The Congo tropical forest is simply different

Date:
February 7, 2022
Source:
ETH Zurich
Summary:
Until now, research assumed that the vast forest area of the Congo
Basin, like other tropical forests, releases large amounts of
nitrous oxide and binds methane. Researchers have now shown that
it behaves differently: methane is released, while nitrous oxide
emissions are smaller than thought.



FULL STORY ==========================================================================
he Congo Basin is the second largest contiguous tropical forest area
on earth.

Even though it is so vast and plays such a major role in the global
climate system, there is no empirical data on the forest's gas exchange
with the atmosphere. Especially with regard to the greenhouse gases
nitrous oxide and methane, research is still in the dark.


========================================================================== Other tropical forests are known to be a major source of nitrous oxide
and a sink for methane. But because comparable data are lacking for the
Congo, the study of how these greenhouse gases behave there has so far
been limited to modelling . The data used for the models comes from the
Amazon, Indonesia and the tropical part of Australia; thus scientists
assumed until today that the Congo Basin behaved similarly to these
other tropical forests.

Now, an international research team led by ETH Professor Johan Six has
partly filled this blank spot. In an elaborate and difficult measurement campaign lasting several years, the scientists determined how much
methane and nitrous oxide the tropical forest of the Congo Basin absorbs
or releases. Their study - - one of the first available on the subject
for that part of the world -- has just been published in the journal
Nature Communications.

Between 2016 and 2020, the researchers measured gas fluxes at multiple
sites in three different forest types in the Congo Basin. The forest types studied include montane forest, lowland tropical forest and periodically flooded swamp forest.

Divergent gas fluxes This shows for the first time that the gas
fluxes of the tropical forests of the Congo Basin differ from those
of other tropical regions of the world. For example, nitrous oxide
emissions from the forest are comparatively low. "This was unexpected,"
says Matti Barthel, a research associate with Six and the study's lead
author. "Our measurements from both the first short measurement campaigns
and subsequent long-term studies didn't confirm the model assumptions,"
he says.



==========================================================================
The situation is different for methane. According to the models, the
Congo Basin should be a methane sink. The ETH researchers were able
to confirm this for the montane and lowland tropical forests. However,
because the much smaller swamp forests of the Congo Basin periodically
emit enormous amounts of this greenhouse gas, the Congolese tropical
forest as a whole seem to be a source of methane. During the rainy season, swamp forests emit up to 1,500 times more methane than during the dry
season, negating the sink capacity of the other two forest types.

To understand why the Congo Basin forest behaves differently with respect
to these climate gases, the researchers "looked into the soil," as Six
says. In one study, they studied microorganisms and their functions,
and for another, the isotopic composition of the nitrogen in the
nitrous oxide. Both studies suggest that soil microbes convert most
nitrous oxide to gaseous nitrogen (N2) in order to generate energy
for their metabolism. This removes this potent greenhouse gas from
the atmosphere. N2 itself is harmless -- it makes up 80 percent of
the atmosphere.

Research under different conditions For this study, the ETH researchers conducted the first ever gas measurements in the Congo Basin. "Africa is underrepresented in such climate gas measurements, and the Congo Basin
is particularly poorly researched," Barthel says. To his knowledge,
there is only one study for this area, dating from 1963, by Belgian researchers. Not only did research in Zaire, now the Democratic
Republic of Congo, collapse almost completely after Mobutu took power,
but the huge equatorial country is still rather poorly accessible and
its infrastructure is in a parlous state.

Professor Six began the preliminary work for this study in 2008/09,
taking part in two expeditions to the Congo River. Given the results
from these two expeditions, he asked Barthel to study the flow of gas
from this tropical forest -- a missing piece in the global carbon cycle
puzzle. Barthel conducted initial measurements in 2016 as part of a
two-month scouting expedition. This expedition took him to the Kivu
region in the eastern Democratic Republic of Congo. The data analyses
showed him that surprisingly little nitrous oxide was escaping into the atmosphere from the montane forest.



========================================================================== Permanent observations carried out for the first time In subsequent years, Barthel and local scientists set up two permanent monitoring stations,
one in the region around Kisangani, the other in Kahuzi- Bie'ga National
Park, eastern Congo. There, gas exchange between the atmosphere and
the ground was then measured during several 7- to 14-day measurement
campaigns throughout the year.

From 2019 to 2020, the researchers operated an additional measurement
station in the western part of the country, in the swamp forests of
what is known as the Cuvette Centrale. "Methane emissions there were
exorbitant at times," Barthel says. These swamp forests occupy only
about 7 percent of the total tropical forest area in the Congo Basin,
but emit such a large amount of methane that it more than compensates
for the negative emissions of the rest of the forest, the researcher says.

"During the first scouting expedition, we quickly realised that we
wouldn't be able to use high-tech measuring equipment," Barthel says,
adding that the general supply situation is too poor, with no way to
obtain spare parts if anything broke. The power supply is also not stable,
he says, "so we worked with devices that are as easy to use, reliable and robust as possible, and that can run on batteries." Not a single probe
was lost Researchers from local universities supervised the measuring
stations. Local employees also took gas samples, filled them into vials
and sent them to Zurich for laboratory analysis. Of the more than 6,500 samples, not a single one was broken or lost during transport. "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" the
ETH researcher says with a smile.

========================================================================== Story Source: Materials provided by ETH_Zurich. Original written by
Peter Ru"egg. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Matti Barthel, Marijn Bauters, Simon Baumgartner, Travis W. Drake,
Nivens
Mokwele Bey, Glenn Bush, Pascal Boeckx, Clement Ikene Botefa,
Nathanae"l De'riaz, Gode Lompoko Ekamba, Nora Gallarotti, Faustin
M. Mbayu, John Kalume Mugula, Isaac Ahanamungu Makelele, Christian
Ekamba Mbongo, Joachim Mohn, Joseph Zambo Mandea, Davin Mata
Mpambi, Landry Cizungu Ntaboba, Montfort Bagalwa Rukeza, Robert
G. M. Spencer, Laura Summerauer, Bernard Vanlauwe, Kristof Van Oost,
Benjamin Wolf, Johan Six. Low N2O and variable CH4 fluxes from
tropical forest soils of the Congo Basin. Nature Communications,
2022; 13 (1) DOI: 10.1038/s41467-022-27978-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220207083459.htm

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