Rhizosphere processes induce changes in dissimilatory iron reduction in a tidal marsh soil: a rhizobox study

Abstract

Although the role of microbial iron respiration in tidal marshes has been recognized for decades, the effect of rhizosphere processes on dissimilatory ferric iron reduction (FeR) is poorly known. Herein, we examined the FeR surrounding the root zone of three tidal marsh plants. Using in situ rhizoboxes, we accurately separated rhizobox soil as one rhizosphere zone, and three bulk soil zones. Dissimilatory and sulfidic-mediated FeR were quantified by accumulation of non-sulfidic Fe(II) and Fe sulfides over time, respectively. The rates of dissimilatory FeR attained 42.5 μmol Fe g−1 d−1 in the rhizosphere, and logarithmically declined by up to 19.1 μmol Fe g−1 d−1 in the outer bulk soil. The rates of sulfidic-mediated FeR were less than 2 μmol Fe g−1 d−1 among all zones. Poorly crystalline Fe(III), DOC and DON, porewater Fe2+, and SO42− were all enriched in the rhizosphere, whereas non-sulfidic Fe(II) and Fe sulfides gradually accumulated away from the roots. Iron reducers (Geobacter, Bacillus, Shewanella, and Clostridium) had higher populations in the rhizosphere than in the bulk soil. Higher rates of dissimilatory FeR were observed in the Phragmites australis and Spartina alterniflora rhizoboxes than in the Cyperus malaccensis rhizoboxes. The radial change pattern of dissimilatory FeR rates were determined by allocation of poorly crystalline Fe(III) and dissolved organic carbon. The interspecies difference of rhizosphere dissimilatory FeR was associated with the root porosity and aerenchyma of the tidal marsh plants.