Deep ocean microorganisms consume particulate organic matter that is produced in the surface ocean and exported to deeper depths. Such consumption not only enriches inorganic carbon in the deep ocean but also transforms organic carbon into recalcitrant forms, creating an alternative type of carbon sequestration. However, estimates of deep microbial carbon demand substantially exceed the available particulate organic carbon exported from the euphotic zone, resulting in an unbalanced dark ocean carbon budget. Here, we combined field‐based microbial activity parameters, integrated multiyear particle export flux data, sinking particle fluxes measured by sediment traps, and optical data from Biogeochemical‐Argo floats to quantify the main sources of organic carbon to the dark ocean. Laterally transported particles (including sinking and suspended particles) serve as a major energy source, which directly provide organic carbon and enhance new organic carbon production by dark carbon fixation, reconciling the mismatch in the regional carbon budget.

Here, we present the first combined results of N₂ fixation rates (¹⁵N₂ assay), dissolved iron (dFe, < 0.2 μm), and primary production (PP) (¹⁴C assay) in the northwestern South China Sea (NWSCS) coastal upwelling region during summer. Surface N₂ fixation rate ranged between 0.1 nmol N L⁻¹ d⁻¹ and 5.6 nmol N L⁻¹ d⁻¹ (average 1.0 nmol N L⁻¹ d⁻¹, n = 50) under nonbloom conditions. At a Trichodesmium bloom station, N₂ fixation rate was ∼ 3 orders of magnitude higher. Depth‐integrated N₂ fixation rate ranged between 7.5 μmol N m⁻² d⁻¹and 163.1 μmol N m⁻² d⁻¹ (average 46.4 μmol N m⁻² d⁻¹). Our results indicate that N₂ fixation is unlikely limited by Fe availability in the NWSCS continental waters, instead, the coastal upwelling‐induced combined effects of physical and biological processes may have played a decisive role. With the upwelled cold, dFe‐rich, nutrient‐replete waters, nondiazotrophic phytoplankton growth would be preferentially enhanced while N₂ fixation was hindered due to relative deficiency of phosphate caused by massive phytoplankton utilization in the coastal upwelling. By comparison, N₂ fixation was notably elevated along with decreased PP in the offshore waters, probably due to a shift from P‐deficiency to N‐deficiency. Consistently, the contribution of N₂ fixation to PP (0.01–2.52%) also increased toward the open waters. As a significant external N source, summertime N₂ fixation is estimated to contribute a flux of 1.4 Gmol N to this area under nonbloom conditions. This study adds to the knowledge of N₂ fixation in the rarely studied subtropical coastal upwellings, and highlights the necessity of future comprehensive studies in such highly dynamic environments.