• Groffman, Peter
  Cary Institute of Ecosystem Studies
  
• Fernandez, Ivan
  University of Maine School of Forest Resources
  

Understanding the nitrogen (N) cycle at landscape and regional scales is a great challenge in environmental science. Uncertainties about the N cycle arise from large amounts of “missing N” that dominate N balances at all scales. This uncertainty has led to increased interest in N gas production as an important fate of N. Gas fluxes are perhaps the least well understood component of the N cycle due to problematic measurement techniques, high spatial and temporal variability, and a lack of methods for scaling point measurements to larger areas. We tested the hypothesis that denitrification, the anaerobic reduction of nitrate to nitrous oxide (N2O) and dinitrogen (N2) is a significant flux (equal to atmospheric deposition) in ecosystem, landscape and regional scale N budgets in northeastern North America. We measured N2O and N2 fluxes at three sites across northeastern North America that differ in N richness, soil and vegetation properties (Turkey Lakes (Ontario), Bear Brook (Maine), Hubbard Brook (New Hampshire)). We characterized soil O2 dynamics and N gas fluxes at each of these sites and used relationships between soil, vegetation, O2 and flux to produce preliminary estimates of the importance of denitrification at each site. Our results suggest that denitrification returns a significant portion of reactive atmospheric N deposition back to the atmosphere as unreactive N2. At Hubbard Brook and Bear Brook, denitrification accounts for approximately 40% of deposition. These ecosystem-scale estimates are also relevant at the landscape scale as there was little variation among sampling locations at these sites. At Turkey Lakes, rates at the ecosystem scale were lower and require accounting for the variation among wetland and upland locations that were sampled to make evaluations at the landscape scale. Still, none of the ecosystems at Turkey Lakes were producing significant amounts of N gas so the landscape-scale implications of these fluxes will be minor. These are preliminary estimates that will be refined and expanded to the regional scale with further analyses. N2 fluxes were much greater (10x) than N2O fluxes suggesting that N2 is the dominant end-product of denitrification in the region and that this process is returning reactive N to the atmosphere with a relatively low yield of radiatively active N2O.