The ocean plays a vital role in regulating Earth's climate by absorbing a substantial portion of anthropogenic carbon dioxide (CO2) and emitting a significant fraction of global nitrous oxide (N2O). The marine biological pump, a biologically driven process transferring carbon from the surface to the ocean interior, is crucial for long-term CO2 regulation. While the biological pump's carbon sequestration capacity has been extensively studied, the potential counter-effect of N2O emissions in offsetting this benefit has been largely overlooked. In subtropical oceans, a considerable amount of organic matter remineralizes in the epipelagic zone (upper 200m), leading to rapid carbon and nitrogen transformations. This study investigates the hypothesis that in these regions of low carbon export efficiency, the counteracting effect of N2O emissions on the climate benefit of the biological pump could be substantial. Understanding this interplay between carbon sequestration and N2O production is crucial for accurately assessing the ocean's role in the global climate system and developing effective climate mitigation strategies.

N2O in the marine nitrogen cycle is primarily produced during nitrification (in oxygenated waters) and denitrification (in anaerobic environments). While nitrification was initially considered the dominant source in the oxygenated ocean, recent research indicates a more complex picture, including contributions from aerobic nitrification and anaerobic nitrate/nitrite reduction within marine aggregates or zooplankton guts. Ammonia-oxidizing archaea, prevalent in open oceans, utilize a hybrid N2O production pathway distinct from bacteria. However, shallow-water N2O sources in the epipelagic zone are less studied despite intensive nitrogen remineralization occurring there, partly due to the light inhibition of ammonia oxidation and the relatively low abundance of ammonia-oxidizing archaea in surface waters. The interplay between primary productivity and N2O production in this region remains unclear, hindering accurate assessment of the ocean's N2O budget. Nitrogen often limits phytoplankton growth in low-latitude oceans, with new nitrogen coming from subsurface NO3-, N2 fixation, and atmospheric deposition. Remineralization within the epipelagic zone also contributes significantly to regenerated production. Previous geoengineering and nitrogen deposition studies have touched upon the offsetting effects of N2O emissions on CO2 sequestration, but the sources of N2O and their link to biological CO2 sequestration need further quantification.

This study involved measuring N2O production and carbon export rates in the epipelagic zone across the South China Sea (SCS) and the North Pacific Subtropical Gyre (NPSG) over eight years. High-resolution vertical profiles were collected to capture vertical variations in N2O concentrations. On-board incubations were conducted at multiple stations using 15N isotope tracers (15NH4+, 15NO2-, 15NO3-) to determine N2O production rates from various nitrogen sources and nitrification rates. Additionally, 13C-bicarbonate tracers were used to measure primary production rates. Water samples were collected for nutrient (NH4+, NOx-, Si), particulate organic carbon (POC), and particulate nitrogen (PN) analyses. N2O concentrations were measured using purge and trap systems coupled with gas chromatography and GC-IRMS. N2O isotopic compositions (δ15N-N2O and δ18O-N2O) were also determined using GC-IRMS. The thorium-deficit method was used to estimate export production, using 234Th measurements. Surface N2O saturation and air-sea fluxes were calculated using Henry's law and wind speed data. A two-endmember mixing model was used to estimate the fraction of N2O originating from shallow in-situ production in the isotope minimum layer. Statistical analyses were performed to correlate N2O production rates with other measured parameters, like POC and PN inventories. Global warming potential (GWP100) was used to compare the radiative warming offset due to N2O production relative to the carbon export's climate mitigation.

N2O concentrations in surface waters were often above air-saturation, with higher saturation in coastal regions. Distinctive N2O concentration peaks were observed at several stations, coinciding with the primary nitrite maximum (PNM) and NO3-/Si maximum layers, suggesting local N2O production. High-resolution vertical profiles of N2O stable isotopes (δ15N-N2O and δ18O-N2O) showed minima near the PNM, further supporting in-situ production. A two-component isotope mass balance model indicated that shallow N2O production contributed 31.3–41.6% to the air-sea flux. Isotope tracer incubations revealed that multiple nitrogen precursors (NH4+, NO2-, NO3-) contributed to N2O production via nitrification, nitrifier denitrification, and denitrification in micro-anoxic niches. Depth-integrated N2O production rates accounted for a significant portion (29.5–61.3%, average 40%) of the air-sea N2O flux. There was a positive correlation between N2O production and ammonia oxidation rates, indicating a link to nitrogen regeneration. Both ammonia oxidation rates and N2O production rates were correlated with POC and PN inventories, suggesting that the intensity of nitrogen recycling and N2O production scales with the availability of organic nitrogen. The study estimates that N2O production offset 5.6–27.2% (median 9%) of the greenhouse warming mitigation achieved by carbon export, though uncertainties exist due to varying remineralization timescales and water mass ventilation. The short distance from the isotope minimum layer to the air-sea interface in the epipelagic zone, along with more vigorous physical dynamics, suggests a potentially greater climatic impact of nitrogen recycling in this region.

The findings highlight the importance of considering epipelagic N2O production when assessing the net climate impact of the marine biological pump. In subtropical oceans, where export production is low and remineralization is rapid, the offsetting effect of N2O production is substantial. The positive correlation between N2O production and nitrogen recycling emphasizes the need for integrated models that couple carbon and nitrogen cycles. The multiple pathways for N2O production observed in the study suggest that current biogeochemical models may underestimate N2O sources, and hence, the extent of the offset. The study's quantification of the N2O/CO2 offset contributes significantly to our understanding of the ocean's role in climate regulation. Future studies should focus on expanding spatial and temporal coverage to improve the generality of these findings.

This study demonstrates that epipelagic N2O production, driven by intense organic matter remineralization and nitrogen recycling, counteracts a significant portion of the climate benefits associated with carbon sequestration by the marine biological pump, particularly in oligotrophic subtropical regions. Accurate assessment of the ocean's net climate effect requires incorporating the coupled carbon and nitrogen cycling processes and the multiple pathways of N2O production. Future research should explore the variability of the N2O/CO2 offset across diverse oceanographic regimes and incorporate these findings into improved biogeochemical models for climate predictions and mitigation strategies.

The study's conclusions are based on a limited number of stations and years of sampling, which may not fully capture the large spatial and temporal variability of carbon export and nitrogen regeneration in the ocean. The GWP100 used for comparing N2O production and CO2 sequestration assumes a 100-year time horizon for both processes, which may oversimplify the complexities of long-term climate change. Further research with broader spatial and temporal coverage is needed to confirm the generality of these findings across various oceanographic regions.