NITROGEN LEACHING FROM EUROPEAN FORESTS IN RELATION TO NITROGEN DEPOSITION

Forests in Europe currently receive inorganic nitrogen deposition (wet and dry) ranging from less than 1 to more than 75 kg N ha(-1) year(-1). A survey of nitrogen output from 65 forested plots and catchments throughout Europe indicated a three-stage response to these inputs. Below a deposition threshold of about 10 kg N ha(-1) year(-1) no significant nitrogen leaching occurred from the forests; at intermediate levels of 10-25 kg N ha(-1) year(-1) leaching occurred at some sites; above 25 kg ha(-1) year(-1) significant leaching occurred at all sites. NITREX sites followed this larger European pattern. Among 41 different variables tested in the survey, output-N (seepage or runoff) was most highly correlated with input-N (r(2)=0.69), followed by input-SO4, soil pH (negative correlation), percent slope, bedrock type and latitude (negative correlation). The latter three, however, were highly intercorrelated with input-N. A combination of input-N and soil pH explained 87% of the variability in output-N (N=20) and predicted output-N successfully from NITREX sites and other European test sites.