Developing a Redox Network for Coastal Saltmarsh Systems in the PFLOTRAN Reaction Model

Abstract

Coastal ecosystems have been largely ignored in Earth system models but are important zones for carbon and nutrient processing. Interactions between water, microbes, soil, sediments, and vegetation are important for mechanistic representation of coastal processes and ecosystem function. To investigate the role of these feedbacks, we used a reactive transport model (PFLOTRAN) that has the capability to be connected to the Energy Exascale Earth System Model (E3SM). PFLOTRAN was used to incorporate redox reactions and track chemical species important for coastal ecosystems as well as define simple representations of vegetation dynamics. Our goal was to incorporate oxygen flux, salinity, pH, sulfur cycling, and methane production along with plant-mediated transport of gases and tidal flux. Using porewater profile and incubation data for model calibration and evaluation, we were able to create depth-resolved biogeochemical soil profiles for saltmarsh habitat and use this updated representation to simulate direct and indirect effects of elevated CO₂ and temperature on subsurface biogeochemical cycling. We found that simply changing the partial pressure of CO₂ or increasing temperature in the model did not fully reproduce observed changes in the porewater profile, but the inclusion of plant or microbial responses to CO₂ and temperature manipulations was more accurate in representing porewater concentrations. This indicates the importance of characterizing tightly coupled vegetation-subsurface processes for developing predictive understanding and the need for measurement of plant-soil interactions on the same time scale to understand how hotspots or moments are generated.

Plain Language Summary

Earth system models typically do not represent interactions between plants, water, and soil in coastal systems. We used a model, PFLOTRAN, to test how changing temperature and CO₂ can alter the connections between plants, water flow and soil processes in a saltmarsh. We were primarily focused on how the daily cycles associated with tides and photosynthesis would affect nutrient cycling in the soil. We found that including daily fluctuations instead of constant rates influenced rate estimates resulting in a higher or lower nutrient content or gas emission than anticipated. Our results show the importance of including variation in space and time to capture daily changes that influence coastal environments.

Key Points

• Energy Exascale Earth System Model (E3SM) does not fully characterize coastal plant-soil-water interactions in the land or ocean module

• Representation of coastal biogeochemical processes requires spatiotemporal heterogeneity, which can be oversimplified in large-scale models

• Plant-soil-water interactions play an important role in regulating coastal ecosystems