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Nitrogen Retention and Release in Atlantic White Cedar Wetlands

Louis Calder Center, Fordham Univ., Box K, Armonk, NY 10504;

Joan G. Ehrenfeld

Dep. Ecology, Evolution and Natural Resources, Cook College, Rutgers Univ., New Brunswick, NJ 08901.

^* Corresponding author (weixing.zhu{at}asu.edu).

ABSTRACT

The fate of N inputs to freshwater wetlands from enriched runoff, ground water, or atmospheric deposition reflects a variety of processes that determine the net retention of N or its release to downstream ecosystems. The history of N enrichment may affect the relative importance of different pathways. We compared the ability of sediments from Atlantic cedar wetlands in suburban and undisturbed watersheds to remove added inorganic N in laboratory incubations. Three pulses of NH₄NO₃, at 50 mg N L^–1 concentration, were added at consecutive 2-wk intervals as an analog of pulsed stormwater inputs. After each incubation, soils were leached with CaCl₂ and inorganic N concentrations in the leachate were analyzed. The peats retained added N during the first incubation, but after the second and third, all soils released more N than had been added. Nitrate concentrations were always lower than ammonium concentrations. Soils from hollows and most hummocks retained added nitrate throughout the study, but all soils released ammonium. Soil extractable ammonium increased four to 10 times after the 6-wk incubation, while extractable dissolved organic N (DON) decreased. There also was a net decrease of soil total Kjeldahl N (TKN). The patterns of retention and release of nitrate and ammonium were correlated positively with the N mineralization and nitrification rates of the soils. Our results suggest that wetland peats in suburban drainages may have limited ability to retain frequent, pulsed N inputs from runoff and high intrinsic N mineralization in N-saturated sediments can become a cause of water quality degradation.

W.-X. Zhu current address: Center for Environmental Studies, PO Box 873211, Arizona State Univ., Tempe, AZ 85287.

Received for publication February 22, 1999.