Nitrogen processing by treatment wetlands in a tropical catchment dominated by agricultural landuse

Agriculture is a major contributor to marine nitrogen pollution, and treatment wetlands can be a strategy to reduce it. However, few studies have assessed the potential of treatment wetlands to mitigate nitrogen pollution in tropical regions. We quantify the nitrogen removal rates of four recently constructed treatment wetlands in tropical Australia. We measured denitrification potential (D_t), the inflow-outflow of nutrients, and tested whether the environment in these tropical catchments is favourable for nitrogen removal. D_t was detected in three of the four systems with rates between 2.0 and 12.0 mg m^-2 h^-1; the highest rates were measured in anoxic soils (ORP -100 to 300 mV) that were rich in carbon and nitrogen (>2% and >0.2%, respectively). The highest nitrogen removal rates were measured when NO₃^--N concentrations were >0.4 mg L^-1 and when water flows were slow. Treatment wetlands in tropical regions can deliver high removal rates of nitrogen and other pollutants when adequately managed. This strategy can reduce nutrient loads and their impacts on sensitive coastal zones such as the Great Barrier Reef.