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.
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We thank Don Buso, Tammy Wooster, Lisa Martel, Kate Shepard, Erica Morgan, Robin Schmidt, and James Burtis for excellent field, laboratory, and data analysis work. We thank Klaus Butterbach-Bahl and two anonymous reviewers, whose comments greatly improved this manuscript. This research was supported by the National Science Foundation (DEB #0949664 and DEB #0919047). J. D. was supported by a Fulbright fellowship of the Spanish Ministry of Education and by a FCT Research Fellowship of the Portuguese Ministry of Education and Science (SFRH/BDP/87966/2012). This research was conducted at the Hubbard Brook Experimental Forest, which is operated by the Northeastern Research Station, USDA Forest Service, Newtown Square, PA. This paper is a contribution to the Hubbard Brook Ecosystem Study.
JLM designed the study, performed research, analyzed data, and wrote the paper. JD designed the study, performed research, wrote the paper. PMG conceived of and designed the study, contributed methods, and wrote the paper.
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Morse, J.L., Durán, J. & Groffman, P.M. Soil Denitrification Fluxes in a Northern Hardwood Forest: The Importance of Snowmelt and Implications for Ecosystem N Budgets. Ecosystems 18, 520–532 (2015). https://doi.org/10.1007/s10021-015-9844-2
- forest soil
- nitrogen saturation
- soil respiration
- nitrous oxide
- elevation gradient