Effects of salinity on the microbial removal of nitrate under varying nitrogen inputs within the marshland upwelling system

The marshland upwelling system (MUS) utilizes the natural properties of wetland soils to treat domestic wastewater injected into the marsh subsurface as the wastewater moves upwards and outwards from the injection site. The system is different from coarse media based wetland treatment systems common in Europe, though it relies on the same principles. A laboratory study was designed to simulate field conditions in order to investigate and quantify the removal of nitrogen from the wastewater by pumping wastewater into the bottom of cores and observing the changes as the wastewater moved upward to the surface. Two nitrogen treatments (100 mg NH(4)-N L(-1) and 80 mg NH(4)-N L(-1)/20 mg NO(3)-N L(-1)) and two salinities (2 and 20‰) for each N treatment were studied. Dissolved organic carbon (DOC) demonstrated a removal efficiency of 90%, while NO(3)-N had a removal efficiency of > 99% throughout the 84 days of the study. Higher salinity had a temporary, significant lower removal of DOC, while nitrate removal was high and consistent over time. Microbial biomass C (MBC) and denitrification enzyme activity (DEA) were measured to determine the role of microbial processes within the MUS. Wastewater introduction increased microbial growth at the column surface, which led to increases in denitrification/nitrification coupling and net N loss, as estimated by DEA. Salinity and organic matter were found to have significant negative and positive impacts, respectively, on DEA rates and MBC. An understanding of the impacts of salinity on specific microbially-mediated N transformations is critical for improving the efficiency of the MUS in coastal environments to determine the long-term sustainability.