Nitrogen Transformations in Ponds Receiving Polluted Water from Nonpoint Sources

A laboratory study was conducted to determine the role of N transformations in ponds receiving inorganic N‐rich effluents from septic fields, agricultural, and pasture watersheds. Undisturbed sediment columns were obtained from three ponds. Floodwater in the columns was enriched with either 15NH+4‐N or 15NO−3‐N. Ammonium removal rates ranged from 55 to 85 mg N m−2 d−1, while NO−3‐N removal rates ranged from 48 to 71 mg N m−2 d−1. Twenty‐three to 49% of floodwater 15NH+4‐N diffused into the sediment during a 22‐d incubation period and was recovered in 15NH+4‐N (6–12% of the floodwater 15NH+4‐N) and organic 15N fractions (14–37% of the floodwater 15NH+4‐N). When 15NO−3‐N was added to the floodwater, about 7% was tied up in the sediment. Ponds receiving effluents from septic tanks and pastured areas retained less floodwater N in the sediment compared to sediments of the pond receiving runoff from agricultural watershed.