The vast swamps and peatlands of the tropics play an important role in the global carbon cycle and consequently in the global climate. The Amazon basin, the Congo basin, and the tropical wetlands of Southeast Asia accumulate carbon in the form of dead, undecomposed plant material, storing around 100 gigatonnes of carbon in the process.
One of the largest and most important of these tropical carbon stores is situated in the Congo Basin in the heart of Africa, home to the mighty Congo River and its numerous tributaries. Although the swamps and peatlands of the Congo Basin cover only 0.3 per cent of the Earth's land surface, they hold one third of the carbon stored in tropical peatlands.
Just how great the impact of these peat ecosystems is on the global carbon cycle and climate has hardly been researched, partly because the central Congo Basin remains difficult to access. Boats and pirogues are often the only means of transport for reaching the remote swamps and lakes.
Research uncovers surprises
A research team headed by ETH Zurich has taken a closer look at the Congo Basin in the last decade. In the process, the researchers uncovered several surprises, such as one of the darkest blackwater rivers in the world, the Ruki River (ETH News reported).
In the latest study, which has just been published in the journal Nature Geoscience, the researchers once again focused on water that has been darkened by the leaching of plant debris: Africa's largest blackwater lake, Lac Mai Ndombe, and its smaller neighbour, Lac Tumba – and they once again met with a surprise.
More than four times the size of Lake Constance, the water of Lake Mai Ndombe resembles black tea. The lake is surrounded by extensive swamp forests and virtually untouched lowland rainforest growing on thick peat. Organic matter washed out of decaying plant and soil material from the surrounding swamp and lowland rainforests colours the lake water dark brown.
Ancient carbon released
Now, researchers have shown that large amounts of carbon in the form of CO₂ are emitted into the atmosphere by way of the two lakes.
Contrary to the researchers' expectations, however, only some of the carbon is from recently produced plant matter. Up to 40 per cent of the carbon stems from peat that has accumulated in the surrounding ecosystems over thousands of years. This is shown by age determinations (radiocarbon dating) of the CO₂ dissolved in the lake water.
"We were surprised to find that ancient carbon is being released via the lake," explains lead author Travis Drake, a scientist in the Sustainable Agroecosystem (SAE) group led by ETH Professor Johan Six. "The carbon reservoir has a leak, so to speak, from which ancient carbon is escaping," adds co-author Matti Barthel, research technician in SAE.
Just how is the carbon released?
Until now, research assumed that carbon stored in the peat of the Congo Basin remained bound for a very long time and was only released under certain conditions, such as prolonged droughts.
It remains unclear just how the carbon is mobilised from the undecomposed plant material. The pathways by which the carbon enters the lake water are also still unknown.
Consequently, it is crucial for researchers to find out whether the release of old carbon indicates a destabilising turning point or a natural state of equilibrium that is balanced by new peat deposits.
Is there a risk of the peatlands drying up?
The release of old carbon could indicate a larger problem, namely that environmental changes triggered by climate change are leading to a chain reaction.
If the climate becomes drier, for example, more carbon could be mobilised because the peat dries out more often and for longer periods of time, allowing oxygen to penetrate deeper the peat layers. This promotes the decomposition of once-stable organic matter by microorganisms, with consequences for the global climate as more CO₂ from this huge carbon store is released into the atmosphere.
"Our results help to improve global climate models, because tropical lakes and wetlands have been underrepresented in these models so far," as Six stated.
Water levels have a massive influence on degassing
In addition to investigating the age and origin of the degassed CO₂, the researchers also examined emissions of two other important greenhouse gases from Lake Mai Ndombe, namely nitrous oxide and methane.
In this parallel study, which was published in the Journal of Geophysical Research, the researchers found that water levels, for example, have a strong influence on the volume of methane escaping into the atmosphere.
The higher the water level of the lake, the more effectively microorganisms break down methane. If the water levels are low(er), as is usual during the dry season, methane is broken down less effectively and escapes from the lake in larger quantities.
"Our fear is that climate change will also upset this balance. If droughts become longer and more intense, the blackwater lakes in this region could become significant sources of methane that impact on the global climate," says ETH Professor Jordon Hemingway. "At present we do not know when the tipping point will be reached."
Changes in land use could prove to be serious
But it is not only climate change that could affect the balance. Changes in land use could incur even more serious consequences. According to estimates, the population of the Democratic Republic of Congo is set to triple by 2050. In order to gain arable land, more forest land will be cleared in future.
Deforestation, in turn, promotes drought, which could keep lake levels permanently low. "We all know the analogy whereby forests are the green lungs of the Earth," says Barthel. "They are not only responsible for gas exchange like our lungs, however, but they also evaporate water through their leaves, thereby enriching the atmosphere with water vapour. This promotes cloud formation and precipitation, which in turn feeds rivers and lakes."
The results help to clarify the role of tropical peatlands and blackwater lakes in global climate change. The research is also vital for developing strategies to reduce greenhouse gases and protect wetlands in the Congo Basin and around the equator belt.
These studies were conducted as part of the TropSEDs project led by ETH Zurich and funded by the Swiss National Science Foundation, in collaboration with scientists from the University of Louvain in Belgium and the Democratic Republic of Congo.
