ORGANIC-CARBON OXIDATION AND SUPPRESSION OF METHANE PRODUCTION BY MICROBIAL FE(III) OXIDE REDUCTION IN VEGETATED AND UNVEGETATED FRESH-WATER WETLAND SEDIMENTS

High concentrations (20-75 mu mol cm(-3)) of amorphous Fe(III) oxide w ere observed in unvegetated surface and Juncus effusus rhizosphere sed iments of a freshwater wetland in the southeastern United States. Incu bation experiments demonstrated that microbial Fe(III) oxide reduction suppressed sulfate reduction and methanogenesis in surface sediments and mediated greater than or equal to 40% of depth-integrated (0-10 cm ) unvegetated sediment carbon metabolism, compared to less than or equ al to 10% for sulfate reduction. In situ CO2 and CH4 flux measurements verified that nonmethanogenic pathways accounted for similar to 50% o f unvegetated sediment carbon metabolism. Lower (similar to 10-fold) r ates of dark/anaerobic CH4 flux from experimental vegetated cores rela tive to unvegetated controls suggested that methanogenesis was inhibit ed 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 fo r 65% of total carbon metabolism in rhizosphere sediment incubations, compared to 22% for methanogenesis. In contrast, methanogenesis domina ted carbon metabolism (72% of total) in experimental unvegetated sedim ent cores. The high Fe(III) oxide concentrations and reduction rates o bserved in unvegetated surface and Juncus rhizosphere sediments were p erpetuated by rapid Fe(III) regeneration via oxidation of Fe(II) compo unds coupled to O-2 input from the overlying water and plant roots, re spectively. The results indicate that Fe(III) oxide reduction could me diate a considerable amount of organic carbon oxidation and significan tly suppress CH4 production in freshwater wetlands situated within glo bally extensive iron-rich tropical and subtropical soil regimes.