Total DIN typically increases over an order of magnitude during the fall-winter transition at the Shingobee River Headwaters, a complex of lakes and wetlands with high regional groundwater input.  We examined DIN transport-retention (415 m reach) Sept 1997 and Jan 1998 in surface water and in the hyporheic zone (10 cm depth) using mass balance and chloride-nitrate co-injections.  Surface water nitrate-N and ammonium-N concentrations were 32±8 and 14±0.1 µg/L respectively in September, but increased to 114±7 and 368±27 µg/L respectively by January.  Fall nitrate mass nearly doubled (7.6-13.1 mg/s) along the reach, whereas ammonium mass balance was conservative.  Chloride-nitrate co-injection indicated little surface water exchange with the bed due to high groundwater pressure gradients.  Ammonium-rich groundwater presumably served as the nitrification source.  In January upstream-downstream nitrate mass balance was conservative, whereas ammonium retention of 7 mg/s was observed.  The retention mechanism is unknown but is likely abiotic since nitrate mass was conservative and ammonium uptake was likely minimized by low temperature (1ºC).  At the few hyporheic sites where chloride penetrated the bed, co-injected nitrate was depleted.  Biotic cycling of nutrients in surface water was likely confined to the bed surface, however nutrients in upwelling groundwater were effectively processed within the reach