Nitrate removal in two relict oxbow urban wetlands: a 15N mass-balance approach

A N-15-tracer method was used to quantify nitrogen (N) removal processes in two relict oxbow wetlands located adjacent to the Minebank Run restored stream reach in Baltimore County (Maryland, USA) during summer 2009 and early spring 2010. A mass-balance approach was used to directly determine the flow of (NO3)-N-15 (-) to plants, algae, and sediments, with unaccounted for N-15 assumed to be denitrified. During the summer, plant and algal uptake accounted for 42%, of the added (NO3)-N-15 (-) in oxbow 1, less than 1% remained in the water column and 57% was unaccounted for. In oxbow 2 during the summer, plant and algal uptake accounted for 63% of the added (NO3)-N-15 (-), with < 1% remaining in the water column and 38% unaccounted for. During the early spring, plant and algal uptake were much lower in both oxbows, ranging from 0.05 to 13.3% of the N-15 added, with 97 and 87% was unaccounted for in oxbow 1 and 2, respectively. The amount of unaccounted for N-15 was equivalent to estimated areal denitrification rates of 12 and 6 mg N m(-2) d(-1) in the summer and 78 and 15 mg N m(-2) d(-1) in the spring, in oxbow 1 and oxbow 2, respectively. However, the uncertainty of these estimates is high as it was difficult to detect accumulation of N-15 in the sediments which could have accounted for a very large percentage of the added N-15. Our results suggest that the two relict oxbow wetlands are sinks for NO3 (-) during both summer and spring but that the pathways of removal vary with plants and algae playing a major role in summer but not in spring.