Sulfate reduction and porewater chemistry in a gulf coast Juncus roemerianus (Needlerush) marsh

Sulfate reduction rates were measured over the course of a year in the sediments of a Juncus roemerianus marsh located in coastal Alabama. Sulfate reduction rates were typically highest in the surface 0-2 cm and at depths corresponding to peak belowground biomass of the plants. The highest volume-based sulfate reduction rate measured was 1,350 mumol liter-sediment(-1) d(-1) in September 1995. Areal sulfate reduction rates (integrated to 20 cm depth) were strongly correlated to sediment temperature and varied seasonally from 15.2 mmol SO42- m(-2) d(-1) in January 1995 to 117 mmol SO42- m(-2) d(-1) in late August 1995. Despite high sulfate reduction rates porewater dissolved sulfide concentrations were low (< 73 muM), indicating rapid sulfide oxidation or precipitation. Sulfate depletion data indicated that net oxidation of sediment sulfides occurred in March through May, following a period of infrequent tidal flooding and during a period of high plant production. Porewater Fe(II) reached very high levels (maximum of 969 muM; mean for all dates was 160 muM), particularly during periods of high sulfate reduction. The annual sulfate reduction rate integrated over the upper 20 cm of sediment was 22.0 mol SO42- m(-2) yr(-1), which is among the highest rates measured in a wetland ecosystem. Based on literature values of net primary production in J. roemerianus marshes, we estimate that an amount equivalent to 16% to 90% of the annual belowground production may be remineralized through sulfate reduction.