Evaluation of Denitrification Rates and Mechanisms in Microcosm Experiments with Sediments and Plants

Five sets of microcosm batch experiments were performed to evaluate mechanisms of nitrate removal in constructed wetlands. The study evaluated biological denitrification activity as a function of 1) electron donors carbon and sulfur 2) wetland sediment depth 3) oxygen and abiotic reactions and 4) carbon addition and attached populations from wetland bulrushes. Wetland-water-only controls amended with nitrate showed negligible denitrification rates. Nitrate-only amended wetland sediments demonstrated nitrate removal rates of 17.0-25.8 mg N/L*day (13.6-20.6 kg N/acre*day) for two wetland systems. Autotrophic denitrification occurred simultaneously with heterotrophic nitrate reduction. The addition of carbon via glucose or plants increased rates 24% to 82%; whereas, adding sulfide to batch tests resulted in variable effects dependent on sediment conditions. Sulfide oxidizers appeared to scavenge available oxygen creating reduced conditions when the headspace was purged with air. Bulrushes contained attached populations of denitrifying bacteria and increased nitrate reduction rates by supplying electron equivalents via carbon.