Substrate acidity, nutritional status and growth of young Norway spruce (Picea abies [L.] Karst.) trees in subalpine regions of the Bavarian Forest (Bavaria, Germany) as influenced by repeated NH4NO3 application - results ofa container experiment

Clonal Norway spruce seedlings were cultivated at 1300 and 1150 m a.s.l. in the Inner Bavarian Forest in large (40 L) containers filled with very acid subsoil material low in humus and highly saturated with aluminum, originat ing from a dystric cambisol derived from granite. They were fertilized for 4 years with 3 doses of NH4NO3 (N+, N++, N+++). The control treatment was a combination of NH4NO3 and MgSO4(N+Mg+). Repeated fertilization with N alone decreased the exchangeable Mg concentra tions and the Mg saturation of the soil as well as the Mg levels in needles and roots and induced typical symptoms of Mg deficiency, whereas the N+Mg treatment showed balanced nutrition. The biomass production of needles, sh oot axes and root fractions, however, was not affected by the treatments. O bviously the lowest N dose (during the period of fertilization) was suffici ent to support optimum growth under the harsh climatic conditions at high e levation. Even in the two treatments with very high application of N and co nsiderable activity of Al in the soil no morphological damage to the fine a nd finest roots could be detected. With regard to the causes of the "Norway spruce disease on acid soils at hi gh altitudes" these experimental results lead to the conclusion that Al tox icity in the strict sense can be ruled out. The same holds true for the gro wth acceleration under discussion as induced by improved N nutrition, along with Mg deficiency (dilution effect). The decisive predisposing factor of this disease type is the natural scarcity of Mg (Ca) in the referring soils , with may be aggravated by both the atmospheric input of acids and increas ed Mg export via biomass harvest.