The Congo forest does not behave like other rainforests

The Congo Basin is home to the world’s second-largest contiguous tropical forest region. Despite its vast size and significance in the global climate system, very limited data is available on how the gases generated within the forest interact with the atmosphere. Research into the greenhouse gases, nitrous oxide and methane, in particular, is still in the early stages.

Several tropical forests have been identified as a substantial source of nitrous oxide and a methane sink. However, because comparable data for the Congo is unavailable, research into how these greenhouse gases react in that region has thus far been confined to modeling. Since the data for these models came from the Amazon, Indonesia, and the tropical region of Australia, scientists assumed the Congo Basin behaved similarly to these other tropical forests.

Now, this knowledge gap has been partially filled by an international research team led by ETH Zurich Professor Johan Six. The scientists measured how much methane and nitrous oxide the Congo Basin tropical forest absorbs or releases after an extensive and laborious measurement campaign that lasted several years. 

The researchers measured gas fluxes at several sites in three different forest types in the Congo Basin between 2016 and 2020. Montane forest, lowland tropical forest, and occasionally flooded marsh forest are among the forest types studied. 

This is the first time that the gas fluxes of the Congo Basin’s tropical forests are distinguished from those of other tropical locations throughout the world. Nitrous oxide emissions from the forest, for example, are quite minimal.

“This was unexpected. Our measurements from both the first short measurement campaigns and subsequent long-​term studies didn’t confirm the model assumptions,” said study lead author Matti Barthel.

For methane, the situation is different. The Congo Basin should be a methane sink, according to the models. For the montane and lowland tropical forests, the ETH researchers were able to confirm that this is the case. However, the Congolese tropical forest as a whole appears to be a source of methane since the much smaller swamp forests of the Congo Basin periodically exhale vast amounts of this greenhouse gas. Swamp forests generate up to 1,500 times more methane during the rainy season than during the dry season, negating the sink capacity of the other two forest types.

The researchers “looked into the soil” to understand why the Congo Basin forest behaves differently in response to certain climate gases, as Six explains it. They looked at microorganisms and their functions in one study and the isotopic composition of nitrogen in nitrous oxide in another. The experts found that soil bacteria convert the majority of nitrous oxide to gaseous nitrogen (N2) to create energy for their metabolism. As a result, this powerful greenhouse gas is removed from the atmosphere. N2 is a non-toxic gas that comprises about 80 percent of the atmosphere.

“Africa is underrepresented in such climate gas measurements, and the Congo Basin is particularly poorly researched,” said Barthel. He is aware of only one such study, which dates back to 1963 and was conducted by Belgian academics. Research in Zaire, now the Democratic Republic of Congo, virtually collapsed after Mobutu assumed power. Furthermore, the vast tropical country remains inaccessible and its infrastructure is in a deplorable state.

Professor Six began the basic research for this paper in 2008, participating in two Congo River missions. He invited Barthel to investigate the movement of gas from this tropical forest, a missing piece in the global carbon cycle puzzle, based on the findings of these two missions. In 2016, as part of a two-month reconnaissance excursion, Barthel took the first measurements. This journey took him to the Kivu region of the Democratic Republic of Congo’s eastern region. The data analysis revealed that the montane forest emits very little nitrous oxide into the atmosphere.

In the following years, Barthel and local scientists established two permanent monitoring stations, one in the Kisangani region and the other in the eastern Congo’s Kahuzi-Béga National Park. During many 7- to 14-day measurement campaigns throughout the year, gas exchange between the atmosphere and the ground was measured.

The researchers established an additional measurement station in the western portion of the country, in the swamp forests of the Cuvette Centrale, from 2019 to 2020. These swamp forests make up just approximately seven percent of the Congo Basin’s total tropical forest area, but they produce so much methane that it more than compensates for the rest of the forest’s negative emissions, according to the study.

“Methane emissions there were exorbitant at times. During the first scouting expedition, we quickly realized that we wouldn’t be able to use high-​tech measuring equipment. So we worked with devices that are as easy to use, reliable and robust as possible, and that can run on batteries,” said Barthel, adding that the whole supply situation is too bad and there was no way to get spare components if something goes wrong. In addition, the power supply is unreliable.

The measuring stations were supervised by researchers from local universities. The volunteers also collected gas samples, placed them in bottles, and shipped them to Zurich for analysis. During the transfer of the over 6,500 samples, not a single one was broken or lost.

“That’s almost a miracle, because often the cardboard boxes in which the tubes were sent on their journey definitely looked the worse for wear,” said Barthel.

The study is published in the journal Nature Communications.

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By Ashikha Raoof, Earth.com Staff Writer