Sulfur Driven Autotrophic Denitrification in Constructed Wetlands

Denitrification in constructed wetlands was studied by obtaining water column samples and by testing sediments for denitrification activity. Samples were obtained from four different wetlands at the Tres Rios Demonstration Constructed Wetlands located adjacent to the City of Phoenix. The wetlands all received nitrified/denitrified effluent with a nitrate-N concentration of less than 10 mg-N/L with sulfate concentrations typically exceeding 100 mg/L. During batch tests, denitrification rates were described as zeroorder with respect to substrate concentration. Sulfate concentration increases were accompanied by decreases in nitrate concentrations. The stoichiometric relationship between nitrate reduction and sulfate production was consistent with autotrophic denitrification. The addition of sulfide as an electron donor had no significant effects on denitrification activity and the sulfide was stored effectively in the sediments. Field studies during times of vegetative die-off demonstrated that simultaneous denitrification and sulfate reduction can occur in wetlands when large quantities of organic carbon are present. During a vegetative growth cycle when carbon addition to the aqueous system is low, denitrification was commensurate with sulfate production and the stoichiometric relationship was consistent with autotrophic denitrification. Autotrophic denitrification was an important mechanism for denitrification in the wetland systems studied and provides a plausible explanation for observed denitrification rates that are independent of vegetative growth cycles. Organic carbon from plants appears to drive denitrification in wetlands through the production of sulfide during sulfate reduction. The sulfide is stored in sediments where it is available for denitrification during periods of low organic carbon addition.