PREDICTING THE EFFECTS OF ATMOSPHERIC NITROGEN DEPOSITION IN CONIFER STANDS - EVIDENCE FROM THE NITREX ECOSYSTEM-SCALE EXPERIMENTS

The NITREX project, which encompasses seven ecosystem-scale experiment s in coniferous forests at the plot or catchment level in northwestern Europe, investigates the effect of atmospheric nitrogen (N) depositio n in coniferous forests. The common factor in all of the experiments i s the experimentally controlled change in N input over a period of 4-5 years. Results indicate that the status and dynamics of the forest fl oor are key components in determining the response of forests to alter ed N inputs. An empirical relationship between the carbon-nitrogen (CI N) ratio of the forest floor and retention of incoming N provides a si mply measured tool through which the likely timing and consequences of changes in atmospheric N deposition for fresh waters may be predicted . In the terrestrial ecosystem, a 50% increase in tree growth is obser ved following the experimental reduction of N and sulfur inputs in a h ighly N-saturated site, illustrating the damaging effects of acidifyin g pollutants to tree health in some locations. Few biotic responses to the experimental treatments were observed in other NITREX sites, but the rapid response of water quality to changes in N deposition, and th e link to acidification in sensitive areas, highlight the need for N-e mission controls, irrespective of the long-term effects on tree health . The observed changes in ecosystem function in response to the experi mental treatments have been considered within the framework of the cur rent critical-load approach and thus contribute to the formulation of environmental policy.