NITROGEN CRITICAL LOADS FOR NATURAL AND SEMINATURAL ECOSYSTEMS - THE EMPIRICAL-APPROACH

One of the major threats to the structure and the functioning of natur al and semi-natural ecosystems is the recent increase in air-borne nit rogen pollution (NHy and NOx). Ecological effects of increased N suppl y are reviewed with respect to changes in vegetation and fauna in terr estrial and aquatic natural and semi-natural ecosystems. Observed and validated changes using data of field surveys, experimental studies or , of dynamic ecosystem models (the 'empirical approach'), are used as an indication for the impacts of N deposition. Based upon these data N critical loads are set with an indication of the reliability. Critica l loads are given within a range per ecosystem, because of spatial dif ferences in ecosystems. The following groups of ecosystems have been t reated: softwater lakes, wetlands & bogs, species-rich grasslands, hea thlands and forests. In this paper the effects of N deposition on soft water lakes have been discussed in detail and a summary of the N criti cal loads for all groups of ecosystems is presented. The nitrogen crit ical load for the most sensitive ecosystems (softwater lakes, ombrotro phic bogs) is between 5-10 kg N ha(-1) yr(-1), whereas a more avenge v alue for the range of studied ecosystems is 15-20 kg N ha(-1) yr(-1). Finally, major gaps in knowledge with respect to N critical loads are identified.