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The use of short rotation forest tree species is increasing worldwide. The hybrid aspen (Populus tremuloides Michx. x P. tremula L.) is one of the suitable tree species under the climatic conditions of the Baltic region. The cultivation of these trees on former agricultural soils differs from agricultural practices with reduced soil tillage and is characterized by increased demand of nutrients, which in long term can cause changes in the soil microbial populations. The aim of our investigation was to compare soil microbial populations in hybrid aspen and European aspen (P. tremula L.) stands in four sampling plots with aspen age ranging from 10 to 46 years. The abundance and diversity of soil microbial populations were estimated by enumeration of microorganisms (plate counts on three microbiological media) and by molecular methods (PCR, ARDRA, molecular identification of fungal isolates). Results showed that during long cultivation periods hybrid aspens reduced the number of culturable bacteria. The number of culturable filamentous fungi was statistically significantly increased only in one sampling plot in soil samples from hybrid aspen clones at a depth of 16.30 cm and only on one microbiological cultivation medium. The same was detected also with molecular methods in the case of fungal diversity estimated by Shannon. Weaver diversity indices in this sampling plot. None of the other characteristics of soil microbial populations, such as the number of yeasts and maltose utilizing bacteria on MEA, the number of yeasts and filamentous fungi on RBA, the total amount of soil DNA, fungal and bacterial diversity estimated by molecular biology methods, and species composition of filamentous fungi, was significantly affected by hybrid aspen. The identified filamentous fungi represented the following genera: Acremonium, Exophiala, Geomyces, Gibellulopsis, Gibberella, Hypocrea/Trichoderma, Leptosphaeria, Metarhizium, Mortierella, Nectria, Paecilomyces, Penicillium, Trichosporon, and others. The main conclusion was that cultivation of hybrid aspen as a short rotation forest tree in the Baltic region would not significantly affect the abundance and diversity of saprophytic soil microorganisms. [ABSTRACT FROM AUTHOR]