Methane fluxes in tidal marshes of the conterminous United States

Abstract

Methane (CH₄) 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 CH₄ emissions, yet the magnitude and factors controlling CH₄ fluxes in tidal wetlands remain uncertain. We synthesized CH₄ 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 CH₄ 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 CH₄ m⁻² year⁻¹, with a median of 3.9 g CH₄ m⁻² year⁻¹, and only 25% of sites exceeding 18 g CH₄ m⁻² year⁻¹. 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 CH₄ flux and porewater biogeochemistry revealed that the 90th percentile of fluxes fell below 5 ± 3 nmol m⁻² s⁻¹ 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 CH₄ 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 CH₄ flux changes, while the opposite was observed at the seasonal scale. Water levels influenced the timing and pathway of diel CH₄ fluxes, with pulsed releases of stored CH₄ at low to rising tide. This study provides data and methods to improve tidal marsh CH₄ emission estimates, support blue carbon assessments, and refine national and global GHG inventories.