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

A ¹⁵N-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 ¹⁵NO₃ ^? to plants, algae, and sediments, with unaccounted for ¹⁵N assumed to be denitrified. During the summer, plant and algal uptake accounted for 42%, of the added ¹⁵NO₃ ^? 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 ¹⁵NO₃ ^?, 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 ¹⁵N added, with 97 and 87% was unaccounted for in oxbow 1 and 2, respectively. The amount of unaccounted for ¹⁵N 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 ¹⁵N in the sediments which could have accounted for a very large percentage of the added ¹⁵N. 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.