A tropical hotspot

The ocean is an important source of the potent greenhouse gas N₂O. Measurements in the tropical South Pacific have revealed a massive efflux of N₂O from the coastal upwelling zone.

Coastal upwelling zones are highly productive; Peru was the most productive fishing nation in the world until the 1970s, owing to upwelling along the coast¹. This high-productivity area creates conditions favourable to the microbial production of nitrous oxide (N₂O) — a powerful greenhouse gas, now the leading cause of stratospheric ozone depletion². Although agriculture and soils under natural vegetation are thought to be the main sources of N₂O, oceans contribute about a third of natural emissions³. However, our understanding of oceanic N₂O emissions is still incomplete. Writing in Nature Geoscience, Arévalo-Martínez and colleagues find evidence for extremely high emissions of N₂O from the Peruvian coastal region, suggesting that oceans are a larger source of N₂O than previously thought⁴.

In upwelling systems, deep water masses are advected to the surface. Substantial quantities of organic matter are respired by microbes in deep water masses, depleting oxygen, producing N₂O and releasing high concentrations of nutrients such as nitrogen. In upwelling systems, nutrients fuel biological productivity in the upper layers of the ocean. The deep water N₂O can be released to the atmosphere, but the highly productive environment can also result in local production of N₂O during microbial nitrification and denitrification, particularly when oxygen is present in low concentrations.

The organic matter produced in these high-productivity systems sinks below the surface layers of the ocean and is subsequently consumed by microbes, along with oxygen. These deeper layers cannot be supplied with oxygen from the atmosphere, and ocean circulation is too slow to replenish the depleted oxygen. Thus oxygen minimum zones (OMZs, where little or no oxygen is present) typically form at depths of 100 to 900 m (ref. 5), and extend for hundreds or thousands of kilometres in area. The high concentrations of sinking organic matter in the OMZs, combined with nutrients from the upwelling water, provide abundant nitrogen to microbes. With low oxygen and high nitrogen concentrations, conditions in upwelling zones are favourable for the microbially mediated reactions that produce N₂O.

The tropical ocean as seen from the research vessel Meteor. Arévalo-Martínez and colleagues⁴ report unprecedented levels of N₂O emissions from the Peruvian upwelling zone in the tropical South Pacific. The emissions of this powerful greenhouse gas from a single upwelling region represent 5–22% of previous estimates of global N₂O emissions from oceans and may be the result of coupled nitrification and partial denitrification at the edge of the OMZ.

Arévalo-Martínez and colleagues⁴ combined vertical profiles and continuous surface measurements of N₂O concentrations along the coast of Peru in 2012 and 2013 to estimate the magnitude of emissions from the region. Water samples from vertical profiles were collected every couple of nautical miles. Continuous measurements — in this case using a laser-based instrument that was only introduced a couple of years ago — are limited to surface waters, but are conducted with a much higher frequency while the ship is moving. The continuous measurements can fill in the gaps between vertical profiles, detecting areas of high N₂O emissions that might have been overlooked otherwise.

What stands out about the sea-to-air flux of 0.3–1.4 TgN₂O yr⁻¹ observed by Arévalo-Martínez and colleagues is that it is remarkably high. These emissions represent 5–22% of previous global estimates of N₂O emissions from the ocean, whereas 5% of the marine N₂O emissions were previously attributed to all of the coastal upwelling regions around the world³.

To understand where these large N₂O fluxes were coming from, Arévalo-Martínez and colleagues measured variables such as oxygen concentrations and utilization, nutrient concentrations, sea surface temperatures, and the abundance of specific microbial genes involved in nitrogen cycling processes such as nitrification and denitrification. These various lines of evidence suggest that N₂O is not produced in a remote location and then transported by the upwelling waters, nor is it produced in the surface layer of the ocean, where most biological production occurs. Instead, it seems that most of the N₂O observed in the Peruvian upwelling region is actually produced below the surface layer, at the boundaries of the OMZ.

Both microbial nitrification and denitrification seem to be responsible for the production of N₂O. Nitrification requires oxygen, but is enhanced under low oxygen concentrations⁶ such as those found at the edges of the OMZ. In contrast, denitrification only occurs under anoxic conditions, and N₂O is an intermediate in a reaction that ultimately produces N₂. Under the extremely low oxygen concentrations in the centre of the OMZ, N₂O produced by denitrification would then be reduced to N₂. But at the edges of the OMZ, oxygen concentrations are slightly higher, which can lead to incomplete denitrification reactions where no reduction of N₂O to N₂ takes place.

Measurements of N₂O in the upwelling region at a range of depths down to 1,000 m provide support for the presence of denitrification, with high concentrations of N₂O at the boundaries, but not in the centre, of the OMZ. There were also higher abundances of genes markers for nitrification and denitrification at the boundaries, providing additional evidence that N₂O was produced by both pathways. The upwelling waters then transport the N₂O produced at the OMZ boundary to the surface, resulting in the remarkably high sea-to-air flux.

One point to note is that the observations of Arévalo-Martínez and colleagues cover the months of November 2012 to March 2013. The limited duration of the measurements leaves us guessing as to what might be happening in other times of the year, or in other years — especially under El Niño or La Niña conditions. Previous studies of the Peruvian upwelling and other ocean regions share this shortcoming, which can only be partly addressed by modelling studies.

The results of Arévalo-Martínez and colleagues⁴ highlight the large uncertainty in current global estimates of marine N₂O emissions. Furthermore, they show how an interdisciplinary approach that includes geoscience and molecular techniques can resolve complicated marine processes. The use of continuous N₂O measurements, a relatively new method for research on marine trace gases, has clear promise for the large-scale resolution of variable fluxes and could be a valuable tool for the observation of complete seasonal cycles. Owing to ongoing expansion of OMZs⁷ and the expected intensification of coastal upwelling in the future⁸, more detailed observations of these regions are crucial for constraining how N₂O emissions will respond to the changing marine environment⁹.