Nitrogen Uptake and Denitrification in Restored and Degraded-Urban Streams: Impacts of Organic Carbon and Integrated Stormwater Management
Newcomer Johnson, Tamara Ann
Kaushal, Sujay S.
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Managing the N cycle and restoring urban infrastructure are major challenges especially in urban ecosystems. Organic carbon is important in regulating ecosystem function and its source and abundance may be altered by urbanization. My research focused on urban-degraded, restored, and forested watersheds at the Baltimore LTER in the Chesapeake Bay watershed. In Chapter 2, I investigated shifts in organic carbon quantity and quality associated with urbanization and ecosystem restoration, and its potential effects on denitrification at the riparian-stream interface. Denitrification enzyme assay experiments showed carbon was limiting in hyporheic sediments and variable carbon sources (grass clippings, decomposing leaves, and periphyton) stimulated denitrification differently. Evidence from stable isotopes, molar C:N ratios, and lipid biomarkers suggested that urbanization can influence organic carbon sources and quality in streams, which may have substantial downstream impacts on ecosystem services such as denitrification. In Chapter 3, I investigated whether stormwater best management practices (BMPs) integrated into restored and degraded urban stream networks can influence watershed N loads. I hypothesized that hydrologically connected floodplains and stormwater BMPs are “hot spots” for N retention through denitrification because they have ample organic carbon, low dissolved oxygen levels, and high residence time. I used reach-scale nitrogen mass balances, in-stream tracer injection studies, and 15N in situ denitrification to measure N retention in stormwater BMPs and their larger stream networks. There were high rates of in situ denitrification in both stormwater BMPs and floodplain features. Hydrologically connected floodplains can be important “hot spots” for N retention at a watershed and stream network scale because these areas likely receive perennial flow through the groundwater-surface water interface during both baseflow and storm events, while BMPs only receive intermittent flow associated with storm events. In Chapter 4, I conducted a literature review of N retention within hydrologically reconnected streams and floodplains. I reviewed 79 stream and floodplain restoration empirical studies from North America, Europe, and Asia and found that methods for measuring N retention varied considerably. I found many diverse strategies for promoting the ecosystem function of N retention in urban and agricultural watersheds.