Simulating redox and soil geochemical interactions with ecosystem carbon cycling in a land surface model

Background/Question/Methods

Redox cycling, mineralogy, and pH are recognized as key drivers of soil organic matter cycling, but are typically not included in terrestrial carbon cycle models. These omissions may introduce errors when simulating carbon cycling and greenhouse gas emissions in systems such as wetlands and coastal interfaces where redox interactions and pH fluctuations are important. Integrating cycling of redox-sensitive elements such as iron, sulfur, and manganese could therefore allow models to better represent key elements of carbon cycling and greenhouse gas production. We coupled the Energy Exascale Earth System Model (E3SM) Land Model (ELM) with the geochemical reaction network model PFLOTRAN, allowing geochemical processes to be integrated with land surface model simulations. We used this model framework to simulate reaction networks including organic matter decomposition coupled to iron reduction, sulfate reduction, and manganese-dependent degradative enzymes as well as pH-dependent precipitation and dissolution of iron and manganese bearing minerals across scenarios of varying hydrology, soil mineralogy, and vegetation interactions.

Results/Conclusions

Simulations of interactions between iron reduction and methanogenesis in permafrost soils showed that iron cycling can significantly increase CO2 production and decrease methane emissions in iron-rich systems subject to frequent redox cycling. Simulations of interactions between manganese and leaf litter decomposition in temperate forests showed that interactions between pH and redox cycling in the subsurface can drive lignin decomposition rates in the litter layer by modifying bioavailability of micronutrients that are used in microbial degradative enzymes. Simulations of coastal wetlands showed that the sensitivity of vegetation to salinity and sulfide toxicity can control net carbon balance of coastal wetlands subject to rising sea levels. Overall, our results demonstrate how including more detailed chemical processes can improve ecosystem model simulations of key ecosystem processes in systems where redox cycling is important.