Baltimore Ecosystem Study Institute of Ecosystem Studies

2011 BES Annual Meeting Presentation and Poster Abstracts

Effects of stormwater management and stream engineering on nitrogen uptake and denitrification in streams
Newcomer, Tamara
Co-Authors: Tamara A. Newcomer, Sujay S. Kaushal, Paul M. Mayer, Peter M. Groffman, and Melissa M. Grese

Abstract: Managing the N cycle and restoring urban infrastructure represent major challenges for biogeochemistry and society. We investigated how stormwater management integrated into ecologically engineered stream networks may alter coupled N & C biogeochemical cycles. Stormwater management areas may serve as "hot-spots" for nitrate removal by denitrification because they may have ample organic carbon, low dissolved oxygen levels, and high residence time. We hypothesized that N and C loads will increase with discharge in a downstream direction, but that stormwater management areas may alter this pattern by decreasing N loads while increasing C loads. We examined impacts of stormwater management areas on coupled N and C cycles in 2 ecologically engineered stream networks with stormwater management areas and 1 forested reference watershed at the Baltimore Long-Term Ecological Research (LTER) site. We used a combination of: (1) stream network scale mass balances of N and C conducted monthly for 2 years and (2) 15N in situ push-pulls to measure N removal via denitrification. At Spring Branch (an ecologically engineered stream with inline stormwater management areas), mass balances showed total dissolved nitrogen (TDN) retention of 18.4 5.9% (mean SE, N=25) and dissolved organic carbon (DOC) retention of -21.8 7.9% in stormwater management areas. At Gwynns Run (an ecologically engineered stream with oxbow stormwater management areas), mass balances showed TDN retention of 48.7 6.6% and DOC retention of -21.8 15.7%. Concentration reduction in ecologically engineered streams appeared to be inversely dependent upon flow, thus the stormwater management areas may be less effective at reducing N pollution during high flows when there is less time for processing. In contrast, the forested reference site (stream with inline pond) showed little percent change in TDN due to consistently low concentrations along the stream network and DOC retention of -21.1 4.8%. Denitrification rates ranged from 0.6 to 1,135.7 g Nkg soil-1day-1 across all sites and seasons (N=48). Denitrification rates averaged across summer and winter 2008 increased from forested Pond Branch (74.9 19.9 g Nkg soil-1day-1) to suburban Spring Branch (92.1 25.2 g Nkg soil-1day-1) to urban Gwynns Run (167.5 74.5 g Nkg soil-1day-1). Surprisingly, denitrification rates measured in the stream reaches were higher than those measured in stormwater management areas and ponds. The highest and most variable rates were observed in urban Gywnns Run. In conclusion, we found that high rates of denitrification and N removal in the stormwater management areas, but their effectiveness may be flow dependent.