Modeling Redox Biogeochemistry Influences on Carbon Cycling at Site to Continental Scales

Redox cycles, geochemistry, and pH are recognized as key drivers of subsurface biogeochemical cycling in the critical zone but are typically not included in land surface models. These omissions may introduce errors when simulating carbon cycling and greenhouse gas emissions in systems where redox interactions, and pH fluctuations are important, such as coastal regions where sulfate concentrations associated with saltwater influence can drive biogeochemical contrasts across salinity gradients or upland systems where redox-active micronutrients contribute to litter decomposition. Here, we coupled the Energy Exascale Earth System Model (E3SM) Land Model (ELM) with geochemical reaction network simulator PFLOTRAN, allowing geochemical processes and redox interactions to be integrated with land surface model simulations. We implemented a reaction network including aerobic decomposition, fermentation, iron oxide reductive dissolution and dissolved iron oxidation, sulfate reduction, sulfide oxidation, methanogenesis, methane oxidation, and pH dynamics and simulated biogeochemical cycling and methane production across coastal gradients of salinity and elevation. Model simulations were parameterized using laboratory incubations and literature values and evaluated using measured porewater concentrations and surface gas emissions from wetland field sites across coastal regions of the United States. In addition, we demonstrate that interactions between manganese redox cycling and nitrogen availability can influence litter decomposition and organic matter cycling in temperate forest ecosystems. These results demonstrate how directly simulating biogeochemical reaction networks can improve land surface model simulations of subsurface biogeochemistry and carbon cycling, and highlight the value of porewater biogeochemical data for evaluating process-based biogeochemical models.