COS 55-2 - Denitrification, riparian heterogeneity and nitrogen fluxes in a forested watershed

Wednesday, August 10, 2011: 8:20 AM
6A, Austin Convention Center
Peter M. Groffman, Cary Institute of Ecosystem Studies, Millbrook, NY, Jonathan M. Duncan, Department of Geography, University of North Carolina, Chapel Hill, NC and Lawrence E. Band, Institute for the Environment, University of North Carolina, Chapel Hill, NC

Understanding the nitrogen (N) cycle at landscape, regional and global scales is a great current challenge in environmental science.  Excess “reactive” N has caused degradation of air and water quality and coastal ecosystems in many areas.  The development of solutions to N pollution problems has been hindered by large amounts of “missing N” that dominate N balances at all scales.  Uncertainty about N balances has led to increased interest in N gas production as a fate of N.  However, gas fluxes are difficult to quantify because of problematic measurement techniques (especially for N2), high spatial and temporal variability, and a lack of methods for scaling point measurements to larger areas.  A particular challenge is that small areas (hotspots) and brief periods (hot moments) frequently account for a high percentage of N gas flux activity. Here, we have applied a new soil core gas flow method to quantify N2 and N2O fluxes and relationships between flux and soil O2 levels, along with in situ soil O2 sensors in a forested watershed at the Baltimore urban LTER site.  Long-term ecohydrology research in this watershed has focused on upland – riparian interactions, and on variation between hummocks and hollows within the riparian zone.


Flux of N2 was consistently highest in the wet riparian “hollows” relative to other landscape positions in the watershed; dry riparian “hummocks,” upland ephemeral drainage channels or uplands.  These results support the idea that wet riparian zones are “hotspots” of denitrification in watersheds.  Denitrification rates were significantly higher at 5% and 10% O2 than under strictly anaerobic conditions.  These results suggest that denitrification in the hollows is strongly coupled to nitrate supply by mineralization and nitrification (an aerobic process).  Potential net N mineralization was negative in the hollows and positive in other sites, supporting this idea.  N2O fluxes were much lower than N2 fluxes (3.5% of total N gas flux at upland sites, but less than 0.1% at all other sites) and were highest from upland sites and at higher O2 levels, suggesting that nitrification was the dominant source of N2O in this landscape.  These results suggest that focusing on topographic heterogeneity is a robust approach to measuring and scaling N gas fluxes.  Strong relationships between flux and soil Osuggest that continuous measurements of soil O2 will be a powerful tool for temporal scaling of measured rates.

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