Carbon Sequestration in Wetlands: a cross Comparison of Intact and Restored, Tidal and Non-tidal Freshwater Wetlands

The role of wetlands in mitigating the effects of climate change and in sequestering atmospheric CO₂ depends on the efficiency with which organic carbon (C) is incorporated into soils, versus exported to adjacent waterbodies and/or offset by methane (CH₄) emissions. The net rate of C accumulation or loss from an ecosystem conforms the net ecosystem carbon balance (NECB), which varies considerably and depends on a wide range of wetland parameters such as elevation, vegetation, inundation, salinity, soil type, tidal influence, soil accretion and/or wetland age. Reference information on NECB from different wetland types is scarce, although it is an extremely useful tool to advise how greenhouse gas (GHG) fluxes may change if wetlands are restored and how to optimally design wetland restoration projects to maximize C uptake and minimize GHG production. Here we present the progress in building the NECB of wetlands in the Sacramento-San Joaquin Delta that encompass intact and restored, tidal and impounded freshwater wetland types. More than a decade of eddy covariance measurements of CO₂ and CH₄ fluxes shows that freshwater, restored non-tidal wetlands in the Delta are sinks of C, with average net annual ecosystem exchange (NEE) of -220 to -450 g C-CO₂ m^-2 yr^-1. However, their CH₄ production due to their permanently inundated conditions is high, 40 - 50 g C-CH₄ m^-2 yr^-1 making them net GHG sources to the atmosphere over decadal timescales considering the global warming potential (GWP₂₀) of CH₄ of 87 CO₂eq. Soil cores were collected from these restored wetlands and from intact wetlands in the Delta along with surface water samples to measure century-scale C burial rates and dissolved organic and inorganic C fluxes, respectively. With these results we will be able to compare C sequestration in soils to the aquatic C flux and CH₄ emissions using direct field observations relying on eddy-covariance, chamber measurements, and century scale C burial rates.