NITROGEN DEPOSITION AND THE FOREST NITROGEN-CYCLE - ROLE OF DENITRIFICATION

The fate and effects of anthropogenic N deposition in forest ecosystems are discussed, and a conceptual hypothesis for the ecosystem response to elevated N deposition is presented. The concept of 'nitrogen saturation' is discussed and defined. The interactions of effects of excess N (nutrient deficiencies, loss of mycorrhiza, soil acidification, etc.) are described; these may destabilize the forest ecosystem and contribute to forest decline. Estimates of long-term critical N loads for forest ecosystems of 2-20 kg N ha-1 year-1, depending on biomass utilization are derived from simple mass balance considerations. Denitrification is supposed to be an insignificant process in the N-cycling of most forests. It may not alleviate the impact of deposition inputs, but it may be an important process in relation to atmospheric stability.