Kinetics of Chemical and Microbial Iron Reduction Along an Inundation Gradient in a Tidal Marsh of the Min River Estuary, Southeastern China

Chemical (via buffered ascorbate-citrate) and microbial (via Shewanella decolorationis S12) iron reduction kinetics in the surficial tidal marsh sediments of the Min River Estuary were compared at six locations along the inundation gradient. A reactive continuum model was successfully applied to describe the kinetic results. Along the inundation gradient, an enhanced tidal inundation frequency towards the sea resulted in changes in the pH, Eh, salinity, as well as the abundances and fractions of reactive Fe(II) and Fe(III). Significant differences were observed between the chemical and microbial iron reductions among the six locations. Larger reducible Fe(III) pools, higher initial rates, and lower mineral heterogeneity were observed in chemical assays when compared to microbial assays. The microbial and chemical reducible Fe(III) as well as their initial reduction rates generally decreased along the inundation gradient. The chemical mineral heterogeneity gradually increased along the inundation gradient, while the microbial mineral heterogeneities were not significantly different at the six locations. Consequently, a higher iron reduction potential was observed in the uplands when compared to that in the lowlands. The result highlights the role of tidal hydrology and topography in iron reduction and suggest that both the pool and mineral heterogeneities of highly reactive Fe(III) should be considered in iron biogeochemical investigations.