A Biogeochemical Compromise: The High Methane Cost of Sequestering Carbon in Restored Wetlands

Abstract Peatland drainage is an important driver of global soil carbon loss and carbon dioxide (CO 2 ) emissions. Restoration of peatlands by reflooding reverses CO 2 losses at the cost of increased methane (CH 4 ) emissions, presenting a biogeochemical compromise. While restoring peatlands is a potentially effective method for sequestering carbon, the terms of this compromise are not well constrained. Here we present 14 site years of continuous CH 4 and CO 2 ecosystem‐scale gas exchange over a network of restored freshwater wetlands in California, where long growing seasons, warm weather, and managed water tables result in some of the largest wetland ecosystem CH 4 emissions recorded. These large CH 4 emissions cause the wetlands to be strong greenhouse gas sources while sequestering carbon and building peat soil. The terms of this biogeochemical compromise, dictated by the ratio between carbon sequestration and CH 4 emission, vary considerably across small spatial scales, despite nearly identical wetland climate, hydrology, and plant community compositions.