Optimizing Wetlands Water Table Depth for GHG Emissions Reduction: Water Level Just Below the Surface with Space for an Oxygen-Rich Zone Above, Limits Methane Emissions and Favors CO2 Emissions, Which Have a Lower Global Warming Potential (GWP)

     New research points to a sweet spot for optimizing greenhouse gas emissions reduction by finding the optimum water table level in wetlands that inhibits methane release. Since methane has a much higher short-term global warming potential than CO2, reducing those emissions is more impactful. Wetlands need not be flooded. Many are seasonally flooded, but there are also wetlands that are not flooded but do have very shallow water tables. The conditions for delineating a wetland include the presence of wetland soils, wetland plants, and connection to local hydrology, either surface water and/or shallow groundwater.   

     It was generally thought that flooding wetlands leads to less greenhouse gas emission since the carbon in the soil below is sequestered and doesn’t reach the atmosphere. However, new research suggests that the release of methane from flooded wetlands makes total greenhouse gas emissions from those flooded wetlands exceed those of less flooded wetlands. This is especially true in shorter time frames since methane has a higher global warming potential (than CO2) in shorter time frames. Thus, comparing effects depends on the time frames analyzed.

     New research from the University of Copenhagen on wetlands in Denmark was recently published in the journal Communications Earth & Environment that quantifies the emissions effects of wetland flooding and suggests an optimized water table depth for the lowest emissions. The Science X staff at Phys.org explains CO2 sequestration in wetlands:

“Wetlands make up only about 6% of the land area but contain about 30% of the terrestrial organic carbon pool. Therefore, CO2 emissions from wetlands are central to the global climate balance. In Denmark, the plan is to flood 140,000 hectares of low-lying land such as bogs and meadows as part of the Green Tripartite Agreement. Flooding such areas will slow down the decomposition of organic material in the soil and keep the CO2 in the soil rather than allowing it to be released to the atmosphere and contribute to the greenhouse effect. At least, that has been the rationale until now.”

     The implications of the study suggest that large-scale flooding is not a good idea:

"Most people currently expect that converted Danish low-lying soils will be flooded on a large scale. But our research shows that this is not a good idea. By keeping the water level slightly below ground level, methane produced can be partly converted to the less harmful greenhouse gas CO2 before it is released, thereby limiting methane emissions," says Professor Bo Elberling from the Department of Geosciences and Natural Resource Management, who led the study.

     This makes sense to me as someone who has evaluated and designed household sewage treatment systems. For them to function properly, they need to be built above the water table, including any seasonal water tables. When we evaluated soil, we looked for indications of submerged conditions, such as the typical grey color of submerged soils and redoximorphic soil features such as mottling, and what are known as redox concentrations and redox depletions, zones where mainly iron has been leached out and accumulated, respectively. Keeping the septic system leach field above the water table, even by a few inches, means that oxygen will be available to supply the aerobic bacteria that decompose the sewage effluent, effectively treating it.

     Soils below a water table will develop redoximorphic features, and eventually, the submerged organic matter will produce methane through anaerobic bacterial decomposition. That methane will then be released into the atmosphere.

     The new study explains that the presence of an oxygenated zone above the wetland water table contains microbes that can oxidize much of the methane arising from the submerged soils below. Those microbes are known as aerobic methane-oxidizing bacteria (MOB) or methanotrophs. These MOBs counteract the release of methane into the atmosphere.  

“Here, Elberling and his colleagues have measured CO2 and methane emissions from the soil continuously for several years and have now modeled a 16-year period from 2007 to 2023. The researchers also monitored the water level, plant life and soil and air temperatures. This large database was then used in a model to simulate observations and to investigate the most optimal water level in relation to the emission of both CO2 and methane.”

"Based on our data from 2007 to 2023, we can see that the most climate-friendly water level in Maglemosen is around 10 centimeters below ground level. This is the level that overall provides the best balance between methane and CO2 emissions," says Elberling.

“The researchers emphasize that the precise recommendation of the depth of the water level will vary from wetland to wetland and will probably be somewhere between 5 and 20 centimeters below ground level. But the main point is clear: "A stable water level below ground level will almost always provide the greatest climate benefit," says Elberling.    

    

 

     While ensuring an optimal water table level in Denmark will be an engineering challenge, the professor notes that the Netherlands has done it quite well. In fact, parts of the country would be underwater if they did not manage the water table. Solar-powered pumps will likely be utilized to manage the water table.

     Other variables were identified in the study. Plant communities matter. Some plants, like canary grass, are very good at taking up soil gases like oxygen and methane through their roots and releasing them into the atmosphere. It is estimated that about 80% of the methane is released by plants. Another variable is nitrous oxide, another potent greenhouse gas. If the water table is allowed to fluctuate, there will be more nitrous oxide released into the atmosphere.  

     Similar studies of greenhouse gas releases from wetlands show that partially flooding previously drained wetlands can lower CO2 emissions more than it will increase methane and nitrous oxides. Thus, each area is slightly different depending on water table depths, past actions, and effects of changes. One study of the methane emissions of wetlands noted that while water level was the main factor affecting emissions, other factors affected seasonal emissions. These include fluctuations in temperature, dissolved oxygen levels, and gross primary production.

  

 

References:

 

Wetlands do not need to be flooded to provide the greatest climate benefit, shows study. Science X staff. Phys.org. January 29, 2026. Wetlands do not need to be flooded to provide the greatest climate benefit, shows study

Optimized wetland rewetting strategies can control methane, carbon dioxide, and oxygen responses to water table fluctuations. Bingqian Zhao, Wenxin Zhang, Peiyan Wang, Adrian Gustafson, Christian J. Jørgensen & Bo Elberling. Communications Earth & Environment volume 7, Article number: 109 (2026). Optimized wetland rewetting strategies can control methane, carbon dioxide, and oxygen responses to water table fluctuations | Communications Earth & Environment

To reduce greenhouse gas emissions from wetlands, just add (fresh) water. Laura Oleniacz, North Carolina State University. Phys.org. November 9, 2022. To reduce greenhouse gas emissions from wetlands, just add (fresh) water

Measuring and modeling methane emissions in wetlands. Aaron Sidder, American Geophysical Union. Phys.org. March 1, 2024. Measuring and modeling methane emissions in wetlands

Bacteria in lakes fight climate change: The role of methanotrophs as biological methane filter. Max Planck Society. Phys.org. August 12, 2024. Bacteria in lakes fight climate change: The role of methanotrophs as biological methane filter