• Coastal wetlands store large amounts of carbon and are sensitive to chemical interactions driven by salinity and tidal fluctuations • We coupled a land surface model to a reactive transport model to simulate biogeochemical cycling in saline and fresh tidal wetlands • Sulfate availability in saline wetlands lowered simulated methane emissions, which compared well with site measurements Notice: This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes.

Redox processes, aqueous and solid-phase chemistry, and pH dynamics are key drivers of subsurface biogeochemical cycling in terrestrial and wetland ecosystems but are typically not included in terrestrial carbon cycle models. These omissions may introduce errors when simulating systems where redox interactions and pH fluctuations are important, such as wetlands where saturation of soils can produce anoxic conditions and coastal systems where sulfate inputs from seawater can influence biogeochemistry. Integrating cycling of redox-sensitive elements could therefore allow models to better represent key elements of carbon cycling and greenhouse gas production. We describe a model framework that couples the Energy Exascale Earth System Model (E3SM) Land Model (ELM) with PFLOTRAN biogeochemistry, allowing geochemical processes and redox interactions to be integrated with land surface model simulations. We implemented a reaction network including aerobic decomposition, fermentation, sulfate reduction, sulfide oxidation, and methanogenesis as well as pH dynamics along with iron oxide and iron sulfide mineral precipitation and dissolution. We simulated biogeochemical cycling in tidal wetlands subject to either saltwater or freshwater inputs driven by tidal hydrological dynamics. In simulations with saltwater tidal inputs, sulfate reduction led to accumulation of sulfide, higher dissolved inorganic carbon concentrations, lower dissolved organic carbon concentrations, and lower methane emissions than simulations with freshwater tidal inputs. Model simulations compared well with measured porewater concentrations and surface gas emissions from coastal wetlands in the Northeastern United States. These results demonstrate how simulating geochemical reaction networks can improve land surface model simulations of subsurface biogeochemistry and carbon cycling.

Incorporating a vegetation response to tidal fluctuations in salinity and water level for salt marsh representation in land surface modelingAuthors: Sophie LaFond-Hudson, Ben Sulman, Teri O’Meara, Inke Forbrich, Zoe CardonTarget journal: Ecological modeling or JAMESTimeline goal:Send to co-authors mid MayOrganize code, archive code/data, reviseResend to co-authors if needed late JuneFinal revisions , submit late summer 2023

The fate of organic carbon (C) in permafrost soils is important to the climate system due to the large global stocks of permafrost C. Thawing permafrost can be subject to dynamic hydrology, making redox processes an important factor controlling soil organic matter (SOM) decomposition rates and greenhouse gas production. In iron (Fe)-rich permafrost soils, Fe(III) can serve as a terminal electron acceptor, suppressing methane (CH4) production and increasing carbon dioxide (CO2) production. Current large-scale models of Arctic C cycling do not include Fe cycling or pH interactions. Here, we coupled Fe redox reactions and C cycling in a geochemical reaction model to simulate the interactions of SOM decomposition, Fe(III) reduction, pH dynamics, and greenhouse gas production in permafrost soils subject to dynamic hydrology. We evaluated the model using measured CO2 and CH4 fluxes as well as changes in pH, Fe(II), and dissolved organic C concentrations from oxic and anoxic incubations of permafrost soils from polygonal permafrost sites in northern Alaska, United States. In simulations of higher frequency oxic-anoxic cycles, rapid oxidation of Fe(II) to Fe(III) during oxic periods and gradual Fe(III) reduction during anoxic periods reduced cumulative CH4 fluxes and increased cumulative CO2 fluxes. Lower pH suppressed CH4 fluxes through its direct impact on methanogenesis and by increasing Fe(III) bioavailability. Our results suggest that models that do not include Fe-redox reactions and its pH dependence could overestimate CH4 production and underestimate CO2 emissions and SOM decomposition rates in Fe-rich, frequently waterlogged Arctic soils.

Soil organic carbon (SOC) stocks represent a large component of the global carbon cycle that is sensitive to warming. Modeling and empirical studies often assume that temperature responses of microbial physiological functions and extracellular enzymatic reactions are predictive of ecosystem-scale SOC decomposition responses to warming. However, temperature-dependent soil trophic interactions such as predation of microbial decomposers by other organisms have not yet been incorporated into quantitative SOC models. Here, we incorporated a microbial predator into a tri-trophic population ecology model and a global-scale predictive SOC model to determine how predation would affect soil community population dynamics and temperature sensitivity of SOC stocks. Predators increased SOC stocks and their dependence on substrate input rates. Top-down controls of predators on microbial biomass caused SOC warming responses to diverge from microbial temperature responses, with warming-induced SOC losses reduced or reversed when predators were more temperature-sensitive. Our results suggest that higher trophic levels can reduce the sensitivity of SOC to warming, and that differences in temperature sensitivity across trophic levels may be a key determinant of SOC warming responses.