From missing source to missing sink: long-term changes in the nitrogen budget of a northern hardwood forest.
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.
College of Environmental Science and Forestry, State University of New York , Syracuse, New York 13210, United States.
Ruth D
RD
Yanai
Matthew A
MA
Vadeboncoeur
Steven P
SP
Hamburg
Mary A
MA
Arthur
Colin B
CB
Fuss
Peter M
PM
Groffman
Thomas G
TG
Siccama
Charles T
CT
Driscoll
D001272:Atmosphere / Q000737:chemistry; D018533:Biomass; D002244:Carbon / Q000378:metabolism; D017753:Ecosystem; D009513:New Hampshire; D009584:Nitrogen / Q000378:metabolism*; D058458:Nitrogen Cycle; D045483:Rivers / Q000737:chemistry; D013997:Time Factors; D014197:Trees / Q000378:metabolism*; D014934:Wood / Q000378:metabolism*
D002244:Carbon; D009584:Nitrogen