Methane fluxes in tidal marshes of the conterminous United States.

Methane (CH) is a potent greenhouse gas (GHG) with atmospheric concentrations that have nearly tripled since pre-industrial times. Wetlands account for a large share of global CH emissions, yet the magnitude and factors controlling CH fluxes in tidal wetlands remain uncertain. We synthesized CH flux data from 100 chamber and 9 eddy covariance (EC) sites across tidal marshes in the conterminous United States to assess controlling factors and improve predictions of CH emissions. This effort included creating an open-source database of chamber-based GHG fluxes (https://doi.org/10.25573/serc.14227085). Annual fluxes across chamber and EC sites averaged 26 ± 53 g CH m year, with a median of 3.9 g CH m year, and only 25% of sites exceeding 18 g CH m year. The highest fluxes were observed at fresh-oligohaline sites with daily maximum temperature normals (MATmax) above 25.6°C. These were followed by frequently inundated low and mid-fresh-oligohaline marshes with MATmax ≤25.6°C, and mesohaline sites with MATmax >19°C. Quantile regressions of paired chamber CH flux and porewater biogeochemistry revealed that the 90th percentile of fluxes fell below 5 ± 3 nmol m s at sulfate concentrations >4.7 ± 0.6 mM, porewater salinity >21 ± 2 psu, or surface water salinity >15 ± 3 psu. Across sites, salinity was the dominant predictor of annual CH fluxes, while within sites, temperature, gross primary productivity (GPP), and tidal height controlled variability at diel and seasonal scales. At the diel scale, GPP preceded temperature in importance for predicting CH flux changes, while the opposite was observed at the seasonal scale. Water levels influenced the timing and pathway of diel CH fluxes, with pulsed releases of stored CH at low to rising tide. This study provides data and methods to improve tidal marsh CH emission estimates, support blue carbon assessments, and refine national and global GHG inventories.