Inorganic nitrogen transformations in the bed of the Shingobee River, Minnesota: Integrating hydrologic and biological processes using sediment perfusion cores

Inorganic N transformations were examined in streambed sediments from the Shingobee River using sediment perfusion cores. The experimental design simulated groundwater–stream water mixing within sediment cores, which provided a well-defined one-dimensional representation of in situ hydrologic conditions. Two distinct hydrologic and chemical settings were preserved in the sediment cores: the lowermost sediments, perfused with groundwater, remained anaerobic during the incubations, whereas the uppermost sediments, perfused with oxic water pumped from the overlying water column, simulated stream water penetration into the bed. The maintenance of oxic and anoxic zones formed a biologically active aerobic-anaerobic interface. Ammonium (NH ) dissolved in groundwater 4 was transported conservatively through the lower core zone but was removed as it mixed with aerated recycle water. Concurrently, a small quantity of nitrate (NO ) equaling ;25% of the NH loss was produced in the upper sedi2 1 3 4 ments. The NH and NO profiles in the uppermost sediments resulted from coupled nitrification-denitrification, 1 2 4 3 because assimilation and sorption were negligible. We hypothesize that anaerobic microsites within the aerated upper sediments supported denitrification. Rates of nitrification and denitrification in the perfusion cores ranged 42– 209 and 53–160 mg N m22 day21, respectively. The use of modified perfusion cores permitted the identification and quantification of N transformations and verified process control by surface water exchange into the shallow hyporheic zone of the Shingobee River. Nitrogen transformations in soil and sediments have been a focus of research for many years, with nitrification and denitrification most commonly studied. When these reactions co-occur, the result is a net loss of N that can have significant biological consequences in both aquatic and terrestrial systems. The capacity of stream beds to process N has been an area of recent interest, especially in the midwestern United States, where high nitrate (NO ) in agricultural runoff has 3 been implicated as a cause of hypoxia in the Gulf of Mexico (Turner and Rabalais 1994). Nitrification and denitrification studies have most commonly involved the use of enzyme assays with sediment slurries, revealing ‘‘potential’’ reaction rates (Rysgaard et al. 1 Present address: Department of Biology, Arizona State University, Tempe, Arizona 85287 ([email protected]).