Nitrogen Distribution in Soils of Constructed Wetlands Treating Lagoon Wastewater

Constructed wetlands have the potential to be used for treatment of N-rich livestock wastewater. Our objectives were to evaluate both the time effect and increasing N loading rates on soil N distribution and NH+4-N concentration in surface-pore water of constructed wetlands. A 5-yr study in North Carolina investigated two wetland systems that treated swine lagoon wastewater. Wetland System 1 was planted to a Schoenoplectus americanus (Pers.) Volkart ex Schinz & R. Keller, S. tabernaemontani (K.C. Gmel.) Palla, Scirpus cyperinus (L.) Kunth, and Juncus effusus L. plant community, and Wetland System 2 was planted to a Typha angustifolia L., T. latifolia L., and Sparganium americanum Nutt. plant community. Nitrogen loading rates were increased annually from 0.6 to 2.7 g m^(-2) d^(-1). Soils were analyzed for total N annually. Surface-pore water was sampled with equilibrators and analyzed for NH+4-N. Although the total N accumulation significantly increased with time in both systems, total soil N accumulation by depth did not differ significantly between systems. Distribution profiles in the surface-pore water column showed that NH+4N was transported upward into surface water at N loading rates from 1.2 to 2.7 g m^(-2) d^(-1). As total N loading rates increased annually in both wetland systems, soil pore water had higher levels of NH+4-N but N removal efficiency of the wetlands sharply decreased. Accumulation of high levels of NH+4-N (>200 mg L^(-1)) in soil pore water could negatively affect long-term ability of wetland systems to treat wastewater with high N levels.