The Fate of Groundwater Ammonium in a Lake Edge Wetland

Abstract

The removal of ammonium—nitrogen (NH 4 ‐N) from groundwater flowing through a grey willow ( Salix cinerea ) and native raupo ( Typha orientalis ) lake edge wetland was investigated over 10 mo. Monthly groundwater sampling showed that more than 95% of incoming NH 2 ‐N was removed by the wetland. Laboratory assays found that the potential for NH 4 ‐N removal by nitrification and subsequent denitrification was significantly higher in the organic surface sediments than in deeper, sandy, root zone sediments. In a laboratory 15 N‐tracer experiment, Salix cinerea and Typha orientalis readily assimilated NH 2 ‐N from the root zone and were found to enhance the natural diffusion of NH 4 ‐N up through the organic sediment layer and into the overlying water. Estimated rates of removal of NH 2 ‐N by coupled nitrification‐denitrification and plant assimilation were 8 to 9 mg of N m −2 d −1 and 9 to 46 mg of N m −2 d −1 , respectively. Assimilated N, returned to the overlying water‐sediment surface via litter fall may be subject to sequential mineralization, nitrification, and denitrification, resulting in gaseous N loss. Sixty‐five percent of the 15 N in Salix leaf litter was transformed in such a way. The results of this study indicate that plants play a central role in the NH 4 ‐N processing capacity of wetlands. The relevance of our findings to constructed wetland design is discussed.