COS 37-1 - From missing source to missing sink: Long-term nitrogen dynamics in the northern hardwood forest

Tuesday, August 4, 2009: 1:30 PM
La Cienega, Albuquerque Convention Center
Ruth D. Yanai, Forest and Natural Resources Management, SUNY College of Environmental Science and Forestry, Syracuse, NY, Steven P. Hamburg, Environmental Defense Fund, New York, NY, Mary A. Arthur, Department of Forestry, University of Kentucky, Lexington, KY, Matthew A. Vadeboncoeur, Earth Systems Research Center, University of New Hampshire, Durham, NH, Colin B. Fuss, Civil and Environmental Engineering, Syracuse University, Syracuse, NY and Thomas G. Siccama, School of Forestry and Environmental Studies, Yale, New Haven, CT

Atmospheric nitrogen deposition in the northeastern United States has been highly elevated over pre-industrial levels over the last century. These chronic N inputs were predicted to lead eventually to N saturation, in which forest ecosystems can no longer sequester the added N, with consequent losses of dissolved N to aquatic ecosystems and reduced forest productivity. At the Hubbard Brook Experimental Forest in New Hampshire, ecosystem nutrient cycling has been monitored for over forty years in a watershed dominated by second-growth northern hardwoods originating after logging and released by the 1938 hurricane.


For the period from 1965 to 1977, N was accumulating in vegetation and the forest floor at rates (19 kg/ha/yr) exceeding the difference between atmospheric N deposition and streamflow export of dissolved inorganic N (4 kg/ha/yr).  The missing source was attributed to N fixation.  More recently, N accumulation in vegetation and the forest floor has slowed to near zero; surprisingly, streamwater export of N has fallen from 5.4 kg/ha/yr in the early period to about 2 kg/ha/yr in recent years. With atmospheric N deposition still about 10 kg/ha/yr, this means that the ecosystem is now a significant sink for N. Repeated sampling of the forest floor shows that N is not accumulating there; in fact, the C:N ratio is increasing (P = 0.05). Repeated sampling of forest floors in a chronosequence of stands in the White Mountain region confirms that the forest floor is not the missing sink for N. The mineral soil is a large and poorly quantified N pool that could account for some part of the missing sink. Denitrification losses of unknown magnitude may also contribute to the lack of expected increase in stream N concentrations throughout the region. Predicting the long-term capacity of the forested landscape to process chronic atmospheric N deposition without exporting N to aquatic systems depends on a better understanding of the controls on N sinks and sources.

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