Greenhouse Gas Emissions from Agricultural Land Before and After Permanent Flooding with Seawater or Freshwater

Sea level rise driven by global warming is threatening low-lying and reclaimed agricultural areas near coasts. The most marginalized of these with low crop yield can be converted into new valuable wetland ecosystems with high CO 2 mitigation capacity by removing drainage systems or performing managed realignment. This study assessed CO 2 and CH 4 dynamics before and after forming two adjacent wetlands by flooding reclaimed agricultural land. The Gyldensteen Coastal Lagoon (214 ha) is an open system flooded with seawater, and the neighboring Lake Engsø (144 ha) is a closed system flooded with freshwater. Before flooding in 2014, the total area was a net source of about 10,350 Mg CO 2 yr −1 due to aerobic microbial soil respiration. The temporal and spatial pattern of CO 2 and CH 4 release changed after flooding in a pattern that depended on soil biogeochemistry, temperature, and other environmental factors. Thus, there were strong exponential temperature dependencies of CO 2 and CH 4 emissions. Slow anaerobic microbial action in the Coastal Lagoon soil and the presence of sulfate prevented CO 2 and CH 4 emission, leading to a slight net uptake of CO 2 in 2019 (−70 Mg CO 2 yr −1 ). Conversely, methanogenesis near the soil–water interface after freshwater flooding of Lake Engsø drove rapid emission of CH 4 (diffusive and ebullitive) that doubled its greenhouse gas emissions. In conclusion, CH 4 emissions in Lake Engsø therefore counteracted the CO 2 mitigation effect by flooding and the total area remains a net source of greenhouse gases with an emission of 8330 CO 2 -equivalents yr −1 .