Riparian wetlands have been shown to be eff ective "sinks" for nitrate N (NO 3 − ), minimizing the downstream export of N to streams and coastal water bodies. However, the vast majority of riparian denitrifi cation research has been in agricultural and forested watersheds, with relatively little work on riparian wetland function in urban watersheds. We investigated the variation and magnitude of denitrifi cation in three constructed and two relict oxbow urban wetlands, and in two forested reference wetlands in the Baltimore metropolitan area. Denitrifi cation rates in wetland sediments were measured with a 15 N-enriched NO 3 − "push-pull" groundwater tracer method during the summer and winter of 2008. Mean denitrifi cation rates did not diff er among the wetland types and ranged from 147 ± 29 μg N kg soil −1 d −1 in constructed stormwater wetlands to 100 ± 11 μg N kg soil −1 d −1 in relict oxbows to 106 ± 32 μg N kg soil −1 d −1 in forested reference wetlands. High denitrifi cation rates were observed in both summer and winter, suggesting that these wetlands are sinks for NO 3 − year round. Comparison of denitrifi cation rates with NO 3 − standing stocks in the wetland water column and stream NO 3 − loads indicated that mass removal of NO 3 − in urban wetland sediments by denitrifi cation could be substantial. Our results suggest that urban wetlands have the potential to reduce NO 3 − in urban landscapes and should be considered as a means to manage N in urban watersheds.

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Although wetlands are known to be sinks for nitrogen (N) and phosphorus (P), their function in urban watersheds remains unclear. We analyzed water and nitrate (NO3−) and phosphate (PO43−) dynamics during precipitation events in two oxbow wetlands that were created during geomorphic stream restoration in Baltimore County, Maryland that varied in the nature and extent of connectivity to the adjacent stream. Oxbow 1 (Ox1) received 1.6‐4.2% and Oxbow 2 (Ox2) received 4.2‐7.4% of cumulative streamflow during storm events from subsurface seepage (Ox1) and surface flow (Ox2). The retention time of incoming stormwater ranged from 0.2 to 6.7 days in Ox1 and 1.8 to 4.3 days in Ox2. Retention rates in the wetlands ranged from 0.25 to 2.74 g N/m2/day in Ox1 and 0.29 to 1.94 g N/m2/day in Ox2. Percent retention of the NO3−‐N load that entered the wetlands during the storm events ranged from 64 to 87% and 23 to 26%, in Ox1 and Ox2, respectively. During all four storm events, Ox1 and Ox2 were a small net source of dissolved PO43− to the adjacent stream (i.e., more P exited than entered the wetland), releasing P at a rate of 0.23‐20.83 mg P/m2/day and 3.43‐24.84 mg P/m2/day, respectively. N and P removal efficiency of the oxbows were regulated by hydrologic connectivity, hydraulic loading, and retention time. Incidental oxbow wetlands have potential to receive urban stream and storm flow and to be significant N sinks, but they may be sources of P in urban watersheds.