The concentrations of inorganic nitrogen, including nitrate (NO3-), are fundamental controls on the trophic state of aquatic ecosystems. Excess NO3- degrades drinking water quality, and therefore there is a need to understand processes that remove inorganic nitrogen. Controls on NO3- removal at the sediment-water interface (SWI) of aquatic ecosystems include both biogeochemical and hydrologic conditions, however the relative importance and interactions of these controls are poorly understood. This thesis explores these controls on NO3- removal using a series of in-situ experiments involving both biogeochemical and hydrologic manipulations of the SWI in both lake and stream settings. Specifically, manipulative experiments altered dissolved organic carbon (DOC) and NO3- concentrations, as well as physical hydrologic residence times. The fate of NO3- in these manipulation experiments was traced by pairing isotopically labeled 15N-NO3- tracer experiments with controlled variable-head infiltrometer rings to isolate the sediment-water system and control the hydrology of the SWI. With these experiments, I was able to isolate biogeochemical versus hydrologic controls on rates of NO3- removal and denitrification rates. I found that increasing NO3- and DOC concentrations increased NO3- removal and denitrification rates in the SWI, but that increases in physical residence time had a stronger effect on increasing NO3- removal and denitrification rates, especially under conditions where DOC and NO3- availability were not limiting.
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