EFFECTS OF CARBON AND NITRATE ON DENITRIFICATION IN BOTTOM SEDIMENTS OF AN EFFLUENT-DOMINATED RIVER

Nitrogen and carbon limitation of denitrification in the bed sediments of an effluent-dominated stream were investigated by quantifying the effects of nitrate and glucose additions on the rate of sediment N2O production. Bed sediment samples were collected from a 30-km stretch of the South Platte River where up to 95% of the base flow discharge consists of effluent from a water treatment plant in Denver, Colorado. The rate of denitrification in upstream sediment samples incubated under in situ nitrate and carbon conditions was primarily limited by nitrate supply. The stimulatory effect of nitrate additions on the rate of bed sediment denitrification decreased with increasing distance downstream of the treatment plant. Approximately 35 km downstream of the treatment plant, denitrification in the bed sediment samples was carbon limited. The observed decreases in the concentration of total inorganic nitrogen (as NH4 + NO3) dissolved in the river and the organic carbon content of the bed sediments with increasing distance downstream of the treatment plant suggest that bed sediment denitrification is a significant sink for nitrogen in this stretch of the river.