Wetland Soil Characteristics Influence the Kinetics of Dissolved Organic Carbon Sorption

Sorption processes at the soil-water interface are observed to be rapid and dominant pathways of dissolved organic carbon (DOC) exchange. However, kinetics data for sorption in any ecosystem are sparse, and specifically non-existent for temperate tidal marshes. In this study, sorption rate kinetics experiments were designed to constrain new formulations of a sediment flux model coded to include explicit sorption between soil organic carbon and DOC pools. Batch incubations for marsh soil samples from Taskinas Creek (VA, USA) and Jug Bay Wetlands Sanctuary (MD, USA) were performed anaerobically under four sets of initial conditions: permutations of two salinities (0, 35) and two DOC concentrations (0 mg L^-1, 275 mg L^-1). Rates were measured at seven time points over 24 h. These results are the first DOC sorption kinetics data for tidal marsh soils, revealing that 76% of total sorption occurred within 15 min. The results also revealed higher capacity for adsorption under high DOC concentrations and salinity, with differences in magnitude between soil types. Numerical models simulating processes from these experiments provided a range of rates by fitting linear first order and non-linear ordinary differential equations to the kinetic change in DOC concentration curves over time. The outputs suggested that introducing soil adsorption capacity improved model fits across all cases. These results provide a deeper understanding of the biogeochemical controls on sorption kinetics and suggest that it is crucial to incorporate sorption processes into sediment flux models to accurately represent DOC fluxes from tidal marshes.