Many countries are considering or have already planned to convert lowland agricultural lands into wetlands and thus save carbon dioxide2 emissions. The idea is that oxygen-depleted soils in wetlands emit fewer greenhouse gases than anoxic soils, benefiting the climate.
But such transfers can come with a surprisingly high price for the climate, warn biologists from the University of Southern Denmark:
“There is a risk that the new wetlands will end up emitting more greenhouse gases than they consume,” says Erik Christensen, professor of ecology at the University of Southern Denmark.
The problem is specifically related to freshwater lakes with standing water. When the flooded area is kept wet, such as a swamp, methane emissions drop dramatically.
Together with his colleagues from the Department of Biology, he followed for several years a large tract of land called the Gyldensteen Strand in Denmark. In 2014, as part of a nature restoration project implemented by the Aage V. Jensen Nature Foundation, cultivated lowlands were flooded.
Because the project is unique in Europe, the Foundation has funded a series of research projects aimed at learning more about what happens when former lowland farmlands are converted into wetlands.
This knowledge can help legislators and authorities make the best and smartest decisions possible when farmland is taken out of wetland use.
“We’ve been doing this experiment on a large scale for several years now, and it shows that large amounts of methane are released into the atmosphere when the Earth is flooded with fresh water, creating shallow lakes. That’s why we say, if you want to turn coastal low-lying areas into wetlands, do it with salt water if you can,” says Erik Christensen.
The study, co-authored by Christensen and fellow biologists Susan Goldberg-Petersen and Cynthia Quintana of the Department of Biology, was published in the journal, estuaries and coasts.
The Gyldensteen Strand was submerged into two parts: part with sea water after the dikes were removed, and another part with fresh water. Studies of greenhouse gas emissions from these two different wetland regions are now leading researchers to warn against the creation of shallow freshwater lakes.
Swamps and wet meadows are better
“We see that when fresh water is stagnant in a wetland – even if it’s only 10 cm of water – there are significant methane emissions. The same does not happen in an area flooded with salt water,” says Erik Christensen.
Therefore, he recommends dumping sea water as much as possible. Alternatively, if there is only fresh water for flooding, standing water should be avoided; Instead, the area should be a moist meadow with no standing water.
“The problem is standing water. Our studies show that when a freshwater area is only wet, like a swamp or a wet meadow without standing water, oxygen from the atmosphere will penetrate and help bacteria in the wet surface consume all the methane that escapes from deeper, oxygen-free layers,” explains Eric Christensen.
According to the study, large amounts of methane play a role.
The researchers stress that their calculations are extrapolated from measurements and data from the freshwater area of the Gyldensteen Strand, a lake called Engsø, which covers 144 hectares and has an average depth of one meter.
The calculation assumption is that other regions behave somewhat similarly to Engsø.
“Some regions will emit less methane, while others will emit more, but overall, we think we can extrapolate,” says Erik Christensen.
Extrapolated calculations show that if 100,000 hectares were converted to a freshwater wetland with stagnant water, it would release methane equivalent to 6 million tons of carbon dioxide.2 equivalent per year.
Of course, there is also CO2 Benefit from non-cultivation of these areas, which amounts to about 3 million tons of carbon dioxide2 synonym. This must be subtracted from 6 million tons. Therefore, the net result is 3 million tons.
In other words, 100,000 hectares will produce an additional 3 million tons of carbon dioxide2 than if they were still cultivating. This is equivalent to the emissions of approximately 750,000 cows. There are about 1.5 million cows in Denmark.
The other flooded area on the Gyldensteen Strand is a coastal lake that was created after dams were removed and seawater was allowed to flood the area.
“The numbers are much better in the coastal lagoon. There is no CO.”2 It emits, which is an improvement over when it was farmland,” says Erik Christensen.
The reason lies in the oxygen-depleted mud under salt and fresh water, where various bacteria come into play.
Methane-producing bacteria thrive in fresh water but are inhibited in salt water. They are also active when the soil is only moist, but then atmospheric oxygen helps the oxygen-consuming bacteria near the surface to consume the methane. Therefore, only small amounts of methane are released from the moist soil.
Future plans for the Lowlands
However, the much lower oxygen content of the standing water inhibits methane-consuming bacteria, allowing large amounts of methane to be released, seeping or bubbling through the water.
“When we’re on field trips to Engsø and wading through the water, we can see bubbles forming where we’re walking. And that methane is released from the mud where we’re going,” explains Erik Christensen.
According to him, these methane bubbles must be taken into account when measuring methane emissions from freshwater wetlands. “If you just measure seepage, you get much lower values. But bubbles need to be taken into account to get an accurate picture. I think the knowledge we gained from the Gyldensteen Strand should be included in future hydro-work plans so that we can design new wetlands in an intelligent way.”
Basis of our accounts: We base our calculations on data from the freshwater flooded Engsø, which covers 144 hectares. According to the researchers’ measurements, Nigso emits 303 tons of methane each year, which is equivalent to 8,400 tons of carbon dioxide.2 equivalent (about 58 tons per hectare).
About Gyldensteen Strand: 616 hectares of former farmland have been acquired by the Aage V. Jensen Nature Foundation and converted into a sea lake, a freshwater lake and a reed wetland, among other things. Biologists from the University of Southern Denmark have been working on several research projects since the floods in 2014.