AMMONIUM SORPTION TO CHANNEL AND RIPARIAN SEDIMENTS - A TRANSIENT STORAGE POOL FOR DISSOLVED INORGANIC NITROGEN

Sediment (0.5 mm-2.0 mm grain size) was incubated in nylon bags (200 m u m mesh) below the water table in the channel and in two transects of shallow wells perpendicular to the banks (to 18 m) of a third-order s tream during August, 1987. One transect of wells drained steep old-gro wth forest, and the other a steep 23 year-old clear-cut partially rege nerated in alder. At approximately 6-week intervals between October, 1 987, and June, 1988, bags were retrieved. Total exchangeable ammonium was determined on sediment, and dissolved oxygen, nitrate and ammonium were determined in stream and well water. Exchangeable ammonium range d from 10 mu eq/100 g of sediment in the stream where nitrification po tential and subsurface exchange with stream water were high, to 115 mu eq/100 g sediment 18 m inland where channel water-groundwater mixing and nitrification potential were both low. Sorbed ammonium was highest during summer/autumn base flow and lowest during winter storm flow. B oth channel and well water contained measurable dissolved oxygen at al l times. Ammonium concentration was typically < 10 mu g-N/L in channel water, increased with distance inland, but did not exceed 365 mu g-N/ L at any site. Nitrate concentration was typically higher in well wate r than channel water. Nitrate levels increased dramatically in wells a t the base of the clear-cut following the onset of autumn rains. The r esults indicate a potential for temporary storage of ammonium on ripar ian sediments which may influence biotic nitrogen cycling, and alter t he timing and form of dissolved inorganic nitrogen transport from the watershed.