Wetlands cover only around six percent of Earths land area but hold about 30 percent of the terrestrial organic carbon pool. In Denmark, authorities plan to rewet 140,000 hectares of low-lying land, including bogs and meadows, under the Green Tripartite Agreement to keep carbon locked in the soil instead of releasing it as CO2.
Flooding peatlands slows the decomposition of organic matter and has been seen as an efficient way to cut CO2 emissions. The new study, published in Communications Earth and Environment, finds that permanently saturated soils can instead create ideal conditions for methane generation, a greenhouse gas up to 30 times more powerful than CO2 over a 100 year period.
Lead author Professor Bo Elberling from the Department of Geosciences and Natural Resource Management explains that large scale permanent flooding of Danish low-lying soils is not a good idea. He says that by keeping the water level slightly below the surface, some of the methane produced in deeper, wetter layers can be converted to CO2 before it reaches the atmosphere, reducing the overall climate impact.
The work focuses on Maglemosen, a peat rich wetland about 20 kilometers north of Copenhagen that has remained undisturbed for more than a century. The site is considered representative of typical Danish peat wetlands and provided a natural laboratory to test how greenhouse gas emissions respond to water table changes.
Researchers monitored Maglemosen over many years, continuously measuring soil CO2 and methane emissions. They also tracked water levels, plant communities and soil and air temperatures. Using this extensive dataset, they modelled a 16 year period from 2007 to 2023 to explore how different water table positions affect combined CO2 and methane fluxes.
The modelling indicates that the most climate friendly water level at Maglemosen is about 10 centimeters below the soil surface. At this depth, the team found the best overall balance between methane and CO2 emissions, with methane production limited and enough oxygen present in upper soil layers to support methane consuming microbes.
A key factor is the activity of methane oxidizing microorganisms in the aerated upper soil. These microbes use oxygen to convert methane rising from deeper waterlogged layers into CO2, preventing some methane from reaching the atmosphere. When the surface soil is flooded, it quickly becomes oxygen free and this methane filter effectively switches off.
The authors stress that the optimal water depth will differ among wetlands depending on local conditions, but is likely to fall between 5 and 20 centimeters below ground level in many cases. The central message is that a stable water table below the surface, rather than full inundation, will almost always provide the greatest climate benefit.
Achieving this balance will require active water management. Elberling notes that ensuring a stable water level in new Danish wetlands is an engineering challenge, since conditions must be wet but not fully saturated. Managing dry summers and heavy autumn rains will demand infrastructure and continuous monitoring to avoid large fluctuations.
The study points to the Netherlands as an example of how to maintain a constant water table in low lying landscapes. The country relies on a network of pumps and canals to keep large areas from flooding, and similar approaches powered by green energy such as solar systems could help stabilize water levels in restored Danish peatlands.
Vegetation changes in rewetted lowlands add another layer of complexity. In Maglemosen, Canary grass dominates and, like rice, can transport oxygen down into the soil and methane up through its tissues. The researchers estimate that about 80 percent of methane at the site is released via plants, and they expect Canary grass to become more common in future lowland restorations.
Greater dominance of such species could increase direct methane transport from soil to air, leaving less time for microbial oxidation in the upper layers. This makes fine control of the water table even more important so that methane consuming microbes have a chance to act before gases escape through plant stems.
The water level also strongly influences nitrous oxide emissions, another potent greenhouse gas around 300 times more powerful than CO2 over a century. If water tables in rewetted areas are allowed to rise and fall freely with weather, nitrous oxide pulses could reduce or even negate the intended climate benefits.
Elberling warns that simply flooding lowlands and walking away is not enough. Without careful regulation of water levels, methane and nitrous oxide emissions could undermine gains from reduced CO2 release. He argues that successful peatland restoration must treat hydrology as an active management task, not a one time intervention.
The research was carried out by scientists from the University of Copenhagen, Lund University and Aarhus University. Co authors include Bingqian Zhao, Wenxin Zhang, Peiyan Wang, Adrian Gustafson and Christian J. Jorgensen, who together combined long term field measurements with process based modelling to refine wetland rewetting strategies.
Their paper, titled Optimized wetland rewetting strategies can control methane, carbon dioxide, and oxygen responses to water table fluctuations, provides guidance for policymakers designing large scale peatland restoration projects. The work suggests that targeting specific subsurface water levels and stabilizing them over time can maximize climate benefits.
The findings are particularly relevant as countries look to land based carbon solutions to meet climate targets. In Denmark and beyond, the authors argue that wetlands can play a significant role in climate mitigation, but only if rewetting projects are designed to manage the trade offs between different greenhouse gases over decades.
Research Report:Optimized wetland rewetting strategies can control methane, carbon dioxide, and oxygen responses to water table fluctuations
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