Climate gas hotspots in the Elbe estuary

Measurements in the port of Hamburg showed comparatively high nitrous oxide emissions. © Hereon/ Sina Bold

Targeting a mega-greenhouse gas: laughing gas is 300 times more intense than carbon dioxide. Researchers have now examined its production in the estuaries of rivers using the Elbe as an example. They discovered two sections with particularly intensive release: in the port of Hamburg and in the particularly coastal area. It also becomes apparent what causes these hotspots. The results can now contribute to a better understanding of global nitrous oxide emissions and the role of agriculture in the climate problem.

When it comes to climate change, the focus is usually on the two greenhouse gases carbon dioxide and methane, which are increasingly accumulating in the earth's atmosphere as a result of human activities. But there is another factor that contributes to climate change that has not been studied so well to date: the increasing release of the particularly potent greenhouse gas nitrous oxide (N2O), which is also known as laughing gas due to its euphoric effect when inhaled. The most important reason for the rising values ​​in the atmosphere is the over-fertilization with nitrogen (N) by agriculture. Because what the plants cannot use remains in the soil or gets into water bodies, where it is converted into nitrous oxide either directly or indirectly through biological processes. Soils and rivers are already known to be important sources. When, how and where exactly nitrous oxide escapes into the air has not yet been adequately researched.

On the trail of N2O in the Elbe estuary

The scientists around Gesa Schulz from the University of Hamburg have now targeted the role of the estuaries. They used the Elbe as an example of a system with significant inputs from agriculture: During nine research trips between 2017 and 2022, they measured the concentrations of N2O and dissolved nutrients at various locations in the estuary region of the river. The researchers then placed their results in the context of other relevant information about the water system.

As they found out, the Elbe estuary releases nitrous oxide throughout the year - albeit with certain temporal fluctuations and peculiarities in certain sections of the river. The data again show that the main basis of nitrous oxide production is agricultural fertilization. "Especially in winter, when the plants aren't growing and hardly take up any nitrogen, a lot of nitrogen gets into the river with the precipitation," says Schulz. It is then partly converted into nitrous oxide by the biochemical processes. Although this happens to a lesser extent in cool winter water than in summer, the emissions are surprisingly high even at this time of year. According to the researchers, this is probably due to the fact that strong winds often mix the Elbe in winter, releasing a relatively large amount of nitrous oxide into the atmosphere.

What causes hotspots

As the team reports, the results also revealed two special nitrous oxide hotspots in the Elbe estuary region: The port of Hamburg releases a particularly large amount. In addition, there is the estuary section from Brunsbüttel to Cuxhaven, in which the Elbe water mixes with the water from the North Sea at high tide. As the researchers explain, a particularly large number of microalgae thrive in this coastal area. These are transported upstream at high tide. When the algae then die there, their nitrogenous components are decomposed, which explains the high emissions in the area from Brunsbüttel to Cuxhaven.

In the port of Hamburg, on the other hand, another cause is emerging, the researchers say: In spring and summer, the Elbe transports large quantities of freshwater algae to the port of Hamburg, which were able to grow particularly well due to the agricultural nitrogen inputs in the course of the river. In the port of Hamburg, these algae then fall into a dark hole, so to speak. Because that is where the Elbe has been dredged deep for cargo ship traffic. The organisms from the light-flooded sections of the Elbe die off in the dark water. When this biomass breaks down, a lot of nitrous oxide is produced, the scientists explain.

According to them, the information could now benefit efforts in the Elbe area to counteract the increased emission of nitrous oxide. In addition, the data are important for a fundamental understanding of nitrous oxide production in estuaries and for better global assessment and modelling. “We are making a contribution to answering the open questions on the subject. This could also make it possible to tackle the problem regionally through targeted measures," concludes Schulz.

Source: Helmholtz Center Hereon, specialist article: Biogeosciences, doi: 10.5194/bg-20-3229-2023

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