidly in the euphotic ocean. Oxidation by nitrifiers is a competing pathway for phytoplankton assimilation of regenerated N (NH₄⁺ and urea). Sharing the overlapping substrates may result in competitive exclusion, thus, niche separation for the two assemblages. Both pathways are sensitive to light, but whether light intensity will intensify or alleviate such resource competition in the euphotic zone remains poorly explored in the field at the community level. By using ¹⁵N labeling techniques, paired kinetic responses of uptake and oxidation were conducted in single bottles under manipulated light intensities for both NH₄⁺ and urea. We found light stimulated the maximum rate (R_m) and specific affinities (α_U) of both NH₄⁺ and urea uptake. In contrast, light effects were opposite for oxidation kinetics (R_m and α_O). As irradiance increased, the rapid increase in α_U and concomitant decrease in α_O imply a distinctive competition advantage of photosynthetic organisms over oxidizers under substrate‐limited environments. The ratio of α_U/α_O for NH₄⁺ ranged from 0.8 to 3089 (5.8–46,788 for urea) showing a distinct increasing pattern as ambient light increases, demonstrating that phytoplankton overwhelms nitrifiers throughout the oligotrophic euphotic zone, driving down concentrations and maintaining short turnover times of the two regenerated N substrates. Moreover, phytoplankton relied equally on NH₄⁺ and urea; yet, nitrifiers preferred NH₄⁺ to urea. In the nitrate‐depleted euphotic ocean, light acts as a crucial driver for utilization pathways of regenerated N and vertical niche separation.