A New Coupled Biogeochemical Modeling Approach Provides Accurate Predictions of Methane and Carbon Dioxide Fluxes Across Diverse Tidal Wetlands.

Tidal wetlands provide valuable ecosystem services, including storing large amounts of carbon. However, the net exchanges of carbon dioxide (CO2) and methane (CH4) in tidal wetlands are highly uncertain. While several biogeochemical models can operate in tidal wetlands, they have yet to be parameterized and validated against high‐frequency, ecosystem‐scale CO2 and CH4 flux measurements across diverse sites. We paired the Cohort Marsh Equilibrium Model (CMEM) with a version of the PEPRMT model called PEPRMT‐Tidal, which considers the effects of water table height, sulfate, and nitrate availability on CO2 and CH4 emissions. Using a model‐data fusion approach, we parameterized the model with three sites and validated it with two independent sites, with representation from the three marine coasts of North America. Gross primary productivity (GPP) and ecosystem respiration (Reco) modules explained, on average, 73% of the variation in CO2 exchange with low model error (normalized root mean square error (nRMSE) <1). The CH4 module also explained the majority of variance in CH4 emissions in validation sites (R2 = 0.54; nRMSE = 1.15). The PEPRMT‐Tidal‐CMEM model coupling is a key advance toward constraining estimates of greenhouse gas emissions across diverse North American tidal wetlands. Further analyses of model error and case studies during changing salinity conditions guide future modeling efforts regarding four main processes: (a) the influence of salinity and nitrate on GPP, (b) the influence of laterally transported dissolved inorganic C on Reco, (c) heterogeneous sulfate availability and methylotrophic methanogenesis impacts on surface CH4 emissions, and (d) CH4 responses to non‐periodic changes in salinity. Plain Language Summary: Tidal wetlands play a crucial role in storing carbon, but there is uncertainty in how they absorb and release carbon dioxide and methane, two of the most important greenhouse gases. Existing models for tidal wetlands have yet to be thoroughly tested using high‐frequency data across diverse sites. The new model presented here was parameterized and validated using 13 years of data from five different tidal wetland sites with representation from the three marine coasts of North America. The model explained on average more than half of the variation in carbon dioxide and methane exchange with low model error across diverse tidal wetlands used in model validation. This is strong performance given the high variation in greenhouse gas fluxes in tidal wetlands, which are known to be biogeochemically complex environments. Our model helps in assessing the greenhouse gas budgets from diverse tidal wetlands and the ability of wetlands to combat climate change. An analysis of model results highlights the need for more research on how tidal pumping and pulse changes in soil and water chemistry, not just at the surface but also across soil layers, influence carbon dioxide and methane production and exchange. Key Points: We present a process‐based modeling approach to estimate greenhouse gas flux in tidal wetlands with representation from the three marine coasts of North AmericaModel validation showed that primary productivity, respiration and methane modules explained >50% of the variation in CO2 and CH4 exchangeThe PEPRMT‐Tidal‐CMEM model is a key advance toward constraining estimates of greenhouse gas emissions across diverse tidal wetlands [ABSTRACT FROM AUTHOR]