Many countries are considering or already planning to convert lowland farmland to wetlands to save CO.2 emissions. The idea is that the oxygen-depleted soil in wetlands emits fewer greenhouse gases than oxygenated cultivated land, thereby benefiting the climate.
But such conversions may come with a surprisingly high price for the climate, warn biologists from the University of Southern Denmark.
“There is a risk that new wetlands may release more greenhouse gases than they take in,” says Erik Kristensen, professor of ecology at the University of Southern Denmark.
The problem is especially tied to freshwater lakes with standing water. If the flooded area remains moist, such as a marsh, methane emissions drop significantly.
Together with colleagues from the Department of Biology, Kristensen followed a large area named Gyldensteen Strand in Denmark for several years. In 2014, as part of a nature restoration project carried out by the Aage V. Jensen Nature Foundation, the area of cultivated lowland was flooded.
Because the project is unique in Europe, researchers are conducting a series of research projects aimed at learning more about what happens when former low-lying agricultural land is turned into wetlands. This knowledge will help policy makers and authorities to make the best possible and wisest decisions when agricultural land is taken out of use for wetlands.
“We’ve been doing this big experiment for a few years now, and it’s showing us that a lot of methane is released into the atmosphere when the land is flooded with fresh water, creating shallow lakes. That’s why we say, low-lying coastal areas to wetlands, do it with salt water if possible,” Kristensen said.
The study, co-authored by Kristensen and fellow biologists Susan Guldberg Petersen and Cintia Quintana from the Department of Biology at the University of Southern Denmark, was published in Estuary and Coast.
The Gyldensteen Strand was flooded in two parts: one part by seawater after the removal of the dikes, and another part by fresh water. It is studies of greenhouse gas emissions from these two different wetland areas that are now leading researchers to warn against the creation of shallow freshwater lakes.
Swampy and moist pastures are better
“We found that when there is standing fresh water in a wetland, even if the water is only 10 centimeters, there is a significant emission of methane. The same does not happen in an area flooded with salt water ,” said Kristensen.
Therefore, he recommends flooding with sea water if possible. On the other hand, if fresh water is available for flooding, standing water should be avoided; instead, the area should be kept as a wet meadow without standing water.
“The problem is standing water. Our studies show that if an area of fresh water is just wet, like a bay or a wet meadow without standing water, oxygen from atmosphere seeps in and helps the bacteria on the wet surface eat all the methane that’s leaking from the deeper oxygen-free layers,” Kristensen explained.
According to the study, large amounts of methane are at play.
The researchers emphasized that their calculations were taken from measurements and data from the freshwater area of Gyldensteen Strand, a lake called Engsø, which covers 144 hectares and has an average depth of 1 meter. The assumption for the calculation is that the other areas behave similarly to Engsø.
“Some areas emit less methane, while others emit more, but in general, we believe we can extrapolate,” Kristensen said.
Extrapolated calculations show that if 100,000 hectares were converted into freshwater wetlands with standing water, they would release methane equivalent to nearly 6 million tons of CO.2 equivalent per year.
Of course, there is also a CO2 would benefit from not cultivating these areas, amounting to approximately 3 million tons of CO2 equivalents. It should be subtracted from 6 million tons. So, the net result is 3 million tons.
In other words, 100,000 hectares produce 3 million tons of CO2 equal to if they were still being farmed. This is equivalent to the emissions of approximately 750,000 cows. There are about 1.5 million cows in Denmark.
The other flooded area of Gyldensteen Strand is a coastal lagoon that was created after the dikes were removed and seawater was allowed to flood the area.
“The numbers are better in the coastal lagoon. There is no CO2 emitted, which is an improvement compared to agricultural land,” said Kristensen.
The reason is in the oxygen-depleted mud under salt water and fresh water, where different bacteria enter.
Methane-producing bacteria can survive in fresh water but are inhibited in salt water. They are also active when the soil is just wet, but then, the oxygen in the atmosphere helps the oxygen-consuming bacteria near the surface that eat the methane. Therefore, very little methane is released from wetlands.
Future plans for the lowlands
However, the extremely low oxygen content of standing water inhibits methane-consuming bacteria, allowing large amounts of methane to be released, seeping or seeping into the water.
“When we are on field trips in Engsø and wade in the water, we see bubbles forming where we walk. That’s methane released from the mud where we walk,” explains Kristensen.
He emphasized that such methane bubbles should be taken into account when measuring methane emissions from freshwater wetlands.
“If you only measure the seepage, you get lower values. But the bubbles have to be taken into account to get an accurate picture. I believe that the knowledge we got from Gyldensteen Strand should be included in the future water action plan so that we can design new wetlands in an intelligent way. Otherwise, there is a risk of creating wetlands that are not climate friendly,” said Kristensen.
Susan Guldberg G. Petersen et al, Greenhouse Gas Emissions from Agricultural Land Before and After Permanent Flooding with Seawater or Freshwater, Estuary and Coast (2023). DOI: 10.1007/s12237-023-01218-6
Provided by the University of Southern Denmark
Citation: Study shows salt water or freshwater is an important climate consideration when flooding low-lying areas (2023, July 18) retrieved on July 18, 2023 from https://phys.org /news/2023-07-saltwater-freshwater-important-climate-consideration.html
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