Monitoring nitrogen processing in constructed wetlands: two stable isotope approaches

Keryn Roberts1, Md Moklesur Rahman2, Wei Wen Wong2, Perran Cook2, Michael Grace2

1 Water Studies Centre, Monash University, Wellington, Clayton, Victoria, 3800, Australia, [email protected]

2 Water Studies Centre, Monash University, Wellington, Clayton, Victoria, 3800, Australia

Abstract

The increased pressure of anthropogenic nitrogen inputs on our waterways has increased the importance of nitrogen management strategies such as constructed wetlands. Several studies have assessed the effectiveness of wetland operation by examining wetland components in the design process, however, long term management of operational wetlands is often reliant on monitoring only nutrient concentrations. Stable isotopes of nitrogen can be a useful tool in investigating nitrogen processing and are often applied to detailed mechanistic studies but rarely utilised in ongoing monitoring strategies. Here we present a conceptual model for two stable isotope approaches, one quantitative and one qualitative, to assess their feasibility as management tools to improve our current understanding of nitrogen removal over the lifetime of a constructed wetland system. The first approach is the direct measurement of NO3- reduction pathways (denitrification) to assess nitrogen removal whilst the second approach uses the dual isotopic composition (δ15N and δ18O) of NO3- at the natural abundance level in the surface water to qualitatively assess denitrification and assimilation. We propose the natural abundance stable isotope approach, with further research, would complement current monitoring programs providing further information on the behaviour of nitrogen in wetlands.