From missing source to missing sink: Long-term changes in the nitrogen budget of a northern hardwood forest

Ruth D. Yanai, Matthew A. Vadeboncoeur, Steven P. Hamburg, Mary A. Arthur, Colin B. Fuss, Peter M. Groffman, Thomas G. Siccama, Charles T. Driscoll

Research output: Contribution to journalArticlepeer-review

70 Scopus citations


Biogeochemical monitoring for 45 years at the Hubbard Brook Experimental Forest in New Hampshire has revealed multiple surprises, seeming contradictions, and unresolved questions in the long-term record of ecosystem nitrogen dynamics. From 1965 to 1977, more N was accumulating in living biomass than was deposited from the atmosphere; the "missing" N source was attributed to biological fixation. Since 1992, biomass accumulation has been negligible or even negative, and streamwater export of dissolved inorganic N has decreased from ∼4 to ∼1 kg of N ha-1 year-1, despite chronically elevated atmospheric N deposition (∼7 kg of N ha-1 year-1) and predictions of N saturation. Here we show that the ecosystem has shifted to a net N sink, either storing or denitrifying ∼8 kg of N ha-1 year-1. Repeated sampling over 25 years shows that the forest floor is not detectably accumulating N, but the C:N ratio is increasing. Mineral soil N has decreased nonsignificantly in recent decades, but the variability of these measurements prevents detection of a change of <700 kg of N ha-1. Whether the excess N is accumulating in the ecosystem or lost through denitrification will be difficult to determine, but the distinction has important implications for the local ecosystem and global climate.

Original languageEnglish
Pages (from-to)11440-11448
Number of pages9
JournalEnvironmental Science and Technology
Issue number20
StatePublished - Oct 15 2013

ASJC Scopus subject areas

  • General Chemistry
  • Environmental Chemistry


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