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 (CO2) emissions. Restoration of peatlands by reflooding reverses CO2 losses at the cost of increased methane (CH4) 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 CH4 and CO2 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 CH4 emissions recorded. These large CH4 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 CH4 emission, vary considerably across small spatial scales, despite nearly identical wetland climate, hydrology, and plant community compositions.

Seasonal Controls of CO2 and CH4 Dynamics in a Temporarily Flooded Subtropical Wetland

“…Accurately representing the factors that control CH 4 fluxes in process models is important, especially in the context of emerging GHG markets as CH 4 emissions could decrease environmental and economic benefits of wetland conservation and restoration for C credits (Hansen, ). CH 4 emissions from wetlands also make these ecosystems sources of GHGs despite their substantial C sequestration rates (Hemes et al, ). Biogeochemical models assume that an increase in plant productivity or GPP will result in increased CH 4 emissions from wetlands.…”

Net Ecosystem Carbon Balance of a Peat Bog Undergoing Restoration: Integrating CO2 and CH4 Fluxes From Eddy Covariance and Aquatic Evasion With DOC Drainage Fluxes

“…Our results show that if the water table in Burns Bog is raised enough to remain high during the growing season, the study area could switch to a C source. A recent study by Hemes et al () suggests that wetland restoration could focus on intermittent water table drawdowns to control greenhouse gas emissions, which tend to increase following rewetting. However, a raised water table is also accompanied by expansion of wetland vegetation through paludification (i.e., the in‐filling of shallow water bodies through peat formation, Hebda et al, ), so that even though there is an initial increase in C emissions due to the presence of open water, they will decline over time and C uptake would be more dominant, thus resembling a natural bog ecosystem.…”

“…Knowing the contribution of dissolved evasion as CO 2 and CH 4 can help to understand the processes that contribute to NECB. This is of particular importance for heterogeneous landscapes that integrate freshwater, marine, and terrestrial ecosystems (Chapin et al, ), and even more so for an ecosystem in the process of restoration (Hemes et al, ; Jarveoja et al, ; Strack & Zuback, ).…”

Where old meets new: An ecosystem study of methanogenesis in a reflooded agricultural peatland

“…Drained peatlands typically emit large quantities of carbon (C; Leifeld & Menichetti, ) in contrast to intact peat‐forming wetlands that typically function as soil C sinks (Bridgham, Megonigal, Keller, Bliss, & Trettin, ). Rewetting of drained peatland and organic wetland soils is therefore being explored to resequester atmospheric C, however, it may come at the cost of very high methane (CH 4 ) emissions (Hemes, Chamberlain, Eichelmann, Knox, & Baldocchi, ; Wilson et al, ) which could offset the climate mitigation of C sequestration for decades to centuries, due to the potency of CH 4 as a greenhouse gas (Neubauer & Megonigal, ).…”