Isotopically resolved fate and processes of nitrogen in a constructed wetland treating domestic effluent

Constructed wetlands (CWs) are recognised as effective wastewater purification systems for the removal of contaminants, particularly nitrogen (N). However, the performance of a CW in N removal relies on complex biogeochemical interactions within and between various compartments. We assessed the concentrations and isotopic compositions of multiple N and C species in the water column, sediment, plants and soil in the riparian and adjacent terrestrial zones within a CW in South Australia. The major source of N supplied to the wetland is domestic effluent delivered via a municipal wastewater treatment plant resulting in high NH₄⁺ concentrations (54 mg L⁻¹) at the inlet of the wetland. The decrease in NH₄⁺ and increase in NO₃⁻ concentrations, coupled with an increase in δ¹⁵N-NH₄ in the water along the flow path of the wetland, strongly indicates the significance of nitrification and assimilation processes. We also observed an increase in δ¹⁵N-NO₃⁻_, however, the lack of increase in δ¹⁸O-NO₃⁻ suggested denitrification was either not a prominent process, or the δ¹⁸O-NO₃⁻ signature was masked by the extent of nitrification. There was evidence of an influence of the wastewater on the plants in the riparian zone as their δ¹⁵N increased along the flow path, unlike the adjacent terrestrial plants. Similarly, this occurred with suspended solids, indicating progressive plant and algal assimilation of N in the CW. Further research is required to account for changes with seasonality and sediment processes and interactions.