Critical loads for nitrogen deposition: Case studies at two northern hardwood forests

A critical load of a pollutant is the level of input below which no harmful ecological effects occur to a complex ecosystem. Critical loads are being used in policy decisions regarding air pollution emissions. In this paper, we applied four mass and charge balance methods of calculating critical loads to two northern hardwood forests in the northeastern United States. Critical loads for nitrogen deposition with respect to acidity ranged from 0-630 eq/ha-yr. Critical loads for nitrogen deposition with respect to effects of elevated nitrogen (eutrophication and nutrient imbalances) ranged from 0-1450 eq/ha-yr. At both the Hubbard Brook Experimental Forest (HBEF) and Huntington Wildlife Forest (HWF), the critical load for nitrogen with respect to acidity was exceeded. At the HBEF, due to reduced forest growth, the critical load for nitrogen with respect to nutrient imbalances and eutrophication was exceeded in recent years. At Huntington Wildlife Forest, the critical load with respect to nitrogen effects was also exceeded. This analysis demonstrated that the calculated critical load of nitrogen varies in response to changes in environmental conditions such as variations in atmospheric deposition of sulfate or changes in forest biomass accumulation.