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River sediment nitrogen removal and recycling within an agricultural Midwestern USA watershed

The Lower Great Miami River (LGMR) lies within the Mississippi River watershed and contributes to nutrient loads that promote seasonal hypoxia in the northern Gulf of Mexico. Moreover, the LGMR has recently experienced algal blooms caused by excess N and P. To investigate N cycling in the LGMR, we incubated intact sediment cores with stable 15N isotope additions in a continuous-flow system. We measured sediment nutrient fluxes, N sinks and sources (denitrification/anammox and N fixation, respectively), and potential dissimilatory NO3− reduction to NH4+ (DNRA) in spring and summer at 6 locations along a river reach (~50 km) influenced by urban and agricultural nutrient loads. LGMR sediments were a source of bioavailable NH4+ and orthophosphate to river water. However, high denitrification rates resulted in LGMR sediments being a net sink for N in the river reach. NO3− amendments did not consistently stimulate denitrification, a result suggesting that denitrifiers were functioning at or near maximum rates. Anammox and DNRA were not consistently observed, and N fixation occurring simultaneously with denitrification was not observed. We estimate that denitrification in LGMR sediments removed 8 to 33% of external N loads to the river from the watershed, with the remainder exported downstream. This result indicates that denitrification can be an important N sink in the LGMR, but further decrease of external N inputs will be required to minimize eutrophication in the LGMR and N export to downstream systems.