Soil Denitrification Fluxes in a Northern Hardwood Forest: The Importance of Snowmelt and Implications for Ecosystem N Budgets

Nitrogen (N) is the nutrient that most frequently limits the productivity of forest ecosystems. Understanding N cycling and forest response to altered N inputs and climate change is an ongoing research challenge. In several intensively studied forests in northeastern North America, well-characterized N inputs are not balanced by measured N losses, suggesting that an unmeasured N loss pathway such as denitrification may be important. We studied soil denitrification gas fluxes in northern hardwood forests at the Hubbard Brook long-term ecological research site in New Hampshire, USA, and found that denitrification in apparently oxic soils could account for N losses greater than half of annual atmospheric N inputs. Denitrification rates were strongly affected by elevation and season, with higher rates occurring at high elevation plots and during snowmelt. These results suggest that denitrification accounts for a major portion of the increasing amounts of missing N reported for this site, and that a significant amount of the anthropogenic N that enters terrestrial ecosystems in northeastern North America is returned to the atmosphere as N-2. These dynamics are highly vulnerable to change, however, as soil moisture levels and conditions during snowmelt are changing rapidly along with climate.