Seasonal and storm dynamics of the hyporheic zone of a 4th-order mountain stream. II: Nitrogen cycling

The objective of this study was to quantify subsurface nitrogen fluxes between a riparianforest and a 4th-order mountain stream, McRae Creek, for each season of the year and during storms.A network of wells was installed on a gravel bar and a portion of the adjacent floodplain between1989 and 1992. Water samples were collected to monitor dissolved nitrogen concentrations. Advectedchannel water and ground water were enriched in nitrogen relative to the stream; thus, subsurfaceflow was a net source of nitrogen to the stream in all seasons of the year and during both base-flowperiods and storms. Estimates of the flux of advected channel water and the discharge of groundwater were combined with changes in mean nitrogen concentrations along subsurface flow paths toestimate nitrogen inputs to the stream. Discharge of ground water from the conifer-dominated flood-plain was the largest source of nitrogen added to the stream; however, more than 50% of this nitrogenwas dissolved organic nitrogen. In contrast, two-thirds of the nitrogen from the alder-dominatedgravel bar was inorganic. Net nitrogen fluxes from the gravel bar to the stream were lowest duringthe summer when water table elevations were low. Net fluxes of nitrogen from the gravel bar to thestream were largest during the fall, especially at peak flow during storms when interstitial water inthe gravel bar was enriched in NO,-. The estimated annual flux of nitrogen from the riparian forestto McRae Creek was 1.9 g/m2 of streambed, of which 1.0 g/m2 was inorganic. Estimated net annualflux was large relative to the estimated input of nitrogen in litterfall, or the nitrogen required tosupport estimated rates of primary productivity.
Key words: hyporheic zone, nutrient dynamics, nitrogen cycling, riparian forest, ground water,hydrologic exchange, hydrology.