Denitrification Hotspots and N20 Flux in Fluvial Systems

a)	Background/Question/Methods
Watersheds have a profound capacity to retain and remove nitrogen (N) inputs before entering coastal waters thereby minimizing eutrophication and hypoxia in estuarine waters. Research suggests that much of this N cycling occurs within aquatic ecosystems, e.g., lakes, streams and wetlands, which may become “hotspots” of nitrous oxide (N2O) production. We examined the effects of woody debris on nitrate removal and denitrification rates in streams of different N enrichment levels. Three different standardized substrates (fresh wood blocks, bundles of naturally occurring streambed wood, and clay‐fired blocks) that were expected to vary in their pool of labile carbon were incubated for 8‐10 weeks in each stream - a high nitrate agricultural stream in Pennsylvania and a low nitrate forested stream in Rhode Island. Substrates were collected and subjected to a series of lab‐based mesocosm assays in stream water amended with 15N-labeled nitrate-N. Nitrate removal rates were determined by mass balance techniques and denitrification rates were estimated by 15N gas generation over 18 hours. The 15N2:15N2O ratios were calculated and the mass of biofilm growth during in-situ incubation was obtained for all substrates.

b)	Results/Conclusions
Wood block mesocosms had mean N removal and denitrification potential rates of 0.14 and 0.038 ug N/cm2 hr, respectively, for RI and 0.48 and 0.011 ug N/cm2 hr, respectively, for PA. Natural wood mesocosms from RI had 76% higher N removal (0.039 ug N/cm2 hr) than denitrification. The PA natural wood mesocosms had mean N removal (0.169 ug N/cm2 hr) an order of magnitude above the mean denitrification potential. Clay block mesocosms had negligible N removal and denitrification in RI. PA clay mesocosms had N removal rates of 0.2 ug N/cm2 hr and denitrification potential rates of 0.01 ug N/cm2 hr, likely due to the biofilms that had developed while incubating in the high nitrate stream. These results suggest that fresh inputs of natural woody debris may be an important nitrate sink in streams. High variability was observed and there were numerous cases of negligible N2O production at both sites. As a result, the differences in N removal and denitrification potential rates between sites were not significant. The mesocosms from the high nitrate PA stream generally released more N2O. The wood substrates in these mesocosms acted as significant N sinks, but are not necessarily hotspots for N2O generation. 
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