Enhanced nitrate removal efficiency in wetland microcosms using an episediment layer for denitrification

Microbial denitrification is the principal removal mechanism for nitrogen in treatment wetlands. For this study, flow-through wetland microcosms were designed to test whether variations in the macroporous structure of the denitrification zone affected overall nitrate removal. In a sediment-only treatment, carbon as cattail (Typha latifolia) fitter was mixed throughout a porous sediment matrix. A second treatment contained a distinct layer of loosely aggregated litter pieces placed atop the sediment matrix to form an episediment zone. Results showed that across nine conditions (NO3- influent = 7, 36, 65 mg N L-1 and Ire, 2.2, 2.9, 4.4, 7.7 d; n = 48) average denitrification: was 33% greater (p < 0.0001) in the episediment treatment than in the sediment-only treatment. Both sediment-only and episediment treatments followed apparent reaction kinetics close to first order, with k(epi) = 0.21 d(-1) for the episediment treatment and k(sed) = 0.12 d(-1) for the sediment-only treatment. Analysis of vertical nitrate profile data using diffusive and turbulent mixing models indicated that denitrification occurred in a partially mixed episediment layer, as well as in an underlying sediment layer. Approximately 40% of the nitrate removal that occurred in the episediment treatment was estimated to occur in the episediments themselves. We conclude that enhancement of an episediment layer can increase denitrification in treatment wetlands which receive nitrate in overlying water.