Coastal wetlands are major carbon sinks and vital landscape components that link terrestrial, aquatic, and ocean ecosystem processes. Incorporating these systems into process-based biogeochemical modeling is critical to improve coastal biogeochemistry modeling in response to climate extremes, sea level rise (SLR), and human landscape modifications for improved scientific understanding and policy guidance. Yet, coastal wetlands are often minimized or excluded from both the terrestrial and oceanic components of land surface models due to their complex terrestrial and aquatic interface. Furthermore, there is limited distinction in current models between different growth forms and functional traits of coastal wetland plants. Mangroves, which are major nutrient cyclers and sequesterers of “blue carbon”, are not currently represented in the Energy Exascale Earth System Model (E3SM) Land Model (ELM). Thus, this research aims to 1) define subtropical mangrove hydrology and plant functional types (PFTs) and 2) compare carbon & nitrogen cycling in Florida and Texas mangroves under extreme climate scenarios in ELM. Observational data and modeling methods are used to define hydrology and mangrove PFTs based on growth forms, biomass allocation, and abiotic factors (e.g., salinity and temperature). Preliminary results indicate that in ELM, a modified evergreen broadleaf tree PFT with mangrove-specific response functions to salinity and sub-freezing temperatures is representative of Gulf Coast mangroves. Future research includes quantifying biogeochemical responses of the new ELM mangrove PFT under three climate scenarios – a freeze event, short-term flooding, and long-term SLR – with the future goal of scaling up to regional simulations of mangrove die-offs and expansion.