The Ocean's Greenhouse Gas Puzzle: Unraveling Nitrous Oxide Emissions in the Northern Indian Ocean

Nitrous oxide (N2O) might not be as famous as CO2, but it's a powerful greenhouse gas. It's also an ozone-depleting substance. While human activities contribute to atmospheric N2O, the ocean is a significant source, accounting for nearly one-fifth of global emissions. Tropical oceans, especially those with upwelling and oxygen minimum zones (OMZs), are major hotspots for these emissions. The northern Indian Ocean, home to a major OMZ, is one such region, known for being a strong source of N2O to the atmosphere.

            But here's a puzzle: How does the N2O produced deep within the ocean make its way to the surface to be emitted? The connection between where N2O is produced and where it's released is often unclear. Understanding this link is crucial for predicting how N2O emissions might change in the future as climate and ocean conditions shift.  

N₂O is mainly produced in the ocean through two biological processes:

1. Nitrification: in the presence of molecular oxygen, ammonium (NH4+) is oxidized to nitrite (NO2−) via hydroxylamine (NH2OH) by ammonia oxidizing bacteria, with N2O as a by-product. This happens in most oceanic regions.

2. Denitrification: this occurs intensely in oxygen minimum zones, where N2O is an intermediate product. N2O can also be consumed by denitrification in oxygen-poor waters.  

Previous studies using ocean models often assumed that the relative contributions of nitrification and denitrification to N2O emissions were the same as their contributions to N2O production. However, this assumption doesn't hold true at regional scales, particularly in OMZ areas, because it doesn't account for how ocean circulation transports N2O. Field observations also suggest that only a small amount of subsurface N2O is emitted locally.             Postdoc Yangyang Zhao from the High Meadows Environmental Institute, Princeton University, led an international group from Princeton’s Resplandy Lab and Ward Lab, along with colleagues from Xiamen University and Shanghai Jiao Tong University, in China, to shed light on this complex link in the northern Indian Ocean by implementing a mechanistically based N2O cycling module into a regional ocean model. This model simulates the physical and biogeochemical dynamics, including the production and consumption of N2O via nitrification and denitrification, and its transport. The model was able to reproduce observed patterns of ocean properties, oxygen levels, and N2O distribution in the region.

            Their research findings reveal a significant decoupling between N2O production and its emission to the atmosphere in the northern Indian Ocean. Here are some of the key takeaways:

• The total simulated N2O emissions from the northern Indian Ocean is estimated at 286 ± 152 Gg N yr⁻¹, which falls within the lower range of observation-based estimates (391 ± 237 Gg N yr⁻¹). The Arabian Sea accounts for about 70% of these emissions, while the Bay of Bengal contributes the remaining 30%.

• N2O emissions in this region are primarily fueled by nitrification (~57%), followed by denitrification (~30%), and then N2O transported from outside the region (~13%). These proportions are similar in both the Arabian Sea and the Bay of Bengal.

• Crucially, the model shows that about 74% of the N2O emitted to the atmosphere is produced below the surface layer and then transported upwards. Only about 26% is produced directly in the surface mixed layer, where it can be immediately released.

•This transport of subsurface N2O into the surface mixed layer happens in areas with vigorous vertical exchange, such as coastal upwelling regions, areas of winter convection, and through turbulent mixing.

While denitrification accounts for a large portion (~89%) of the total gross production of N2O in the subsurface, much of it is consumed (reduced to N2) within the OMZs. This results in denitrification contributing only a smaller fraction (about 20%) to the total net production of N2O below the surface. In contrast, nitrification, while contributing less to gross production (~11%), dominates net production (~83% in the subsurface) because the N2O it produces is not consumed as effectively.

            Despite nitrification dominating net production, the model shows that denitrification-sourced N2O has a higher "emission fraction" (ratio of emission to net production) than nitrification-sourced N2O in the northern Indian Ocean (43% vs. 22%), particularly in the Arabian Sea (64% vs. 19%). This is because denitrification-sourced N2O is often produced deeper but is then efficiently transported to the surface by the strong vertical mixing and circulation in areas like coastal upwelling systems. Nitrification-sourced N2O, produced shallower, can be trapped by stratification.

            This spatial decoupling, where N2O is produced in the subsurface OMZ areas but emitted primarily in surface upwelling and mixing zones, highlights the critical role of ocean transport in determining N2O emissions.

            For future projections of N2O emissions, this means we need to consider not just changes in factors that affect N2O production (like oxygen levels and primary productivity), but also shifts in ocean circulation that control transport and emissions. Climate change is expected to affect both production factors (e.g., potential decline in productivity, changes in OMZs) and transport pathways (e.g., changes in stratification and upwelling). Predicting the net effect on N2O emissions is challenging, especially as many current Earth system models have simplified N2O cycling and don't fully resolve the critical transport processes in regions like upwelling systems.

            In summary, this study underscores that understanding ocean circulation and transport is just as important as understanding biological N2O production pathways when assessing and predicting N2O emissions, particularly in dynamic regions like the northern Indian Ocean. More observations and improved models are needed to reduce uncertainties.

Zhao, Yangyang, Laure Resplandy, Xianhui Sean Wan, Fan Yang, Enhui Liao, and Bess Ward. "Decoupling of N2O production and emissions in the northern Indian Ocean." Global Biogeochemical Cycles 39, no. 4 (2025): e2024GB008481. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024GB008481