NITROGEN CYCLING IN POTOMAC RIVER TRANSITION-ZONE SHALLOW-WATER SEDIMENTS (AMMONIUM, ADSORPTION, ION EXCHANGE)

A modified diffusion-controlled sampler was used during a 3-year study to collect water samples from the water-column, at the interface between the water-column and sediment, and several tens of centimeters into the sediment. Speciation of nitrogen in the system showed nitrate to be the predominant dissolved species in the water-column. Organic nitrogen and adsorbed ammonium were the predominant nitrogen forms in the sediment. Organic nitrogen concentrations ranged from 0.007 to 0.01 weight percent (0.5 to 73.3 micromoles per gram). Adsorbed ammonium concentrations ranged from non-detectable to 3.7 micromoles per gram of sediment. Concentrations of adsorbed ammonium were one-to-three orders of magnitude more than the interstitial-water concentrations. Fluxes of nitrate were directed into the sediment from the water-column approximately 75 percent of the sampling periods and ranged from 0.02 to 0.80 millimole meter('-2) day('-1). Flux of nitrate from the sediment into the water-column, due apparently to nitrification in surficial sediment, occurred only during 2 summer sampling periods and never exceeded 0.5 millimole meter('-2) day('-1). Ammonium fluxes were less than 0.1 millimole meter('-2) day('-1) during 90 percent of the sampling periods. Of the ammonium fluxes which were greater than 0.05 millimole meter('-2) day('-1), all were into the sediment during sampling periods experiencing sediment resuspension, and all were into the water-column during periods of calm. Resuspension occurred during approximately one-half of the sampling periods. One resuspension event could mix more ammonium as interstitial-water and adsorbed ammonium with the water-column than could be delivered to the water-column by diffusive flux in 10 to 1000 days. The magnitude of the input from resuspension depended on the concentration of adsorbed ammonium in the sediment at the time of the event. Laboratory experiments suggest that up to 100 percent of adsorbed ammonium can desorb during resuspension. The sediment solids have a cation exchange capacity of 4.7 (+OR-) 0.9 milliequivalents per 100 grams. Ammonium adsorption appears to be related to the presence of cation-exchanging minerals and amorphous silica but not organic matter. Experimental evidence suggests that adsorption and desorption of ammonium involves more than simple ion exchange. Interstitial-water samples from 2 sampling periods were found to be in equilibrium with struvite, MgNH(,4)PO(,4).