Dissimilatory Iron Reduction And Its Effect On Anaerobic Organic Matter Oxidation In The Tidal Marsh Of Min River Estuary In Southeast China

Dissimilatory iron reduction plays an important role in organic matter degradation in estuarine tidal marshes. Iron reduction was studied via in-situ observation and laboratory incubation experiments in the subtropical Cyperus malaccensis tidal marsh of Min River Estuary (MRE). The goals of this dissertation are:ⅰ) to examine the abundance and speciation of iron, ⅱ) to determine the kinetics of chemical and dissimilatory iron reduction, and ⅲ ) to figure out the rates and contributions of dissimilatory iron reduction along the inundation gradient.There were different reactive iron species in the tidal marsh of MRE. The abundance orders of these iron species were:non-sulfidic Fe(Ⅱ)> Fe(Ⅲ)> FeSx. The spatial distribution of these iron species were impacted by pH, redox condition, salinity, SO42-and organic matter. These environment factors created a unique iron geochemical zonation across the tidal marsh of MRE. Amorphous Fe(Ⅲ) and crystalline Fe(Ⅲ) were enriched in the surface of the high tide zone. FeS and FeS2 accumulated closer to the lower extremity where the salinity was higher across the tidal marsh. The non-sulfidic Fe(Ⅱ) could be detected throughout the transect and was enriched in the deep layer of upland.The results of kinetics experiment indicated that iron reduction was a dynamic process. The initial reduction rates were impacted by the abundance of Fe(Ⅲ). The mineral heterogeneity of Fe(Ⅲ) controlled the change in iron reduction rate. The microbially reducible Fe(Ⅲ) accounted for only～46% of reactive Fe(Ⅲ) pool determined by the traditional chemical extraction. The chemical mineral heterogeneities of Fe(Ⅲ) were much lower than dissimilatory mineral heterogeneities of Fe(Ⅲ). However, the chemical mineral heterogeneities exhibited highly spatial heterogeneity across the tidal marsh, while dissimilatory mineral heterogeneities were statistically similar at six locations.The iron reduction included dissimilatory and chemical iron reduction pathway in the tidal marsh of MRE. The dissimilatory iron reduction was the main diagenetic path of iron reduction. The rate ratios of anaerobic organic carbon and nitrogen oxidation ranged from 10.5 to 12.3. The dissimilatory iron reduction contributed～51% to the anaerobic organic matter oxidation, while dissimilatory sulfate reduction contributed～38% to the anaerobic organic matter oxidation. The pathways of anaerobic organic matter oxidation were controlled by spatial distribution of Fe(Ⅲ) and SO42- across the tidal marsh. The dominated organic matter degradation pathway was switched from dissimilatory iron reduction to sulfate reduction along the inundation gradient.This study highlighted the effect of tidal hydrology and topography on the iron biogeochemical cycling.