THE EFFECTS OF EXPERIMENTAL NITROGEN ADDITION ON THE ECTOMYCORRHIZAL FUNGUS FLORA IN AN OLIGOTROPHIC SPRUCE FOREST AT GARDSJON, SWEDEN

Changes in the ectomycorrhizal fungus flora in connection with increased, ambient and decreased levels of nitrogen deposition were studied in transect plots at Gardsjon, Sweden, during 1990-1993 as part of the NITREX programme. From 1991, ammonium nitrate was added (about 35 kg N ha(-1) year(-1)) to catchment G2 NITREX, while at adjacent catchment G1 ROOF ambient N deposition was excluded. Both catchments are in mature, oligotrophic Norway spruce forest. After 1.5 years of treatment, the fruitbody production of mycorrhizal fungi at G2 NITREX was reduced to levels considerably below those of G1 ROOF and Fl CONTROL. The reduction was especially pronounced for the dominant genera Cortinarius and Russula, but their diversity was less affected. The genus Lactarius showed little change, and a few species such as Paxillus involutus and Lactarius rufus showed increased production, with year-to-year fluctuations. The species-rich genus Cortinarius emerges as an indicator group of macrofungi sensitive to enhanced N levels. The changes in fruitbody productivity appear to be one of the first biological responses to the N additions at the NITREX catchment at Gardsjon.