Organic carbon oxidation and suppression of methane production by microbial Fe(III) oxide reduction in vegetated and unvegetated freshwater wetland sediments

Abstract

High concentrations (20–75 µmol cm‒3) of amorphous Fe(III) oxide were observed in unvegetated surface and Juncus effusus rhizosphere sediments of a freshwater wetland in the southeastern United States. Incubation experiments demonstrated that microbial Fe(III) oxide reduction suppressed sulfate reduction and methanogenesis in surface sediments and mediated ≥40% of depth‐integrated (0–10 cm) unvegetated sediment carbon metabolism, compared to ≤10% for sulfate reduction. In situ CO2 and CH4 flux measurements verified that nonmethanogenic pathways accounted for ∼50% of unvegetated sediment carbon metabolism. Lower (∼10‐fold) rates of dark/anaerobic CH4 flux from experimental vegetated cores relative to unvegetated controls suggested that methanogenesis was inhibited in the Juncus rhizosphere, in which active Fe(III) oxide reduction was indicated by the presence of low but readily detectable levels of dissolved and solid‐phase Fe(II). Fe(III) oxide reduction accounted for 65% of total carbon metabolism in rhizosphere sediment incubations, compared to 22% for methanogenesis. In contrast, methanogenesis dominated carbon metabolism (72% of total) in experimental unvegetated sediment cores. The high Fe(III) oxide concentrations and reduction rates observed in unvegetated surface and Juncus rhizosphere sediments were perpetuated by rapid Fe(III) regeneration via oxidation of Fe(II) compounds coupled to O2 input from the overlying water and plant roots, respectively. The results indicate that Fe(III) oxide reduction could mediate a considerable amount of organic carbon oxidation and significantly suppress CH4 production in freshwater wetlands situated within globally extensive iron‐rich tropical and subtropical soil regimes.