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

Abstract

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 N2. These dynamics are highly vulnerable to change, however, as soil moisture levels and conditions during snowmelt are changing rapidly along with climate.