Effects of nitrogen and phosphorus fertilization on mycorrhizal formation of two poplar clones (Populus trichocarpa and P-tremula x tremuloides)

Mineral fertilization is a common management practice in short rotation for estry. The mycorrhizal formation of trees can be affected by fertilizer app lications, however, very little is known on such effects in arable soils. T he effects of a nitrogen (N) and phosphorus (P) fertilization on mycorrhiza l formation of two poplar clones (Populus trichocarpa and P. tremula x trem uloides) were investigated at the plantation Abbachhof (South Germany). We determined the ectomycorrhizal colonization and the abundance of VAM spores in the soil during three years, and the species richness of sporocarps dur ing one growing season. Approximately 26 to 73% of the fine roots of P. tri chocarpa and 41 to 82% of the fine roots of P. tremula x tremuloides were c olonized with ectomycorrhizal fungi. The percentage of ectomycorrizal colon ization on P. tremula x tremuloides was significantly reduced after both fe rtilization treatments. On P. trichocarpa only the P-fertilization reduced the ectomycorrhizal colonization. The composition of ectomycorrhizal morpho types was significantly affected by the N and P fertilization on P. tremula x tremuloides, but not on P. trichocarpa. Sporocarps of 12 ectomycorrhizal fungi species were found at the plantation. Cortinarius uliginosus, Lactar ius controversus and Krombholziella aurantiaca produced sporocarps only on control plots, whereas Cortinarius croceocaeruleus, Inocybe umbrina, Laccar ia tortilis, Paxillus involutus and Rhizopogon roseolus produced sporocarps only on fertilized plots. Inocybe geophylla, I. glabripes, Laccaria laccat a and Tuber borchii produced sporocarps on both control and fertilized plot s. The density of VAM spores was lower in the rooting zone of Populus trich ocarpa than under P. tremula x tremuloides. In an efficient management of t hese short rotation plantations mineral fertilizer applications must be low enough to avoid undesired suppressions of mycorrhizal formation.