Coastal Wetland Microbial Community Response to Restoration.

Wetland restoration has been proposed as a potential long-term carbon sequestration strategy, however wetland site selection and management practices are critical for ensuring restored wetlands sequester more greenhouse gases (GHG) than they emit. In an effort to better understand the underlying factors that shape the balance of carbon flux in wetland soils, we targeted the microbial communities along a salinity gradient in the San Francisco Bay-Delta region. Using 16S rRNA gene sequencing and shotgun metagenomics, coupled with greenhouse gas monitoring and soil biogeochemical characterization, we sampled sixteen sites capturing a range of salinities and restoration status. As expected, freshwater wetland soils produced more methane on average than brackish and saline sites, since sulfate in seawater discourages methane production. However, restoration status also significantly affected GHG cycling: notably, restored freshwater wetlands produced orders of magnitude more methane than their historic counterparts, possibly due to differences in trace metal and organic carbon content in younger wetlands. However, unrestored former industrial salt ponds produced methane at rates rivaling those of brackish restored wetlands, and reconnection to the Bay resulted in a decrease in methane production.  Across all systems, overall methanogen abundance was only weakly correlated with methane production, but sequence data allowed for the identification of hallmark species associated with methane production.  Our study links belowground microbial communities with their aboveground greenhouse gas production and provides a benchmark for predicting wetland soil microbial response in the face of both natural and unnatural disturbances.