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.

Results/Conclusions

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.