Our understanding of the N-cycle in urban streams is affected by how accurately we can define the hydrologic, geomorphic and biological characteristics of the stream channel and stream channel features that can affect N-cycling. In urban restored streams, the effectiveness of stream restoration techniques to re-establish hydro-geomorphic stability and potentially influence biogeochemical processes is unknown. The goal of this study was to extensively characterize microbial variables within relict and engineered wetlands that influence nitrogen retention, uptake, and removal. This study focused on a section of Minebank Run, a small urban stream in Baltimore County, MD, draining approximately 2.06 mi2.
Restored in 1998 and 2005 to re-establish stability of the stream channel, the restoration straightened a fairly sinuous channel leaving behind relict wetland features. In this study, we measured and compared denitrification potential and a suite of microbial variables (microbial biomass [C] and N content, potential N mineralization, nitrification and respiration) in relict wetland and pool-riffle features. The potential for nitrate removal in these features is particularly important; where water is slow moving they can function as “hotspots” for nitrogen removal by denitrification, an anaerobic microbial process that converts NO3- into N2.
We measured denitrification potential (DEA) using the acetylene inhibition technique. Microbial biomass C and N content were measured with the chloroform fumigation–incubation method. In addition, inorganic N and CO2 production were measured to determine soil respiration and potential net N mineralization and nitrification.
Results/Conclusions
Preliminary results show sediment denitrification rates were high in the wetland features, ranging from 2.5 to 6.5 ug N g soil – 1 hr – 1, compared with pool and riffles features in the stream channel ranging from 0.068 to 2.9 ug N g soil – 1 hr – 1. Similar to DEA, microbial biomass C and N were also high within the wetland features; ranging from 104 to 6643 ug C g soil and 56 to 457 ug N g dry soil– 1 respectively. Wetland sediments were high in moisture content and microbial respiration and low in mineralization and nitrification rates compared with pool-riffle features.
Given the importance of nitrogen removal in urban watersheds, the high denitrification potential, and the tight cycling of N in the wetlands that we observed; this suggest that if a portion of the stream channel flow can be routed through these areas, these sites could function as important sinks for NO3- in urban restored streams.
