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The Fate of Groundwater Ammonium in a Lake Edge Wetland

The National Institute of Water & Atmospheric Research, P.O. Box 11-115, Hamilton, NZ;

Keith Thompson

Dep. of Biological Sciences, Univ. of Waikato, Private Bag 3105, Hamilton, NZ.

^* Corresponding author (m.gibbs{at}niwa.cri.nz).

ABSTRACT

The removal of ammonium—nitrogen (NH₄-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 NH₂-N was removed by the wetland. Laboratory assays found that the potential for NH₄-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 ¹⁵N-tracer experiment, Salix cinerea and Typha orientalis readily assimilated NH₂-N from the root zone and were found to enhance the natural diffusion of NH₄-N up through the organic sediment layer and into the overlying water. Estimated rates of removal of NH₂-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 ¹⁵N in Salix leaf litter was transformed in such a way. The results of this study indicate that plants play a central role in the NH₄-N processing capacity of wetlands. The relevance of our findings to constructed wetland design is discussed.

F.E. Lusby, present address, Dep. of Geography, Univ. of Durham, South Road, Durham DH1 3LE, United Kingdom.

Received for publication May 30, 1997.