Methane Fluxes in Tidal Wetlands Across the Contiguous United States: Annual Conversion Factors and the Interplay of Salinity and Temperature on CH4 flux

Tidal wetlands have the potential to provide major feedback to the Earth system as they exchange greenhouse gasses with the atmosphere including methane (CH4). However, global atmospheric CH4 contributions from tidal wetlands are not well constrained due to the lack of systematic observations, data quality, uncertainties associated with the area they cover, and the risk of double counting ecosystem types. Unlike wetlands inland, tidal wetlands experience spatial and temporal variations in tidal inundation, redox potential, and seawater influence, which yields spatial gradients in plant species and traits, microbial communities, and processes such as CH4 production. Salinity and sulfate concentrations, temperature, soil properties, sediment accretion, organic matter quality and quantity, tidal pumping, dominant vegetation type, and hydrology have been identified as important drivers of CH4 emissions in initial syntheses. However, the relative influence of overlapping predictors has yet to be resolved in a quantitative fashion. Here, we present the results of the Coastal Carbon Research Coordination Network's tidal wetland CH4 flux synthesis. This includes the creation of an open-source database of > 10,000 disaggregated chamber-based CH4 flux measurements along with ancillary covariates, and the synthesis and analysis of chamber and eddy covariance CH4 flux measurements across tidal wetlands in the contiguous United States. We review annual CH4 budgets, predictors of CH4 variability, and the CH4-salinity/sulfate relationship. Results provide i) conversion factors to scale discrete chamber measurements to annual estimates, ii) a new classification scheme of tidal wetland CH4 fluxes based on salinity and mean annual temperature, and iii) a revised salinity and sulfate concentration threshold above which CH4 production is limited. Our work highlights the power of community syntheses to improve understanding of tidal wetland CH4 flux predictors, advance science-informed blue carbon policy, and enhance tidal wetland ecosystem management. We acknowledge the CCRCN data contributors for the data provided.