Methane fluxes in tidal marshes of the conterminous United States

Abstract

Abstract Methane (CH 4 ) 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 4 emissions, yet the magnitude and factors controlling CH 4 fluxes in tidal wetlands remain uncertain. We synthesized CH 4 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 4 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 4 m _2 year _1 , with a median of 3.9 g CH 4 m _2 year _1 , and only 25% of sites exceeding 18 g CH 4 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 CH 4 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 CH 4 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 4 flux changes, while the opposite was observed at the seasonal scale. Water levels influenced the timing and pathway of diel CH 4 fluxes, with pulsed releases of stored CH 4 at low to rising tide. This study provides data and methods to improve tidal marsh CH 4 emission estimates, support blue carbon assessments, and refine national and global GHG inventories.