Managing Restored Wetlands in the Sacramento-San Joaquin Delta to Reduce Methane Emissions and Increase Carbon Uptake Laurie Koteen, Sara Knox, Cove Sturtevant, Joseph Verfaillie, Jaclyn Hatala, Dennis Baldocchi

The Sacramento-San Joaquin Delta of California is a region transformed by more than a century of agricultural practices. Beginning in the 19th century, substantial regions were first drained of water and then converted to cropland in order to take advantage of the area's rich peatland soils. In the intervening time period, soil oxidation and subsidence have led to huge peat losses of up to 10 m in some places, and river water now threatens to topple the levees that were erected to keep fields from flooding. Within this region, we have been monitoring greenhouse gas exchange of several agricultural sites, a degraded pasture, and two restored wetlands. Of these land use types, restoration of wetlands is of particular interest to Delta managers as these sites attain many of the region's most pressing ecological goals, including improved water quality, increased wildlife habitat, and soil accretion. In our current investigation, we hope to assess if wetland management activities can be implemented to achieve greenhouse gas management goals as well. While we find that the restored wetlands are able to take up and store a substantial amount of carbon via rapid growth rates, they also emit methane; a greenhouse gas 25 times more potent than CO¬2. We are currently in the process of implementing two management activities with the goals of reducing methane emissions and increasing carbon uptake. Evidence from the wetland literature indicates that periodic lowering of the water table below the soil surface can reduce wetland methane emissions by: 1. Reintroducing oxygen into the soil column. This both supports growth of the methanotrophic bacteria that consume methane produced in the anaerobic zones of the soil column, and suppresses the methanogens that produce it. 2. Re-oxidization of formerly reduced compounds in the soil, (i.e. NO3-, SO42-) which can serve as alternative terminal electron acceptors of the decomposition byproducts (i.e. H2 and acetate) that lead to methane formation. Under these conditions, it becomes more energetically favorable for alternative chemical transformations to occur in which CO2 and not CH4¬ is released. A second management activity would be implemented to see if wetland carbon uptake could be increased. As wetlands mature, perennial wetland vegetation often develops a significant thatch layer which can reduce photosynthesis of growing shoots by blocking radiation from leaf surfaces. Here we propose to remove the stalks of established vegetation. This would serve two goals: 1. Decomposing vegetation would be incorporated into the soil, leading to soil accretion, and 2. Thatch removal would liberate fledgling shoots, potentially increasing carbon uptake through subsequent seasons. A third investigation would compare CO2 and CH4 fluxes at an existing tower atop low salinity sediments with a new tower where site salinity is relatively high. This effort would inform new site selection efforts for wetland restoration projects. Sites located closer to the San Francisco Bay Area are tidally-influenced, and therefore have higher salinity than the impounded freshwater systems of our current study within the Delta region.